T cell immunoglobulin and mucin domain-containing-3 (Tim-3) is an inhibitory receptor expressed on exhausted T cells during HIV-1 and HCV infection. By contrast, Tim-3 expression and function are defective in multiple human autoimmune diseases. However, the molecular mechanisms governing Tim-3 function remain poorly understood. Here we show that HLA-B-associated transcript 3 (Bat3) binds to, and represses the function of Tim-3. Bat3-deficient T cells display elevated expression of exhaustion markers, and knocking down Bat3 in myelin antigen-specific CD4+ T cells dramatically inhibits the development of experimental autoimmune encephalomyelitis while promoting the expansion of a dysfunctional Tim-3hiIFNγlo CD4+ cell population. Furthermore, exhausted Tim-3+ T cells from murine tumors and HIV-1-infected individuals display substantially reduced Bat3 expression and targeted deletion of Bat3 induces an exhausted phenotype in T cells. These data indicate that Bat3 acts as a molecular safety catch that inhibits Tim-3-dependent cell death/exhaustion, suggesting that Bat3 may represent a viable therapeutic target in autoimmune disorders, chronic infections and cancers.

Experimental autoimmune myocarditis (EAM) appears after infectious heart disease, the most common cause of dilated cardiomyopathy in humans. Here we report that mice lacking T-bet, a T-box transcription factor required for T helper (Th)1 cell differentiation and interferon (IFN)-γ production, develop severe autoimmune heart disease compared to T-bet −/− control mice. Experiments in T-bet −/− IL-4−/− and T-bet −/− IL-4Rα−/− mice, as well as transfer of heart-specific Th1 and Th2 cell lines, showed that autoimmune heart disease develops independently of Th1 or Th2 polarization. Analysis of T-bet −/− IL-12Rβ1−/− and T-bet −/− IL-12p35−/− mice then identified interleukin (IL)-23 as critical for EAM pathogenesis. In addition, T-bet −/− mice showed a marked increase in production of the IL-23–dependent cytokine IL-17 by heart-infiltrating lymphocytes, and in vivo IL-17 depletion markedly reduced EAM severity in T-bet −/− mice. Heart-infiltrating T-bet −/− CD8+ but not CD8− T cells secrete IFN-γ, which inhibits IL-17 production and protects against severe EAM. In contrast, T-bet −/− CD8+ lymphocytes completely lost their capacity to release IFN-γ within the heart. Collectively, these data show that severe IL-17–mediated EAM can develop in the absence of T-bet, and that T-bet can regulate autoimmunity via the control of nonspecific CD8+ T cell bystander functions in the inflamed target organ.

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Genetic regulators and environmental stimuli modulate T-cell activation in autoimmunity and cancer. The enzyme co-factor tetrahydrobiopterin (BH4) is involved in the production of monoamine neurotransmitters, the generation of nitric oxide, and pain1,2. Here we uncover a link between these processes, identifying a fundamental role for BH4 in T-cell biology. We find that genetic inactivation of GTP cyclohydrolase 1 (GCH1, the rate-limiting enzyme in the synthesis of BH4) and inhibition of sepiapterin reductase (SPR, the terminal enzyme in its synthetic pathway) severely impair the proliferation of mature mouse and human T cells. BH4 production in activated T cells is linked to alterations in iron metabolism and mitochondrial bioenergetics. In vivo blockade of BH4 synthesis abrogates T-cell-mediated autoimmunity and allergic inflammation, while enhancing BH4 levels through GCH1 overexpression augments responses by CD4- and CD8-expressing T cells, increasing their antitumour activity in vivo. Administration of BH4 to mice markedly reduces tumour growth and expands the population of intratumoral effector T cells. Kynurenine—a tryptophan metabolite that blocks antitumour immunity—inhibits T-cell proliferation in a manner that can be rescued by BH4. Finally, we report the development of a potent SPR antagonist for possible clinical use. Our data uncover GCH1, SPR and their downstream metabolite BH4 as critical regulators of T-cell biology that can be readily manipulated to either block autoimmunity or enhance anticancer immunity.

