Cigarette Smoke Modulates Inflammation and Immunity via Reactive Oxygen Species-Regulated Trained Immunity and Trained Tolerance Mechanisms

Abstract

Significance:

Cigarette smoke (CS) is a prominent cause of morbidity and death and poses a serious challenge to the current health care system worldwide. Its multifaceted roles have led to cardiovascular, respiratory, immunological, and neoplastic diseases.

Recent Advances:

CS influences both innate and adaptive immunity and regulates immune responses by exacerbating pathogenic immunological responses and/or suppressing defense immunity. There is substantial evidence pointing toward a critical role of CS in vascular immunopathology, but a comprehensive and up-to-date review is lacking.

Critical Issues:

This review aims to synthesize novel conceptual advances on the immunomodulatory action of CS with a focus on the cardiovascular system from the following perspectives: (i) the signaling of danger-associated molecular pattern (DAMP) receptors contributes to CS modulation of inflammation and immunity; (ii) CS reprograms immunometabolism and trained immunity-related metabolic pathways in innate immune cells and T cells, which can be sensed by the cytoplasmic (cytosolic and non-nuclear organelles) reactive oxygen species (ROS) system in vascular cells; (iii) how nuclear ROS drive CS-promoted DNA damage and cell death pathways, thereby amplifying inflammation and immune responses; and (iv) CS induces endothelial cell (EC) dysfunction and vascular inflammation to promote cardiovascular diseases (CVDs).

Future Directions:

Despite significant progress in understanding the cellular and molecular mechanisms linking CS to immunity, further investigations are warranted to elucidate novel mechanisms responsible for CS-mediated immunopathology of CVDs; in particular, the research in redox regulation of immune functions of ECs and their fate affected by CS is still in its infancy.

Introduction

Cigarette smoke (CS) accounts for 480,000 adult death annually (1/5 all the deaths/year; 1300 deaths/day) (Warren et al, 2014) and 41,000 deaths from secondhand smoke exposure in the United States (Jamal et al, 2016). Annual economic costs of diseases associated with CS exceed $289 billion (Kohut, 2017). CS contributes significantly to the pathogenesis of several types of major immune-inflammatory diseases including coronary heart disease (CHD) (Centers for Disease Control (CDC), 1989), lung cancers, chronic obstructive pulmonary disease (COPD) (Ishii, 2013), end-stage renal disease (Choi et al, 2019), multiple sclerosis (Alrouji et al, 2019), stroke, and oral inflammatory disease (Table 1).

Table 1.

Cigarette Smoke Promotes Several Types of Major Immunoinflammatory Diseases

Diseases	Effect of cigarette smoke	PMID
CVD	Increased neutrophils, lymphocytes, and monocytes Increased inflammatory cytokines and chemokines such as IL-1β and TNF-α Induced endothelial vascular dysfunction and inflammation Increased oxidative stress and ROS production Increased total serum cholesterol, VLDL, LDL, and triglyceride concentrations Increased risk of thrombosis, atherosclerosis, stroke, and CHD Increased cardiovascular morbidity and mortality	17485580 21285293 15145091 26174518
Respiratory diseases	Increased production of immune mediators and ROS by lung macrophages Increased incidence of COPD and airway inflammation Increased risk of interstitial lung diseases, bronchial asthma, and lung cancer	22196881 24507834 23631228
CKD	Increased production of proinflammatory mediators Increased EC dysfunction Increased oxidative stress and ROS production Increased cardiovascular morbidity and mortality in CKD patients Increased risk of incident ESRD	18003763 28339863 22158113 31862942
RA	Increased risk for development and severity of RA Increased rheumatoid nodule formation with multiple joint involvements	24594022 25479074
MS	Increased incidence and severity of the disease	32905534
SLE	Increased inflammatory markers associated with decreased anti-inflammatory IL-10 Increased oxidative stress Elevated titers of anti-dsDNA Increased risk of developing SLE	29724134
IBD	Impaired function of inflammatory cells Increased recruitment of CD4+ and CD8+ T cells, and of CD11b+ DCs Increased proinflammatory chemokines/cytokines (CCR6, CCL20, IL-8) increased in the expression of MHC-II and costimulatory molecules Increased incidence of Crohn's disease Protected against UC by decreased inflammatory mediators and cytokine and decreased gut permeability by nicotine	20333390 21112082 24691114 21537330 32823518
Oral inflammatory diseases	Impaired function of inflammatory cells Increased inflammatory cytokines Promoted periodontal diseases	20361572

CS increases ROS production, inflammatory cytokine secretion, inflammatory and immune cells, and impairment of inflammatory cells' function. Complex roles of CS have resulted in several diseases, including CVDs, respiratory diseases, CKD, rheumatoid arthritis, multiple sclerosis, systemic lupus erythematosus, and inflammatory bowel disease.

anti-dsDNA, anti-double-stranded DNA; CCL20, chemokine (C-C motif) ligand 20; CCR6, C-C motif chemokine receptor 6; CHD, coronary heart disease; CKD, chronic kidney disease; COPD, chronic obstructive pulmonary disease; CS, cigarrette smoke; CVD, cardiovascular diseases; DC, dendritic cell; EC, endothelial cell; ESRD, end-stage kidney disease; IBD, inflammatory bowel disease; IL, interleukin; LDL, low-density lipoprotein; MHC-II, MHC class II molecules; MS, multiple sclerosis; RA, rheumatoid arthritis; ROS, reactive oxygen species; SLE, systemic lupus erythematosus; TNF-α, tumor necrosis factor-α; UC, ulcerative colitis; VLDL, very-low-density lipoprotein.

The mechanisms responsible for these effects include increased reactive oxygen species (ROS), membrane receptor (clusters of differentiation)-mediated cell–cell interaction (Horvathova et al, 2009; Xu et al, 2021), noncoding RNA-containing exosome secretion (Maccani and Knopik, 2012; Yang et al, 2017b), and proinflammatory cytokine secretion (Lu et al, 2022; Ni et al, 2021; Zhang et al, 2020a; Zhang et al, 2020b), which lead to both an upregulation in the numbers of leukocytes and an impairment of the function of inflammatory cells. CS induces chronic inflammation in the lungs (Duaso and Duncan, 2012; Wood and Stockley, 2007), which is apparent at the level of tissue damage, and may result in COPD (Jaspers, 2014).

Tobacco smoke (TS) is a carcinogenic and immunomodulatory toxic mixture of more than 5000 chemicals (Talhout et al, 2011). Chronic exposure to CS leads to impaired T cell function (Valiathan et al, 2014), reprogramming of Treg transcriptomes (Shao et al, 2022; Shao et al, 2021b), a heightened level of systemic inflammation (Stämpfli and Anderson, 2009), and depressed antiviral immune responses, particularly in the lungs (Bauer et al, 2013). CS may increase end-organ injury (including atherosclerosis) by activating endothelial cells (ECs), a new innate immune cell type as we proposed (Drummer et al, 2021a; Mai et al, 2013; Shao et al, 2020), and promoting EC–monocyte interactions and increasing leukocyte tissue infiltration. CS induces chronic inflammation by increasing the numbers of neutrophils and macrophages (Jaspers, 2014).

CS administration leads to decreased CD4⁺ forkhead box P3 (Foxp3)⁺ T regulatory cells (Tregs, the major anti-inflammation cell type) and reduced expression of anti-inflammatory/immunosuppressive cytokine interleukin (IL)-10 and Treg-specific transcription factor Foxp3. For example, the frequency of Treg cells is lower in CS-exposed mice compared with the control group. More importantly, the frequency of Treg cells is negatively correlated with T helper cell 17 (Th17 cells) and CD8⁺ IL-17 producing T cells (Tc17 cells) (Duan et al, 2016).

CS is a major reversible risk factor for cardiovascular disease (CVD). CS-induced vascular inflammation and vascular pathology play important roles in the development and progression of subsequent clinical outcomes (Ambrose and Barua, 2004). CS has various cytotoxic effects in a wide range of vascular cell types including vascular ECs and vascular smooth muscle cells (VSMCs), in part, by increasing ROS production, activating nuclear factor kappa B (NF-κB), upregulating adhesion molecules, inducing endothelial activation and endothelial dysfunction, and promoting VSMC phenotypic switching and VSMC matrix degradation. The adhesion of leukocytes on the surface of ECs is an early event in the atherogenesis process and the increased levels of proinflammatory cytokines promote leukocyte-EC adhesion and leukocyte recruitment (Xu et al, 2022; Xu et al, 2021).

CS increases the expression of vascular cell adhesion molecule 1 (VCAM-1), intercellular adhesion molecule 1 (ICAM-1), and E-selectin on human umbilical vein endothelial cells (HUVECs), and increased transendothelial migration of monocytes and neutrophils (Bermudez et al, 2002; Shen et al, 1996). In addition, CS induces ferroptosis cell death in VSMCs (Sampilvanjil et al, 2020) and reduces carotid artery VSMC numbers due to increased cell necrosis (Ambalavanan et al, 2001).

Nicotine exposure promotes VSMC migration from the tunica media to atheromatous plaques in the vascular intima and induces VSMC transformation from contractile to synthetic phenotype (VSMC phenotype switching) via nicotinic acetylcholine receptors and G protein-coupled receptors (Yoshiyama et al, 2014). Electronic cigarettes (e-cigarettes), also known as electronic nicotine delivery systems, have become popular as substitutes for conventional tobacco cigarettes. While e-cigarettes contain fewer toxicants than tobacco cigarettes, they still emit detectable levels of volatile organic compounds, aldehydes, and nitrosamines that induce oxidative stress and inflammation (Cheng, 2014). E-cigarette-exposed alveolar macrophages show a significant increase in the production of proinflammatory cytokines/chemokines, apoptosis and necrosis, and ROS production (Scott et al, 2018). Furthermore, e-cigarette exposure significantly increases oxidative stress, oxidized low-density lipoprotein (ox-LDL), and cardiovascular risk (Carnevale et al, 2016; Moheimani et al, 2017).

Chronic e-cigarette exposure induces vascular endothelial dysfunction (Shao et al, 2014), cardiac dysfunction, and atherosclerosis in mice (Espinoza-Derout et al, 2019). In addition, e-cigarette exposure increases the risk for COPD (Bowler et al, 2017) and CVDs (D'Amario et al, 2019).

