Natural killer cells in infection and inflammation of the lung

Authors


F. J. Culley, Respiratory Medicine, National Heart and Lung Institute, Faculty of Medicine, Imperial College London, St Mary’s Campus, London W2 1PG, UK. Email: f.culley@imperial.ac.uk.

Summary

The lungs are a major site of entry of pathogens into the body and thus require rapid and effective innate responses to prevent pathogens establishing infection and to limit their spread. Additionally, the immune response in the lung must be tightly regulated such that pathogens are cleared, but immunopathology and chronic inflammation are prevented. In this review, I consider the role of natural killer (NK) cells in pulmonary infection and inflammation, specifically their contributions to influenza, tuberculosis, asthma and chronic obstructive pulmonary disease (COPD), which are major causes of morbidity and mortality world-wide. Despite evidence of the importance of NK cells in these diseases, there are still major gaps in our understanding of how their function is regulated in this unique tissue environment. Understanding how different beneficial and detrimental effector functions of NK cells are triggered will be crucial if NK cells are to be exploited therapeutically in respiratory disease.

Introduction: natural killer cells in the lung

Natural killer (NK) cells are innate lymphocytes which are a first line of defence against infection and cancer.1,2 NK cells form synapses with diseased cells, but also other leucocytes, including macrophages and dendritic cells, in which they integrate activating and inhibitory signals from a multitude of germline-encoded receptors.3–7 Activating receptors include the natural cytotoxicity receptors (NCRs), such as NKp46 and NKp44, the Fc receptor CD16 and NKG2D.7 The ligands for NK cell-activating receptors include both host and pathogen glycoproteins; for example, NKG2D recognizes the stressed-induced ligand MHC class I polypeptide-related sequence A (MICA).8,9 Inhibitory receptors, such as killer immunoglobulin-like receptors (KIRs) and the NKG2A:CD94 dimer, generally recognize classical and non-classical class I major histocompatibility complex (MHC) molecules, and NK cell activation can also be triggered by loss of inhibitory ligands from the cell surface.10,11 In addition, NK cells are activated by cytokines, including type I interferons, interleukin (IL)-12 and IL-18.12–14 Once activated, NK cells can direct cytolytic granules towards the synapse to directly kill a target cell.1,15 Our understanding of NK cells is evolving rapidly and their functions clearly go beyond those of innate killer cells. Importantly, NK cells are a potent and early source of cytokines, particularly interferon (IFN)-γ, but they can also produce T helper type 2 (Th2)-associated cytokines, such as IL-5 and IL-13, and the regulatory cytokine IL-10.16 NK cells also specialize their function at different tissue locations: recently, a novel IL-22-secreting subset of NK cells has been described in the gut and tonsils.17–19 The interrelationships and functions of different NK cell subsets are not fully understood, but in humans, NK cells expressing high levels of CD56, the predominant subset in lymph nodes, exhibit higher cytokine production but diminished cytotoxicity relative to CD56 dim cells, which are the major subset in the periphery.20 NK cells can be activated by interactions with dendritic cells and macrophages and profoundly influence the generation of the adaptive response.1,2,21–23 The existence of memory in NK cells, that is long-term alteration of NK cell responses according to previous experience, has also been recently described.24–26

Here, I review the contribution of NK cells to respiratory infections and inflammatory disorders of the lung. The airways are a major route of entry of many important pathogens into the body and the ability of NK cells to respond rapidly to infection suggests an important role for these cells in acute pulmonary infection. However, evidence is emerging that NK cells are also important in regulating chronic infection and inflammation, and thus may play important roles in chronic infections, such as tuberculosis, and chronic inflammatory disorders of the airways, such as asthma.

