Endogenous inhibitory mechanisms in asthma

Abstract

Endogenous inhibitory mechanisms promote resolution of inflammation, enhance tissue repair and integrity, and promote homeostasis in the lung. These mechanisms include steroid hormones, regulatory T cells, IL-10, prostaglandin E₂, prostaglandin I₂, lipoxins, resolvins, protectins, maresins, glucagon-like peptide-1 receptor, adrenomedullin, nitric oxide, and carbon monoxide. Here we review the most recent literature regarding these endogenous inhibitory mechanisms in asthma, which remain a promising target for the prevention and treatment of asthma.

Since the publication of a review summarizing the known endogenous inhibitory mechanisms in asthma more than 20 years ago,¹ our understanding of these pathways has deepened significantly. Here we present an updated review of the endogenous inhibitory mechanisms that orchestrate the resolution of inflammation and tissue repair in asthma (Table I).2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 Impairment of endogenous inhibitory mechanisms may amplify the underlying inflammation in asthma, thus increasing disease severity. Understanding endogenous inhibitory mechanisms may also identify novel targets for therapy.

Table I.

Endogenous inhibitory mechanisms in asthma

Endogenous inhibitory mechanism	Cellular sources	Mediator-specific effects
Steroid hormones	•Adrenal glands and gonads	•Endogenous glucocorticoids have broad anti-inflammatory effects, mainly by binding intracellular receptors and modulating gene transcription2 •11β-HSD1 and 11β-HSD2 regulate local levels of endogenous glucocorticoids3, 4, 5, 6, 7, 8 •Testosterone decreases the number of ILC22s and inhibits airway inflammation9, 10, 11, 12, 13, 14
IL-10	•Treg cells, dendritic cells, macrophages, mast cells, eosinophils, neutrophils, B cells, and ILC210s	•Treg cells modulate the resolution of airway inflammation15, 16, 17, 18, 19, 20 •IL-10 acts on multiple cells to inhibit inflammatory pathways, including antigen presentation, eosinophil recruitment and activation, and cytokine production21, 22, 23, 24, 25, 26, 27
Prostaglandins	•Epithelial cells, endothelial cells, airway smooth muscle, and monocytes/macrophages	•PGE2 acts via the EP2 to regulates 5-lipoxygenase function and cysteinyl leukotriene production28 •PGE2 inhibits eosinophil migration and ILC2 activation and proliferation28, 29, 30, 31, 32, 33, 34, 35 •PGI2 suppresses TH2 cytokine expression, eosinophilia, and mucus production36
Lipoxins	•Epithelial cells, polymorphonuclear leukocytes, and platelets	•LXA4 promotes the resolution of inflammation and inhibits neutrophil-endothelial cell interactions37,38
Resolvins	•Epithelial cells, endothelial cells, and polymorphonuclear leukocytes	•RvE1 suppresses the nuclear translocation of NF-κB and cytokine production39 •RvE1 inhibits TH17 cell inflammation40 •RvD1 promotes allergen clearance41
Protectins	•Eosinophils	•PD1 elicits apoptotic signals in neutrophils and T cells42 •PD1 inhibits TNF-α and IFN-γ production42,43
Maresins	•Macrophages, neutrophils, and platelets	•MaR1 increases Treg cells and inhibits ILC2 cytokine production44,46 •MaR1 induces Treg formation44
GLP-1R	•GLP-1 is secreted by L cells in the gastrointestinal tract	•GLP-1R agonists reduce airway inflammation via a protein kinase A–dependent inactivation of NF-κB47 •GLP-1R agonists decrease IL-33 release, numbers of ILC2s expressing IL-5 and IL-13, and expression of type 2 cytokines48
AM	•Epithelial cells, endothelial cells, airway smooth muscle, macrophages, and parasympathetic neural cells	•AM increases cyclic AMP in airway smooth muscle49 and decreases AHR50, 51, 52, 53, 54, 55
NO	•Epithelial cells, endothelial cells, macrophages, neutrophils, and mast cells	•NO functions via the stimulation of soluble guanylate cyclase to increase the production of the secondary messenger cGMP •Low levels of endogenous S-nitrosothiols act as a bronchodilator56
CO	•Epithelial cells, endothelial cells, macrophages, and fibroblasts	•HO-1–derived CO activates cGMP and promotes airway smooth muscle relaxation57 •CO decreases the alarmin response from airway epithelial cells58 •Administration of low-dose CO can reverse AHR59

cGMP, Cyclic guanosine monophosphate; ILC2₁₀, IL-10–producing ILC2; MaR1, maresin-1; NF-κB, nuclear factor-κB; RvE1, resolvin E1.