Signals of Danger-Associated Molecular Pattern Receptors Contribute to the Modulation of Inflammation and Immunity by CS

TS contains several chemicals that are harmful. Among the more than 5000 chemicals identified, at least 250 are reported to be harmful, including hydrogen cyanide, carbon monoxide (CO1), and ammonia (Centers for Disease Control and Prevention et al, 2010; National Center for Chronic Disease Prevention and Health Promotion Office on Smoking and Health, 2014). Based on chemical binding capacities, we tentatively classify them into five groups as follows:

• (i)At least 72 chemicals cause cancer (carcinogens) and some of these covalently bind to DNA directly (Goodson et al, 2015) and form DNA adducts. Twenty-eight of the carcinogenic chemicals including polycyclic aromatic hydrocarbons (benzo[a]pyrene), N-nitrosamines, heavy metals (nickel, cadmium, chromium, and arsenic), alkaloids (nicotine and its major metabolite, cotinine), and aromatic amines have immunomodulatory effects on distinct innate and adaptive immune cells and can lead to impaired immunity. These compounds can either downregulate (anti-inflammatory) or upregulate (proinflammatory) the immune response, leading to either an increased susceptibility to the development of cancers or infectious diseases, or immunopotentiation resulting in an increased secretion of inflammatory mediators (Table 2).
• (ii)The second group consists of more than 400 odorants. There are distinctive and clinically relevant odorants that evokes a response from 144 odorant receptors (ORs) and 3 trace-amine-associated receptors. The recognition of CS is accomplished by a broad receptor response pattern, and 1-pentanethiol (the odorant most critical for perception of the artificial mimic) is responsible for a small subset of the responsive ORs in this combinatorial code (McClintock et al, 2020). The ratios of >400 odorants over 144 ORs suggest significant degeneracy of CS chemicals in binding to their cellular receptors, about 2.7 odorants/receptors in nonhigh odorant-specific manners.
• (iii)Danger-associated molecular pattern (DAMP)/pathogen-associated molecular pattern (PAMP) receptors are the third group. These include toll-like receptors (TLRs), C-type lectin receptors, NOD-like receptors (NLRs including NLRP3), retinoic acid-inducible gene-like receptors, cytosolic DNA sensors, receptor for advanced glycation end products, triggering receptors expressed on myeloid cells, G-protein-coupled receptors, ion channels (Gong et al, 2020), and conditional DAMP receptors (Wang et al, 2016b). Compared with wild-type (WT) mice, CS-induced pulmonary inflammation is unaltered in DAMP/PAMP receptors and TLR2-deficient (Tlr2^−/−) (Yang et al, 2008) and TLR4-deficient (Tlr4^−/−) mice. CS-induced airway fibrosis, characterized by increased collagen deposition around small airways, is not altered in Tlr2^−/− mice, but is attenuated in Tlr4^−/− mice compared with CS-exposed WT controls (Haw et al, 2018). In addition, CS amplifies inflammatory reactions and atherogenesis through activation of the H1R-TLR2/4-cyclooxygenase-2 (COX-2) axis (Barua et al, 2015).
• (iv)CS inhibits the NLRP3 inflammasome and leads to caspase-1 activation via the TLR4-toll-like receptor adaptor molecule 1 (TICAM1, TRIF)-caspase-8 axis in human macrophages (Buscetta et al, 2020).
• (v)The compounds induced by CS and shared with endogenous metabolites (Zhang et al, 2022) as well as chemokines induced by morphine (Rogers, 2020) can use their intrinsic receptors [conditional DAMP receptors as we proposed (Wang et al, 2016b)] to drive or modulate inflammatory signals (Muri and Kopf, 2021; Voss et al, 2021).

Table 2.

Carcinogenic Constituents from Cigarette Smoke Induce Reactive Oxygen Species Production and Oxidative DNA Damage, Inhibit Immune Cell Generation and Proliferation, Modulate Cytokine Secretion, and Suppress Immune Functions

Carcinogen	Immune cells/tissue	Effects	Proinflammatory (+) Anti-inflammatory (*)	Receptor/pathway	PMID
Benz[j]aceanthrylene	Macrophages	DNA damage			9054606
	Lymphocytes	DNA damage			9744557
Benz[a]anthracene	Lymphocytes	Increased ROS production and oxidative DNA damage			21888224
	T cells	Inhibited T cell proliferation Inhibited IL-2 production Induced immunosuppression	*	Signal transduction mediated by TCR and IL-2R	8931739
	T cells	Suppressed helper T cell activation			6219909
Benzo[b]fluoranthene	HPBECs	Increased innate and adaptive immune signals Increased expression of oxidative stress genes	+		30090392
	Macrophages	Increased secretion of inflammatory cytokines	+		18830893
Benzo[j]fluoranthene	Mouse skin	A potent tumor initiator with high tumorigenic activity			3677067
B[k]F	Mouse	Reduced T cells in thymus and spleen Reduced CD4+ IL-2+ cells	*	Immunosuppression through IL-2 production	16326422
BaP	BMDMs	Impaired proliferation and decreased number of mature cells Increased expression of F4/80 and MHC-II Increased TNF-α and IL-10	+	AhR	30099064
	Mouse/mouse epidermal cells	Increased neutrophil and macrophage accumulation Increased secretion of IL-5, IL-13, IL-33, MCP-1 Increased expression of MHC-II and CD86 expression Increased Th1/Th2 responses and allergic airway inflammation Induced VEGF PI-3K/AP-1 activation	+		26918773 16461351
	T cells	Increased expression of AhR and CYP1 expression		Activation of AhR/CYP1-metabolizing enzymes	28461126 24412381
	BMDMs	Decreased proinflammatory cytokines Increased expression of IL-10, MHC-II, CD14, Fcγ receptor I (FcγRI/CD64) Increased NO production and phagocytosis	*	AhR activation	30053493
	DCs and keratinocytes	Increased secretion of cytokines	+	AhR activation	30748024
	T cells	Inhibited T cell proliferation Decreased production of IFN-γ, IL-2, and IL-4	*	Ca2+/CaM/NF-κB and Ca2+/CaM/CaN/NFAT signal transduction pathways	28290727
Cyclopenta[c,d]pyrene	Leukocytes	Increased DNA adducts			8200070
5-Methylchrysene	Mouse skin	Increased DNA adducts			1643254
BD	Mouse	Suppressed cytotoxic T cell generation			3787617 2401263
Benzene	Human and mouse inhalation	Decreased proliferation of B/T cells Suppressed antibody production by of B cells Oxidative stress imbalance Decreased p53 expression			2941900 29883905 9129168
Naphthalene	Macrophages	Increased lipid peroxidation Increased cytochrome c reduction and oxidative stress	+		9667488
Styrene	Mouse	DNA damage in lymphocytes, liver, bone marrow, and kidney			9783323
	Human	Reduced proportion of T helper cells Increased T suppressor cells Increased NK cells			1630405
	Human	Increased expression of adhesion molecules on lymphocytes, monocytes, and granulocytes	+		12141393
N-Nitrosodimethylamine	Mouse	Suppressed cell-mediated immunity and humoral immunity	*		1433375 6716271 3156199
	PMNs	Increased expression of iNOS, phospho-PI3K, phospho-IκBα and NF-κB, phospho-Akt (T308), phospho-Akt (S473), and phospho-IKKαβ, c-Jun and FosB Increased apoptosis		PI3K-Akt/PKB pathway	30722700 23971717
N-Nitrosodiethanolamine	Lymphocytes	Increased mutagenicity and genotoxicity			2808483 3204103
Catechol	Human PBMCs	Inhibited production of IL-2 and IL-1β	*		10932071
NNN	Rat oral and esophageal mucosa and keratinocytes	Alteration of immune regulation genes (Aire, Ctla4, and CD80) and inflammation (Ephx2 and Inpp5d)			26785143
NNN	Rat oral and esophageal mucosa and keratinocytes	Alteration of immune regulation genes (Aire, Ctla4, and CD80) and inflammation (Ephx2 and Inpp5d)			26785143
NNK	CTLs	Increased expression of adhesion molecule CD62L Impaired expansion capacity of CTLs Reduced memory programming			23673295
	Epithelial cells/macrophages	Inhibited secretion of IL-8, IL-6, MCP-1, and TNF	*		15762874 17096151 14764458
	Macrophages	Increased release of soluble TNF Decreased IL-10 synthesis	+		11258792
o-Anisidine	Hepatocytes/macrophages	Increased proinflammatory cytokines Induced ROS generation	+	eIF2- and Nrf2-mediated oxidative stress response	28089782
Trp-P-1	DCs	Increased expression of costimulatory receptors (CD80, CD86, and MHC) Inhibited DC maturation Inhibited IL-12 and TNF-α production Attenuated T cell proliferation/activation induced by DCs	+	signaling pathways mediated through p38 kinase	21078543
	Macrophages	Inhibited IL-8 expression Inhibited phosphorylation of p38 MAP kinase	*	Intracellular calcium/p38 MAP kinase-dependent	18281166
Benzo[b]furan	Cancer cells	Increased expression of proapoptotic genes (TNFRSF 10A, TNFRSF 10B, CASP8, BAX, BID, NOXA, APAF1) Activated JNK and p38 kinase		PI3K/Akt/mTOR signaling and mitochondrial-mediated apoptosis	30465514 27149364
FA	Splenocytes	Increased T cell differentiation into regulatory T cells Suppressed Th1-, Th2-, and Th17-related splenic cytokines Suppressed effector T cell activity and decreased T cell-related cytokines	*	Calcineurin-NFAT signaling	33046725
	Human exposure	Increased CD19+ B cells and CD56+ NK cells Increased IL-10 and IL-4 Decreased IL-8 and IFN-γ	*		25157974
Acetaldehyde	Splenocytes	Decreased cytokine production Inhibited aerobic glycolysis-related signal in T cells Inhibited NK activity and CTL-mediated lysis	*		30121625 2690659
	Human exposure	Degranulation of human mast cells and histamine release Induced GMCSF production and NF-κB activation Increased allergic inflammation	+		17590989
	PMN/monocytes	Increased ROS production Decreased PMN phagocytic functions	+
	PBMC	Inhibited T cell and B cell proliferation Decreased release of IFN-γ			8953156
Cadmium	Macrophages Mast cells	Increased ROS production Increased TNF-α and nitrite production Decreased mast cell TNF-α and IgE-mediated histamine release	+		30828855
	Peripheral blood neutrophils and splenocytes	Increased expression of cytochrome P450s enzymes Increased ROS production Induced autophagy and apoptosis in splenocytes		miR-216a-PI3K/AKT	32058096 30985881 32563067
Cobalt	HMEC-1 Macrophages	Increased inflammatory markers such as IL-6, IL-8, ICAM-1, and sICAM-1	+	TLR4	27835611
	Murine macrophages	Increased oxidative stress and ROS production Decreased OCR and induced mitochondrial dysfunction			30144138
Lead (inorganic)	Macrophages	Increased cell death Increased the antioxidant enzymatic activity of catalase Decreased macrophage phagocytic index, nitric oxide production, endosomal/lysosomal system stability			16757190 17959351
N-nitrosopiperidine	Esophageal epithelium	Increased cell death			16816872

Twenty-eight out of 72 carcinogens in cigarette smoke (PMID: 21324834) have been characterized to affect the activity and function of different innate and adaptive immune cells.