NK cells make up 10% of resident lymphocytes in the lung, in numbers second only to those in the spleen,27–29 and their survival may be promoted by bronchial epithelial cells which spontaneously produce IL-15.30 Within days of infection, or hours after inflammatory stimulation, large numbers of NK cells are recruited to the lung from the blood and become activated to secrete cytokines, particularly IFN-γ.28,31–35 The airways are a unique environment in which the immune response must function. In homeostasis, the upper airways must tolerate continuous exposure to environmental antigens and commensal organisms. During infection, innate responses in the lung must be induced rapidly, but inflammation must be balanced to avoid damage to airway structures and airway occlusion, leading to impaired gaseous exchange. Inflammation in the lung is restrained, chiefly by IL-10 and transforming growth factor (TGF)-β produced by alveolar macrophages, which raise the threshold of activation which needs to be overcome before immune responses can occur.36 In homeostasis, pulmonary NK cells from bronchoalveolar lavage (BAL) or from lung tissue are suppressed; they can form conjugates with target cells, but are profoundly impaired in their cytotoxic capacity.29,37,38 Lung NK cells regain their activity after 24 hr in culture or stimulation with type I IFN, and, conversely, peripheral blood NK cells can be suppressed by culture with BAL fluid or alveolar macrophages, an effect unique to this type of macrophage.38–41 Soluble factors present in the lung that can regulate NK cell activity include TGF-β,42 prostaglandins produced by alveolar macrophages28,43 and pulmonary surfactant.44 Human leucocyte antigen (HLA)-G has also been reported to be expressed on pulmonary macrophages and dendritic cells during lung cancer; however, the role of HLA-G in regulating pulmonary NK cells during inflammation is unknown.45 The importance of regulation of NK cells in the lung is illustrated by the fatal lung pathology caused when NK cells are systemically activated by exogenous IL-18 and IL-2.46 Thus the extent of NK cell activation in the lung will depend on the balance of pro-inflammatory and regulatory factors.

Genetic deficiencies that effect NK cell function are rare, but have important implications for pulmonary health. In transporters associated with antigen processing-2 (TAP2)-deficient patients, class I MHC expression is defective and NK cells are poorly regulated. Early in life, NK cells are believed to protect patients against infection in the absence of effective T-cell immunity; however, later in life chronically activated NK cells are recruited to the skin and respiratory tract via chemokine (C-C motif) receptor 2 (CCR2), where they form lethal granulomatous lesions.47–49 Furthermore, genetic deficiencies that result in loss of NK cell function are associated with recurrent viral and bacterial infections, including those of the upper and lower respiratory tract.50–55 Next, I discuss our current state of knowledge of the role of NK cells in the acute respiratory viral infection influenza and the chronic bacterial infection tuberculosis. I also discuss the role of NK cells in the inflammatory disorders asthma, chronic obstructive pulmonary disease (COPD) and other cases of fibrosing airway disease.

NK cells in influenza infection

There is an urgent need for a better understanding of the immune response to influenza, with the goals of reducing pathology during infection and enhancing protection by vaccination.56 The adaptive immune response, particularly that involving cytotoxic T cells and antibody, can protect against influenza.57,58 The cytotoxic lymphocyte response must be sufficiently rigorous to aid clearance of the virus, as illustrated by cases of severe influenza infection in infants characterized by a deficiency of NK and cytotoxic T lymphocytes in the lung,59,60 but dysregulation of the innate response results in a ‘cytokine storm’ and correlates with severity of symptoms.61–65

NK cells are recruited to the lung within the first few days of influenza infection in humans and in murine models28,66 and depletion of lung NK cells leads to increased morbidity and mortality, within days of infection.35,67,68 NK cells reciprocally regulate the adaptive response in influenza: NK cells are required for activation of the cytotoxic T lymphocyte (CTL) response69 and T-cell IL-2 production augments NK cell IFN-γ production in recall responses.70

NKp46 is a key activating receptor which is critical for protecting mice against lethal influenza infection,71 and is one of the few known examples of direct binding of viral glycoprotein to an NK cell-activating receptor. Influenza haemagglutinin (HA) binds to both NKp46 and NKp44, largely via the α-2,6-linked terminal sialic acid, which is present on residue Thr225 of NKp46.72–74 The ability of NK cells to be activated by different influenza strains is influenced by levels of HA expression, HA affinity for sialic acid and HA glycosylation.73,75,76 However, it is not clear how the specificity of this interaction is conferred, that is, why other related receptors such as NKp30, which are likely to be similarly glycosylated, do not exhibit the same interaction with HA. NK cells are activated by influenza-infected monocytes and dendritic cells, via both contact-dependent mechanisms and cytokines.77 Enhanced cytotoxicity of NK cells is stimulated by IFN-α secretion; CD69 up-regulation is induced by IFN-α, NKG2D recognition of the ligands UL16-binding protein (ULBP)1–3, and NKp46 ligation of HA, and IFN-γ secretion is stimulated by IL-12, NKG2D and NKp46.79