Steroid hormones

Endogenous glucocorticoids suppress inflammation in asthma through multiple molecular mechanisms, including activation of anti-inflammatory genes, repression of proinflammatory genes, and posttranscriptional modifications.² The diurnal patterns of endogenous glucocorticoids, such as cortisone and cortisol, are abnormal in asthmatic patients at baseline and during an exacerbation. For instance, asthmatic patients with a persistent asthma exacerbation have increased levels of serum cortisone in the afternoon compared with the levels in asthmatic patients in remission.⁶⁰ Asthmatic patients also have higher cortisol levels and cortisol-to-cortisone ratios after an inhaled allergen challenge than healthy control subjects do. In asthmatic patients, higher baseline cortisol levels were correlated with rapid recovery in FEV₁ value. Conversely, asthmatic patients with late-phase reactions had lower cortisol levels.⁶¹ An increase in late-phase reactions was also observed after cortisol depletion in an experimental dog model of biphasic bronchoconstriction.⁶² Interestingly, cortisol suppression in humans has been shown to enhance IgE-dependent inflammatory processes.⁶³

Local factors in the human lung can also regulate endogenous glucocorticoid activation and inactivation.³ For instance, 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) catalyzes the conversion of inactive cortisone to active cortisol.⁶⁴ In contrast, 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) catalyzes the inactivation of cortisol to cortisone.^4,5 Interestingly, exposure of human airway smooth muscle cells to IL-13 increased mitogen-activated protein kinase (MAPK) signaling and expression of 11β-HSD1. In turn, this led to an increase in the conversion of cortisone to cortisol and downstream glucocorticoid receptor transcriptional activity.⁶ Others have demonstrated that the upregulation of epithelial 11β-HSD2 leads to impaired endogenous glucocorticoid activation, which enables the proinflammatory effects of IL-13.⁷ The enzymatic regulation of the cortisone/cortisol shuttle occurs in the lung’s structural cells and immune cells relevant to the pathogenesis of asthma. In murine T lymphocytes, the activation of 11β-HSD1 and the downstream increase in glucocorticoid production can modulate their differentiation, effector functions, and apoptosis.⁸

Sex steroids can also regulate airway inflammation in asthmatic patients. In humans, lower levels of testosterone are correlated with higher asthma prevalence and lower FEV₁ value.9, 10, 11 Furthermore, the administration of nebulized dehydroepiandrosterone-3-sulfate improves disease control in patients with moderate-to-severe asthma.¹² Mechanistically, testosterone has been shown to decrease the number of lung type 2 innate lymphoid cells (ILC2s) and the expression of IL-33 and thymic stromal lymphopoietin (TSLP) in mice.¹³ In addition, testosterone promotes the relaxation of airway smooth muscle via an epithelial and nitric oxide (NO)-dependent pathway.¹⁴ Finally, progesterone can inhibit airway remodeling, glucocorticoid resistance, and mast cell degranulation in animal studies.^65,66

Treg cells and IL-10

Regulatory T (Treg) cells are a specialized population of T cells that promote peripheral tolerance, maintain homeostasis, and limit chronic inflammation in asthma and other allergic diseases (Fig 1, A).15, 16, 17, 18 Treg cells regulate airway inflammation by releasing several cytokines, including IL-10, IL-35, TGF-β, fibrinogen-like protein 2, CD39, and CD73.¹⁹ Treg cells can also modulate inflammation via cell contact–mediated suppression (cytotoxic T-lymphocyte antigen-4 [CTLA-4], lymphocyte activating gene-3 [LAG-3], galectin-1, and T-cell immunoglobulin and mucin domain 3 [TIM-3]).¹⁵ Patients with moderate-to-severe asthma have a higher percentage of CD4⁺FoxP3⁺ Treg cells in their bronchoalveolar lavage fluid than do patients with mild asthma and healthy controls. Similarly, CD4⁺ CTLA-4 and CD103 expression levels were higher in patients with moderate-to-severe asthma.²⁰

Fig 1.

Endogenous inhibitory mechanisms in asthma. A, Treg cells promote tolerance and limit lung inflammation via the regulation of T_H cells, antigen-presenting cells, mast cells, eosinophils, and ILC2s. B, The biosynthesis of lipoxins from arachidonic acid by epithelial cells, polymorphonuclear leukocytes, and platelets leads to the resolution of lung inflammation. C, Anti-inflammatory effects of GLP1R agonism on epithelial cells, ILC2s, eosinophils, and goblet cells. D, Low levels of endogenous S-nitrosothiols act as a bronchodilator.