AhR, aryl hydrocarbon receptor; B[k]F, benzo[k]fluoranthene; BaP, benzo[a]pyrene; BD, 1,3-butadiene; BMDMs, bone marrow-derived macrophages; CTLs, CD8⁺ cytotoxic T lymphocytes; CYP1, cytochrome P450 family 1; FA, formaldehyde; GMCSF, granulocyte/macrophage-colony-stimulating factor; HMEC-1, human microvascular endothelial cell 1; HPBECs, human primary bronchial epithelial cells; ICAM-1, intercellular adhesion molecule 1; IFN-γ, interferon-gamma; JNK, c-Jun N-terminal kinase; mTOR, mammalian target of rapamycin; NF-κB, nuclear factor kappa B; NK, natural killer; NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone; NNN, N′-nitrosonornicotine; NO, nitric oxide; Nrf2, nuclear factor erythroid 2-related factor 2; OCR, oxygen consumption rate; PBMCs, peripheral blood mononuclear cells; PHA, phytohemagglutinin; PI3-AKT, phosphoinositide-3-kinase–protein kinase B; PMNs, polymorphonuclear neutrophils; sICAM-1, soluble intercellular adhesion molecule 1; Th, T helper cell; TLR, toll-like receptor; VEGF, vascular endothelial growth factor.

Trained immunity is a relatively new concept where signaling from DAMP receptors can modulate immunity. TLR agonists have been shown to trigger trained immunity through metabolic reprogramming and epigenetic modifications, which drive profound augmentation of antimicrobial functions (Owen et al, 2020). In addition, inflammasome NLRP3 was reported to mediate trained immunity following the use of Western diet (high-fat diet) and could thereby mediate the potentially deleterious effects of trained immunity in inflammatory diseases. Taken together, although we do not know whether signals triggered by all the DAMPs/PAMPs modulate trained immunity, CS activates signaling of TLRs and inflammasomes, which could lead to the establishment of chronic inflammation via trained immunity (Christ et al, 2018).

Cigarette Smoke Reprograms Immunometabolism and Trained Immunity-Related Metabolic Pathways in Innate Immune Cells and T Cells, Which Can Be Sensed by the Cytoplasmic (Cytosolic and Non-Nucleus Organelles) ROS System

CS influences both innate and adaptive immunity and plays dual functions in regulating immunity by promoting antipathogen immune responses or suppressing defensive immunity. Noncarcinogenic DAMPs from CS activate both innate and adaptive immune cells including macrophages, neutrophils, dendritic cells (DCs), natural killer (NK) cells, B cells, CD4⁺ T helper cells (T helper cell 1, Th1) (Th1/Th2/Th17), CD4⁺Foxp3⁺ Tregs, CD8⁺ T cells, and memory T/B lymphocytes (Table 3).

Table 3.

Noncarcinogenic Danger-Associated Molecular Patterns from Cigarette Smoke Activate Both Innate and Adaptive Immune Cells

Cell type	Effects	PMID
Macrophages	Increased cell activation Increased expression of TLR4 Increased secretion of proinflammatory mediators (TNF-α) Increased ROS production Impaired bactericidal and phagocytotic processes Activation of IL-1R-associated kinase, p38, and NF-κB	12033743 17630319 16004610 16620395 19409098 17947684
Neutrophils	Increased cell number Increased IL-8 production Increased ROS production Enhanced degradation of IκB-α/β proteins and activity of p65 and p50	6556892 12960242
DCs	Increased number of immature DCs and decreased number of mature DCs Decreased T cell stimulatory capacity and Th1-cell polarization Increased expression of MHC-II molecules and costimulatory molecules CD40 and CD86 Increased CD4+ cells rather than CD8+ cells	18337593 25338516 16055867
NK cells	Decreased NK cell numbers and activity Decreased cytotoxic activity and cytokine production Decreased IFN-γ, TNF-α secretions, and perforin expression	26201093 18055568
B lymphocytes	Increased levels of memory B cells and memory IgG+ B cells Increased levels of circulating IgE Decreased IgG, IgM, and IgA Increased expression of nicotinic receptors	19909533 24502245 14500745 25011477
T cells	Increased lung CD3+, CD8+, and CD4+ T cells Increased the percentage of CD8+ NKG2D+ cells Enhanced percentage of CD8+ CD69+ cells and cell surface expression of CD69 Increased IFN-γ and CCR6 expression Increased Th17 cells and Th17-related cytokines (IL-17A, IL-6, and IL-23) Increased percentages of Treg cells Increased activation of cytotoxic CD8+ T cells Increased production of IL-1β, IL-6, IL-10, IL-12p70, TNF-α, and IFN-γ Decreased CD4+ CD25+ Treg cells, Foxp3, TGF-β, and IL-10	18706444 21763119 20863413 20646637 23044435 23319833 28745532 22070100

CS affects both innate and adaptive immunity and plays dual functions in regulating immunity by promoting pathogenic immune responses or suppressing defensive immunity. Innate immune cells affected by CS include macrophages, neutrophils, DCs, and NK cells, while adaptive immune cells affected by CS mainly include T helper cells (Th1/Th2/Th17), CD4⁺CD25⁺ regulatory T cells, CD8⁺ T cells, B cells, and memory T/B lymphocytes.

Foxp3, forkhead box P3; TGF-β, transforming growth factor-β.

Innate immune cells can develop exacerbated long-term immune responses and inflammatory phenotype following brief exposure to endogenous or exogenous DAMPs, which results in a primed and significantly enhanced inflammatory response toward a second challenge after the return to a nonactivated state. This phenomenon is known as innate immune memory or trained immunity (Drummer et al, 2021a; Lu et al, 2019; Zhong et al, 2020). As shown in Figure 1, the establishment of trained immunity involves metabolic reprogramming and epigenetic modification of the innate immune cells, allowing qualitatively and quantitatively adjusted responses of innate immune cells to subsequent time-delayed heterologous stimulations (Netea et al, 2020; Netea et al, 2019; Netea et al, 2016).

FIG. 1.

The concept of trained immunity and trained tolerance. Trained immunity involves metabolic reprogramming and epigenetic modification of the innate immune cells, allowing qualitatively and quantitatively adjusted responses of innate immune cells to subsequent time-delayed heterologous stimulation. Inappropriate trained immunity response can contribute to disease progression, resulting in either a chronic hyperinflammatory state or a persistent state of immunological tolerance.

Trained immunity is not only important for host defense and vaccine responses but also for promotion of the pathogenesis of chronic inflammation including metabolic CVDs such as atherosclerosis (Shao et al, 2021a) and synergies among the disease risk factors (Drummer et al, 2021b; Fagenson et al, 2020).

In contrast to the memory function in adaptive immune system with specialized cell subsets to carry out memory function (i.e., memory T cells and memory B cells) (Shen et al, 2019), trained immunity can occur in innate immune cells, including monocytes/macrophages (Lai et al, 2019), DCs (Netea et al, 2020), NK cells, aortic cells (Lu et al, 2022), innate immune functions of T cells, and Treg cells (Lee et al, 2020; Ni et al, 2021; Seyda et al, 2016; Zhang et al, 2020b), and nontraditional immune cells, such as ECs (Lu et al, 2022; Lu et al, 2019; Mai et al, 2013; Shao et al, 2020), VSMCs (Flores-Gomez et al, 2021; Lu et al, 2022; Schnack et al, 2019), fibroblasts (Drummer et al, 2021a), and hepatocytes (Drummer et al, 2021b; Fagenson et al, 2020). Furthermore, after the second challenge, these innate cells can alternatively respond less strongly than to the primary response, and the anti-inflammatory mechanisms are promoted resulting in a state of innate immunological tolerance to prevent tissue damage and limit the inflammatory response (Netea et al, 2020).

It has been reported that T cells play important roles in innate immunity and in antigen nonspecific protection (Berg and Forman, 2006). On the contrary, as adaptive immune cells, T cells are regulated by cytokines and cell–cell communication signals of the innate immune system (Kwesi-Maliepaard et al, 2021; Stäger and Kaye, 2004). Along the same line, we recently reported that CD4⁺Foxp3⁺ Treg cells have many active innate immune pathways (Ni et al, 2021; Zhang et al, 2020b), and Tregs can sustain their immunosuppressive functions (Shao et al, 2021b) although Treg cell plasticity in atherosclerosis with a chronic inflammatory environment has been reported (Shao et al, 2021b; Xu et al, 2018).

Our reports indicate that Treg cells and other adaptive immune cells not only respond to antigen stimulation (Yan et al, 2008) but also respond to the stimulation by DAMPs/PAMPs similar to that of innate immune cells (Ke et al, 2008; Lopez-Pastrana et al, 2015; Ni et al, 2021; Yang et al, 2015; Yin et al, 2013). Evidence supporting this new model is that DAMPs/PAMPs sensor inflammasome-dependent release of cytokines, and antigen can activate, shape, or even inhibit adaptive immune responses (Deets and Vance, 2021). TLRs, cytokine receptors, and T cell and B cell antigen receptors have been shown to activate trained immunity-related metabolic pathways, which include phosphoinositide-3-kinase–protein kinase B, mammalian target of rapamycin (Bekkering et al, 2021), liver kinase B1 (also known as serine/threonine kinase 11) (Timilshina et al, 2019), and AMP-activated protein kinase signaling pathways (Cheng et al, 2014; Chou et al, 2022).

CS constituents including nicotine can bind to the intracellular and outer membrane receptors and induce metabolic reprograming and increase disease risk (Fig. 2). CS exposure in several disease conditions, which would include COPD (Pauwels et al, 2011), hyperlipidemia (high-fat diet feeding) (Wu et al, 2018), and CHD (Mao et al, 2021), enhances the innate immune response and promotes disease progression (Table 4). It has been reported that increased energy metabolism and the electron transport chain (Yin and O'Neill, 2021) via (i) glycolysis, (ii) acetyl-CoA generation, (iii) mevalonate synthesis (part of cholesterol biosynthesis), (iv) glutaminolysis (converting glutamine into tricarboxylic acid [TCA] cycle metabolites through the activity of multiple enzymes), and (v) epigenetic modification (Ferreira et al, 2022; Lu et al, 2019) contribute significantly to the establishment of trained immunity. Trained immunity can contribute to disease progression, resulting in a chronic hyperinflammatory state (Shao et al, 2021a).

FIG. 2.

Several DAMP receptor pathways induce innate immune memory (trained immunity) and sustained inflammation, tissue remodeling, and damage. CS promotes disease risk factors by binding to its receptors and induces bioenergetic metabolic reprogramming to induce trained immunity pathology. CS, cigarette smoke; DAMP, danger-associated molecular pattern.

Table 4.