To counter recognition by NK cells, influenza causes reorganization of MHC I into aggregates within GM1 ganglioside (GM-1) rich lipid microdomains, which increases binding of the inhibitory receptors KIR2DL1 and LIR, increasing inhibition of NK cell function.80,81 Responses of NK cells to influenza-infected monocytes were dependent on the KIR/HLA compound genotype, providing evidence that KIR/HLA-C interactions have a significant role in cytotoxicity and represent a mechanism by which these genotypes may influence influenza and other viral infections.82

Antibodies to influenza matrix protein 2 (M2), which is expressed on the surface of infected cells, required NK cells to confer protection in vivo, suggesting that, once an antibody response to influenza has developed, antibody-dependent cell-mediated cellular cytotoxicity (ADCC) is mediated by NK cells and contributes to viral clearance.83 NK cells express tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) early in influenza infection, but blockade of TRAIL unexpectedly decreased viral titre at this time-point.84 The role of IL-18 in influenza infection is controversial. One group reported that, in the absence of IL-18, NK cell activity and IFN-γ production were reduced and early viral replication was poorly controlled,85 whereas others reported decreased viral load, with no difference in pathology or NK cell IFN-γ production in the same IL-18-deficient mice.86

In conclusion, during influenza infection, NK cells are recruited to the lung where they could potentially interact with virally infected epithelial cells, monocytes, dendritic cells and T cells (Fig. 1). They contribute to protection against influenza, limiting early viral replication and promoting an effective CTL response. Yet, the mechanisms involved in achieving this, for example the relative importance of NK cell cytokine production versus cytotoxicity, over the time–course of influenza infection are unclear.

Figure 1.

 Potential activating and inhibitory interactions of natural killer (NK) cells in the lung. NK cell function in the lung is regulated by both contact-dependent interactions and soluble mediators. Note that, although many of these interactions have been demonstrated in vitro, their timing, location and relative importance in vivo are not known. HA, haemagglutinin; IFN, interferon; IL, interleukin; KIR, killer immunoglobulin-like receptor; MICA, MHC class I polypeptide-related sequence A; MHC, major histocompatibility complex; TGF, transforming growth factor; ULBP, UL16-binding protein.

NK cells in tuberculosis

One third of the world’s population are currently infected with Mycobacterium tuberculosis (MTb), and this infection results in almost 2 million deaths annually.87 In the majority of people, the infection remains in a chronic latent state, in which the immune response prevents bacterial dissemination, but is not so vigorous as to cause immunopathology. Mycobacteria survive within macrophages, which can kill the bacteria if sufficiently activated, so induction of a Th1-type response, and in particular IFN-γ production, is key to protection against infection.88,89 The importance of the innate response in disease is still unclear.90

NK cell NKp46 expression and cytotoxicity are reduced in freshly isolated peripheral blood mononuclear cells (PBMCs) from tuberculosis patients, which may be attributable to suppression by monocytes and IL-10.91–93 NK cells in the pleural effusion, the excess fluid that collects around the lungs of patients with tuberculosis, are enriched for CD56hi cells with reduced expression of CD16 and perforin, which may be attributable to selective apoptosis of CD56dim cells induced by as yet unidentified soluble factors in pleural fluid.94 In accordance with the CD56hi subset of NK cells being associated with high cytokine production, NK cells from pleural effusions spontaneously produced IFN-γ and responded strongly to re-exposure to MTb by producing IFN-γ, and this IFN-γ production correlated with disease severity.94 Thus, in active disease, NK cells exhibit reduced cytotoxicity but increased IFN-γ production, perhaps because of selective activation of NK cell subsets.

