Lung‐resident γδ T cells and their roles in lung diseases

Summary

γδ T cells are greatly enriched in mucosal and epithelial sites, such as the skin, respiratory, digestive and reproductive tracts, and they are defined as tissue‐resident immune cells. In these tissues, the characteristics and biological roles of γδ T cells are distinguished from each other. The lungs represent the most challenging immunological dilemma for the host, and they have their own effective immune system. The abundance of γδ T cells, an estimated 8–20% of resident pulmonary lymphocytes in the lung, maintains lung tissue homeostasis. In this review, we summarize the recent research progress regarding lung‐resident γδ T cells, including their development, residency and immune characteristics, and discuss the involvement of γδ T cells in infectious diseases of the lung, including bacterial, viral and fungal infections; lung allergic disease; lung inflammation and fibrosis; and lung cancer.

Introduction

γδ T cells are a subset of T cells with a T‐cell receptor (TCR) composed of γ and δ chains; however, this TCR does not engage MHC–antigen complexes. Compared with αβ T cells, γδ T cells often express higher levels of activated markers and memory markers early in their development. γδ T cells can rapidly recognize conserved non‐peptide antigens that are up‐regulated by stressed cells and induce effector functions. γδ T cells have been termed non‐conventional and innate‐like T cells because of features that they share with innate immune cells. Additionally, γδ T cells exhibit some degree of immunological memory formation, which is a classic feature of adaptive immune cells. Hence, γδ T cells are considered to be a bridge between innate and adaptive immune responses. γδ T cells are involved in protective immunity against pathogens, tumour surveillance, innate and adaptive immune response regulation, tissue healing, and epithelial cell maintenance.1 Additionally, γδ T cells are involved in a variety of diseases, such as infection, autoimmune disorders (experimental autoimmune encephalomyelitis, collagen‐induced arthritis) and cancer.2

γδ T cells account for a small proportion (1–5%) of peripheral blood lymphocytes. Interestingly, γδ T cells are greatly enriched in mucosal and epithelial sites, such as the skin, respiratory, digestive and reproductive tracts, and approximately 25–60% of the lymphocytes in the gut are γδ T cells.3 In the murine epidermis, all T cells express γδ TCRs.4 They migrate into these tissues early in their development and persist as tissue‐resident cells. In these epithelium‐rich tissue sites, γδ T cells frequently express invariant or closely related γδ TCRs, which results in different biological roles of γδ T cells from one tissue to another. The lungs represent the most challenging immunological dilemma for the host, not only due to the environment, which is usually the first site of pathogen exposure, but also due to their critical physiological function of gas exchange. Consequently, the lungs have their own effective immune system. Herein, we review the recent progress in lung‐resident γδ T cells and their roles in lung diseases.

Characteristics of lung‐resident γδ T cells

The TCR γ chain is made up of V, J and C elements and displays little diversity, whereas the TCR δ chain is composed of V, D, J and C elements and is deleted during α chain recombination. In the mouse thymus, γδ T cells branch off from common thymocyte precursor cells at the DN 2 and DN 3 stage, which then commit to produce either interleukin‐4 (IL‐4), interferon‐γ (IFN‐γ) (γδT1) or IL‐17 (γδT17).5, 6 Scart‐2 and CCR6 segregate with the commitment of γδT17 cells that use a strongly restricted TCR‐γ4 chain, whereas NK1.1 and CD27 expression is observed on γδT1 cells during thymic development.7, 8, 9 In mice, signalling through the γδ TCR and CD27 is required for γδT1 cell differentiation, whereas signalling via the lymphotoxin‐b receptor, transforming growth factor‐β (TGF‐β), IL‐7, B lymphoid kinase (Blk) and the Notch‐transcription factor Hes1 is required for γδT17 cell differentiation. These natural γδT17 cells in the thymus are identified as CCR6⁺ CD27^– CD44^hi IL‐23R⁺ cells and can persist as a self‐renewing pool in peripheral tissues.10, 11, 12

