Invariant NKT Cells: Regulation and Function during Viral Infection

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.     

Human invariant NKT cells display alloreactivity instructed by invariant TCR-CD1d interaction and killer Ig receptors

Invariant NKT (iNKT) cells are a subset of highly conserved immunoregulatory T cells that modify a variety of immune responses, including alloreactivity. Central to their function is the interaction of the invariant TCR with glycosphingolipid (GSL) ligands presented by the nonpolymorphic MHC class I molecule CD1d and their ability to secrete rapidly large amounts of immunomodulatory cytokines when activated. Whether iNKT cells, like NK and conventional T cells, can directly display alloreactivity is not known. We show in this study that human iNKT cells and APC can establish a direct cross-talk leading to preferential maturation of allogeneic APC and a considerably higher reactivity of iNKT cells cultured with allogeneic rather that autologous APC. Although the allogeneic activation of iNKT cells is invariant TCR-CD1d interaction-dependent, GSL profiling suggests it does not involve the recognition of disparate CD1d/GSL complexes. Instead, we show that contrary to previous reports, iNKT cells, like NK and T cells, express killer Ig receptors at a frequency similar to that of conventional T cells and that iNKT cell allogeneic activation requires up-regulation and function of activating killer Ig receptors. Thus, iNKT cells can display alloreactivity, for which they use mechanisms characteristic of both NK and conventional T cells.     

Efficient activation of Valpha14 invariant NKT cells by foreign lipid antigen is associated with concurrent dendritic cell-specific self recognition

A burst release of cytokines by Valpha14 invariant NKT (iNKT) cells upon their TCR engagement critically regulates innate and adaptive immune responses. However, it remains unclear in vivo why iNKT cells respond efficiently to microbial or intracellular lipid Ags that are at low levels or that possess suboptimal antigenicity. We found that dendritic cells (DCs) potentiated iNKT cells to respond to a minimal amount of ligand alpha-galactosylceramide (alphaGalCer) through CD1d-dependent autoreactive responses that require endosomal processing and CD1d trafficking. The ability of potentiation of NKT cells was DC specific and did not depend on costimulatory signals and IL-12 production by DCs. However, DCs that failed to synthesize a major endogenous lipid Ag isoglobotrihexosylceramide were unable to potentiate NKT cells for efficient activation. Further analysis showed that differences in the level and pattern of endogenous lipid Ag presentation differentiate DCs and B cells for effective potentiation and subsequent activation of iNKT cells in the presence of an exogenous Ag. Thus, CD1d-dependent potentiation by DCs may be crucial for iNKT cell-mediated immunity against infectious agents.     

Dendritic cell function can be modulated through cooperative actions of TLR ligands and invariant NKT cells

The quality of signals received by dendritic cells (DC) in response to pathogens influences the nature of the adaptive response. We show that pathogen-derived signals to DC mediated via TLRs can be modulated by activated invariant NKT (iNKT) cells. DC maturation induced in vivo with any one of a variety of TLR ligands was greatly improved through simultaneous administration of the iNKT cell ligand alpha-galactosylceramide. DC isolated from animals treated simultaneously with TLR and iNKT cell ligands were potent stimulators of naive T cells in vitro compared with DC from animals treated with the ligands individually. Injection of protein Ags with both stimuli resulted in significantly improved T cell and Ab responses to coadministered protein Ags over TLR stimulation alone. Ag-specific CD8(+) T cell responses induced in the presence of the TLR4 ligand monophosphoryl lipid A and alpha-galactosylceramide showed faster proliferation kinetics, and increased effector function, than those induced with either ligand alone. Human DC exposed to TLR ligands and activated iNKT cells in vitro had enhanced expression of maturation markers, suggesting that a cooperative action of TLR ligands and iNKT cells on DC function is a generalizable phenomenon across species. These studies highlight the potential for manipulating the interactions between TLR ligands and iNKT cell activation in the design of effective vaccine adjuvants.     