Human NK cells can be activated by and induce apoptosis in mycobacteria-infected monocytes and macrophages in vitro,95,96 mediated by NKp46 recognition of vimentin and NKG2D recognition of its ligand ULBP-1.93,97,98 NK cells can also be activated by direct binding of NKp44 to the mycobacterial cell wall, although the ligand remains undetermined.99,100 MICA is the gene most strongly associated with susceptibility to the opportunistic Mycobacterium avium and is expressed in the epithelium, macrophages, epitheloid cells and multinucleated giant cells in infected tissues, suggesting a potential role for this NKG2D ligand in mycobacterial infection.101 As well as direct killing of infected cells, NK cells may also regulate the T-cell response to MTb. In mixed PBMC cultures stimulated with MTb, NK cell IFN-γ production and CD40:CD40L interactions with infected monocytes stimulated IL-15 and IL-18 production by monocytes and promoted expansion and cytotoxicity of CD8+ cells.102 In similar mixed cultures, NK cells lysed activated regulatory T cells (Tregs) via NKp46 and NKG2D:ULBP1 interactions.103 Thus, overall, many cell types express ligands that could activate NK cells in the lung during mycobacterial infection (Fig. 1).

Are NK cells important in MTb infection in vivo? Animal models do not give a clear answer to this question. NK cells are activated and produce IFN-γ in the lung following mycobacterial infection.104–107 In T-cell-deficient mice, a protective role for IL-12-induced IFN-γ production by NK cells has been demonstrated.108 However, depletion of NK cells had no effect on bacterial replication in the lung of immunocompetent mice,104 suggesting that NK cells may be redundant in the presence of intact adaptive immunity. In fact, surprisingly, IFN-γ knockout (KO) mice, which are impaired in their ability to clear mycobacteria, cleared them as effectively as wild-type mice when NK cells were depleted, suggesting that NK cells can inhibit protective immunity.105 It should be borne in mind that murine models may poorly reflect the situation in humans; for example, although lymphocyte aggregates form in the lung, the classical granuloma does not.109

To reconcile the data and understand the importance of NK cells in MTb infection it may be necessary to differentiate their contributions at different stages of disease. Recently, it was shown that during chronic infection with Leishmania donovani, another pathogen that inhabits macrophages, NK cells are recruited to liver granulomas where they produce IL-10, which suppresses cell-mediated immunity.16 Such a mechanism may explain the apparent suppressive role of NK cells observed in the murine model of mycobacterial infection.105 Thus, to fully understand the role of NK cells in tuberculosis, it may be necessary to define their roles in limiting early infection, inducing protective adaptive immunity and maintaining latency, and during re-activation of infection.90

Other murine models of pulmonary infection

The contribution of NK cells to a number of other pulmonary infections has been studied in murine models (Table 1). The requirement for NK cells in respiratory infection and inflammation can be demonstrated by depletion, but cases where this is the only evidence for NK cell involvement in infection must be interpreted with caution, as the commonly used markers for depletion, NK1·1 and asialo-GM1, are also expressed on other lymphocyte subsets.

Table 1.   The role of natural killer (NK) cells in murine models of pulmonary infection
PathogenProtective effect of NK cells?Possible protective functions of NK cellsProposed mechanism of NK cell activationNotesReferences
  1. DC, dendritic cell; IFN, interferon; IL, interleukin; PMN, polymorphonuclear cell; scid, severe combined immunodeficiency; Th, T helper; TNF, tumour necrosis factor.

Fungi
 Cryptococcus neoformansYes – promote clearanceIFN-γ productionIL-18NK cells are a major source of IFN-γ in IL-12−/− mice176
 Aspergillus fumigatusYes – promote survival and clearance of pathogenIFN-γ enhances macrophage fungicidal activity and induces chemokine production in the lung Opportunistic. NK cell IFN-γ is sufficient to mediate clearance33,177
Bacteria
 Bordetella pertussisYes – promote bacterial clearanceIFN-γ activates macrophages and suppresses Th2 responseProduction of IL-12 by activated DCs 178
 Streptococcus pneumoniaeNo – detrimental effect on clearance of bacteriaMajor source of IFN-γPneumolysin activated monocytesExperiments performed in scid−/− mice179
 Francisella tularensisYes – promote survivalEarly source of IFN-γ promotes clearance of bacteria and Th1 responses  180
 Legionella pneumophilaYes – mediate pathogen clearanceEarly source of IFN-γNK cell activation is dependent on MyD88 in NK cells 181
 Haemophilus influenzaeYes – required for pathogen killingStimulate killing of intracellular bacteria by PMNsActivation requires IL-15 production by Gr-1hi PMNs 182
 Pseudomonas aeruginosaYes – critical for bacterial clearanceIFN-γ productionNKG2DOpportunistic173,183
 Staphylococcus aureusYesIFN-γ and TNF production, augmentation of phagocytosisby macrophagesActivation by infected macrophages and bacterial superantigenOpportunistic184–186
Viruses
 Herpes simplex virus (HSV)Yes – mediate viral clearanceIFN-γ secretion and cytotoxicityNK cell activation is IL-18, but not IL-12, dependentHSV can cause pneumonia in neonates and immune-compromised patients187,188
 Respiratory syncytial virus (RSV)Yes – viral clearanceEarly IFN-γ secretionRecruitment to the lung depends on macrophages 135,150,189,190