IL-10, in particular, has multiple inhibitory properties that regulate the resolution of inflammation in asthmatic patients.²¹ For instance, IL-10 suppresses the function of antigen-presenting cells.²² Myelomonocytic cells exposed to IL-10 upregulate the IL-1 receptor antagonist, decreasing airway hyperreactivity and eosinophil chemotaxis.²³ IL-10 also inhibits nuclear factor-κB (NF-κB) activation in monocytes,²⁴ cytokine generation from bone marrow–derived mast cells,²⁵ pathogenic IFN-γ responses to allergens,²⁶ and allergen-induced neutrophil and eosinophil recruitment to the airways.²⁷ Although Treg cells are an important source of IL-10, other cells, including dendritic cells, macrophages, mast cells, eosinophils, neutrophils, and B cells, can also produce this inhibitory cytokine.⁶⁷ Interestingly, IL-10–producing ILC2s (ILC2₁₀s) have recently been identified.⁶⁸ In mice, ILC2₁₀s suppress proinflammatory ILC2 effector functions and regulate allergic lung inflammation.⁶⁹

Bronchoalveolar lavage fluid from asthmatic patients has lower levels of IL-10 than in bronchoalveolar lavage fluid from nonasthmatic controls⁷⁰; similarly, PBMCs of asthmatic patients produce less IL-10 at baseline and when simulated.⁷¹ In addition, alveolar macrophages from asthmatic patients release a lower amount of IL-10 at baseline and after IL-1β stimulation than do those from nonasthmatic patients.⁷² Glucocorticoid-induced IL-10 release from CD4⁺ T lymphocytes is also defective in asthmatic patients.⁷³ Finally, specific IL-10 polymorphisms are associated with an increased asthma prevalence in adults.⁷⁴ Furthermore, compared with controls. patients with severe asthma are less likely to have the putative high–IL-10–producing haplotype and more likely to have the putative low–IL-10–producing haplotype.⁷⁵

Multiple studies have proved the importance of Treg cells in regulating immune tolerance in the lung. For instance, the expansion of Treg cells via a TNF receptor superfamily member 25 (TNFR25)-specific mAb led to the suppression of allergic lung inflammation in mice, including inhibition of T_H2 cytokine production.⁷⁶ Treg cells are also important in regulating the priming responses to allergens, as demonstrated by an article showing that depletion of Treg cells during the sensitization phase led to augmented allergic airway inflammation in mice.⁷⁷ Interestingly, depletion of Treg cells during the elicitation phase did not have a similar effect.⁷⁸ These results contrast with those of other murine studies showing that the adoptive transfer of Treg cells during an allergen challenge attenuates airway inflammation.⁷⁹

Prostaglandins and lipoxins

Certain prostaglandins and lipoxins can also promote the resolution of allergic lung inflammation (Fig 1, B). Prostaglandin E₂ (PGE₂) is a naturally occurring prostaglandin that acts via the E prostanoid receptor 2 (EP₂) to inhibit 5-lipoxygenase function and cysteinyl leukotriene (CysLT) production.²⁸ In a murine model of allergic asthma, the PGE₂-EP₂ axis, specifically in T cells, inhibited allergic airway inflammation. EP₂ knockout mice had worse airway inflammation than wild-type controls did. Furthermore, administering a PGE₂ analog protected wild-type mice against airway inflammation but not EP₂ knockout mice.²⁹ Specifically, PGE₂ protects against allergic lung inflammation by regulating macrophage polarization,³⁰ inhibiting eosinophil migration and activation,^31,32 and limiting ILC2 proliferation and activation.^33,34 Moreover, human PMNs exposed to PGE₂ switched eicosanoid biosynthesis from leukotriene E₄ (LTE₄) to lipoxin A4 (LXA₄), which was associated with resolution of inflammation.³⁵ In addition, LXA₄ inhibits PMN-endothelial cell interactions.³⁷ Finally, in a rat model of allergic asthma using Angiostrongylus costaricensis, cyclooxygenase-2–derived PGE₂ and LXA₄ promoted the resolution of allergic edema.³⁸

Sputum PGE₂ level is higher in asthmatic patients than in healthy controls. A decreased PGE₂-to– cysteinyl leukotriene ratio may promote airway remodeling and worse lung function in humans.⁸⁰ Level of LXA₄ in exhaled breath condensate was elevated in all severities of asthma, but the LXA₄/LTB₄ ratio in exhaled breath concentrate was lower in patients with severe asthma than in patients with moderate asthma.⁸¹ Notably, there have been several clinical trials aimed at administering arachidonic acid–derived inhibitory mediators for the treatment of asthma,⁸² including PGE₂,83, 84, 85, 86 and LXA₄.^87,88 For instance, LXA₄ inhalation in asthmatic patients decreased LTC₄-induced bronchoconstriction.⁸⁷ Novel synthetic compounds to regulate these pathways, such as the selective formyl peptide receptor type 2 (FPR2)/LXA₄ receptor agonist ACT-389949,⁸⁹ are also under development.

