Brandon M. Sullivan

Systemically dispersed innate IL-13–expressing cells in type 2 immunity

Type 2 immunity is a stereotyped host response to allergens and parasitic helminths that is sustained in large part by the cytokines IL-4 and IL-13. Recent advances have called attention to the contributions by innate cells in initiating adaptive immunity, including a novel lineage-negative population of cells that secretes IL-13 and IL-5 in response to the epithelial cytokines IL-25 and IL-33. Here, we use IL-4 and IL-13 reporter mice to track lineage-negative innate cells that arise during type 2 immunity or in response to IL-25 and IL-33 in vivo. Unexpectedly, lineage-negative IL-25 (and IL-33) responsive cells are widely distributed in tissues of the mouse and are particularly prevalent in mesenteric lymph nodes, spleen, and liver. These cells expand robustly in response to exogenous IL-25 or IL-33 and after infection with the helminth Nippostrongylus brasiliensis, and they are the major innate IL-13–expressing cells under these conditions. Activation of these cells using IL-25 is sufficient for worm clearance, even in the absence of adaptive immunity. Widely dispersed innate type 2 helper cells, which we designate Ih2 cells, play an integral role in type 2 immune responses.

The Transcription Factor T-bet Regulates Mucosal T Cell Activation in Experimental Colitis and Crohn's Disease

The balance between pro and antiinflammatory cytokines secreted by T cells regulates both the initiation and perpetuation of inflammatory bowel diseases (IBD). In particular, the balance between interferon (IFN)-γ/interleukin (IL)-4 and transforming growth factor (TGF)-β activity controls chronic intestinal inflammation. However, the molecular pathways that evoke these responses are not well understood. Here, we describe a critical role for the transcription factor T-bet in controlling the mucosal cytokine balance and clinical disease. We studied the expression and function of T-bet in patients with IBD and in mucosal T cells in various T helper (Th)1- and Th2-mediated animal models of chronic intestinal inflammation by taking advantage of mice that lack T-bet and retroviral transduction techniques, respectively. Whereas retroviral transduction of T-bet in CD62L+ CD4+ T cells exacerbated colitis in reconstituted SCID mice, T-bet–deficient T cells failed to induce colitis in adoptive transfer experiments suggesting that overexpression of T-bet is essential and sufficient to promote Th1-mediated colitis in vivo. Furthermore, T-bet–deficient CD62L− CD4+ T cells showed enhanced protective functions in Th1-mediated colitis and exhibited increased TGF-β signaling suggesting that a T-bet driven pathway of T cell activation controls the intestinal balance between IFN-γ/IL-4 and TGF-β responses and the development of chronic intestinal inflammation in T cell–mediated colitis. Furthermore, TGF-β was found to suppress T-bet expression suggesting a reciprocal relationship between TGF-β and T-bet in mucosal T cells. In summary, our data suggest a key regulatory role of T-bet in the pathogenesis of T cell–mediated colitis. Specific targeting of this pathway may be a promising novel approach for the treatment of patients with Crohns disease and other autoimmune diseases mediated by Th1 T lymphocytes.

Loss of T-bet, But Not STAT1, Prevents the Development of Experimental Autoimmune Encephalomyelitis

The transcription factors signal transducer and activator of transcription (STAT)1 and T-bet control the differentiation of interferon (IFN)-γ–producing T helper type (Th)1 cells. Here we compare the role of T-bet and STAT1 in the initiation and regulation of experimental autoimmune encephalomyelitis (EAE), a disease initiated by Th1 cells. T-bet–deficient mice immunized with myelin oligodendrocyte glycoprotein (MOG) were resistant to the development of EAE. This protection was also observed when T-bet−/− mice were crossed to the MOG-specific 2D2 T cell receptor transgenic strain. In contrast, although T-bet is downstream of STAT1, STAT1−/− mice were highly susceptible to EAE and developed more severe and accelerated disease with atypical neuropathologic features. The function of T-bet was dominant as mice deficient in both T-bet and STAT1 were also protected from EAE. CD4+ CD25+ regulatory T cells from these two mice strains were fully competent and do not explain the difference in disease susceptibility. However, enhanced EAE in STAT1−/− mice was associated with continued generation of IFN-γ–producing Th1 cells and up-regulation of selective chemokines responsible for the increased recruitment of macrophages and neutrophils in the central nervous system. Although the two transcription factors, STAT1 and T-bet, both induce IFN-γ gene transcription, our results demonstrate marked differences in their function in regulating pathogenic Th1 cell responses.

Antigen-driven effector CD8 T cell function regulated by T-bet

Type 1 immunity relies on the differentiation of two major subsets of T lymphocytes, the CD4+ T helper (Th) cell and the CD8+ cytotoxic T cell, that direct inflammatory and cytotoxic responses essential for the destruction of intracellular and extracellular pathogens. In contrast to CD4 cells, little is known about transcription factors that control the transition from the CD8 naïve to effector cell stage. Here, we report that the transcription factor T-bet, known to regulate Th cell differentiation, also controls the generation of the CD8+ cytotoxic effector cell. Antigen-driven generation of effector CD8+ cells was impaired in OT-I T cell receptor transgenic mice lacking T-bet, resulting in diminished cytotoxicity and a marked shift in cytokine secretion profiles. Furthermore, mice lacking T-bet responded poorly to infection with lymphocytic choriomeningitis virus. T-bet is a key player in the generation of type 1 immunity, in both Th and T cytotoxic cells.