Cigarette Smoke Exacerbates Inflammatory and Immune Responses

Smoke component	Cell type/tissue/disease condition	Effect	Receptor/pathway	PMID
CS-exposed patients	Lung tissue of COPD patients	Increased expression of IL-1α and IL-1β Promoted inflammation and activated NLRP3-related pyroptosis pathway	IL-1α and Nlrp3/caspase-1/IL-1β	21622588
Nicotine	HFD-fed ApoE−/− mice	Increased inflammatory cytokine secretion Increased cleavage of caspase-1, IL-1β, and IL-18 secretion Increased atherosclerotic plaques Increased activation of NLRP3-ASC inflammasome Increased pyroptosis in HAECs	ROS-NLRP3-mediated EC pyroptosis	29416034
Nicotine	BMDM/ECs from CHD patients	Increased plasma IL-1β and IL-18 Increased TXNIP expression Increased mitochondrial ROS production Increased activation of NLRP3 and caspase-1 inflammasome Increased GSDMD expression Increased monocyte/macrophage dysfunction	TXNIP/NLRP3-mediated pyroptotic pathway	33626512
Nicotine	Macrophages from HFD-fed ApoE−/− mice	Increased expression of cleaved caspase1, NLRP3, IL-1β, IL-18 Increased LDH release Induced pyroptosis	HDAC6/NF-κB/NLRP3 signaling pathway	33321327
Smoke exposure	Pseudomonas aeruginosa infected mouse lung	Delayed rate of bacterial clearance Increased neutrophils and mononuclear cells infiltration Increased proinflammatory cytokines and chemokines		15317669
CS	Angiotensin-II induced hypertension in mice	Induced mitochondrial oxidative stress and endothelial dysfunction Induced oxidation of cardiolipin (mitochondrial oxidative stress biomarker) Increased blood pressure		30608177
Chronic nicotine exposure	Renal IRI in mice	Exacerbated renal IRI and oxidative stress-induced injury Aggravate acute renal ischemia- or oxidative stress-induced stress kinase signaling		21511693 23892062 22552933
CSE	Human bronchial epithelial cell	Increased expression of TLR4 Increased binding of LPS Increased release of IL-8 by LPS stimulated cells Increased neutrophil chemotaxis Amplification of inflammation	Activation of ERK pathway	18217953
CS	Mice with poly (I:C) administration	Increased infiltration of macrophages, neutrophils, and eosinophils in the bronchoalveolar lavage Induced inflammatory response		28468623
CS	NTHI	Exacerbated NTHI-mediated chronic respiratory inflammation Increased proinflammatory IL-1β, IL-6, and TNF-α Decreased IFN-γ and IL-4 secretion		24752444

CSE, cigarette smoke extract; GSDMD, gasdermin D; HAECs, human aortic endothelial cells; HDAC6, histone deacetylase 6; HFD, high-fat diet; IRI, ischemia–reperfusion injury; LDH, lactate-dehydrogenase; LPS, lipopolysaccharide; NLRP3, NOD-, LRR- and pyrin domain-containing protein 3; NTHI, nontypeable Haemophilus influenzae; TXNIP, thioredoxin-interacting protein.

We previously reported that the anti-inflammatory cytokine IL-35 is an inflammation-induced cytokine (Li et al, 2018; Li et al, 2012), in contrast to housekeeping anti-inflammatory cytokines such as transforming growth factor-β (Li et al, 2012). We reported that anti-inflammatory cytokines IL-35 and IL-10 can inhibit EC activation and vascular inflammation but spare the modulation of trained immunity gene expression (Li et al, 2020). Similarly, concomitant to a proinflammatory response, anti-inflammatory mechanisms such as trained innate immune tolerance, with a feature of upregulated immune responsive gene 1 (IRG1) itaconate synthesis enzyme (Domínguez-Andrés et al, 2019), and homeostasis molecular patterns (Wang et al, 2016b) are induced to prevent excessive inflammation and tissue damage and to limit the inflammatory response in time.

Overall, CS has the capacity to induce opposing effects on the immune system, with chronic exposure increasing inflammation on the one hand, and potentially reducing inflammation by trained immune tolerance on the other (Table 5).

Table 5.

Cigarette Smoke Can Induce Trained Innate Immune Tolerance

	Cell type/disease	Effect	Receptor/pathway	PMID
CSE	MDMs and THP-1 cells	Inhibited LPS- and LPS plus ATP-induced IL-1β and IL-18 release and pro-IL-1β, pro-IL-18, and NLRP3 expression Decreased LPS-induced lactate release and basal glycolytic flux and impaired glycolytic burst	NLRP3-independent and TLR4-TRIF-caspase-8-dependent pathway	31914643
A single injection of nicotine	Renal I/R injury in rats	Attenuated renal dysfunction and tubular necrosis induced by I/R injury Ameliorated acute tubular damage following renal I/R injury Reduced TNF-α protein expression and leukocyte infiltration of the kidney	α7nAChR	18401335
A single injection of nicotine	Renal I/R injury in rats	Reduced the α7nAChR protein after I/R injury Inhibited renal I/R-induced STAT3 activation	α7nAChR	18614620
Nicotine (500 μM)	LPS-stimulated primary human macrophages	Inhibited TNF release from LPS-stimulated macrophages		12508119
Nicotine	IL-18-stimulated PBMC	Inhibited IL-18-enhanced expression of ICAM-1 and CD40 Inhibited production of IL-12, TNF-α, and IFN-γ by IL-18-stimulated cells	α7nAChR	16966384
Pretreatment with CSE	LPS stimulated bronchial epithelial cells	Inhibited LPS-induced GM-CSF and IL-8 protein release Suppressed neutrophil chemotaxis induced by LPS Downregulated the activity of LPS-induced transcription factor AP-1 Attenuated inflammatory response induced by LPS	Suppression of AP-1 activation	15356167
Pretreatment with CSE	Anti-CD3 and PMA stimulated PBMCs	Suppressed production of IL-1β, TNF-α, IL-2, and IFN-γ		10932071
CS	Lung macrophages of Apoe−/− mice	Increased itaconate metabolite Increased expression of IRG1 Prevents overshooting macrophage activation in the lungs May counteract the oxidative challenge from the activated immune system as well as directly from CS exposure.		32419906

CS increases the expression of anti-inflammatory cytokines and decreases the expression of proinflammatory cytokines and chemokines.

α7nAChR, α7 nicotinic acetylcholine receptor; IRG1, immune responsive gene 1; MDMs, monocyte-derived macrophages; PMA, phorbol myristate acetate; STAT3, signal transducer and activator of transcription 3.

Trained innate immune tolerance leads to a persistent state of innate immunological tolerance, a new mechanism that dampens the inflammatory response of the host to maintain homeostasis and prevent tissue damage and organ failure, but with the subsequent risk of secondary infections and other diseases related to decreased activity of the immune system (Netea et al, 2020). Itaconate accumulation has been shown to inhibit the expression of proinflammatory mediators such as IL-6, IL-1β, and IL-12p70 in lipopolysaccharide (LPS)-stimulated macrophages (Lampropoulou et al, 2016).

As shown in Figure 3, itaconate is upregulated in macrophages activated by LPS via increased cis-aconitate decarboxylase/IRG1 transcription. Overproduction of itaconate activates the antioxidant transcription factor nuclear factor erythroid 2–related factor 2 (Nrf2) pathway by alkylation of Kelch-like ECH-associated protein 1, which induces the transcription of various Nrf2-dependent antioxidant and anti-inflammatory genes. This would include heme oxygenase-1, glutamate cysteine synthase, and glutathione synthetase (for glutathione biosynthesis). Itaconate can also inhibit succinate dehydrogenase and reduce ROS generation and IL-1β secretion. Itaconate promotes the transcription of activating transcription factor 3, which directly inhibits the inhibitor of nuclear factor kappa B zeta (IκBζ) expression and leads to decreased expression of IL-6. In addition, itaconate directly alkylates the cysteine residues of glyceraldehyde-3-phosphate dehydrogenase, aldolase, fructose-bisphosphate A (leading to reduced glycolysis), and downregulates IL-1β secretion, thereby depressing the inflammatory response (O'Neill and Artyomov, 2019).

FIG. 3.

The concept of trained immune tolerance. Upon LPS stimulation, itaconate production is increased by CAD/IRG1 transcription. Overproduction of itaconate activates the antioxidant transcription factor Nrf2 by alkylation Keap1, which induces the transcription of various Nrf2-dependent antioxidant and anti-inflammatory genes. Itaconate can also inhibit SDH and reduce ROS generation and IL-1β secretion. Itaconate promotes the transcription of ATF3, which directly inhibits the IκBζ expression to reduce IL-6 secretion. In addition, itaconate directly alkylates the cysteine residue 22 of GAPDH and ALDOA to inhibit glycolysis, and reduces IL-1β secretion, thereby alleviating the inflammatory response. CS exposure can significantly increase the expression of IRG1 and the abundance of itaconate metabolites. ALDOA, aldolase, fructose-bisphosphate A; ATF3, activating transcription factor 3; CAD, cis-aconitate decarboxylase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; IκBζ, inhibitor of nuclear factor-kappa B zeta; IL, interleukin; IRG1, immune responsive gene 1; LPS, lipopolysaccharide; Keap1, kelch-like ECH-associated protein 1; Nrf2, nuclear factor erythroid 2-related factor 2; ROS, reactive oxygen species; SDH, succinate dehydrogenase.

CS can increase the anti-inflammatory gene expression and decreases the expression of proinflammatory cytokines and chemokines, leading to an immunosuppressive effect. CS exposure significantly increases the abundance of itaconate metabolites and the expression of IRG1 (Table 5) (Titz et al, 2020).

Recent progress on trained innate immune tolerance may lead to the development of a new anti-inflammatory therapy such as ItaCORMs (Krause et al, 2021). Although it is unclear whether CS promotes functionally trained immunity or trained immune tolerance, CS has divergent effects on innate immunity in promoting inflammatory bowel disease but inhibiting ulcerative colitis (AlQasrawi et al, 2020). These findings demonstrate that innate immune cells and adaptive immune cells are equipped with innate immune signaling pathways, presumably including both innate immune memory (trained immunity) and innate immune tolerance, in response to stimulation by various DAMPs, PAMPs, and other environmental stimuli derived from CS.

Based on extensive analysis, we recently proposed that ROS systems are a new integrated network for sensing homeostasis and alarming stress in organelle metabolic processes (Sun et al, 2020). In addition to critical functions in signaling (Li et al, 2016a; Li et al, 2013) and damaging DNAs (nuclear ROS), ROS contribute to a large number of human diseases (Hybertson et al, 2011) and aging (Kowalska et al, 2020). Thus, cytoplasmic ROS and reactive nitric species (RNS) (Michel and Vanhoutte, 2010) can be used as the indicators of metabolic reprogramming. CS contributes to metabolic dysregulation and a shift of metabolism in innate immune cells (Table 6). CS inhibits alveolar macrophage mitochondrial respiration while inducing glycolysis and ROS (Aridgides et al, 2019).

Table 6.

Metabolic Reprogramming Induced by Cigarette Smoke May Be the Novel Mechanisms Underlying the Establishment of Trained Innate Immunity and Trained Tolerance

Smoke component	Cell type/tissue/species	Effect	PMID
Chronic CS	Lung cells	Increased expression of TCA cycle enzymes Increased expression of enzymes involved in glutamine metabolism, fatty acid degradation, and lactate synthesis Induced mitochondrial metabolic reprogramming	29042306
CS and electronic cigarette vaping		Increased sphingolipid metabolites Decreased TCA cycle metabolites	34072305
Short-term CS	Alveolar type-II cells	Increased glycolysis Increased mitochondria β-oxidation Decreases levels of phosphatidylcholine	24625219
Short-term CS	Mouse lung (mitochondria)	Increased pentose phosphate pathway Increased expression and activity of complexes II, III, IV, and V Upregulation of genes involved in glycolysis, TCA cycle, mitochondrial fatty acid oxidation, and redox regulation	23064950
CSE	Pulmonary microvascular ECs	Increased glycolysis Decreased mitochondrial respiration Decreased fatty acid oxidation	31555131
CS	Alveolar macrophages	Increased glycolysis Decreased mitochondrial respiration Increased ROS and metabolic dysfunction	31270372
Chewing tobacco	Epithelial cells	Increased oxidative phosphorylation Decreased expression of enzymes involved in the glycolytic pathway Increased expression of mitochondrial proteins involved in the electron transport chain and enzymes of the TCA cycle	31438645
CS	Human lymphocytes	Decreased mitochondrial complex IV activity Increased lipid peroxidation	10383908
CSE	Monocyte-derived macrophages and THP-1 cells	Decreased basal glycolytic flux Impaired glycolytic burst in response to LPS	31914643

CS induces metabolic reprogramming depending on many variables, including the type of tobacco and duration of use, and type of exposed cells.