Activation of invariant NKT cells by the helminth parasite schistosoma mansoni

Mouse CD1d-restricted NKT cells, including invariant (i)NKT cells, are innate cells activated by glycolipid Ags and play important roles in the initiation and regulation of immune responses. Through their ability to promptly produce large amounts of Th1 and/or Th2 cytokines upon TCR engagement, iNKT cells exert crucial functions in the immune/inflammatory system during bacterial, protozoan, fungal, and viral infections. However, their roles during metazoan parasite infection, which are generally associated with strong Th2 responses, still remain elusive. In this study, we show that during the course of murine schistosomiasis, iNKT cells exhibit an activated phenotype and that following schistosome egg encounter in the liver, hepatic iNKT cells produce both IFN-gamma and IL-4 in vivo. We also report that schistosome egg-sensitized dendritic cells (DCs) activate, in a CD1d-dependent manner, iNKT cells to secrete IFN-gamma and IL-4 in vitro. Interestingly, transfer of egg-sensitized DCs promotes a strong Th2 response in recipient wild-type mice, but not in mice that lack iNKT cells. Engagement of TLRs in DCs is not necessary for iNKT cell stimulation in response to egg-sensitized DCs, suggesting an alternative pathway of activation. Finally, we propose that self, rather than parasite-derived, CD1d-restricted ligands are implicated in iNKT cell stimulation. Taken together, our data show for the first time that helminths can activate iNKT cells to produce immunoregulatory cytokines in vivo, enabling them to influence the adaptive immune response.     

Activation of nonclassical CD1d-restricted NK T cells induces airway hyperreactivity in beta 2-microglobulin-deficient mice

Allergic asthma is characterized by Th2-driven eosinophilic airway inflammation and by a central feature called airway hyperreactivity (AHR), development of which requires the presence of classical type I invariant NK T (iNKT) cells. Allergen-induced AHR, however, develops in beta(2)-microglobulin (beta(2)m)(-/-) mice, which lack classical iNKT cells, suggesting that in some situations iNKT cells may be dispensable for the development of AHR. In contrast, our studies now suggest that a CD1d-restricted, NK1.1(+) noninvariant TCR NKT cell population is present in beta(2)m(-/-) mice and is responsible for the development of AHR but not for Th2 responses. Furthermore, treatment of beta(2)m(-/-) mice with anti-CD1d mAb or anti-NK1.1 mAb unexpectedly abolished allergen-induced AHR. The CD1-restricted NKT cells in these mice, which failed to respond to alpha-galactosylceramide and which therefore were not classical type I iNKT cells, appear to represent an NKT cell subset restricted by a beta(2)m-independent form of CD1d. These results indicate that, although classical type I iNKT cells are normally required for the development of AHR, under different circumstances other NKT cell subsets, including nonclassical NKT cells, may substitute for classical iNKT cells and induce AHR.     

IFN-gamma-producing human invariant NKT cells promote tumor-associated antigen-specific cytotoxic T cell responses

CD1d-restricted invariant NKT (iNKT) cells can enhance immunity to cancer or prevent autoimmunity, depending on the cytokine profile secreted. Antitumor effects of the iNKT cell ligand alpha-galactosylceramide (alphaGC) and iNKT cell adoptive transfer have been demonstrated in various tumor models. Together with reduced numbers of iNKT cells in cancer patients, which have been linked to poor clinical outcome, these data suggest that cancer patients may benefit from therapy aiming at iNKT cell proliferation and activation. Herein we present results of investigations on the effects of human iNKT cells on Ag-specific CTL responses. iNKT cells were expanded using alphaGC-pulsed allogeneic DC derived from the acute myeloid leukemia cell line MUTZ-3, transduced with CD1d to enhance iNKT cell stimulation, and with IL-12 to stimulate type 1 cytokine production. Enhanced activation and increased IFN-gamma production was observed in iNKT cells, irrespective of CD4 expression, upon stimulation with IL-12-overexpressing dendritic cells. IL-12-stimulated iNKT cells strongly enhanced the MART-1 (melanoma Ag recognized by T cell 1)-specific CD8(+) CTL response, which was dependent on iNKT cell-derived IFN-gamma. Furthermore, autologous IL-12-overexpressing dendritic cells, loaded with Ag as well as alphaGC, was superior in stimulating both iNKT cells and Ag-specific CTL. This study shows that IL-12-overexpressing allogeneic dendritic cells expand IFN-gamma-producing iNKT cells, which may be more effective against tumors in vivo. Furthermore, the efficacy of autologous Ag-loaded DC vaccines may well be enhanced by IL-12 overexpression and loading with alphaGC.     