Prostaglandin I₂ (PGI₂), also known as prostacyclin, is another prostaglandin that promotes the resolution of inflammation. PGI₂ suppresses T_H2 cytokine expression, eosinophilia, and mucus production in an ovalbumin murine model of asthma. Also in mice, cicaprost (a PGI₂ analog) inhibits IL-33–driven T_H2 cytokines by CD4⁺ T cells in a PGI₂ receptor–specific manner.³⁶

Resolvins, protectins, and maresins

Docosahexaenoic acid–derived lipid mediators, such as resolvins, protectins, and maresins, also promote tissue homeostasis and the resolution of lung inflammation. For instance, in a murine model of asthma, resolvin E1 (RvE1) inhibits the production of IL-6 and IL-23, both of which are integral components of T_H17 cell inflammation and the response to allergens.⁴⁰ Resolvin D1 (RvD1) promotes the resolution of lung eosinophilia in a murine model of airway inflammation. Furthermore, aspirin-triggered RvD1 (AT-RvD1) enhances murine macrophage phagocytosis and allergen clearance.⁴¹ Protectin D1 (PD1) is generated by T_H2 cell–skewed PBMCs via a 15-lipoxygenase-1–dependent pathway and elicits apoptotic signals in human neutrophils and T cells.⁴² Finally, maresin-1 promotes the resolution of inflammation by increasing Treg cells, resulting in the downstream inhibition of ILC2 activation and cytokine production.⁴⁴ In addition, maresin-1 suppresses proinflammatory signals from human bronchial epithelial cells, enhances tissue regeneration, and favors a return to homeostasis.^45,46

Glucagon-like peptide-1 receptor

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted by L cells in the gastrointestinal tract. GLP-1 receptor (GLP-1R) agonists potentiate glucose-dependent insulin secretion and were initially developed for managing type 2 diabetes mellitus (T2DM). More recently, GLP-1R agonism has been recognized as a novel endogenous inhibitory pathway in asthma (Fig 1, C). Zhu et al⁴⁷ initially demonstrated that treatment with a GLP-1R agonist led to a significant reduction in airway inflammation and mucus secretion in a murine model of asthma. Furthermore, Zhu et al⁴⁷ showed that these effects were mediated by the protein kinase A–dependent inactivation of nuclear factor-κB. Similar findings were observed in obese mice, with decreased airway eosinophilic inflammation and airway hyperresponsiveness (AHR) in the group treated with GLP-1R agonist. Hur et al show that these findings were mediated by IL-1β and the NLRP3 inflammasome.⁹⁰ Notably, in an Alternaria extract model of allergic inflammation, GLP-1R agonist treatment led to reductions in IL-33 release, the numbers of ILC2s expressing IL-5 and IL-13, expression of type 2 cytokines and chemokines, airway eosinophilia, mucus secretion, and AHR.⁴⁸

Others have shown that GLP-1Rs are expressed in human eosinophils and neutrophils and that GLP-1R agonist treatment decreases LPS-induced eosinophil activation.⁹¹ Furthermore, the GLP-1R agonist exendin-4 reduced human bronchial hyperreactivity in an epithelium-independent and cyclic AMP–dependent protein kinase A pathway.⁹² In a more recent publication involving patients with asthma and T2DM, Foer et al found that the asthma exacerbation rate and asthma symptoms were lower in those initiating GLP-1 therapy than in those using other medications to manage T2DM, such as insulin or sulfonylureas.⁹³ The effects of GLP-1 therapy were independent of the improvement in glycemic control and weight loss. Others have also shown the beneficial effects of GLP-1 therapy on the lung function of asthmatic patients with T2DM.⁹⁴ These promising studies highlight the use of GLP-1R agonists as a novel treatment strategy for asthmatic patients with metabolic dysfunction.