TCA, tricarboxylic acid.

Furthermore, short-term (4 and 8 weeks) exposure of A/J mice to CS (a model for lung tumorigenesis caused by TS) (Witschi, 2005) induces upregulation of genes encoding glycolysis, TCA cycle, mitochondrial fatty acid oxidation pathway, and redox regulation (Agarwal et al, 2014). Increased expression of nuclear genes in the glycolytic pathway and decreased levels of mitochondrial genes following exposure to either the main stream extract or side stream smoke extract support the notion that CS significantly contributes to the transformation of nonmalignant esophageal epithelial cells into a tumorigenic phenotype via metabolic reprogramming (Kim et al, 2010). In addition, acute CS exposure leads to a reversible airspace enlargement in A/J mice, indicative of alveolar damage. A decrease in respiration rates while metabolizing glucose is observed in the CS-exposed group, indicating altered glycolysis that is compensated by an increase in fatty acid palmitate utilization.

Fatty acid palmitate utilization is accompanied by an increase in the expression of CD36 and carnitine palmitoyl transferase 1 in type II alveolar cells for the transport of palmitate into the cells and into mitochondria, respectively (Agarwal et al, 2014). As we and others reported, fatty acid oxidation is also essential for inflammasome activation in proinflammatory type 1 macrophages (M1) (Batista-Gonzalez et al, 2019; Lai et al, 2019).

Epigenetic alteration, including DNA methylation (Jamaluddin et al, 2007) and histone posttranslational modifications (Shao et al, 2016), has been shown to play a significant role in the progression of CS-related diseases (Table 7). DNA methylation is catalyzed by a protein family known as DNA methyltransferases (DNMTs), which transfer methyl groups from S-adenosyl-l-methionine to the 5-carbon position of cytosine residues in DNA. Several studies demonstrate the associations between smoking and altered DNA methylation at multiple cytosine-phosphateguanine (CpG) sites (Breitling et al, 2011; Shenker et al, 2013; Wan et al, 2012). Some DNA methylation sites associated with tobacco smoking have also been localized to genes related to CHD (Breitling et al, 2012) and pulmonary disease (Wauters et al, 2015). Some studies have found different DNA methylation associated CpGs in smokers versus nonsmokers (Zeilinger et al, 2013). CS increases DNA methylation and inflammation in macrophages (Yu et al, 2016) and CS-exposed individuals (Siedlinski et al, 2012). DNMTs are significantly overexpressed in the lung tissues of the smokers compared with the nonsmokers (Kwon et al, 2007). Histone acetylation, methylation, phosphorylation, and ubiquitination are the most broadly studied and extensively characterized histone posttranslational modifications in terms of the regulation of chromatin structure and transcriptional activity (Li et al, 2018; Shao et al, 2016).

Table 7.

Cigarette Smoke Induces Epigenetic Innate Immune Memory in the Form of Histone Acetylation and Methylation and Provides a Foundation for Sustained Inflammation, in Which Acetyl and Methyl Donations Are Generated by CS-Induced Metabolic Reprogramming

HATs/HDACs	Smoking	Tissues/cells/species	Changes by CS	PMID
I: DNA methylation
DNMT1	CS	Macrophages	Increased DNMT1 expression Increased proinflammatory cytokine production Decreased PPAR-γ	17053888 27530451
JAK3  KRT1  RUNX3	CS	CS-exposed individuals	Hypermethylation of RUNX3, JAK3, and KRT1 genes associated with CRP in COPD increased inflammation	22617718
II: Histone posttranslational modification: HATs and HDACs
HAT, CBP/p300	CSE	Bronchial epithelial cells	Increased expression of HAT, p300/CBP Increase acetylation of histones (H3/H4) and NF-κB in COPD Increased inflammation	27925185 19811389
HDAC1	CSE/CS	Rat lung tissue and macrophages	Decreased expression of HDAC1 Increased MCP-1, IL-8, TNF-α, and MMP9 expressions increased level of acetylated H3K9, Increased inflammation	26033389
HDAC2	CS	Lung tissue of COPD smokers	Decreased cytoplasmic expression of HDAC2 Increased acetylation of histones H3 and H4 Increased expression of IL-12p40 Decreased IκBα expression Increased inflammation	30659954 16574938
HDAC3	CSE	Bronchial epithelial cells Alveolar macrophages	Decreased SIRT1, HDAC2, HDAC3, and FoxO3 Increased TLR4 and survivin, LPS binding, and ERK ½ activation Increased IL8 and IL1β production Increased inflammation	30659954 22613758
HDAC4	Chronic CS	Mouse lung and lung tissue of COPD smokers	Decreased expression of HDAC4 Repressed c-Jun and IL-17A in COPD	27793800
HDAC5/8	CS	Lung tissue and alveolar macrophages of COPD smokers	Decreased HDAC5 expression Increased IL-8 expression Increased histone-4 acetylation	15888697
HDAC6	CSE	Mouse epithelial cells and lung microvascular ECs/chronic smokers with COPD	Increased HDAC6 expression	24200693 26452072
HDAC7	CS	Human exposure	Decreased HDAC7 expression Increased HIF1α	22172637
HDAC10	CS	Lung tissues of smokers with COPD	Decreased HDAC10 expression	30214182
SIRT1	Smoker with COPD	PBMCs	Decreased expression and activity of SIRT1	29861836 27167200
SIRT3	CS	WT mice	Decreased sirtuin-3 expression Increased mitochondrial oxidative stress Induced endothelial dysfunction	30608177
SIRT4	CS	Human pulmonary microvascular ECs	Decreased SIRT4 expression Induced mononuclear cell adhesion Induced VCAM-1, E-selectin, and NF-κB activation Increased proinflammatory cytokines IL-1β, TNF-α, and IL-6	24603126
SIRT5	CSE	Lung epithelial cells	Increased SIRT5 expression and FOXO3 acetylation	25981116
SIRT6	CSE	Human bronchial epithelial cells	Decreased SIRT6 expression Induced cellular senescence	24367027

CS induces hyperacetylation and increases H3 and H4acetylation by increasing HATs and decreasing HDACs as well as modification of DNA methylation.

CS, cigarette smoke; DNMT, DNA methyltransferase; HATs, histone acetyltransferases; VCAM-1, vascular cell adhesion molecule 1; WT, wild type.

Histone posttranslational modifications play a critical role in gene expression by adding or removing the acetyl or methyl group, which is catalyzed by various enzymes (Bannister and Kouzarides, 2011). Also, lysine residues in histones can be oxidized by the lysyl oxidase family of proteins (Serra-Bardenys and Peiroó, 2021). Furthermore, epigenetic modification in the form of histone acetylation and methylation, which are catalyzed by histone acetyltransferases (HATs), histone demethylases, and histone deacetylases (HDACs), is associated with trained immunity. For example, there are increases inmarkers of open chromatin such as histone 3 lysine 4 trimethylation (H3K4me3), H3K4me1, and histone 3 lysine 27 acetylation (H3K27ac). Simultaneously, there is a decrease in histone markers depicting closed chromatin such as histone 3 lysine 9 dimethylation (H3K9me2). Epigenetic alterations include histone methylation and acetylation that work in concert to regulate gene transcription in a heritable manner.

The enzymes that regulate these epigenetic modifications can be activated by smoking, which further mediates the expression of multiple inflammatory genes (van der Heijden et al, 2018). Finally, CS has been shown to increase histone acetylation by increasing the expression of HATs and decreasing the expression of HDACs (Ito et al, 2005; Sundar et al, 2014).

Transgenerational epigenetic inheritance occurs when developmental programming is transmitted across generations that were not exposed to the initial environment, which triggered the change. The transgenerational programming has been described for metabolic and CVD risk (Krauss-Etschmann et al, 2015). A previous study showed significant hypomethylation in the placentas of babies born to mothers who smoked during pregnancy compared with that of nonsmoking mothers. This hypomethylation was found to correlate with increased placental cytochrome P450 family 1 subfamily A member 1 (CYP1A1) expression, which may have implications for xenobiotic metabolism in the offspring (Suter et al, 2011). Moreover, hypermethylation of the brain-derived neurotrophic factor may be responsible for its decreased expression with subsequent behavioral consequences in infants, children, and adolescents exposed in utero to maternal cigarette smoke (Knopik et al, 2012).

Taken together, these results have demonstrated that CS may modulate trained immunity and trained tolerance pathways via regulating energy metabolisms and epigenetic modifications. Since CS has divergent pro- and anti-inflammatory activities (AlQasrawi et al, 2020), we suggest that CS modulates inflammation and immunity via a linked trained innate immunity and trained innate immune tolerance process.

Five Nucleus-Localized ROS Activate CS-Promoted DNA Damage and Cell Death Pathways to Potentially Amplify Inflammation and Immune Responses via Releasing Alarmins

The toxicity of CS is largely attributed to its ability to generate ROS. CS contains a number of highly unstable free radicals, and these free radicals enhance ROS and RNS production leading to oxidative/nitrosative stress. Increased oxidative/nitrosative stress plays an important role in the pathogenesis of several diseases such as diabetes, lung cancer, atherosclerosis, and COPD. Accumulating evidence suggests a key role for smoke-induced ROS and the resulting oxidative stress in inflammation and cancer. CS is also known to stimulate ROS production by activating ROS sources, such as nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and mitochondria (Fetterman et al, 2017; Kim et al, 2014; Li et al, 2016a).

Five ROS are localized in the nucleus including superoxide (O₂⁻), nitrogen dioxide (NO₂⁻), hydroxyl radical (OH⁻), hydrogen peroxide (H₂O₂), and hypochlorous acid (HOCl) (Lu et al, 2021; Sun et al, 2020), and it has been reported that CS extract produces superoxide by reacting with bicarbonate (Park et al, 2021). Nitrogen oxides (NO_x), most notably nitrogen dioxide (NO₂⁻), are among the most hazardous forms of air pollution. TS is a major indoor source of NO_x. Substantial increases in NO_x concentrations are found when smoking only one cigarette (Braun et al, 2021), suggesting that NO_x exposure occurs for secondhand smokers following smoking in indoor rooms. In addition, cigarette tar in aqueous solution contains a quinone–hydroquinone–semiquinone complex that can reduce oxygen to produce superoxide and hence hydrogen peroxide and the hydroxyl radical.