MHC class II-expressing thymocytes suppress invariant NKT cell development

Natural killer T (NKT) cells are positively selected on cortical thymocytes expressing the non-classical major histocompatibility complex (MHC) class I CD1d molecules. However, it is less clear how NKT cells are negatively selected in the thymus. In this study, we investigated the role of MHC class II expression in NKT cell development. Transgenic mice expressing MHC class II on thymocytes and peripheral T cells had a marked reduction in invariant NKT (iNKT) cells. Reduced numbers of iNKT cells correlated with the absence of in vivo production of cytokines in response to the iNKT cell agonist alpha-galactosylceramide. Using mixed bone marrow chimeras, we found that MHC class II-expressing thymocytes suppressed the development of iNKT cells in trans in a CD4-dependent manner. Our observations have significant implications for human iNKT cell development as human thymocytes express MHC class II, which can lead to an inefficient selection of iNKT cells.     

CD1d-independent activation of mouse and human iNKT cells by bacterial superantigens

Invariant NKT (iNKT) cells are infrequent but important immunomodulatory lymphocytes that exhibit CD1d-restricted reactivity with glycolipid Ags. iNKT cells express a unique T-cell receptor (TCR) composed of an invariant α-chain, paired with a limited range of β-chains. Superantigens (SAgs) are microbial toxins defined by their ability to activate conventional T cells in a TCR β-chain variable domain (Vβ)-specific manner. However, whether iNKT cells are directly activated by bacterial SAgs remains an open question. Herein, we explored the responsiveness of mouse and human iNKT cells to a panel of staphylococcal and streptococcal SAgs and examined the contribution of major histocompatibility complex (MHC) class II and CD1d to these responses. Bacterial SAgs that target mouse Vβ8, such as staphylococcal enterotoxin B (SEB), were able to activate mouse hybridoma and primary hepatic iNKT cells in the presence of mouse APCs expressing human leukocyte antigen (HLA)-DR4. iNKT cell-mediated cytokine secretion in SEB-challenged HLA-DR4-transgenic mice was CD1d-independent and accompanied by a high interferon-γ:interleukin-4 ratio consistent with an in vivo Th1 bias. Furthermore, iNKT cells from SEB-injected HLA-DR4-transgenic mice, and iNKT cells from SEB-treated human PBMCs, showed early activation by intracellular cytokine staining and CD69 expression. Unlike iNKT cell stimulation by α-galactosylceramide, stimulation by SEB did not induce TCR downregulation of either mouse or human iNKT cells. We conclude that Vβ8-targeting bacterial SAgs can activate iNKT cells by utilizing a novel pathway that requires MHC class II interactions, but not CD1d. Therefore, iNKT cells fulfill important effector functions in response to bacterial SAgs and may provide attractive targets in the management of SAg-induced illnesses.     

Innate Invariant NKT Cells Recognize Mycobacterium tuberculosis–Infected Macrophages,Produce Interferon-γ, and Kill Intracellular Bacteria

Cellular immunity to Mycobacterium tuberculosis (Mtb) requires a coordinated response between the innate and adaptive arms of the immune system, resulting in a type 1 cytokine response, which is associated with control of infection. The contribution of innate lymphocytes to immunity against Mtb remains controversial. We established an in vitro system to study this question. Interferon-γ is produced when splenocytes from uninfected mice are cultured with Mtb-infected macrophages, and, under these conditions, bacterial replication is suppressed. This innate control of bacterial replication is dependent on CD1d-restricted invariant NKT (iNKT) cells, and their activation requires CD1d expression by infected macrophages as well as IL-12 and IL-18. We show that iNKT cells, even in limiting quantities, are sufficient to restrict Mtb replication. To determine whether iNKT cells contribute to host defense against tuberculosis in vivo, we adoptively transferred iNKT cells into mice. Primary splenic iNKT cells obtained from uninfected mice significantly reduce the bacterial burden in the lungs of mice infected with virulent Mtb by the aerosol route. Thus, iNKT cells have a direct bactericidal effect, even in the absence of synthetic ligands such as α-galactosylceramide. Our finding that iNKT cells protect mice against aerosol Mtb infection is the first evidence that CD1d-restricted NKT cells mediate protection against Mtb in vivo.     

