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Numerical and functional defects of invariant natural killer T cells (iNKT) have been documented in human and mouse cancers, resulting in a defect in IFN production in several malignancies. iNKT cells recognize glycolipids presented on CD1d molecules by dendritic and related cells, leading to their activation and thereby regulating immune reactions. Activated iNKT cells cytokine secretion and cytotoxicity can inhibit existing and spontaneous tumor growth, progression, and metastasis. We have identified functional iNKT cell defects in the murine TRAMP prostate cancer model. We found that iNKT cells show the ability to migrate into TRAMP prostate tumors. This infiltration was mediated through CCL2: CCR5 chemokine: receptor interaction. Prostate tumor cells expressing CD1d partially activated iNKT cells, as appreciated by up-regulation of CD25, PD-1 and the IL-12R. However, despite inducing up-regulation of these activation markers and, hence, delivering positive signals, prostate tumor cells inhibited the IL-12-induced STAT4 phosphorylation in a cell-cell contact dependent but CD1d-independent manner. Consequently, tumor cells did not induce secretion of IFNγ by iNKT cells. Blocking the inhibitory Ly49 receptor on iNKT cells in the presence of α-GalCer restored their IFNγ production in vivo and in vitro. However, Ly49 blockade alone was not sufficient. Importantly, this defect could be also be reversed into vigorous secretion of IFNγ by the addition of both IL-12 and the exogenous CD1d ligand alpha-galactosylceramide, but not by IL-12 alone, both in vivo and in vitro. These data underscore the potential to optimize iNKT-based therapeutic approaches.

Introduction

NKT cells comprise approximately 30% of the hepatic lymphoid population in mice (~50% in humans). Most mouse hepatic NKT cells [invariant (i)NKT cells] express T cell receptors, composed of invariant Vα14Jα18 chains. Unlike conventional T cells, iNKT cells recognize glycolipid molecules presented in association with MHC class Ib (CD1d) molecules. Purportedly, iNKT cells serve a key function in a wide range of immunological events; the precise nature of this function is often unclear. Indeed, the consequences of hepatic iNKT cell activation can be beneficial or detrimental. α-Galactosylceramide, the prototypic glycolipid recognized by the iNKT cell receptor, stimulates the rapid production of both interferon-γ and interleukin-4. The reciprocal suppression exhibited by these cytokines limits the potential therapeutic value of α-galactosylceramide. An extensive research effort is ongoing to develop α-galactosylceramide analogs that modulate iNKT cell activity and selectively promote interferon-γ or interleukin-4.

Areas covered

This review provides a broad overview of hepatic iNKT cells and their purported role in liver disease. Efforts to develop therapeutic agents that promote their beneficial contributions are detailed.

Expert Opinion

While a growing body of literature documents the differential effects of α-GalCer analogs on IFN-γ and IL-4 production, the effects of these analogs on other iNKT cell activities, e.g., cytolysis and the production of other cytokines, remain to be determined. Similarly, an exhaustive examination of the effects of these analogs on inflammation and liver injury in animal models remains prior to considering their utility in clinical trials.

Background

Natural killer T (NKT) cells are a heterogeneous population of innate T cells that have attracted recent interest because of their potential to regulate immune responses to a variety of pathogens. The most widely studied NKT cell subset is the invariant (i)NKT cells that recognize glycolipids in the context of the CD1d molecule. The multifaceted methods of activation iNKT cells possess and their ability to produce regulatory cytokines has made them a primary target for therapeutic studies.

Objective/Methods

This review gives insight into the roles of iNKT cells during infectious diseases, particularly viral infections. We also highlight the different mechanisms leading to iNKT cell activation in response to pathogens.

Conclusions

The iNKT cell versatility allows them to detect and respond to several viral infections. However, therapeutic approaches to specifically target iNKT cells will require additional research. Notably, examination of the roles of non-invariant NKT cells in response to pathogens warrant further investigations.