The cigarette tar radical can penetrate viable cells, bind to DNA, and induce nicks (Pryor, 1997; Zeng et al, 2018). Moreover, nicotine mediates hypochlorous acid-induced nuclear protein damage in mammalian cells (Salama et al, 2014). Chronic CS exposure results in increased oxidative stress leading to diaphragm muscle dysfunction (Barreiro et al, 2012), and increased expression of pro-oxidant genes and proinflammatory markers (Khanna et al, 2013). CS also increases intracellular ROS, cytokine expression, basal mitochondrial ROS, and oxidative stress in alveolar macrophages (Table 8) (Lugg et al, 2022).

Table 8.

Cigarette Smoke Induces Reactive Oxygen Species and Oxidative Stress, Which Promote Inflammation, Inflammatory Cell Death (Pyroptosis), Oxidative DNA Damage, and Cancers

	Cell type/tissue/species	Effect	PMID
CS-exposed humans, mice, and pig	Skeletal muscles Diaphragm	Induced oxidative stress and inflammation leading to muscle dysfunction	20413628 22349133
CS-exposed rat	Serum/brain	Increased expression of pro-oxidant genes (iNOS, NOX4, dual oxidase 1, and p22[phox]) leading to oxidative stress Increased expression of proinflammatory markers (IFN-γ, TNF-α, IL-1α, IL-1β, IL-23, IL-6, IL-23, IL-17, IL-10, TGF-β, T-bet, and FoxP3) and increased inflammation	23031832
CSE	Type II alveolar epithelial cells	Decreased cell viability Decreased expression of VEGF Increased ROS production and oxidative stress	25607112
CSE	Mouse β cells	Increased β cell endoplasmic reticulum and oxidative stress Reduced insulin secretion Impaired β cell survival	32283079
CS	Mouse lung	Increased MDA and MPO activities Decreased SOD activity Induced oxidative stress Reduced the expression of the mitochondrial fusion protein OPA1 and fission protein MFF Induced mitochondrial dysfunction	34221235
Acute CS exposure	Human alveolar epithelial cell	Increased cellular oxidative stress and autophagy	31016633
Electronic cigarette	Rats	Induced oxidative stress	34515107
CS exposure	Alveolar macrophages	Increased intracellular ROS Increased IL-8 cytokine production Increased basal mitochondrial ROS Increased oxidative stress	33986144
Nicotine	Mouse VSMCs/Apoe−/− mice	Induced autophagy and VSMC phenotype switching Increased ROS production and NF-κB signaling pathway Increased aortic atherosclerotic plaque	30856513

iNOS, inducible nitric oxide synthase; MDA, malondialdehyde; MFF, mitochondrial fission factor; MPO, myeloperoxidase; OPA1, optic atrophy protein 1; SOD, superoxide dismutase; VSMCs, vascular smooth muscle cells.

Several new cell death pathways (Xiong et al, 2009; Yan et al, 2008; Yang et al, 2005; Yang et al, 2002; Yang et al, 1997) have been recently reported including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition-driven necrosis, necroptosis, ferroptosis, pyroptosis (inflammatory cell death), parthanatos (excessive oxidative damage to DNA leading to overactivation of poly(ADP-ribose), polymerase [PARP]), entotic cell death, neutrophil extracellular trap (NETotic) cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, mitotic catastrophe (Galluzzi et al, 2018; Shao et al, 2021a; Wang et al, 2019), and alkaliptosis and oxelptosis (Tang et al, 2019).

Among the 16 cell death types, most cell death types are involved in inflammation (Galluzzi et al, 2018; Wang et al, 2019; Yan et al, 2008; Yang et al, 2021). Moreover, a new pathway for proinflammatory programmed cell death, PANoptosis, has been described, which is controlled by a recently identified cytoplasmic multimeric protein complex named the PANoptosome. The PANoptosome can engage, in parallel, three key modes of programmed cell death—pyroptosis, apoptosis, and necroptosis (Samir et al, 2020). The cells undergoing death emit numerous signals that interact with the host to dictate the immunological signature of cellular stress and death. In the absence of reactive (nontolerated) antigenic determinants (which is generally the case for healthy cells), such signals may drive inflammation but cannot engage adaptive immunity.

Conversely, when cells exhibit sufficient reactive nontolerated antigenicity, as in the case of infected or malignant cells with tumor antigens (Yang and Yang, 2005; Yang et al, 2006), their death can culminate with adaptive immune responses that are executed by cytotoxic T lymphocytes and elicit immunological memory (Kroemer et al, 2022; Shen et al, 2019).

In addition to carcinogenic factors of CS exposure, oxidative stress plays significant roles in mediating CS-triggered cell death (Aoshiba and Nagai, 2003). CS exposure induces cellular senescence and seven different types of cell death including apoptosis, necrosis, necroptosis, ferroptosis, pyroptosis (Lopez-Pastrana et al, 2015; Wang et al, 2016a; Yin et al, 2015), parthanatos pathway of cell death, and autophagic cell death (Table 9) (Liu et al, 2021). CS exposure induces mitochondria-to-nuclear translocation of apoptosis-inducing factor and endonuclease G within the first 3 h characteristic of the parthanatos pathway.

Table 9.

Cigarette Smoke Induces Seven Types of Inflammation-Related Cell Death and Senescence

Smoke components	Model/cell type	Effects	PMID
CSE	Bronchial epithelial cells	Induced cell death via repressing PRMT6/AKT signaling	33260152
CS	Bronchial epithelial cells	Activated parthanatos pathway of cell death	31396404
CSS	Olfactory epithelium	Induced apoptotic cell death and TNF expression	29950987
CS	Alveolar epithelial cells	Enhanced oxidative stress-induced apoptosis and/or necrosis	19570263
TS	Human premonocytic	Induced apoptosis and necrosis	9755110
CS	Human bronchial epithelial cell	Induced necroptotic cell death and release of DAMP and proinflammatory cytokines	26719146
CS	Human umbilical vein ECs	Induced prolonged endoplasmic reticulum stress and autophagic cell death	21676957
CSE	Bronchial epithelial cells	Reduced cell proliferation Increased β-galactosidase activity Increased cellular senescence	27237816
CSE	Mice and human bronchial epithelial cells	Increased expression of NLRP3 Increased caspase-1 activity and cleaved GSDMD Increased release of IL-1β and IL-18 Induced pyroptosis	33465393
CSE	Human bronchial epithelial cells	Induced ferroptosis cell death	31316058
Acute cigarette smoke exposure	Normal and COPD human bronchial epithelial cells	Induced apoptosis and CXCL8/IL8 production Induced epithelial to mesenchymal transition	28468623
CSE	Human bronchial epithelial cells/bronchoalveolar lavage fluid of CS-exposed mice	Induced necroptotic cell death Induced release of DAMPs Induced release of proinflammatory cytokines via MyD88 signaling	26719146
CS	Bovine pulmonary artery ECs and rat lung microvascular ECs/mice	Increased EC apoptosis	24853558
CSE	Rat VSMCs	Induced ferroptosis and cell death Increased inflammatory responses	31975626

CS induces seven types of newly characterized cell death pathways in epithelial cells and ECs including apoptosis, necrosis, ferroptosis, pyroptosis, necroptosis, parthanatos pathway cell death, autophagic cell death, and cellular senescence (PMID:29362479; 31355136).

CSS, cigarette smoke solution; CXCL8, C-X-C motif chemokine ligand 8; DAMP, danger-associated molecular pattern; PRMT6, protein arginine methyltransferase 6; TS, tobacco smoke.

These diseases cause oxidative stress in neurons that also produce peroxynitrite (ONOO^−, also a nuclear ROS) from the reaction of superoxide anions and nitric oxide causing DNA damage with subsequent PARP-1 activation (Sun et al, 2020). The use of a specific PARP-1 inhibitor, BMN673, abrogates the effect of smoke-induced activation of the parthanatos pathway. Smoke-mediated activation of the parthanatos pathway is increased in human bronchial epithelial cells obtained from chronic smokers (Künzi and Holt, 2019).

Inflammatory cell death can amplify inflammation via at least three pathways: (i) caspase-1-caspase-4 (human)/caspase-11 (mouse) facilitated N-gasdermin D membrane pore to release big proinflammatory secretomes as we previously reported (Lu et al, 2022; Lu et al, 2021; Ni et al, 2021; Xu et al, 2021); (ii) release of alarmins (DAMPs) from dying cells (Yang et al, 2017a); and (iii) NETotic cell death (Delgado-Rizo et al, 2017).

High-mobility group box protein 1 (HMGB1), a highly conserved nonhistone nuclear protein, binds to DNA to regulate the structure of chromosomes and maintain transcription, replication, DNA repair, and nucleosome assembly. HMGB1 is actively or passively released into the extracellular space during cell activation or necrosis. Extracellular HMGB1 as an alarmin can initiate immune responses alone or combined with other substances such as nucleic acid. It has been reported that HMGB1 is involved in various inflammatory responses, the inflammatory-reparative response (Foglio et al, 2022), and autoimmunity (Dong et al, 2022). In addition to HMGB1, alarmins also include a surprising number of chromatin-binding moieties, such as HMGN1, IL-1α, and IL-33, as well as heat shock proteins, S100 proteins, ATP, and uric acid crystals (Yang et al, 2017a). It has been reported that CS can silence innate lymphoid cell function and facilitates an exacerbated type I IL33-dependent inflammatory response to infection (Kearley et al, 2015).

Taken together, five nucleus-localized ROS can activate CS-promoted DNA damage (Zeng et al, 2018) and potentially trigger cell death. CS-induced cell death can amplify inflammation and immune responses. In the future, more detailed work is needed to determine whether combinations of several cell death pathways mediate trained immunity-promoted CS-triggered cell death.

Cigarette Smoke Increases ROS and EC Activation/Dysfunction to Induce Vascular Inflammation and Subsequently Promote CVDs

Vascular endothelium plays a fundamental role in the regulation of vascular tone, inflammatory response, vascular growth, and thrombotic balance by producing important vasodilators with antiatherosclerotic and antiaggregatory properties such as nitric oxide (NO) and prostacyclin (Vane et al, 1990). Endothelium is among the first line of the body's defense system. The normal vascular endothelium is taken as a gatekeeper of cardiovascular health, whereas abnormality of vascular endothelium is a major contributor to CVDs, such as atherosclerosis, aging, hypertension, obesity, and diabetes. Based on the capacities of ECs in carrying out 11 important innate immune functions that are originally considered to be carried out by prototypic innate immune cell-type macrophages, in 2013, we proposed a novel concept that ECs are innate immune cells (Drummer et al, 2021a; Mai et al, 2013; Shao et al, 2021a; Shao et al, 2020).

To consolidate this new concept, we reported that ECs are equipped with innate immune sensing function including canonical inflammasome and caspase-1 (Li et al, 2017; Li et al, 2016b; Lopez-Pastrana et al, 2015; Lu et al, 2021; Wang et al, 2016a; Yin et al, 2015), immune checkpoint receptor upregulation in response to proinflammatory cytokine stimulation (Shen et al, 2019), upregulation of six types of canonical and noncanonical secretomes (Lu et al, 2022), upregulation of a list of new CDs (clusters of differentiation) (Xu et al, 2021), and upregulation of innate immune regulatome and myelopoiesis factor (Shao et al, 2021a).