In the neonatal and adult thymus, γδ T‐cell subsets express variant Vγ1, Vγ4 and Vγ7 (the Vγ was based on the nomenclature by Heilig and Tonegawa13) chains in combination with multiple Vδ chains, and they home to the peripheral blood and organs.14, 15 The lung is a preferred site for the homing of γδ T cells in the perinatal period. In a study conducted in 1989, an estimated 8–20% of resident pulmonary lymphocytes were demonstrated to be CD4 and CD8 double‐negative γδ T cells. The usage of γδ V gene segments was limited, and Vγ4, Vδ1 and Vδ6 were selectively expressed.16 γδ T cells located in different organs/tissues express different Vγ gene segments. Vγ6/Vδ1‐expressing γδ T cells predominantly migrate to the lung epithelia, reproductive tracts (uterus and vagina) and tongue, whereas mature Vγ5/Vδ1‐expressing fetal thymocytes mainly migrate to the skin as a dendritic epidermal T‐cell precursor.16, 17 After birth, the pattern of Vγ gene usage of lung‐resident γδ T cells changes with age. Vγ6⁺ γδ T cells are the major γδ T‐cell population from birth until 8–10 weeks of age, whereas Vγ4⁺ γδ T cells predominate from that age on.18 Other Vγ‐defined subsets, such as Vγ1⁺ γδ T cells, represent only a minor population in the normal lung. In normal adult C57BL/6 mice, a population of 2 × 10⁴ to 5 × 10⁴ γδ T cells is divided into subsets expressing Vγ4⁺ (~ 45%), Vγ1⁺ (~ 15%), Vγ6⁺ (~ 20%) and Vγ7⁺ (rare); Vγ5⁺ is absent.19, 20 These γδ T cells are present in all regions of the lung, except for the airway mucosa. Interestingly, Vγ4⁺ and Vγ1⁺ populations have a more parenchyma‐biased distribution.19 Evidence of lymphoid precursors present in the lungs indicates that these cells might undergo differentiation and selection in the lung environment. In Vδ1^−/− mice, a significant block in the development of pulmonary resident γδ T cells was observed, including markedly impaired generation of Vγ6⁺ γδ T cells and a relatively limited junctional diversity of Vδ4 TCR δ chains, which indicated that the invariant Vδ1⁺ T cells were crucial for optimal γδ T‐cell expansion and affect the migration or microenvironment for other γδ T cells in the lungs.15

With progress on tissue‐resident immune cell research, the understanding of γδ T cells located in different organs/tissues is clearer. We summarized the similarities and differences in cell phenotype, migration and function between the lung and other organs/tissues, including the skin, intestine, liver, uterus–vagina, spleen and peripheral blood (Table 1). Tissue‐associated subsets of γδ T cells might respond to tissue‐specific signalling and exhibit different immune functions during tissue homeostasis and dysregulation conditions.

Table 1.

Characteristics of tissue‐resident γδ T cells in mice

Tissue site	Frequency of lymphocytes	Phenotype	Chemokine receptor	Cytotoxicity	Cytokine production	Function	Ref
Skin	3%	Vγ5Vδ 1	CCR10	+	IL‐2, IFN‐γ, KGF, EGF, IGF	Promotes tissue repair, cell survival, proliferation, migration and recruitment; protects against skin carcinogenesis; suppresses GVHD	88, 89
IEL	10–50%	Vγ7, Vγ4	CCR9	+	IFN‐γ, KGF‐1	Cytoprotective, immunomodulatory	90, 91
Lung	8–20%	Vγ6Vδ 1 Vγ4	CCR6	−	Il‐17	Eliminates bacteria and prevents inflammation and lung fibrosis	70
Liver	3·10%	Vγ1Vδ 6·3 (77%) Vγ4	CCR6	−	IL‐4, IFN‐γ IL‐17A	Protects against fulminant hepatitis	92 93
Spleen	0·26%	Vγ1Vδ 6·3 (60·8%) Vγ4	CCR2	−	IL‐4, IFN‐γ	Suppresses IgE responses	82, 92, 94
Uterus	~1–5%	Vγ6Vδ 1 (60%) Vγ4 (20%) Vγ1 (20%)	CCR6	−	IL‐17, IFN‐γ	Protects against infection; Prevents rejection during pregnancy	17, 95
Peripheral blood	~1%	Vγ1 Vγ4		+	IFN‐γ, IL‐4, IL‐17	Protects against infection and tumours	3, 93, 96

IEL, intestinal intra‐epithelial lymphocytes; IL‐2, interleukin‐2; IL‐4, interleukin‐4; IL‐17, interleukin‐17; IL‐17A, interleukin‐17A; IFN‐γ, interferon‐γ; KGF, keratinocyte growth factor; KGF‐1, keratinocyte growth factor 1; EGF, epidermal growth factor; IGF, insulin‐like growth factor.