Invariant natural killer T (iNKT) cells are a unique subset of T lymphocytes that recognize glycolipid antigens in the context of the antigen-presenting molecule CD1d. Upon glycolipid antigen stimulation, iNKT cells rapidly produce copious amounts of immunomodulatory cytokines, leading to potent activation of a variety of innate and adaptive immune cells. These immune-potentiating properties of iNKT cells hold great promise for the development of vaccine adjuvants. This review aims to summarize the immunomodulatory activities of iNKT cell ligands and to discuss prospects for developing iNKT cell-based vaccine adjuvants.

Invariant NKT (iNKT) cells are a unique subset of T lymphocytes that rapidly carry out effector functions following activation with glycolipid Ags, such as the model Ag α-galactosylceramide (αGalCer). Numerous studies have investigated the mechanisms leading to Th1- and Th2 cytokine production by iNKT cells, and the effects of the copious amounts of cytokines these cells produce. Less is known, however, about the mechanisms of iNKT cell cytotoxicity. Here we investigated the effect of antigen availability and strength, as well as the molecules involved in iNKT cytotoxicity. We demonstrate that the iNKT cell cytotoxicity in vivo correlates directly with the amount of CD1d expressed by the targets as well as the TCR affinity for the target glycolipid Ag. iNKT cells from spleen, liver and thymus were comparable in their cytotoxicity in vitro. Surprisingly, we show that the antigen-specific cytotoxicity of iNKT cells in vivo depended almost exclusively on the interaction of CD95 (Fas) with CD178 (FasL), and that this mechanism can be efficiently utilized for tumor protection. Therefore unlike NK cells, which rely mostly on perforin/granzyme mediated mechanisms, the antigen-specific cytotoxicity of iNKT cells in vivo is largely restricted to the CD95/CD178 pathway.

Following stimulation through their T cell receptor, invariant natural killer T (iNKT) cells act as innate effector cells by rapidly releasing large amounts of effector cytokines and chemokines and therefore have an important role in modulating the ensuing immune response. iNKT cells recognize and are activated by diverse glycolipid antigens, many of which are found in microorganisms. However, iNKT cells also demonstrate some reactivity to “self”. Here, I outline our current understanding of iNKT cell autoreactivity and propose that several self-lipids are probably involved in the positive selection and autoreactivity of iNKT cells.

The immune system plays a major role in protecting the host against viral infection. Rapid initial protection is conveyed by innate immune cells, while adaptive immunity (including T lymphocytes) requires several days to develop, yet provides high specificity and long-lasting memory. Invariant natural killer T (iNKT) cells are an unusual subset of T lymphocytes, expressing a semi-invariant T cell receptor together with markers of the innate NK cell lineage. Activated iNKT cells can exert direct cytolysis and can rapidly release a variety of immune-polarizing cytokines, thereby regulating the ensuing adaptive immune response. iNKT cells recognize lipids in the context of the antigen-presenting molecule CD1d. Intriguingly, CD1d-restricted iNKT cells appear to play a critical role in anti-viral defense: increased susceptibility to disseminated viral infections is observed both in patients with iNKT cell deficiency as well as in CD1d- and iNKT cell-deficient mice. Moreover, viruses have recently been found to use sophisticated strategies to withstand iNKT cell-mediated elimination. This review focuses on CD1d-restricted lipid presentation and the strategies viruses deploy to subvert this pathway.

Glycolipid ligands for invariant natural killer T cells (iNKT cells) are loaded onto CD1d molecules in the late endosome/lysosome. Accumulation of glycosphingolipids (GSLs) in lysosomal storage diseases could potentially influence endogenous and exogenous lipid loading and/or presentation and, thus, affect iNKT cell selection or function. The percentages and frequency of iNKT cells were reduced in multiple mouse models of lysosomal GSL storage disease, irrespective of the specific genetic defect or lipid species stored. Reduced numbers of iNKT cells resulted in the absence of cytokine production in response to α-galactosylceramide (α-GalCer) and reduced iNKT cell–mediated lysis of wild-type targets loaded with α-GalCer. The reduction in iNKT cells did not result from defective expression of CD1d or a lack of antigen-presenting cells. Although H-2 restricted CD4+ T cell responses were generally unaffected, processing of a lysosome-dependent analogue of α-GalCer was impaired in all the strains of mice tested. These data suggest that GSL storage may result in alterations in thymic selection of iNKT cells caused by impaired presentation of selecting ligands.