NK cells in asthma

300 million people world-wide suffer from asthma, which in the majority of cases is associated with allergy to environmental antigens.110 Acute attacks caused by allergen exposure trigger mast cell degranulation, eosinophilic inflammation, mucus production and bronchoconstriction. In the long term, airway remodelling, characterized by airway thickening caused by extracellular matrix deposition, and muscle and goblet cell hypertrophy, results in diminished airway function.111 Inflammation and pathology in asthma are driven by the production of Th2 cytokines (IL-4, IL-5, IL-13, IL-9 and IL-3), which have pleiotropic effects on leucocytes and airway stromal cells.

NK activity is enhanced in PBMCs from asthmatics.112–114 Immediately after allergen challenge, this activity declines, consistent with extravasation of NK cells to the lung.112 This is also observed in an animal model of allergic airway sensitization,32 and could result from the release of chemoattractants by activated mast cells.115 In contrast, an increased frequency of NK cells was reported in PBMCs from asthmatic children during acute exacerbations, which resolved when children were in a stable condition.116 However, whether these exacerbations were caused by viral infection was not determined. The phenotype of NK cells is also altered in asthma and allergy. Atopic asthmatics were reported to have a slightly higher frequency of IL-4+ and a lower frequency of IFN-γ+ NK cells following ex vivo activation of PBMCs,117,118 and purified peripheral NK cells of patients with atopic dermatitis spontaneously released high amounts of IFN-γ, IL-4, IL-5 and IL-13.119 Thus NK cells may contribute to the balance of Th1 and Th2 cytokines in asthma and allergy.

The mechanisms by which NK cells are stimulated to produce different cytokines are poorly understood. Human and mouse NK cells produce IL-5 and IL-13 (and in some cases IL-4) when activated ex vivo, and production of these cytokines is selectively promoted by IL-4, and inhibited by IL-12 or IL-10.120–129 In freshly isolated peripheral blood NK cells, IL-13 is predominantly produced by the CD56hi subset.120 It has been proposed that cytokine production correlates with NK cell maturation, as culture of immature NK cells with IL-12 results in an irreversible change from IL-5 to IFN-γ production.130–134 So, the phenotype of NK cells in asthma and allergy could be a result of exposure to a Th2 cytokine environment. In support of this hypothesis, there is evidence that, in the lung, the cytokine profile of NK cells can be influenced by the nature of the T-cell response. In a murine model of respiratory syncytial virus (RSV) infection, the proportion of NK cells secreting IFN-γ was augmented during a Th1 response, but reduced in a Th2 response.135 This may be a result of the direct actions of Th1-produced IFN-γ on the NK cell phenotype in vivo.126 However, T cells are not required for activation of IL-13-producing NK cells126,136 and IL-4 can stimulate IFN-γ-producing NK cells,137 suggesting that polarization of NK cells does not simply echo the T-cell cytokine milieu. Other factors that could influence the NK cell phenotype in the lung in asthma include Prostaglandin D2 (PGD2), which is produced predominantly by mast cells138 and can potently inhibit NK cell IFN-γ production and cytotoxicity.139 Asthmatics are also deficient in type I IFN production, which could impact on NK cell activation, particularly during viral exacerbations of asthma.13,140,141