It remains unknown if similar phenomena that occur in mice apply to human γδ T cells. In human fetal blood at around 20 weeks of gestation, nearly 75–80% of peripheral γδ T cells express Vγ9 and Vδ2 chains, which will decrease to 15–20% before delivery; however, Vγ9Vδ1 and Vγ9Vδ3 γδ T cells will increase to 50% during gestation.21 Vδ1 is also expressed in the thymus, spleen, liver, gut epithelia and dermis.1, 22, 23 Fifty to eighty per cent of peripheral Vγ9Vδ2 T cells have a distinctive T helper type 1 (Th1) signature with IFN‐γ and tumour necrosis factor‐α (TNF‐α) production, but < 1% produce IL‐17. Under appropriate culture conditions, Vγ9Vδ2 γδ T cells polarize to Th2, Th17, follicular helper T and regulatory T cells, indicating a broad plasticity of human peripheral γδ T cells influencing the immune response.24

Involvement of γδ T cells in lung diseases

The abundance of γδ T cells in the lung maintains lung tissue homeostasis. Vγ4 and Vγ1 γδ T cells are the major subsets studied in lung diseases, and the roles of γδ T cells are multiple. Here, we discuss the involvement of γδ T cells in infectious diseases of the lung, including bacterial, viral and fungal infections; allergic disease; inflammation and fibrosis; and cancer (Table 2).

Table 2.

The different γδ T‐cell subsets involved in lung diseases

Condition	TCR expression (γ/δ chain)	Expression change	Effector molecule	Function	Example	Ref
Infection	Vγ1,Vγ4,Vγ6 (mouse)	Increase	IFN‐ γ, IL‐17A	Defence against infection; Resolve inflammation	Streptococcus pneumoniae	28
	Vγ4,Vγ6 (mouse)	Increase	IL‐17A	Inhibit bacterial	Mycobacterium tuberculosis	33
	Vγ9Vδ2 (human)	Decrease	NK inhibitory receptors	Inhibit NK cell	M. tuberculosis	35
	Vγ2Vδ2 (human)	Increase	IFN‐ γ	Down‐regulate IL‐22‐producing Th17 cells	M. tuberculosis	36
	Vγ4− γ1‐ or Vγ4 (mouse)	Increase	IL‐17A	Inhibit bacterial	Bordetella pertussis	37
	Vγ1,Vγ7(mouse)	Increase	IL‐4	Protect against LPS‐induced lung injury	Escherichia coli	41
	Vγ1(mouse)	Increase	IL‐17A	Defence against infection	Aspergillus fumigatus	44
	Vγ4(mouse)	Increase	IFN‐ γ, RANTES, IL‐10, IL‐4 and IL‐5	Anti‐viral inflammation	Respiratory syncytial virus	45
	Vγ2Vδ2 (human)	Increase	IL‐17A	Kill influenza virus‐infected lung alveolar epithelial cells; Inhibit virus replication	Influenza virus	50, 51
Allergy	Vγ1 (human)	Increase	IL‐4	Enhance allergic inflammation	Asthma	53
	Vγ1 (mouse)	Increase	IFN‐ γ	Enhance AHR	Asthma (OVA‐induced)	54
	Vγ4 (mouse)	Increase	IL‐17A	Inhibit AHR	Allergic airway disease (OVA‐induced)	58
Fibrosis	Vγ6Vδ1 (mouse)	Increase	IL‐17A or IL‐22	Inhibit inflammation and fibrosis	Lung fibrosis (Bacillus subtilis‐induced)	70, 71, 72
Cancer	Vδ1 (human)	Decrease	IFN‐ γ	Immune surveillance	Lung cancer	82
	Vγ9Vδ2 (human)	Expanded ex vivo	soluble TRAIL, NKG2D	Induce apoptosis	Lung cancer cell lines (H460 and H125)	85
	Vδ2 and Vδ1 (human)	Expanded ex vivo	perforin, granzyme A and B, FasL, TNF‐α and IFN‐ γ	Cytotoxicity	Lung cancer cell lines (H520, GLC‐82 and H446)	86
	Vγ9Vδ2 (human)	Expanded ex vivo	NKG2D	Cytotoxicity	Non small cell lung cancer	87

IFN‐γ, interferon‐γ; IL‐17A, interleukin‐17A; IL‐4, interleukin‐4; IL‐5, interleukin‐5; IL‐10, interleukin‐10; IL‐22, interleukin‐22; TRAIL, tumor necrosis factor‐related apoptosis‐inducing ligand; NKG2D, natural killer group 2, member D; FasL, Fas ligand; TNF‐α, tumor necrosis factor‐α.