Divergent expression patterns of IL-4 and IL-13 define unique functions in allergic immunity

Interleukin 4 (IL-4) and IL-13 are critical for responses to parasitic helminthes. We used genetically engineered reporter mice to assess the temporal and spatial production of these cytokines in vivo. In lymph nodes, IL-4, but not IL-13, was made by follicular helper T cells (TFH cells). In contrast, tissue type 2 helper T cells (TH2 cells) produced both cytokines. There was also divergent production of IL-4 and IL-13 among cells of the innate immune system, whereby basophils produced IL-4, whereas innate helper type 2 cells (Ih2 cells) produced IL-13. IL-13 production by TH2 and Ih2 cells was dependent on the transcription factor GATA-3, which was present in large amounts in these cells, and in contrast to the small amount of GATA-3 in TFH cells and basophils. The distinct localization and cellular expression of IL-4 and IL-13 explains their unique roles during allergic immunity.

Recent developments in the transcriptional regulation of cytolytic effector cells

Transcription factors have a profound influence on both the differentiation and effector function of cells of the immune system. T-bet controls the cytotoxicity of CD8+ T cells and the production of interferon-γ, and it also affects the development and function of natural killer cells and natural killer T cells. Other factors such as eomesodermin, MEF, ETS1 and members of the interferon-regulatory factor family also contribute to the effector function of immune cells. In this review, we focus on recent studies that have shed light on the transcriptional mechanisms that regulate cellular effector function in the immune system.

Genetic analysis of basophil function in vivo.

Contributions by basophils to allergic and helminth immunity remain incompletely defined. Using sensitive interleukin 4 (Il4) reporter alleles, we demonstrate here that basophil IL-4 production occurs by a CD4+ T cell–dependent process restricted to the peripheral tissues affected. We genetically marked and achieved specific deletion of basophils and found that basophils did not mediate T helper type 2 (TH2) priming in vivo. Two-photon imaging confirmed that basophils did not interact with antigen-specific T cells in lymph nodes but engaged in prolonged serial interactions with T cells in lung tissues. Although targeted deletion of IL-4 and IL-13 in either CD4+ T cells or basophils had a minimal effect on worm clearance, deletion from both lineages demonstrated a nonredundant role for basophil cytokines in primary helminth immunity.

IL-18 and IL-33 elicit Th2 cytokines from basophils via a MyD88- and p38α-dependent pathway

IL‐4 and IL‐13 are instrumental in the development and progression of allergy and atopic disease. Basophils represent a key source of these cytokines and produce IL‐4 and IL‐13 when stimulated with IL‐18, a member of the IL‐1 family of cytokines. Comparative analyses of the effects of caspase‐1‐dependent IL‐1 family cytokines on basophil IL‐4 and IL‐13 production have not been performed, and the signaling pathway proteins required for FcεRI‐independent Th2 cytokine production from basophils remain incompletely defined. Using mouse bone marrow‐derived cultured basophils, we found that IL‐4 and IL‐13 are produced in response to IL‐18 or IL‐33 stimulation. IL‐18‐ or IL‐33‐mediated Th2 cytokine production is dependent on MyD88 and p38α signaling proteins. In addition, basophil survival increased in the presence of IL‐18 or IL‐33 as a result of increased Akt activation. Studies in vivo confirmed the potency of IL‐18 and IL‐33 in activating cytokine release from mouse basophils.

Basophils: A Nonredundant Contributor to Host Immunity

The role of basophils, the rarest of blood granulocytes, in host immunity has been a mystery. Long considered the poor relative of mast cells, basophils have received much recent attention because of the availability of new reagents and models that reveal unique properties of these cells. Basophils are known to have distinct roles in allergic hypersensitivity reactions and in the immune response to intestinal helminthes. In this review, we highlight these advances and summarize our current understanding of the repertoire of functions attributed to these cells. Despite these recent insights, we are likely only beginning to gain a full understanding of how and where these cells lend effector functions to vertebrate immunity. Advances are likely to come only with the development of specific reagents that enable the finer study of basophil lineage and function. Although many fundamental aspects of basophil biology remain unanswered, the prospects remain bright for unmasking new contributions by these unusual cells.

Increased Susceptibility of Mice Lacking T-bet to Infection with Mycobacterium tuberculosis Correlates with Increased IL-10 and Decreased IFN-γ Production

A sustained CD4+ Th1-dominated type 1 immune response is required to successfully control Mycobacterium tuberculosis infection. Considerable work has demonstrated that the transcription factor, T-bet, is required for IFN-γ expression and fundamental to the generation of type 1 immunity in multiple cell types. Mice lacking T-bet are susceptible to virulent M. tuberculosis infection. Susceptibility of T-bet-deficient mice is associated with increased systemic bacterial burden, diminished IFN-γ production, and the striking accumulation of eosinophilic macrophages and multinucleated giant cells in the lung. Interestingly, T-bet−/− mice did not develop a fully polarized Th2 response toward M. tuberculosis, but exhibited selective elevation of IL-10 production. These results indicate that T-bet plays a central role in controlling M. tuberculosis disease progression, in part through the regulation of both IFN-γ and IL-10.