Expression of CD1d molecules by human schwann cells and potential interactions with immunoregulatory invariant NK T cells

CD1d-restricted NKT cells expressing invariant TCR alpha-chains (iNKT cells) produce both proinflammatory and anti-inflammatory cytokines rapidly upon activation, and are believed to play an important role in both host defense and immunoregulation. To address the potential implications of iNKT cell responses for infectious or inflammatory diseases of the nervous system, we investigated the expression of CD1d in human peripheral nerve. We found that CD1d was expressed on the surface of Schwann cells in situ and on primary or immortalized Schwann cell lines in culture. Schwann cells activated iNKT cells in a CD1d-dependent manner in the presence of alpha-galactosylceramide. Surprisingly, the cytokine production of iNKT cells stimulated by alpha-galactosylceramide presented by CD1d+ Schwann cells showed a predominance of Th2-associated cytokines such as IL-5 and IL-13 with a marked deficiency of proinflammatory Th1 cytokines such as IFN-gamma or TNF-alpha. Our findings suggest a mechanism by which iNKT cells may restrain inflammatory responses in peripheral nerves, and raise the possibility that the expression of CD1d by Schwann cells could be relevant in the pathogenesis of infectious and inflammatory diseases of the peripheral nervous system.     

TLR-dependent IL-4 production by invariant Valpha14+Jalpha18+ NKT cells to initiate contact sensitivity in vivo

LPS stimulated B-1 cell polyclonal in vivo IgM responses depend on IL-4 release by invariant Valpha14+Jalpha18+ NKT (iNKT) cells. The IgM Abs can recruit effector T cells to mediate contact sensitivity. LPS activates the B-1 cell response just 1 day later, and depends on CD1d, iNKT cells, IL-4, TLR4, and MyD88. LPS in vivo and in vitro stimulates rapid preferential production of IL-4 in hepatic iNKT cells within 2 h. TLR4 were demonstrated in iNKT cells by flow cytometry and functional studies. Thus, innate microbial stimulation via TLR can activate iNKT cell and B-1 cell collaboration. The result is polyclonal IgM Ab responses capable of recruiting Ag-specific T cells into tissues. This may be involved in the promotion of autoimmunity by infectious agents.     

The complementarity determining region 2 of BV8S2 (V beta 8.2) contributes to antigen recognition by rat invariant NKT cell TCR

Invariant NKT cells (iNKT cells) are characterized by a semi-invariant TCR comprising an invariant alpha-chain paired with beta-chains with limited BV gene usage which are specific for complexes of CD1d and glycolipid Ags like alpha-galactosylceramide (alpha-GalCer). iNKT cells can be visualized with alpha-GalCer-loaded CD1d tetramers, and the binding of mouse CD1d tetramers to mouse as well as to human iNKT cells suggests a high degree of conservation in recognition of glycolipid Ags between species. Surprisingly, mouse CD1d tetramers failed to stain a discrete cell population among F344/Crl rat liver lymphocytes, although comprised iNKT cells are indicated by IL-4 and IFN-gamma secretion after alpha-GalCer stimulation. The arising hypothesis that rat iNKT TCR recognizes alpha-GalCer only if presented by syngeneic CD1d was then tested with the help of newly generated rat and mouse iNKT TCR-transduced cell lines. Cells expressing mouse iNKT TCR reacted to alpha-GalCer presented by rat or mouse CD1d and efficiently bound alpha-GalCer-loaded mouse CD1d tetramers. In contrast, cells expressing rat iNKT TCR responded only to alpha-GalCer presented by syngeneic CD1d and bound mouse CD1d tetramers only poorly or not at all. Finally, CD1d-dependent alpha-GalCer reactivity and binding of mouse CD1d tetramers was tested for cells expressing iNKT TCR comprising either rat or mouse AV14 (Valpha14) alpha-chains and wild-type or mutated BV8S2 (Vbeta8.2) beta-chains. The results confirmed the need of syngeneic CD1d as restriction element for rat iNKT TCR and identified the CDR2 of BV8S2 as an essential site for ligand recognition by iNKT TCR.     