Invariant natural killer T (iNKT) cells recognize glycolipid antigens such as the marine sponge-derived glycosphingolipid α-galactosylceramide (αGalCer) presented by the CD1d protein. In vivo activation of iNKT cells with αGalCer results in robust cytokine production followed by the acquisition of an anergic phenotype. Here, we have investigated mechanisms responsible for the establishment of αGalCer-induced iNKT cell anergy. We found that αGalCer-activated iNKT cells rapidly upregulated expression of the inhibitory co-stimulatory receptor programmed death (PD)-1 at their cell surface, and this increased expression was retained for at least one month. Blockade of the interaction between PD-1 and its ligands, PD-L1 and PD-L2, at the time of αGalCer treatment prevented the induction iNKT cell anergy, but was unable to reverse established iNKT cell anergy. Consistently, injection of αGalCer into PD-1-deficient mice failed to induce iNKT cell anergy. However, blockade of the PD-1:PD-L pathway failed to prevent bacterial- or sulfatide-induced iNKT cell anergy, suggesting additional mechanisms of iNKT cell tolerance. Finally, we showed that blockade of PD-1:PD-L interactions enhanced the antimetastatic activities of αGalCer. Collectively, our findings reveal a critical role for the PD-1:PD-L costimulatory pathway in the αGalCer-mediated induction of iNKT cell anergy that can be targeted for the development of immunotherapies.

Objective

In this study, we investigated whether dyslipidemia-associated perturbed invariant natural killer T (iNKT) cell function is due to intrinsic changes in iNKT cells or defects in the ability of antigen-presenting cells to activate iNKT cells.

Methods and Results

We compared iNKT cell expansion and cytokine production in C57BL/6J (B6) and apolipoprotein E-deficient (apoE−/−) mice. In response to in vivo stimulation with α-galactosylceramide, a prototypic iNKT cell glycolipid antigen, apoE−/− mice showed significantly decreased splenic iNKT cell expansion at 3 days after injection, a profile associated with iNKT cell anergy due to chronic stimulation. This decrease in expansion and cytokine production was accompanied by a 2-fold increase in percentage of iNKT cells expressing the inhibitory marker programmed death-1 in apoE−/− mice compared with controls. However, in vivo and in vitro blockade of programmed death-1 using monoclonal antibody was not able to restore functions of iNKT cells from apoE−/− mice to B6 levels. iNKT cells from apoE−/− mice also had increased intracellular T cell receptor and Ly49 expression, a phenotype associated with previous activation. Changes in iNKT cell functions were cell autonomous, because dendritic cells from apoE−/− mice were able to activate B6 iNKT cells, but iNKT cells from apoE−/− mice were not able to respond to B6 dendritic cells.

Conclusion

These data suggest that chronic dyslipidemia induces an iNKT cell phenotype that is unresponsive to further simulation by exogenous glycolipid and that sustained unresponsiveness is iNKT cell intrinsic.

Glycolipids presented by the major histocompatibility complex class I (MHC I) homolog CD1d are recognized by natural killer T (NKT) cells characterized by either a semi-invariant (type I or iNKT) or a relatively variable (type II) T cell receptor (TCR) repertoire. Here we describe the first structure of a type II NKT TCR complexed with CD1d-lysosulfatide (LSF). Both TCR α and β chains contacted the CD1d molecule with a diagonal footprint, typical of MHC-TCR interactions, while the antigen was recognized exclusively with a single TCR chain, similar to the iNKT TCR. Type II NKT cells, therefore, recognize CD1d-sulfatide complexes with a distinct recognition mechanism characterized by features of both iNKT cells as well as conventional peptide-reactive T cells.