Approximately 200 million people throughout the world are infected with hepatitis C virus (HCV). One of the most striking features of HCV infection is its high propensity to establish persistence (∼70–80%) and progressive liver injury. Galectins are evolutionarily conserved glycan-binding proteins with diverse roles in innate and adaptive immune responses. Here, we demonstrate that galectin-9, the natural ligand for the T cell immunoglobulin domain and mucin domain protein 3 (Tim-3), circulates at very high levels in the serum and its hepatic expression (particularly on Kupffer cells) is significantly increased in patients with chronic HCV as compared to normal controls. Galectin-9 production from monocytes and macrophages is induced by IFN-γ, which has been shown to be elevated in chronic HCV infection. In turn, galectin-9 induces pro-inflammatory cytokines in liver-derived and peripheral mononuclear cells; galectin-9 also induces anti-inflammatory cytokines from peripheral but not hepatic mononuclear cells. Galectin-9 results in expansion of CD4+CD25+FoxP3+CD127low regulatory T cells, contraction of CD4+ effector T cells, and apoptosis of HCV-specific CTLs. In conclusion, galectin-9 production by Kupffer cells links the innate and adaptive immune response, providing a potential novel immunotherapeutic target in this common viral infection.

The concept of tumor surveillance implies that specific and nonspecific components of the immune system eliminate tumors in the early phase of malignancy. Understanding the biochemical mechanisms of tumor immunosurveillance is of paramount significance because it might allow one to specifically modulate spontaneous antitumor activity. We report that inactivation of the E3 ligase Casitas B cell lymphoma-b (Cbl-b) confers spontaneous in vivo rejection of tumor cells that express human papilloma virus antigens. Moreover, cbl-b−/− mice develop significantly fewer ultraviolet B (UVB)–induced skin malignancies and reject UVB-induced skin tumors. CD8+ T cells were identified as key players in the spontaneous tumor rejection response. Loss of Cbl-b not only enhances antitumor reactivity of CD8+ T cells but also occurs in the absence of CD4+ T cells. Mechanistically, cbl-b−/− CD8+ T cells are resistant to T regulatory cell–mediated suppression and exhibit enhanced activation and rapid tumor infiltration. Importantly, therapeutic transfer of naive cbl-b−/− CD8+ T cells is sufficient to mediate rejection of established tumors. Even up to 1 yr after the first encounter with the tumor cells, cbl-b−/− mice carry an “anticancer memory.” These data identify Cbl-b as a key signaling molecule that controls spontaneous antitumor activity of cytotoxic T cells in different cancer models. Inhibition of Cbl-b is a novel approach to stimulate long-lasting immunity against cancer.

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) in which myelin becomes the target of attack by autoreactive T cells. The immune components of the disease are recapitulated in mice using the experimental autoimmune encephalomyelitis (EAE) model. EAE is classically induced by the immunization of mice with encephalitogenic antigens derived from CNS proteins such as proteolipid protein (PLP), myelin basic protein (MBP) and myelin oligodendrocyte glycoprotein (MOG). Immunization of susceptible mouse strains with therse antigens will induce autoreactive inflammatory T cell infiltration of the CNS. More recently, the advent of clonal T cell receptor transgenic mice has led to the development of adoptive transfer protocols in which myelin-specific T cells may induce disease upon transfer into naïve recipient animals. When used in concert with gene knockout strains, these protocols are powerful tools by which to dissect the molecular pathways that promote inflammatory T cells responses in the central nervous system (CNS). Further, myelin-antigen-specific transgenic T cells may be cultured in vitro under a variety of conditions prior to adoptive transfer, allowing one to study the effects of soluble factors or pharmacologic compounds on T cell pathogenicity. In this review, we describe many of the existing models of EAE, and discuss the contributions that use of these models has made in understanding both T helper cell differentiation and the function of inhibitory T cell receptors. We focus on the the step-by-step elucidation of the network of signals required for T helper 17 (Th17) cell differentiation, as well as the molecular dissection of the Tim-3 negative regulatory signaling pathway in Th1 cells.

SUMMARY The inhibitory receptor Tim-3 has emerged as a critical regulator of the T cell dysfunction that develops in chronic viral infections and cancers. However, little is known regarding the signaling pathways that drive Tim-3 expression. Here, we demonstrate that IL-27 induces NFIL3, which promotes permissive chromatin remodeling of the Tim-3 locus and induces Tim-3 expression together with the immunosuppressive cytokine IL-10. We further show that the IL-27/NFIL3 signaling axis is crucial for the induction of Tim-3 in vivo. IL-27-conditioned Th1 cells exhibit reduced effector function and are poor mediators of intestinal inflammation. This inhibitory effect is NFIL3 dependent. In contrast, tumor-infiltrating lymphocytes (TILs) from IL-27R−/− mice exhibit reduced NFIL3, less Tim-3 expression and failure to develop dysfunctional phenotype, resulting in better tumor growth control. Thus, our data identify an IL-27/NFIL3 signaling axis as a key regulator of effector T cell responses via induction of Tim-3, IL-10, and T cell dysfunction.