The five cardinal signs of inflammation are redness, swelling, heat, pain, and loss of function. ECs play a major role in the initiation of inflammatory process. Endothelial activation encompasses a range of endothelial responses to inflammatory signals including changes in thromboresistance, altered vasomotor tone, and loss of barrier function. Once activated, the EC facilitates cellular trafficking. Leukocyte activation and transmigration are important for innate and adaptive immunity. Endothelial dysfunction is widely used to describe the nonphysiologic activity of ECs. The pathophysiology of endothelial dysfunction is complex and multifactorial (Shao et al, 2014). These include imbalanced vasodilation and vasoconstriction, decreased bioavailability of NO, reduced activity of endothelial nitric oxide synthase (eNOS), and increased production of ROS, increased proinflammatory factors. The resulting oxidative stress caused by these factors within the vascular wall contributes significantly to the pathophysiology of endothelial dysfunction and, subsequently, vascular inflammation and CVDs (Shao et al, 2014).

Endothelial dysfunction is a central early event in the pathogenesis of most CVDs and provides an important link between diseases such as hypertension, chronic renal failure, and diabetes. Previous reports showed that inflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interact with ECs or VSMCs to induce endothelial nitric oxide dysfunction, ROS production, and VSMC proliferation, resulting in endothelial dysfunction and promotion of CVD. Indications regarding endothelial function are mainly assessed using flow-mediated dilation (FMD). FMD is a noninvasive measurement of brachial artery diameter changes following an increase in shear stress induced by reactive hyperemia (Corretti et al, 2002). Brachial FMD showed a significant predictive value for cardiovascular events (Ras et al, 2013). CS is the most important modifiable risk factor and plays a critical role in the pathogenesis and development of atherosclerotic CVDs by, at least in part, endothelial dysfunction.

CS causes increased oxidative stress by several mechanisms, including (i) direct damage by radical species and (ii) the inflammatory response caused by CS. CS-induced oxidative stress and ROS induce expression of cell adhesion molecules to promote leukocytes—EC adhesion and VSMC proliferation (Table 10) (Teasdale et al, 2014). CS, e-cigarette, and nicotine administration have all been shown to reduce NO bioavailability, which is the central mechanism in the pathophysiology of endothelial dysfunction (DiGiacomo et al, 2018; He et al, 2017).

Table 10.

Cigarette Smoke Induces Endothelial Cell Activation/Dysfunction and Vascular Inflammation via Increasing Reactive Oxygen Species Production, Activating Nuclear Factor Kappa B, and Upregulating Adhesion Molecules

Smoke components	Model/cell type	Effects	PMID
CSE	Mouse cerebral microvascular ECs	Decreased cell viability Increased paracellular permeability Increased proinflammatory cytokines Increased ROS production	31666540
CSE	Pulmonary ECs	Increased endothelial monolayer permeability Induced EC activation and promoted lung inflammation	29351435
CSE	HMVEC-L	Increased PAF which promotes transendothelial migration Increased cell surface expression of adhesion molecules such as P-selectin, E-selectin, ICAM-1, and VCAM-1 Increased polymorphonuclear leukocytes adherence	21984569
CSE	Carotid arteries of rats	Impaired vascular relaxations and induced endothelial dysfunction Increased ROS production Increased expression of adhesion molecules (ICAM-1, VCAM-1, and E-selectin) and monocyte adhesion. Increased expression of inflammatory markers (iNOS, TNF-α, IL-1β, and IL-6) Induced activation of NF-κB	17213480
CSC	HUVEC	Increased expression of ICAM-1 and VCAM-1 Induced activation of NF-κB Increased transendothelial migration of monocytes and neutrophils Induced activation of protein kinase C	8928867 21651795
CSE	HLMVECs/mouse lung tissue cells	Increased expression of CXCR3 Increased responsiveness to EMAP II and IP-10 to induce apoptosis	22936405
CS and e-cigarette	Male C57BL/6 mice	Increases superoxide radical and NADPH oxidase in mouse aorta Decreases eNOS, p-eNOS Ser1177, and BH4 expression in aorta Impaired ACh-induced endothelium-dependent relaxation in mouse aortic segments	32412791 35089811
CSE	HAECs	Decrease in eNOS expression Increase in expression of iNOS, NLRP3, caspase-1p20, and IL-1β	34768128
CS and e-cigarette	Healthy smokers and nonsmoker adults	Increased soluble NOX2-derived peptide and oxidative stress Decrease in NO bioavailability and FMD	27108682

Ach, acetylcholine; CSC, cigarette smoke condensate; CXCR3, C-X-C motif chemokine receptor 3; EMAP II, endothelial monocyte-activating polypeptide II; eNOS, endothelial nitric oxide synthase; FMD, flow-mediated dilation; HLMVECs, human lung microvascular endothelial cells; HuAoSMCs, human aorta primary smooth muscle cells; HUVEC, human umbilical vein endothelial cells; IP-10, interferon-inducible protein–10; NADPH, nicotinamide adenine dinucleotide phosphate; PAF, platelet-activating factor.

eNOS is an enzyme that is responsible to produce NO in ECs. Exposure to CS in EC can reduce the expression of eNOS genes and proteins, resulting EC dysfunction (He et al, 2017; Su et al, 1998). A decrease of eNOS and NO levels will increase vascular tone, increase expression of adhesion molecules, trigger coagulation cascade and inflammation (Kaur et al, 2018), and increases the risk of atherosclerotic CVDs (Fig. 4). Measurement of FMD represents a useful tool to assess the impacts of smoking on the vascular wall and the efficacy of treatment options for early smoking-induced proatherogenic changes in the vasculature. CS showed a significant decrease in FMD (Esen et al, 2004). A possible mechanism for lowering FMD induced by CS is reduced production of NO and increased expression of adhesion molecules, leading to impaired endothelial function.

FIG. 4.

The schematic figure showed that CS and nicotine induce endothelial dysfunction by binding to different types of DAMP receptors and causing direct damage to endothelial cells, decreasing eNOS and NO bioavailability leading to increased oxidative stress and adhesion molecules, therefore increasing inflammatory response and vascular inflammation to promote initiation and progression of cardiovascular diseases. eNOS, endothelial nitric oxide synthase; NO, nitric oxide.

Conclusion

Cigarette smoke (CS) has a major impact on health issues worldwide. Many of the health care consequences of cigarette smoking could be related to its ability to depress the immune system although the mechanisms by which CS alters immunity are not completely understood. The toxic chemicals present in CS have broad immunomodulatory consequences that include altered innate and adaptive immunity and interrupt immunological homeostasis. This leads to several diseases and exerts paradoxical effects on inflammation. We published one such inflammation paradox between proinflammatory cytokines and microRNAs (Liu et al, 2020). We observed increased ROS and oxidative stress generation, increased DNA damage and cell death, as well as metabolic reprogramming and epigenetic modification. In particularly, CS acts as a double-edged sword that either exacerbates pathological immune responses or attenuates the defensive function of the immune system, possibly due to the complexities and functional diversities of CS components.

Even though CS exerts diverse effects on immune responses, the net effect is deleterious rather than beneficial. We have reviewed experimental evidence that showed (i) the signals of DAMP receptors contribute to CS modulation of inflammation and immunity; (ii) CS reprograms immunometabolism and trained immunity-/trained innate immune tolerance-related metabolic pathways in innate immune cells and T cells; (iii) five nucleus-localized ROS activate CS-promoted DNA damage and cell death pathways to potentially amplify inflammation and immune responses; and (iv) CS increases ROS and EC activation/dysfunction to induce vascular inflammation and subsequently promotes CVDs.

Cigarette smoking attributes significantly to CHD deaths and doubles the risk of ischemic stroke. Smoking acts synergistically with other disease risk factors, substantially increasing the risk of CHD, peripheral vascular disease, cancer, chronic lung disease, and many other chronic diseases (Shinton and Beevers, 1989). In the presence of a high-fat diet (hyperlipidemia), a known trained immunity inducer (Christ et al, 2018), nicotine exposure significantly increased vascular inflammation and atherosclerosis in apolipoprotein E-deficient (ApoE^−/−) mice (Wu et al, 2018). CS in angiotensin II-induced hypertension increased mitochondrial oxidative stress, which contributes to endothelial dysfunction and CVDs (Dikalov et al, 2019). Furthermore, smoking substantially increases the risk of CVD in patients with type 2 diabetes (Yang et al, 2022).

Cigarette smoking is known to both increase susceptibility to infection and drives inflammation. Exposure to CS impaired host defense response and immune suppressive effects by suppressing multiple host defense mechanisms such as epithelial cell responses and recruitment and activation of innate immune cells including monocytes/macrophages, neutrophils, and NK cells. Recently, smokers have been found to be at a higher risk of developing severe forms of coronavirus disease 2019 (COVID-19) (Masso-Silva et al, 2021). E-cigarette aerosol inhalation (vaping) has been associated with several inflammatory lung disorders. Previous study demonstrated that mice exposed to e-cigarette aerosol showed a marked reduction in IgA immunoglobulins and CD4 T cells associated with increased neutrophil activation, indicating that e-cigarette vapers may be at a higher risk of increasing the immune response and developing inflammatory disorders of the lungs (Masso-Silva et al, 2021). E-cigarette-exposed alveolar macrophages show a significant increase in the production of proinflammatory cytokines/chemokines, apoptosis and necrosis, and ROS production (Scott et al, 2018), as well as oxidative stress, ox-LDL, and cardiovascular risk (Carnevale et al, 2016; Moheimani et al, 2017).

Chronic e-cigarette exposure induces vascular endothelial dysfunction, cardiac dysfunction, and atherosclerosis in mice (El-Mahdy et al, 2021; Espinoza-Derout et al, 2019) and increases risk for COPD (Bowler et al, 2017) and CVDs (D'Amario et al, 2019). On the contrary, another clinical study has found that CS or vaping e-cigarettes showed a reduction in the expression of immune-related genes. Also, vaping e-cigarettes was associated with suppression of a large number of unique genes and e-cigarette users showed greater suppression of genes common with those changed in cigarette smokers.

Furthermore, vaping e-cigarettes suppressed the expression of transcription factors, such as early growth response 1, which was functionally associated with decreased expression of 5 target genes in cigarette smokers and 18 target genes in e-cigarette users (Martin et al, 2016).

CS modulates the immune functions of ECs. CS exposure increases pulmonary endothelial barrier permeability and causes endothelial activation/dysfunction. CS decreased eNOS and inhibited eNOS activity. In addition, CS increased ROS production, and oxidative stress-mediated reduction in eNOS-NO signaling is a significant contributor to CS-induced vascular endothelial dysfunction. CS stimulates the expression of P-selectin, E-selectin, ICAM-1, and VCAM-1, as well as cytokines and chemokines, including TNF-α, IL-6, and IL-1β, via NADPH oxidase-dependent NF-κB transcriptional activation. CS also increases adherence of monocytes to the endothelium and transendothelial migration, as well as neutrophil transmigration across HUVECs and induced upregulation of the CXCR3 receptor in ECs. CS synergizes with the inflammatory cytokine IL-1β to increase vascular permeability and endothelial dysfunction via ROS/p38/phosphatase and tensin homolog-mediated tyrosinephosphorylation of vascular endothelial cadherin and β-catenin and subsequent β-catenin nuclear translocation and expression of inflammatory genes, such as COX-2 in cardiac ECs.