CD1d-restricted natural killer T cells with invariant T cell receptor α chains (iNKT cells) are a unique lymphocyte subset that responds to recognition of specific lipid and glycolipid antigens. They are conserved between mice and humans and exert various immunoregulatory functions through their rapid secretion of a variety of cytokines and secondary activation of dendritic cells, B cells and NK cells. In the current study, we analyzed the range of functional activation states of human iNKT cells using a library of novel analogs of α-galactosylceramide (αGalCer), the prototypical iNKT cell antigen. Measurement of cytokines secreted by human iNKT cell clones over a wide range of glycolipid concentrations revealed that iNKT cell ligands could be classified into functional groups, correlating with weak versus strong agonistic activity. The findings established a hierarchy for induction of different cytokines, with thresholds for secretion being consistently lowest for IL-13, higher for interferon-γ (IFNγ), and even higher for IL-4. These findings suggested that human iNKT cells can be intrinsically polarized to selective production of IL-13 by maintaining a low level of activation using weak agonists, whereas selective polarization to IL-4 production cannot be achieved through modulating the strength of the activating ligand. In addition, using a newly designed in vitro system to assess the ability of human iNKT cells to transactivate NK cells, we found that robust secondary induction of interferon-γ secretion by NK cells was associated with strong but not weak agonist ligands of iNKT cells. These results indicate that polarization of human iNKT cell responses to Th2-like or anti-inflammatory effects may best be achieved through selective induction of IL-13 and suggest potential discrepancies with findings from mouse models that may be important in designing iNKT cell-based therapies in humans.

Invariant NKT (iNKT) cells are innate-like lymphocytes that recognize glycolipid antigens in the context of the MHC class I–like antigen-presenting molecule CD1d. In vivo activation of mouse iNKT cells with the glycolipid α-galactosylceramide (α-GalCer) results in the acquisition of a hyporesponsive (anergic) phenotype by these cells. Because iNKT cells can become activated in the context of infectious agents, here we evaluated whether iNKT cell activation by microorganisms can influence subsequent responses of these cells to glycolipid antigen stimulation. We found that mouse iNKT cells activated in vivo by multiple bacterial microorganisms, or by bacterial LPS or flagellin, became unresponsive to subsequent activation with α-GalCer. This hyporesponsive phenotype of iNKT cells required IL-12 expression and was associated with changes in the surface phenotype of these cells, reduced severity of concanavalin A–induced hepatitis, and alterations in the therapeutic activities of α-GalCer. These findings may have important implications for the development of iNKT cell–based therapies.

Systemic lupus erythematosus (SLE) is a chronic autoimmune inflammatory disease with complex immunological and clinical manifestations. Multiple organ failure in SLE can be caused by immune dysfunction and deposition of autoantibodies. Studies of SLE-susceptible loci and the cellular and humoral immune responses reveal variable aberrations associated with this systemic disease. Invariant natural killer T (iNKT) cells are a unique subset of lymphocytes that control peripheral tolerance. Mounting evidence showing reductions in the proportion and activity of iNKT cells in SLE patients suggests the suppressive role of iNKT cells. Studies using murine lupus models demonstrate that iNKT cells participate in SLE progression by sensing apoptotic cells, regulating immunoglobulin production, and altering the cytokine profile upon activation. However, the dichotomy of iNKT cell actions in murine models implies complicated interactions within the body's milieu. Therefore, application of potential therapy for SLE using glycolipids to regulate iNKT cells should be undertaken cautiously.

Invariant NKT (iNKT) cells are involved in host defence against microbial infections. While it is known that iNKT cells recognize glycolipids presented by CD1d, how and where they encounter antigen in vivo remains unclear. We used multi-photon microscopy to visualize the dynamics and activation of iNKT cells in lymph nodes. Following antigen administration, iNKT cells become confined in a CD1d-dependent manner in close proximity to subcapsular sinus CD169+ macrophages. These macrophages retain, internalize and present lipid antigen, and are required for iNKT cell activation, cytokine production and expansion. Thus, CD169+ macrophages can act as bona fide antigen presenting cells controlling early iNKT cell activation and favouring fast initiation of immune responses.