Bacterial infection

Klebsiella pneumoniae is a Gram‐negative extracellular bacterium that causes nosocomial and community‐acquired pneumonia. Interleukin‐17 signalling is critical in recruiting neutrophils into the alveolar space by inducing granulocyte colony‐stimulating factor (G‐CSF) and macrophage inflammatory protein‐2 (MIP‐2) and defence against K. pneumoniae infection.25 In a time dynamics research study, γδ T cells were demonstrated to be the major sources of IL‐17A, which was induced by the cytokines IL‐1β and IL‐23 during K. pneumoniae infection.26 The IL‐17‐producing γδ T cells also play a pivotal role in the early host immune defence stages against acute Pseudomonas aeruginosa pulmonary infection by inducing several chemokines, including G‐CSF, keratinocyte chemoattractant, MIP‐1 and MIP‐2.27

During Streptococcus pneumoniae lung infection, a significant increase in the number of Vγ1⁺, Vγ4⁺ and Vγ6⁺ γδ T cells was observed in the lungs but not in the associated lymphoid tissue.28 These γδ T cells were activated with the expression of CD69 and CD25 and expanded locally. Furthermore, these γδ T cells exhibited lung‐homing capacity in both naive and infected recipients.28 Among them, Vγ4⁺ γδ T cells played an important role in neutrophil‐mediated host defence against S. pneumoniae infection by promoting the production of TNF‐α and possibly MIP‐2 in the lungs.29 Other studies showed that increased γδ T cells in the lungs after S. pneumoniae infection expressed high levels of IFN‐γ and IL‐17A. Activated γδ T cells uniquely co‐express Gr‐1, CD14 and CD274 (PD‐L1), and Gr‐1 expression patterns on lung γδ T cells provide a marker to differentiate IFN‐γ (Gr‐1⁺)‐ and IL‐17A (Gr‐1⁻) ‐producing subsets.30 It was also found that the resolution of S. pneumoniae‐induced lung inflammation was associated with a > 30‐fold increase in γδ T‐cell number, which decreased the number of pulmonary dendritic cells and alveolar macrophages.31

During Mycobacterium tuberculosis infection, γδ T cells dominate the early production of IL‐17 in response to IL‐23 that is present in the lungs early in infection, which is produced by M. tuberculosis‐infected dendritic cells. These IL‐17‐producing γδ T cells continue to produce this cytokine throughout the infection and play a central role in the innate protective response to pulmonary infection.32 Further, γδ T cells expressing TCR Vγ4 or Vγ6 were identified as the major IL‐17A‐producing cells.33 In the lung, mycobacterial infection‐induced granulomas play an essential role in the sequestration and killing of mycobacteria. Interleukin‐17A‐producing γδ T cells play a critical role in inducing mature granuloma formation and protecting against M. tuberculosis infection.33 Consistent with the results from mice, IL‐17‐producing γδ T cells in the peripheral blood are markedly increased in patients with tuberculosis (TB), whereas IFN‐γ‐producing γδ T cells are clearly lower in patients with TB.34 However, Vγ9Vδ2 T cells from broncho‐alveolar lavage samples of patients with active TB showed a strong down‐modulation of CD3 expression, high levels of natural killer inhibitory receptors, low expression of CD16 and a dramatically reduced response to antigen stimulation compared with peripheral blood Vγ9Vδ2 T cells, indicating specifically impaired resident γδ T‐cell functions in situ by active TB.35 Interestingly, activated Vγ2Vδ2 T cells produce IFN‐γ, which further down‐regulates the capability of T cells to produce IL‐22 in lung TB granulomas, suggesting that targeting Vγ2Vδ2 T cells is a potential method to balance the T‐cell response in severe TB.36

During Bordetella pertussis infection, γδ T cells, predominantly Vγ4⁻ γ1⁻ cells, produce IL‐17 in the lungs as early as 2 hr after infection to protect against primary infection by inducing antimicrobial peptides. Further, 7–14 days after challenge in the lungs, pathogen‐specific γδ T cells, exclusively Vγ4, were detected and showed a memory immune response. These lung‐resident γδ T cells with a memory T‐cell phenotype (CD69⁺ CD103⁺) were expanded in the lungs and proliferated rapidly after re‐challenge of B. pertussis in convalescent mice, and they produced greater amounts of IL‐17.37 Interestingly, during early chlamydial infection stages, γδ T cells are the major producers of IL‐17A, whereas at later stages of chlamydial infection, Th17 cells are the major producers of IL‐17A. The protection of IL‐17A against chlamydial lung infection is mainly mediated by Th17 cells, which markedly increase the Th1 response. However, γδ T cells promote IL‐1α production by dendritic cells and induce a Th17 response.38 Hence, during different lung infections, the role of IL‐17A‐producing γδ T cells is different (Fig. 1).