AM

Adrenomedullin (AM) is a 33–amino acid vasodilator peptide with multiple biologic effects on health and disease. AM is expressed in several cell populations of the human lung, including epithelial, smooth muscle, endothelial, alveolar macrophage, and parasympathetic neural cells.^50,51 Hagner et al showed that chronic allergic inflammation reduced AM in airway epithelial and endothelial cells in 2 distinct murine models of asthma.⁵² Interestingly, the administration of AM resulted in a decrease in ovalbumin-induced AHR. An AM receptor antagonist reversed these effects of AM. In addition, Hagner et al demonstrated that AM increased wound repair in human and mouse ex vivo studies.⁵² Others have shown that the genetic loss of AM led to an increase in airway smooth muscle hyperplasia and AHR in a murine model of asthma. In this model, AM-deficient mice did not have changes in level of immunoglobulin, level of type 2 cytokines, eosinophilia, or airway secretion.⁵³ In guinea pigs, AM has been shown to induce a dose-dependent inhibition of acetylcholine- and histamine-induced bronchoconstriction.⁵⁴ Moreover, AM can also inhibit the secretion of the cytokine-induced neutrophil chemoattractant from LPS-stimulated rat alveolar macrophages.⁵⁵ Interestingly, plasma AM levels increase as asthma severity progresses and are negatively correlated with FEV₁ value in asthmatic patients.⁹⁵

NO

Endogenous NO is a principal regulator of airway function in humans.⁹⁶ NO is generated during the conversion of l-arginine to l-citrulline by NO synthase (NOS). High levels of NO lead to the detrimental oxidation and nitration of proteins, with downstream effects on airway inflammation. In contrast, low levels of endogenous S-nitrosothiols (SNOs) are beneficial and act as a bronchodilator (Fig 1, D).⁵⁶ In this regard, a study showed that wild-type mice with AHR have low levels of lung SNOs and increased levels of GSNO reductase (GSNOR). Interestingly, GSNOR knockout mice have higher lung SNOs and are protected against AHR.⁹⁷ NO can also have a protective role in the upper airways.⁹⁸ Physiologic levels of endogenous NO have also been shown to suppress plasma leakage in rats, whereas NO production associated with inducible NOS increases plasma leakage.⁹⁹ Evidence of the bronchoprotective effects of NO arise from a study of the effects of NG-nitro-l-arginine methyl ester (L-NAME), a nonselective NOS inhibitor; in which the administration of nebulized L-NAME to subjects increased AHR.¹⁰⁰ In contrast, another human study found that inhibiting endogenous NO does not affect the allergen-induced early and late asthmatic responses.¹⁰¹

CO

The primary source of endogenous carbon monoxide (CO) is the catabolism of hemoglobin by heme oxygenase-1 (HO-1).¹⁰² The HO-1–CO axis has been recognized as having protective roles.¹⁰³ The HO-1 products CO and bilirubin decrease the NOD-like receptor protein 3 (NLRP3)-retinoid X receptor (RXR) axis in human airway epithelial cells. In addition, HO-1 decreases the production of IL-25, IL-33, and TSLP, which are key alarmins in asthma, both in human airway epithelial cells and mice.⁵⁸ The HO pathway has also been shown to have protective effects by inhibiting airway inflammation, oxidate stress, and AHR in a guinea pig model of asthma.¹⁰⁴ Similarly, low-dose CO can reverse AHR in a murine model of asthma, irrespective of the presence of airway inflammation.⁵⁹ HO-1–derived CO can also relax airway smooth muscle by activating guanylyl cyclase and generating cyclic guanosine monophosphate.⁵⁷ Notably, asthmatic patients have higher levels of exhaled CO and HO-1.¹⁰⁵

In conclusion, in the past 2 decades, we have seen a notable advance in our understanding of the endogenous inhibitory pathways in asthma. Novel mechanisms are being explored, such as the transmembrane protein 178, which negatively regulates calcium responses and inflammatory signals in airway epithelial cells.¹⁰⁶ Nevertheless, there are still significant gaps in our knowledge and ability to leverage these pathways for the treatment of asthma and restoration of lung homeostasis. Therefore, further research in this critical aspect of asthma pathogenesis is warranted. These future studies should also explore the interactions and synergy between the various endogenous inhibitory mechanisms.

Disclosure statement

Supported by the National Institute of Allergy and Infectious Diseases, National Institutes of Health (grant K08AI141765) and the 2022 American Academy of Audiology, Asthma & Immunology Foundation and Donald Y.M. Young, MD, FAAAI, Faculty Development Award (to S.E.C.).

Disclosure of potential conflict of interest: The authors declare that they have no relevant conflicts of interest.