NKT cells play an important role in autoimmune diseases, tumor surveillance, and infectious diseases, providing in most cases protection against infection. NKT cells are reactive to CD1d presented glycolipid antigens. They can modulate immune responses by promoting the secretion of type 1, type 2, or immune regulatory cytokines. Pathogen-derived signals to dendritic cells mediated via Toll like Receptors (TLR) can be modulated by activated invariant Natural Killer T (iNKT) cells. The terminal β-(1–4)-galactose residues of glycans can modulate host responsiveness in a T helper type-1 direction via IFN-γ and TLRs. We have attempted to develop a defined immunotherapeutic, based on the cooperative action of a TLR ligand and iNKT cell using a mouse model of visceral leishmaniasis. We evaluated the anti-Leishmania immune responses and the protective efficacy of the β-(1–4)-galactose terminal NKT cell ligand glycosphingophospholipid (GSPL) antigen of L. donovani parasites. Our results suggest that TLR4 can function as an upstream sensor for GSPL and provoke intracellular inflammatory signaling necessary for parasite killing. Treatment with GSPL was able to induce a strong effective T cell response that contributed to effective control of acute parasite burden and led to undetectable parasite persistence in the infected animals. These studies for the first time demonstrate the interactions between a TLR ligand and iNKT cell activation in visceral leishmaniasis immunotherapeutic.

Author Summary

Kala azar (visceral leishmaniasis) is a deadly disease caused by the parasitic protozoa Leishmania donovani. In absence of a suitable vaccine, the incidence of leishmaniasis has increased. The World Health Organization observes that, if the disease is not treated, the fatality rate in developing countries can be as high as 100% within 2 years. Therapy of visceral leishmaniasis can be complicated by toxic side effects, drug resistance, and the need for prolonged treatment regimens. Therefore, improved therapy for leishmaniasis remains desirable. Immunotherapy to selectively induce type 1 immune responses considered essential for resistance to leishmaniasis has shown great promise. CD1d-binding glycolipids stimulate TCR signaling and activation of invariant natural killer T (iNKT) cells. Terminal β-(1–4)-galactose residues in glycoconjugates have been identified as the TLR ligand that induces IFN-γ via TLR signaling. We have used the β-(1–4)-galactose terminal glycosphingophospholipid (GSPL) antigen from L. donovani parasites to treat infected BALB/c mice. We report that immunotherapy with GSPL induced IFN-γ, a type 1 cytokine, through the cooperative action of TLR4 and NKT-cells that contributed to effective control of acute parasite burden in the infected animals.

TLR-mediated signaling and the production of IL-12 by APCs, rather than recognition of microbial antigens, enables rapid iNKT cell responses to diverse microbial infections.

Invariant natural killer T cells (iNKT cells) are critical for host defense against a variety of microbial pathogens. However, the central question of how iNKT cells are activated by microbes has not been fully explained. The example of adaptive MHC-restricted T cells, studies using synthetic pharmacological α-galactosylceramides, and the recent discovery of microbial iNKT cell ligands have all suggested that recognition of foreign lipid antigens is the main driver for iNKT cell activation during infection. However, when we compared the role of microbial antigens versus innate cytokine-driven mechanisms, we found that iNKT cell interferon-γ production after in vitro stimulation or infection with diverse bacteria overwhelmingly depended on toll-like receptor–driven IL-12. Importantly, activation of iNKT cells in vivo during infection with Sphingomonas yanoikuyae or Streptococcus pneumoniae, pathogens which are known to express iNKT cell antigens and which require iNKT cells for effective protection, also predominantly depended on IL-12. Constitutive expression of high levels of IL-12 receptor by iNKT cells enabled instant IL-12–induced STAT4 activation, demonstrating that among T cells, iNKT cells are uniquely equipped for immediate, cytokine-driven activation. These findings reveal that innate and cytokine-driven signals, rather than cognate microbial antigen, dominate in iNKT cell activation during microbial infections.

Weak TCR stimulation of iNKT cells, such as that resulting from self-antigen recognition, induces histone modifications at the IFNG locus that allow the iNKT cells to subsequently produce IFN-γ in response to proinflammatory cytokines alone.