The TLR7 agonist R848 alleviates allergic inflammation by targeting invariant NKT cells to produce IFN-gamma

It has been documented that TLR7 stimulation triggers not only antiviral responses, but also alleviates experimental asthma. Considering the implication of invariant NKT (iNKT) cells in both situations, we postulated that they might contribute to the anti-inflammatory effect of TLR7 ligands. We show in this study that spleen cells activated by the TLR7 agonist resiquimod (R848) attenuate allergic inflammation upon adoptive transfer when they are recovered from wild-type, but not from iNKT cell-deficient Jα18(-/-) mice, which proves the specific involvement of this regulatory population. Furthermore, we provide evidence that IFN-γ is critical for the protective effect, which is lost when transferred iNKT cells are sorted from IFN-γ-deficient mice. In support of a direct activation of iNKT cells through TLR7 signaling in vivo, we observed a prompt increase of serum IFN-γ levels, associated with upregulation of CD69 expression on iNKT cells. Moreover, we demonstrate that iNKT cells effectively express TLR7 and respond to R848 in vitro by producing high levels of IFN-γ in the presence of IL-12, consistent with the conclusion that their contribution to the alleviation of allergic inflammation upon treatment with TLR7 ligands is mediated through IFN-γ.     

Loss of IL-4 secretion from human type 1a diabetic pancreatic draining lymph node NKT cells

Altered frequency and function of peripheral invariant NKT (iNKT) cells have been implicated in the regulation of murine and human type 1a diabetes. To examine regulatory cells from the site of drainage of autoinflammatory tissue and autoantigenic T cell priming in diabetes, we directly cloned iNKT cells from human pancreatic draining lymph nodes (PLN). From 451 T cell clones from control and diabetic PLN, we derived 55 iNKT cells by two methods and analyzed function by cytokine secretion. iNKT cell clones isolated from control PLN secreted IL-4 and IFN-gamma upon TCR stimulation. For type 1a diabetic subjects, PLN iNKT cell clones from three samples secreted IFN-gamma and no IL-4. In a rare recent onset diabetic sample with islet-infiltrating CD4+ T cells, the phenotype of PLN iNKT cell clones was mixed. From normal and diabetic PLN, one-third of CD1d tetramer+-sorted T cell clones were reactive with CD1d transfectants or proliferated/secreted cytokine in response to alpha-galactosylceramide-pulsed PBMCs; tetramer-staining T cell clones from diabetic PLN did not secrete IL-4. This is the first report directly examining iNKT cells from lymph nodes draining the site of autoimmunological attack in humans; iNKT cells were altered in cytokine secretion as previously reported for circulating iNKT cells in human type 1a diabetes.     

Circulating myeloid dendritic cells of advanced cancer patients result in reduced activation and a biased cytokine profile in invariant NKT cells

CD1d-restricted invariant NKT (iNKT) cells play important regulatory roles in various immune responses, including antitumor immune responses. Previous studies have demonstrated quantitative and qualitative defects in iNKT cells of cancer patients, and these defects are clinically relevant as they are associated with poor prognosis. In this study we demonstrate that defects in the iNKT cell population can, at least in part, be attributed to defective interactions between iNKT cells and CD1d-expressing circulating myeloid dendritic cells (mDC), as mDC of patients with advanced melanoma and renal cell cancer reduced the activation and Th1 cytokine production of healthy donor-derived iNKT cells. Interestingly, this reduced activation of iNKT cells was restricted to patients with low circulating iNKT cell numbers and could be reversed by IL-12 and in part by the neutralization of TGF-beta, but it was further reduced by the neutralization of IL-10 in vitro. Additional experiments revealed discordant roles for TGF-beta and IL-10 on human iNKT cells, because TGF-beta suppressed iNKT cell activation and proliferation and IFN-gamma production while IL-10 was identified as a cytokine involved in stimulating the activation and expansion of iNKT cells that could subsequently suppress NK cell and T cell responses.     

iNKT Cells Suppress the CD8(+) T Cell Response to a Murine Burkitt's-Like B Cell Lymphoma

The T cell response to B cell lymphomas differs from the majority of solid tumors in that the malignant cells themselves are derived from B lymphocytes, key players in immune response. B cell lymphomas are therefore well situated to manipulate their surrounding microenvironment to enhance tumor growth and minimize anti-tumor T cell responses. We analyzed the effect of T cells on the growth of a transplantable B cell lymphoma and found that iNKT cells suppressed the anti-tumor CD8(+) T cell response. Lymphoma cells transplanted into syngeneic wild type (WT) mice or Jalpha18(-/-) mice that specifically lack iNKT cells grew initially at the same rate, but only the mice lacking iNKT cells were able to reject the lymphoma. This effect was due to the enhanced activity of tumor-specific CD8(+) T cells in the absence of iNKT cells, and could be partially reversed by reconstitution of iNKT cells in Jalpha 18(-/-) mice. Treatment of tumor-bearing WT mice with alpha -galactosyl ceramide, an activating ligand for iNKT cells, reduced the number of tumor-specific CD8(+) T cells. In contrast, lymphoma growth in CD1d1(-/-) mice that lack both iNKT and type II NKT cells was similar to that in WT mice, suggesting that type II NKT cells are required for full activation of the anti-tumor immune response. This study reveals a tumor-promoting role for iNKT cells and suggests their capacity to inhibit the CD8(+) T cell response to B cell lymphoma by opposing the effects of type II NKT cells.     