In our working model (Fig. 5), we showed that after an initial exposure to the first stimulus, innate immune cells with “memory” traits respond more rapidly and with a high magnitude to secondary stimulation. After the secondary stimulus, cells can respond by increasing the proinflammatory mediator, mediated by (i) metabolic reprogramming involving increased aerobic glycolysis, increased acetyl CoA generation, increased mevalonate synthesis, glutaminolysis, and increased production of lactate and fumarate; and (ii) epigenetic modification including increased H3K27ac and H3K4me3. Also, cell can respond with decreasing the magnitude of immune response (innate immune tolerance) characterized by decreased proinflammatory and increased anti-inflammatory mediators mediated by metabolic reprogramming including decreased aerobic glycolysis, increased production of itaconate metabolites, as well as epigenetic rewiring including increased H3K9me3.

FIG. 5.

Our working model. Representative model of innate immune memory and immune tolerance response. After initial exposure to the first stimulus, innate immune cells with “memory” traits respond rapidly with a high magnitude of immune response to the secondary stimulation, increased proinflammatory mediators mediated by metabolic reprogramming such as increased aerobic glycolysis, increased acetyl CoA generation, increased mevalonate synthesis, glutaminolysis, and increased production of fumarate and lactate as well as epigenetic modification such as increased H3K27ac and H3K4me3. Also, after exposure to the first stimulus, cells respond with a decreased magnitude of innate immune response (tolerance) characterized by decreased proinflammatory and increased anti-inflammatory mediators mediated by metabolic reprogramming such as decreased aerobic glycolysis, increased production of itaconate metabolites, as well as epigenetic rewiring such as increased and H3K9me3. H3K4me3, histone 3 lysine 4 trimethylations; H3K9me3, histone 3 lysine 9 trimethylation; H3K27ac, histone 3 lysine 27 acetylations.

The carcinogenic and immunomodulatory toxins in CS bind to several receptors on the cell membrane, cytosol, and nucleus leading to increases in the cytosolic ROS and five nuclear ROS expression. This leads to the induction of seven types of cell death and subsequent alarmins and NET release, which amplify inflammation and induced trained immunity. Metabolic reprogramming induced by CS can result in the generation of acetyl and methyl groups and induce histone remodeling and exacerbate immune responses to induce trained immunity. CS exposure significantly upregulates the IRG1 expression and increases the abundance of itaconate metabolites (Titz et al, 2020) resulting in immunosuppression.

The anti-inflammatory effect of CS is counteracted by the proinflammatory effect of other CS constituents, resulting in reduced immunosuppression effects and increased inflammation and trained immunity in a phenomenon known as inflammation paradox (Liu et al, 2020; Virtue et al, 2017) or second inflammation wave as we proposed (Johnson et al, 2020; Liu et al, 2020) (Fig. 6).

FIG. 6.

Our working model. The carcinogenic and immunomodulatory toxins in CS bind to different receptors on the cell membrane, cytosol, and nucleus leading to increased cytoplasmic (cytosolic and non-nucleus organelle) ROS, which further induces five different nuclear ROS. Increased ROS production results in the induction of seven types of cell death and subsequent alarmins and neutrophil extracellular trap release, which amplify inflammation and induce trained immunity. Metabolic reprogramming induced by CS can induce epigenetic remodeling and exacerbate immune repose to induce trained immunity. CS exposure significantly upregulates the IRG1 expression and increases the abundance of itaconate metabolites resulting in immunosuppression. The anti-inflammatory effect of CS is counteracted by the proinflammatory effect of other CS constituents resulting in reduced immunosuppression effects and increased inflammation and trained immunity in a phenomenon known as inflammation paradox1 or second inflammation wave. AChR, acetylcholine receptor; CLRs, C-type lectin receptors; GPCRs, G-protein-coupled receptors; NLRs, NOD-like receptors; PAMPs, pathogen-associated molecular patterns; RAGE, receptor for advanced glycation end products; RLRs, retinoic acid-inducible gene-like receptors; TREMs, triggering receptors expressed on myeloid cells; TLRs, toll-like receptors.

In conclusion, the effects of CS on immune responses are wide-ranging and complex, and both potentiation (proinflammatory/trained immunity) and attenuation (anti-inflammatory/trained tolerance) may be induced. The net effect of CS on the immune response and inflammation depends on many variables, including the dose and duration of CS use, cell types, tissue types, and contexture, and the presence of other factors at the time of immune cell stimulation. The recognition of specific mechanisms by which CS affects immune response is an important step toward elucidating mechanisms of CS-induced diseases and may identify novel therapeutic approaches for the management of diseases that afflict smokers.

Abbreviations Used

α7nAChR

α7 nicotinic acetylcholine receptor

Ach

acetylcholine

AChR

acetylcholine receptor

AhR

aryl hydrocarbon receptor

ALDOA

aldolase, fructose-bisphosphate A

ApoE^−/−

apolipoprotein E deficient

ATF3

activating transcription factor 3

B[k]F

benzo[k]fluoranthene

BaP

benzo[a]pyrene

BD

1,3-butadiene

BMDMs

bone marrow-derived macrophages

CAD

cis-aconitate decarboxylase

CCL20

chemokine (C-C motif) ligand 20

CCR6

C-C motif chemokine receptor 6

CDs

clusters of differentiation

CHD

coronary heart disease

CKD

chronic kidney diseases

CLRs

C-type lectin receptors

COPD

chronic obstructive pulmonary disease

COVID-19

coronavirus disease 2019

COX-2

cyclooxygenase-2

CpG

cytosine-phosphate-guanine

CS

cigarette smoke

CSC

cigarette smoke condensate

CSE

cigarette smoke extract

CTLs

CD8⁺ cytotoxic T lymphocytes

CVD

cardiovascular disease

CXCL8

C-X-C motif chemokine ligand 8

CXCR3

C-X-C motif chemokine receptor 3

CYP1A1

cytochrome P450 family 1 subfamily A member 1

DAMPs

danger-associated molecular pattern

DCs

dendritic cells

DNMTs

DNA methyltransferases

dsDNA

double-stranded DNA

e-cigarettes

electronic cigarettes

ECs

endothelial cells

EMAP II

endothelial monocyte-activating polypeptide II

eNOS

endothelial nitric oxide synthase

ESRD

end-stage kidney disease

FA

formaldehyde

FMD

flow-mediated dilation

Foxp3

forkhead box P3

GAPDH

glyceraldehyde-3-phosphate dehydrogenase

GMCSF

granulocyte/macrophage-colony-stimulating factor

GPCRs

G-protein-coupled receptors

GSDMD

gasdermin D

H1R

histamine receptor 1

H₂O₂

hydrogen peroxide

H3K27ac

histone 3 lysine 27 acetylation

H3K4me3

histone 3 lysine 4 trimethylation

H3K9me2

histone 3 lysine 9 dimethylation

H3K9me3

histone 3 lysine 9 trimethylation

HAECs

human aortic endothelial cells

HATs

histone acetyltransferases

HDACs

histone deacetylases

HFD

high-fat diet

HLMVECs

human lung microvascular endothelial cells

HMEC-1

human microvascular endothelial cell 1

HMGB1

high-mobility group box protein 1

HMGN1

high-mobility group nucleosome binding domain 1

HOCl

hypochlorous acid

HPBECs

human primary bronchial epithelial cells

HuAoSMCs

human aorta primary smooth muscle cells

HUVECs

human umbilical vein endothelial cells

IBD

inflammatory bowel disease

ICAM-1

intercellular adhesion molecule 1

IFN-γ

interferon-gamma

IκBζ

inhibitor of nuclear factor-kappa B zeta

IL

interleukin

iNOS

inducible nitric oxide synthase

IP-10

interferon-inducible protein–10

IRG1

immune responsive gene 1

IRI

ischemia–reperfusion injury

JNK

c-Jun N-terminal kinase

Keap1

kelch-like ECH-associated protein 1

LDH

lactate-dehydrogenase

LDL

low-density lipoprotein

LPS

lipopolysaccharide

MDA

malondialdehyde

MDMs

monocyte-derived macrophages

MFF

mitochondrial fission factor

MHC-II

MHC class II molecules

MPO

myeloperoxidase

MS

multiple sclerosis

mTOR

mammalian target of rapamycin

NADPH

nicotinamide adenine dinucleotide phosphate

NETs

neutrophil extracellular traps

NF-κB

nuclear factor kappa B

NK

natural killer

NLRP3

NOD-, LRR-, and pyrin domain-containing protein 3

NLRs

NOD-like receptors

NNK

4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

NNN

N′-nitrosonornicotine

NO

nitric oxide

NO₂⁻

nitrogen dioxide

NO_x

nitrogen oxides

Nrf2

nuclear factor erythroid 2-related factor 2

NTHI

nontypeable Haemophilus influenzae

O₂⁻

superoxide

OCR

oxygen consumption rate

OH⁻

hydroxyl radical

OPA1

optic atrophy protein 1

ORs

odorant receptors

ox-LDL

oxidized low-density lipoprotein

PAF

platelet-activating factor

PAMPs

pathogen-associated molecular patterns

PARP

poly(ADP-ribose) polymerase

PBMCs

peripheral blood mononuclear cells

PI3-AKT

phosphoinositide-3-kinase–protein kinase B

PMA

phorbol myristate acetate

PMA

phorbol myristate acetate

PMNs

polymorphonuclear neutrophils

PRMT6

protein arginine methyltransferase 6

RA

rheumatoid arthritis

RAGE

receptor for advanced glycation end products

RLRs

retinoic acid-inducible gene-like receptors

RNS

reactive nitric species

ROS

reactive oxygen species

SDH

succinate dehydrogenase

sICAM-1

soluble intercellular adhesion molecule 1

SLE

systemic lupus erythematosus

SOD

superoxide dismutase

STAT3

signal transducer and activator of transcription 3

TCA

tricarboxylic acid

TGF-β

transforming growth factor-β

Th

T helper cell

TICAM1

toll-like receptor adaptor molecule 1

TLRs

toll-like receptors

TNF-α

tumor necrosis factor-α

Tregs

T regulatory cells

TREMs

triggering receptors expressed on myeloid cells

TS

tobacco smoke

TXNIP

thioredoxin-interacting protein

UC

ulcerative colitis

VCAM-1

vascular cell adhesion molecule 1

VEGF

vascular endothelial growth factor

VLDL

very-low-density lipoprotein

VSMCs

vascular smooth muscle cells

WT

wild type

Author Disclosure Statement

No competing financial interests exist.

Funding Information

Our research activities are supported by grants from the National Institutes of Health (NIH)/(HL131460, HL132399, HL138749, HL147565, HL130233, DK104116, DK113775, P30 DA13429, RO1 DA040619, and RO1 DA049745). The content in this article is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.