Invariant NKT cells (iNKT cells) are innate T lymphocytes that are thought to play an important role in producing an early burst of IFN-γ that promotes successful tumor immunosurveillance and antimicrobial immunity. The cellular activation processes underlying innate IFN-γ production remain poorly understood. We show here that weak T cell receptor (TCR) stimulation that does not directly activate iNKT cell IFN-γ messenger RNA transcription nevertheless induces histone H4 acetylation at specific regions near the IFNG gene locus. This renders the iNKT cells able to produce IFN-γ in an innate manner (i.e., not requiring concurrent TCR stimulation) upon exposure to IL-12 and IL-18. The iNKT cells retain the capacity for innate activation for hours to days after the initial weak TCR stimulation, although their innate responsiveness gradually declines as a function of histone deacetylation. These results explain how iNKT cells are able to mediate rapid innate IFN-γ secretion in a manner that does not require them to undergo permanent TH1 differentiation. Moreover, our results also indicate that iNKT cell motility is maintained during activation by IL-12 and IL-18. Therefore, iNKT cells activated through this pathway can continue to migrate and may thus disseminate the IFN-γ that they produce, which may amplify its impact.

Invariant natural killer T cells (iNKTs) are innate-like T cells that are highly concentrated in the liver and recognize lipids presented on the MHC-like molecule CD1d. Although capable of a myriad of responses, few essential functions have been described for iNKTs. Among the many cell types of the immune system implicated in metabolic control and disease, iNKTs seem ideally poised for such a role, yet little has been done to elucidate such a possible function. We hypothesized that lipid presentation by CD1d could report on metabolic status and engage iNKTs to regulate cellular lipid content through their various effector mechanisms. To test this hypothesis, we examined CD1d deficient mice in a variety of metabolically stressed paradigms including high fat feeding, choline-deficient feeding, fasting, and acute inflammation. CD1d deficiency led to a mild exacerbation of steatosis during high fat or choline-deficient feeding, accompanied by impaired hepatic glucose tolerance. Surprisingly, however, this phenotype was not observed in Jα18−/− mice, which are deficient in iNKTs but express CD1d. Thus, CD1d appears to modulate some metabolic functions through an iNKT-independent mechanism.

Natural killer T cells (NKT cells) represent a subset of T lymphocytes that express natural killer (NK) cell surface markers. A subset of NKT cells, termed invariant NKT cells (iNKT), express a highly restricted T cell receptor (TCR) and respond to CD1d-restricted lipid ligands. iNKT cells are now appreciated to play an important role in linking innate and adaptive immune responses and have been implicated in infectious disease, allergy, asthma, autoimmunity, and tumor surveillance. Advances in iNKT identification and purification have allowed for the detailed study of iNKT activity in both humans and mice during a variety of chronic and acute infections. Comparison of iNKT function between non-pathogenic simian immunodeficiency virus (SIV) infection models and chronic HIV-infected patients implies a role for iNKT activity in controlling immune activation. In vitro studies of influenza infection have revealed novel effector functions of iNKT cells including IL-22 production and modulation of myeloid-derived suppressor cells, but ex vivo characterization of human iNKT cells during influenza infection are lacking. Similarly, as recent evidence suggests iNKT involvement in dengue virus pathogenesis, iNKT cells may modulate responses to a number of emerging pathogens. This Review will summarize current knowledge of iNKT involvement in responses to viral infections in both human and mouse models and will identify critical gaps in knowledge and opportunities for future study. We will also highlight recent efforts to harness iNKT ligands as vaccine adjuvants capable of improving vaccination-induced cellular immune responses.

Invariant NKT (iNKT) cells have been extensively studied throughout the last decade due to their ability to polarize and amplify the downstream immune response. Only recently however, have the various mechanisms underlying NKT cell activation begun to unfold. iNKT cells have the ability to respond as innate immune cells with minimal TCR involvement as well as through direct TCR recognition of glycolipid antigens. Additionally, the existence of several subsets of iNKT cells creates the potential for other unique pathways, which are not yet clearly defined. Here we provide an overview of the known mechanisms of invariant NKT cell activation, focusing on cytokine driven pathways and the resulting cytokine responses.