The differential activation of NK cells in asthma may have important functional consequences because of their ability to influence the adaptive response. NK cells activated with IL-12 can kill immature dendritic cells and it has been proposed that, through ‘dendritic cell editing’ during an immune response, they remove dendritic cells which would otherwise promote Th2 responses or tolerance.21 NK cells activated with IL-4 do not perform this function, and may therefore promote T-cell anergy or Th2 responses.142 Supporting this hypothesis, in patients with rhinitis and asthma, the proportion of CD56hi NK cells was low, and IFN-γ production and dendritic cell maturation, following co-culture with NK cells, were impaired.143 There may be other consequences of the altered NK cell response in asthma. NK cells from asthmatics also expressed more CD95 (Fas) and affected T-cell activation by cyclic AMP (cAMP),144 and thus may directly influence the T-cell response. Asthma exacerbations are strongly associated with respiratory viral infections and asthmatics experience more severe and longer-lasting symptoms following infection.145,146 Inappropriate or poor activation of NK cells in asthma could enhance susceptibility to these infections. NK cells may also influence sensitizing antibody [immunoglobulin E (IgE)] production directly or indirectly.119

Mouse models support an important role for NK cells in allergic airway inflammation. In a model of allergen sensitization followed by airway challenge, depletion of NK cells inhibited the development of allergic pulmonary inflammation, dramatically decreasing eosinophil numbers in the lung and serum IgE.147 In this model, NK cell depletion during sensitization was necessary to reduce allergic inflammation, suggesting that NK cells were required for initiation of the Th2 response, as has been demonstrated for some Th1 responses.22,23,148 Prior infection with bacteria can activate NK cells such that they inhibit allergic sensitization and subsequent respiratory inflammation,149 and activation of NK cells with IL-12 during sensitization inhibited eosinophilia in a respiratory virus model of airway inflammation.150 NK cells can also influence ongoing allergic inflammation. In peritoneal inflammation, NK cell depletion during allergen challenge could reduce eosinophilia and IL-5 production,151 and IFN-γ-secreting NK cells induced in vivo by IL-2 and IL-18 significantly suppressed airway hyper-responsiveness and eosinophilia after allergen sensitization.152

Taken together, these studies suggest that NK cell function is altered in asthma, towards a Th2-cytokine-producing phenotype. NK cells can promote allergic airway inflammation during sensitization and ongoing inflammation, but stimulation of NK cells towards an IFN-γ-secreting phenotype can reduce allergic airway pathology, at least in animal models. Our knowledge of the signals that stimulate different phenotypes of NK cell cytokine secretion in asthma and allergic responses is still very limited. Are NK cells already polarized and influencing dendritic cell and T-cell activation during sensitization? If so, what causes early differentiation of NK cells? How NK cells promote ongoing allergic sensitization and the relative importance of direct cytokine production, or interactions with T cells and accessory cells, are also areas that deserve further study.

NK cells in fibrotic lung disease

Pulmonary fibrosis occurs as a result of chronic lung inflammation, in diseases including asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF) and idiopathic pulmonary fibrosis (IPF).153 Persistent inflammation results in dysregulation of the normal wound healing responses, and generation of pro-fibrotic cytokines (IL-13 and TGF-β) and growth factors, leading to accumulation of extracellular matrix components, with resulting impairment of airway function. COPD is a chronic inflammation of the lung, the primary risk factor for which is cigarette smoking, which affects 210 million people world-wide.110,154 COPD is associated with destruction of the lung parenchyma (resulting in emphysema), and inflammation and obstructive fibrosis of the bronchioles. The immunological mechanisms underlying COPD are still poorly understood.154 Idiopathic pulmonary fibrosis is the name given to fibrotic lung disease of unknown origin, which is generally fatal within 2–5 years and which is considered a Th2 disease.

NK cell function is impaired in COPD, which can be partially attributed to the effects of smoking, which reduces NK cell function in the lungs and peripheral blood,155–158 possibly by increasing the numbers of immunosuppressive alveolar macrophages.39 However, peripheral blood NK cell cytotoxicity is reduced even in ex-smokers with COPD, compared with control ex-smokers, suggesting a deficiency associated with disease.159,160 In patients with IPF, expression of NKG2D was reduced on NK, NKT and γδ cells in BAL, which may be a consequence of the increased expression of soluble MICA or TGF-β in these patients.161–163 Patients with IPF also strongly express MICA on epithelial cells and fibroblasts in the lung and have a significant increase in the frequency of the MICA*001 allele and a decrease in the frequency of MICA*004, suggesting that this NK cell ligand may play a role in regulating disease progression.163