NKT cells enhance CD4+ and CD8+ T cell responses to soluble antigen in vivo through direct interaction with dendritic cells

Modification in the function of dendritic cells (DC), such as that achieved by microbial stimuli or T cell help, plays a critical role in determining the quality and size of adaptive responses to Ag. NKT cells bearing an invariant TCR (iNKT cells) restricted by nonpolymorphic CD1d molecules may constitute a readily available source of help for DC. We therefore examined T cell responses to i.v. injection of soluble Ag in the presence or the absence of iNKT cell stimulation with the CD1d-binding glycolipid alpha-galactosylceramide (alpha-GalCer). Considerably enhanced CD4(+) and CD8(+) T cell responses were observed when alpha-GalCer was administered at the same time as or close to OVA injection. This enhancement was dependent on the involvement of iNKT cells and CD1d molecules and required CD40 signaling. Studies in IFN-gammaR(-/-) mice indicated that IFN-gamma was not required for the adjuvant effect of alpha-GalCer. Consistent with this result, enhanced T cell responses were observed using OCH, an analog of alpha-GalCer with a truncated sphingosine chain and a reduced capacity to induce IFN-gamma. Splenic DC from alpha-GalCer-treated animals expressed high levels of costimulatory molecules, suggesting maturation in response to iNKT cell activation. Furthermore, studies with cultured DC indicated that potentiation of T cell responses required presentation of specific peptide and alpha-GalCer by the same DC, implying conditioning of DC by iNKT cells. The iNKT-enhanced T cell responses resisted challenge with OVA-expressing tumors, whereas responses induced in the absence of iNKT stimulation did not. Thus, iNKT cells exert a significant influence on the efficacy of immune responses to soluble Ag by modulating DC function.     

ICOS costimulates invariant NKT cell activation

It has been reported that costimulatory molecules, CD80/86-CD28 and CD154-CD40, critically contribute to activation of CD1d-restricted invariant NKT (iNKT) cells. Here we have demonstrated that ICOS, a new member of the CD28 family, plays a substantial role in iNKT cell activation. iNKT cells constitutively expressed ICOS as well as CD28 independently, and ICOS expression was further up-regulated 2-3 days after alpha-galactosylceramide (alpha-GalCer) treatment. Blockade of ICOS-mediated costimulation by administration of anti-ICOS ligand (B7RP-1) mAb or by ICOS gene knockout substantially inhibited alpha-GalCer-induced IFN-gamma and IL-4 production, cytotoxic activity, and anti-metastatic effect. Moreover, blockade of both B7RP-1-ICOS and CD80/86-CD28 interactions mostly abolished the alpha-GalCer-induced immune responses. These findings indicate that iNKT cell activation is regulated by CD28 and IOCS independently.     

Cutting edge: Compartmentalization of Th1-like noninvariant CD1d-reactive T cells in hepatitis C virus-infected liver

Murine intrahepatic lymphocytes (IHL) are dominated by invariant TCR alpha-chain expressing CD1d-reactive NKT cells, which can cause model hepatitis. Invariant NKT (CD56(+/-)CD161(+)) and recently identified noninvariant CD1d-reactive T cells rapidly produce large amounts of IL-4 and/or IFN-gamma and can regulate Th1/Th2 responses. Human liver contains large numbers of CD56(+) NKT cells but few invariant NKT. Compared with matched peripheral blood T cell lines, primary IHL lines from patients with chronic hepatitis C had high levels of CD161 and CD1d reactivity, but the invariant TCR was rare. CD1d-reactive IHL were strikingly Th1 biased. IHL also demonstrated CD1d-specific cytotoxic activity. Hepatocytes and other liver cells express CD1d. These results identify a novel population of human T cells that could contribute to destructive as well as protective immune responses in the liver. CD1d-reactive T cells may have distinct roles in different tissues.