Invariant NKT (iNKT) cells regulate early immune responses to infections, in part because of their rapid release of IFN-γ and IL-4. iNKT cells are proposed to reduce the severity of Lyme disease following Borrelia burgdorferi infection. Unlike conventional T cells, iNKT cells express an invariant αβ TCR that recognizes lipids bound to the MHC class I-like molecule, CD1d. Furthermore, these cells are positively selected following TCR interactions with glycolipid/CD1d complexes expressed on CD4+CD8+ thymocytes. Whereas conventional T cell development can proceed with as few as 4/10 CD3 immunoreceptor tyrosine-based activation motifs (ITAMs), little is known about the ITAM requirements for iNKT cell selection and expansion. We analyzed iNKT cell development in CD3 ζ transgenic lines with various tyrosine-to-phenylalanine substitutions (YF) that eliminated the functions of the first (YF1,2), third (YF5,6), or all three (YF1–6) CD3 ζ ITAMs. iNKT cell numbers were significantly reduced in the thymus, spleen, and liver of all YF mice compared with wild type mice. The reduced numbers of iNKT cells resulted from significant reductions in the expression of the early growth response 2 and promyelocytic leukemia zinc finger transcription factors. In the mice with few to no iNKT cells, there was no difference in the severity of Lyme arthritis compared with wild type controls, following infections with the spirochete B. burgdorferi. These findings indicate that a full complement of functional CD3 ζ ITAMs is required for effective iNKT cell development. The Journal of Immunology, 2010, 184: 6822–6832.

Invariant natural killer T cells (iNKT cells) express a restricted T cell antigen receptor (TCR) repertoire and they respond rapidly to glycolipid antigens presented by CD1d. These glycolipid antigens have hexose sugars in α-linkage to two types of lipids that can bind to CD1d. Recent work has shown that the responses of iNKT cells to antigen-bearing microbes can have a profound impact on the development of inflammatory diseases. iNKT cells overcome the limitation of their limited TCR diversity by also responding in a foreign antigen-independent fashion to some infectious agents, similar to NK cells. Recent results demonstrate several mechanisms for the indirect activation of iNKT cells by viruses or TLR ligands, dependent on self-antigen recognition and/or different cytokines produced by antigen presenting cells. The means by which iNKT cells influence other cell types and overall host defense are likewise diverse, illustrating the flexibility and functional diversity of this T lymphocyte sublineage.

Background & Aims

NK1.1+ TCRαβint CD1-restricted T (NKT) cells are a unique subset of T lymphocytes that are thought to have an immunoregulatory role in a wide range of diseases. Most mouse NKT cells express a T-cell receptor that contains an invariant Vα14Jα18 chain and recognizes antigenic glycolipids presented in association with MHC class Ib (CD1d) molecules. These invariant NKT cells (iNKT) have been implicated in cholestatic liver injury.

Methods

We examined the role of iNKT cells in liver injury associated with biliary obstruction in mice with ligations of the common bile duct.

Results

The number of activated iNKT cells increased markedly in the livers of mice following bile duct-ligation (BDL). Plasma alanine aminotransferase (ALT) levels, an indicator of liver injury, were significantly higher in iNKT cell–deficient (Jα18−/−) mice, compared to wild-type mice, following BDL. Photoimage analysis of histologic sections confirmed that more damage was present in the livers of in Jα18−/−mice; liver damage correlated with increases in keratinocyte-derived chemokine (KC) and macrophage inflammatory protein-2 (MIP-2) production as well as neutrophil sequestration. Liver injury was significantly reduced in Jα18−/−mice treated with anti-KC and anti-MIP-2 or rendered neutrophil-deficient before BDL. Similarly, Jα18−/−mice that were injected with iNKT cells before BDL exhibited significant decreases in neutrophil accumulation and liver damage.

Conclusion

These data document the role of iNKT cells in suppressing the neutrophil proinflammatory response and neutrophil-dependent cholestatic liver damage.

Invariant natural killer T (iNKT) cells are T lymphocytes that behave similarly to cells of the innate immune system. The glycolipid α-galactosylceramide (α-GalCer) is a potent and specific activator of mouse and human iNKT cells and has been used in cancer clinical trials to drive NKT cell–mediated immune responses. However, little is known about the dynamics of the iNKT cell response to α-GalCer in vivo. In this issue of the JCI, Parekh and colleagues demonstrate that administration of α-GalCer causes iNKT cells to become unresponsive, for at least 1 month, in mice. This leads us to ask, should sequential administration of α-GalCer still be used to activate iNKT cells given the anergic state it has been shown here to induce? This intriguing article raises the issue of the avoidance of anergy induction in the design of treatment regimens that use α-GalCer as a specific activator of iNKT cells.