NK cells may mediate a protective effect against fibrosis. In models of bleomycin-induced pulmonary fibrosis, lack of NK cell recruitment, in the absence of chemokine (C-X-C motif) receptor 3 (CXCR3), resulted in an absence of IFN-γ in the lung and enhanced fibrosis, and exogenous IFN-γ treatment had a therapeutic effect, demonstrating the importance of NK cell IFN-γ in regulating pulmonary fibrosis.164,165 However, although initial studies suggested that IFN-γ could be used therapeutically in patients with idiopathic pulmonary fibrosis,166 a recent large multicentred trial did not find any clinical benefit of this treatment.167

By what mechanisms, other than IFN-γ production, could NK cells protect against pulmonary fibrosis? In hepatic fibrosis, NK cells promote disease resolution by selective recognition and killing of collagen-secreting stellate cells.168–171 As the balance of fibroblast proliferation and apoptosis underlies the extent of pulmonary fibrosis,153 it would be interesting to know if NK cells can also regulate numbers of collagen-secreting cells in the lung. Importantly, the ability of NK cells to protect against infection may also limit airway inflammation and consequently fibrosis. Respiratory infections are more prevalent in COPD and most exacerbations of COPD and asthma are caused by infections.146,172 In CF, chronic infections lead to lung fibrosis, and NK cells, activated via NKG2D, secrete IFN-γ which mediates clearance of the principal opportunistic infection in CF, Pseudomonas aeruginosa.173 Taken together, these studies suggest a model in which NK cells shift the balance of lung inflammation away from a pro-fibrotic response, perhaps via cytokine production and protection against infection, and these functions of NK cells are impaired in smokers and patients with fibrotic lung disease.165,166,174 Thus, enhancement of NK cell function may offer novel therapeutic approaches to these debilitating and often fatal diseases.

Finally, although NK cell activation may be beneficial in reducing lung fibrosis, NK cells may contribute to loss of lung parenchymal cells in COPD. MICA is expressed on the airway epithelium of COPD patients, and expression of the murine NKG2D ligand Rae-1 on lung epithelium leads to emphysema-like pulmonary dysfunction in mice, which is blocked by treatment with anti-NKG2D or NK cell depletion.175

Conclusions and future questions

NK cells may tip the balance between health and pathology in the lung, and thus understanding their actions may identify novel targets for immunomodulation in respiratory disease. NK cells are activated by multiple mechanisms in the lung and protect against viral, bacterial and fungal infection, through direct antiviral actions and activation of macrophages, dendritic cells and the adaptive immune response. NK cells are also activated in chronic inflammatory diseases of the lung. Although their role in these diseases is not fully understood, their ability to produce ‘Th2’ cytokines may promote lung inflammation, whereas their production of IFN-γ, and other actions, may reduce lung fibrosis. The activation status of NK cells may have dual implications for chronic inflammatory diseases, such as asthma and COPD, which are exacerbated by respiratory infection.

Many important gaps remain in our understanding of the NK cell response in the lung. NK cells can be deficient or altered in phenotype in respiratory diseases, but whether this is a reflection of the ongoing pathological process or a cause of increased susceptibility to disease is often unclear. Although many potential interactions of NK cells with dendritic cells, macrophages and T cells have been demonstrated in vitro, their location, timing and importance during different phases of an ongoing respiratory infection or inflammatory response are still largely unknown, as is the role of different NK subsets. The lung has unique properties which regulate immune responses and, as NK cells specialize their function in peripheral tissues, it will be interesting to discover whether NK cells also specialize their phenotype to the pulmonary environment in homeostasis and disease. It will also be important to know whether ‘memory’ or long-term changes in NK cell responses can result from or determine respiratory health. Finally, how the NK cell response is down-regulated after a pathogen has been cleared or to prevent pathology during inflammation, is another area that could provide insights into the mechanisms underlying important respiratory diseases.

Acknowledgements

I am grateful to Daniel M. Davis, Philipp Eissmann, Suzie M. Hingley-Wilson and Salim I. Khakoo for critical reading of this manuscript.

Disclosures

The author has no conflicts of interest to disclose.

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