Figure 1.

The roles of lung resident γδT cells in bacterial and viral infections. Lung resident γδ T cells can be activated by antigens, pathogen‐associated molecular patterns (PAMPs), damage‐associated molecular patterns (DAMPs), activating receptor ligands or cytokine signalling, and they are involved in lung infection diseases. (a) During bacterial infection, activated lung γδ T cells produce interleukin‐17 (IL‐17), which recruits neutrophils, induces mature granuloma formation or induces T helper type 17 (Th17) immune responses to perform their defence functions. Moreover, activated lung γδ T cells can produce interferon‐γ (IFN‐γ) to activate pulmonary dendritic cells (DCs) and alveolar macrophages. Additionally, lung γδ T cells induce memory immune response. (b) During viral infections, activated lung γδ T cells produce several types of cytokines, among which some inhibit virus replication and some induce or inhibit lung inflammation.

During polymicrobial severe sepsis, early and highly elevated IL‐17A levels derived from γδ T cells in the peritoneal fluid account for pro‐inflammatory cytokine production, neutrophil infiltration and lung injury.39 Glutamine administration regulates lung γδ T cells by inducing a higher γδ T‐cell percentage; lower γδ T‐cell apoptotic rates; decreased expression of IL‐17A, IFN‐γ and IL‐10 by γδ T cells; as well as lower neutrophil numbers and decreased expression of CXCR2 by neutrophils in the lungs. This immune regulation was shown to be partly responsible for ameliorating acute lung injury induced by sepsis.40 Recently, it was found that γδ T cells protect against lipopolysaccharide‐induced lung injury. In response to Escherichia coli lipopolysaccharide, Vγ1 and Vγ7 γδ T cells expand in the lungs and express IL‐4, which reduces the accumulation of activated M1 macrophages and decreases TNF‐α production by resident alveolar macrophages.41

Fungus infection

Candida albicans, a dimorphic fungus, causes chronic mucocutaneous candidiasis. Widely distributed IL‐17A‐producing γδ T cells in the lungs play a protective role at a very early stage after systemic C. albicans infection.42 γδ T cells can directly recognize pathogen‐derived signals as well as environmental signals through their innate receptors, Toll‐like receptor 1, Toll‐like receptor 2 and Dectin‐1, long before Th17 cells can sense bacterial invasion, indicating that γδ T cells are an efficient first line of defence against pathogens, including C. albicans infection.43

Chronic granulomatous disease causes recurrent bacterial and fungal infections, as phagocytes lack NADPH oxidase activity and do not generate reactive oxygen species. Mice that have defective indoleamine 2,3‐dioxygenase spontaneously develop chronic granulomatous disease‐like syndrome, which associates with strong IL‐17 production from Vγ1⁺ T cells together with enhanced neutrophilic infiltration into the lungs.44

Viral infection

As summarized in Fig. 1, lung‐resident γδ T cells play critical roles in anti‐viral immune responses and are involved in virus‐induced lung inflammation and injury. Respiratory syncytial virus (RSV), one of many (~ 200) viruses known as a common cold virus, predominately affects infants and leads to long‐term lung disease. The contribution of γδ T cells to RSV infection has been tested in mice infected with RSV with or without immunization with a live vaccine vector expressing RSV F protein. Vγ4⁺ γδ T cells were enhanced in the lungs and produced IFN‐γ, RANTES, IL‐10, IL‐4 and IL‐5 in a time‐dependent manner after challenge of sensitized mice. Depletion of γδ T cells reduced lung inflammation and disease severity and slightly increased peak viral replication without compromising viral clearance during secondary challenge in vaccinated mice.45 Using a neonatal mouse model of RSV, it was found that neonates failed to develop IL‐17A responses of the type observed in adult mice. In adults, γδ T cells are the main producers of IL‐17A. Exogenous IL‐17A administration decreases inflammation in RSV‐infected neonates, whereas neutralization of IL‐17A increases lung inflammation and airway mucus in RSV‐infected adults. Hence, RSV disease severity is in part mediated by a lack of IL‐17A⁺ γδ T cells in the lungs of neonates.46 Additionally, RSV infection elevates Th1 cytokine and suppresses Th2 cytokine expression in lung γδ T cells. As described previously, ovalbumin (OVA) challenge induces a large influx of γδ T cells into the lungs. When mice were previously infected with RSV, the OVA‐induced infiltration and activation of γδ T cells were inhibited, suggesting that RSV protected against subsequent OVA‐induced allergic responses by inhibiting Th2‐type γδ T cells.47

During influenza virus infection, RORγt‐positive αβ and γδ T cells, as well as innate lymphoid cells, express enhanced IL‐22 as early as 2 days post‐infection. Although IL‐22 plays no role in the control of influenza A virus replication, IL‐22 is beneficial during sublethal influenza A virus infection but not lethal influenza A virus infection, which limits lung inflammation and injury after a secondary challenge with S. pneumoniae.48 On the contrary, type I interferon induction during influenza virus infection increases susceptibility to secondary S. pneumoniae infection by negative regulation of γδ T cells with decreased IL‐17 production.49 Human Vγ9Vδ2 T cells that are activated in vitro by aminobisphosphonate pamidronate efficiently kill influenza virus‐infected lung alveolar epithelial cells and inhibit virus replication in a cell‐to‐cell contact manner. The cytotoxic activity of Vγ9Vδ2 T cells requires NKG2D activation and involves perforin/granzyme B, TRAIL and FasL.50, 51

Lung allergic disease

Asthma is a common disease that features chronic inflammation of the conducting airways. Approximately 300 million people suffer from asthma worldwide. Asthma had long been considered the hallmark Th2 response of the airways. In an OVA‐induced mouse asthma model, mice that lacked γδ T cells had decreased OVA‐specific IgE and IgG1 expression, pulmonary IL‐5 levels and eosinophil and T‐cell infiltration compared with wild‐type mice, indicating that γδ T cells are essential for the Th2 airway inflammation response to peptide antigens. This process could be restored by administering IL‐4 to γδ T‐cell‐deficient mice during primary immunization, which was demonstrated by increased specific IgE and IgG1 expression, IL‐5 levels and eosinophil influx into the lungs.52 Allergen‐specific CD30⁺ Th2‐type γδ T cells were found in the lungs of patients with asthma, which produced high amounts of IL‐4, but not IFN‐γ, suggesting a role for γδ T cells in the local immune response to inhaled allergens and strongly supporting the notion that asthma is a local rather than a systemic disease.53 Additionally, Vγ1⁺ γδ T cells enhance airway hyperresponsiveness by inhibiting IL‐10‐producing CD4⁺ CD25⁺ T cells in the lung of OVA‐sensitized and challenged mice.54

However, in contrast to the reported enhancement of allergic airway inflammation by γδ T cells, a suppressive effect of γδ T cells on airway hyperresponsiveness was also demonstrated (Fig. 2). In mice that were systemically sensitized to OVA and challenged through the airways to inhaled methacholine, regulation by γδ T cells occurred in an αβ T‐cell‐ and B‐cell‐independent manner, which may reflect their interactions with the innate systems of host defence.55 Now, it is known that neutrophilic asthma is controlled by Th17 cells, and some eosinophilic asthma is controlled by type 2 innate lymphoid cells (ILC2).56 Chitin, a polysaccharide constituent of many allergens and parasites, induces inflammatory responses in lung tissue. A proportion of lung γδ T cells express IL‐17A, but not IFN‐γ, in response to intranasal chitin challenge. Chitin‐activated ILC2 inhibit the activation of γδ T cells in lung tissue, but this cross‐talk is independent of the ILC2‐derived type 2 cytokines IL‐5 and IL‐13.57 Vγ4 subset (IL‐17⁺ γδ T) was also demonstrated to down‐modulate central features of allergic reactions, including Th2‐driven inflammation and lung eosinophilia influx.58, 59 Antigen‐specific regulatory γδ T cells derived from OVA‐tolerant mice selectively suppress the specific IgE response in vivo, with the production of high levels of IFN‐γ in response to antigen stimulation in vitro.60

Figure 2.

The roles of lung resident γδ T cells in allergic disease. (a) Lung resident γδ T cells promote allergic inflammation. By producing interleukin‐4 (IL‐4), lung γδ T cells enhance specific IgE production, T helper type 2 (Th2) responses and eosinophil influx. Additionally, by producing interferon‐γ (IFN‐γ), lung γδ T cells inhibit IL‐10‐producing CD4⁺ CD25⁺ regulatory T cell functions. (b) Lung‐resident γδ T cells suppress allergic inflammation. By producing IL‐17A, lung γδ T cells inhibit Th2‐driven inflammation and eosinophil influx. Antigen‐specific regulatory γδ T cells inhibit specific IgE production. During this process, ILC2 cells might inhibit γδ T‐cell activation to control airway hyperresponsiveness.

In humans, Vγ1⁺ Vδ1⁺ γδ T cells with an IL‐4‐producing phenotype are capable of enhancing allergic inflammation, whereas Vγ2⁺ Vδ2⁺ γδ T cells with IFN‐γ‐production were shown to have a partial ability to modulate allergen‐specific Th2‐skewed immunity.61 The IFN‐γ ⁺/IL‐17⁺ ratio of γδ T cells is substantially decreased in patients with allergic asthma.62 Additionally, cloned human Vγ1⁺ T cells from subjects with allergy were found to recognize tree pollen‐derived phosphatidyl‐ethanolamine in a CD1d‐restricted fashion, and the proliferating clones drove IgE production in vitro and in vivo. CD1d‐restricted γδ T cells act as a regulatory subset and control the early host reactivity against tree pollens.63 γδ T cells derived from bronchoalveolar lavage fluid have the potential to produce IL‐5 and IL‐13 in people with mild atopic asthma. After allergen challenge, the imbalance between Th1 and Th2 cytokines was further accentuated by a reduction in IFN‐γ and IL‐2.64

Lung inflammation and fibrosis

Lung fibrosis is a life‐threatening disease caused by lung inflammation. Using an intense lung inflammatory response model (named as severe sepsis) induced by caecal ligation and puncture, it was found that high expression levels of CCL2, CCL3 and CCL5 enabled migration of γδ T cells into the lungs of mice that had undergone caecal ligation and puncture. Early‐activated Vγ4 T cells dominated IL‐17 production in inflamed lungs and played a beneficial role in host defence, resulting in delayed mortality of the septic mice.65 In an experimental autoimmune model by transfer of scurfy lymphocytes, γδ T cells were demonstrated to prevent hepatitis and pneumonitis in recipient mice. These γδ T cells had a highly activated phenotype CD62L^lo CD44^hi and expressed high levels of CD39 and NKG2D, which enhanced production of the suppressor cytokine IL‐10 from autoreactive T cells in the liver and lung.66

In a mouse model of silica‐induced lung inflammation and fibrosis, it was observed that IL‐17A production by γδ T cells and Th17 cells was required for early lung neutrophilic inflammation and acute tissue injury; however, they were dispensable for the long‐term inflammatory process and lung fibrosis.67 In a mouse model of bleomycin‐induced lung inflammation and subsequent fibrosis, γδ T cells were demonstrated to be required for an organized inflammatory response and epithelial repair in the lungs by producing IL‐17. A lack of γδ T cells correlated with increased lung inflammation and fibrosis.68 Additionally, γδ T cells were demonstrated to promote bleomycin‐induced lung fibrosis resolution through CXCL10 production.69 Consistent with this, in a model of Bacillus subtilis‐induced lung fibrosis, early IL‐17 production by Vγ6Vδ1⁺ T cells prevented excessive inflammation and eventual lung fibrosis, which might associate with the regulatory role of Vγ6Vδ1⁺ T cells in the recruitment of CD4⁺ and CD8⁺ T cells into the lungs.70, 71 Moreover, γδ T cells expanded in the lung and predominantly produced IL‐22, which inhibited lung fibrosis through diminished recruitment of CD4⁺ T cells into the lungs.72 However, inconsistent results showed that IL‐17‐producing γδ T cells aggravated lung fibrosis. It was found that osteopontin is expressed ubiquitously in the lung parenchymal and bone marrow cell components and contributes to pathogenesis by affecting the ratio of pathogenic IL‐17/protective IFN‐γ T cells (Th17 and γδT17 cells/Th1 and γδT1 cells).73

Cystic fibrosis is characterized by cycles of inflammation and infection, mostly with Staphylococcus aureus and Pseudomonas aeruginosa, resulting in significant mortality due to lung disease. Additionally, patients with cystic fibrosis have airway hyperresponsiveness, small intestinal bacterial overgrowth and an altered intestinal microbiome. By using a cystic fibrosis transmembrane conductance regulator‐deficient mouse model, streptomycin treatment reduced the intestinal bacterial overgrowth that principally affected Lactobacillus levels, which resulted in a decrease in the number of cystic fibrosis‐specific respiratory IL‐17‐producing γδ T cells and airway hyperresponsiveness.74 Additionally, increased percentages of pulmonary and mesenteric lymph node Th17, CD8⁺ IL‐17⁺ and CD8⁺ IFN‐γ ⁺ lymphocytes were observed following streptomycin treatment. Interleukin‐17‐producing γδ T cells associated with airway hyperresponsiveness, which was influenced by the intestinal microbiome in patients with cystic fibrosis.

Lung cancer

γδ T cells play important roles in tumour development in the local microenvironment, and they are currently being explored as a target for tumour immunotherapy. It has been reported that γδ T cells act as tumour‐promoting cells by inducing angiogenesis, promoting vascular endothelial growth factor production, enhancing metastasis, reducing the abilities of antigen‐presenting cells, recruiting myeloid‐derived suppressor cells and producing cytokines (IL‐4, IL‐10 and TGF‐β) as γδ regulatory T cells.75, 76, 77, 78 However, beneficial roles for γδ T cells in inhibiting tumour growth have also been reported by direct cytotoxicity (perforin/granzyme, TRAIL, FasL and ADCC pathways), promoting dendritic cell maturation, inducing Th1 and Th17 differentiation, activating invariant natural killer T cells and natural killer cells and promoting B‐cell antibody production.78, 79, 80, 81

For lung cancer, using a preclinical transplantable B16 melanoma model, type 1 cytotoxic γδ T cells were recruited to B16 lesions through the inflammatory CCR2/CCL2 chemokine pathway and displayed their antitumour activity by producing IFN‐γ. In multiple human tumour types, such as lung, prostate, liver or breast cancer tumours, CCL2 expression was strongly deregulated in Vδ1 T cells, which may constitute an evasion strategy against Vδ1 T‐cell‐mediated immune surveillance.82 In our previous work, we also found that γδT17 cells show critical antitumour activity during lung tumour development. When the roles of γδT17 cells were defective in Abt mice, these mice were found to be more susceptible to B16/F10 melanoma and Lewis lung carcinoma development, had larger tumours and more tumour foci in their lungs and exhibited shortened mean survival times.83 The microbiota modulates tumoral immune surveillance in the lungs through a γδT17 cell‐dependent manner. In the lung tumour microenvironment, IL‐1 determines the balance between inflammation and antitumour immunity by inducing the recruitment of γδ T cells and their activation for IL‐17 production, with no involvement of Th17 cells, to enhance antitumour immunity.84 It was suggested that interventions in IL‐1/IL‐17 production could be used therapeutically. Additionally, human lung cancer cells express TRAIL R2 and NKG2D ligands, and γδ T cells use TRAIL and NKG2D to kill lung cancer cells. NKG2D activation can promote γδ T cells to produce soluble TRAIL, which also induces apoptosis in lung cancer cells through TRAIL R2.85

Expanded γδ T cells with immobilized anti‐TCR‐γδ antibody exhibited potent cytolytic activity for squamous lung carcinoma, leading to prolonged survival of tumour‐bearing mice and slowed tumour growth. These expanded γδ T cells had major subset with Vδ2 phenotype, approximately 10% of the Vδ1 subsets and high percentages of CD27⁻ CD45RA⁻ and CD27⁻ CD45RA⁺ effector cells. Further, γδ T cells derived from patients with lung cancer had proliferative activity after γδ TCR ligation, and they displayed marked cytotoxicity to lung cancer cells by expressing perforin, granzyme A and B, FasL, TNF‐α and IFN‐γ. These findings suggest that expanded γδ T cells with anti‐γδ TCR may be used as a cellular therapy in the treatment of lung cancer.86 A phase I clinical study in which ex vivo zoledronate‐expanded γδ T cells were re‐infused into patients with advanced or recurrent non‐small‐cell lung cancer who were refractory to or intolerant of current conventional treatments indicated that adoptive transfer of γδ T cells was safe and feasible in these patients.87

Perspective

Although γδ T cells were identified nearly 30 years ago, much is still unknown regarding their development, migration, activation and function. In particular, the local microenvironment will be a focus of exploration regarding the immune features of tissue‐resident γδ T cells. It is known that γδ T cells can recognize antigens via γδ TCR‐mediated signalling, recognize stress‐induced ligands (such as MIC‐A, MIC‐B or ULBPs) through the NKG2D receptor or be activated by peptide antigen derived by the isoprenoid pathway used by several microorganisms or by the mevalonate pathway in infected or transformed cells. Additionally, γδ T cells can be indirectly activated by pro‐inflammatory cytokines or by Toll‐like receptors and dectin‐1 that bind to viral or bacterial products. However, there are few reports on the local recognition of resident γδ T cells in the lungs, which deserves further investigation. Furthermore, the multiple roles of lung‐resident γδ T cells indicate that local microenvironment factors influence their specific functions in tissue homeostasis and diseases.

Disclosure

The authors have declared no conflict of interests.