Faculty Bibliography

Commensal bacteria are crucial for maturation and function of the mucosal immune system. However, the mechanisms of these interactions are poorly understood. In addition, the role of the composition of the microbiota and the importance of individual species in this community in stimulating different types of immunity are major unanswered questions. We recently showed that the balance between two major effector T cell populations in the intestine, IL-17(+) Th17 cells and Foxp3(+) Tregs, requires signals from commensal bacteria and is dependent on the composition of the intestinal microbiota. Comparison of microbiota from Th17 cell-deficient and Th17 cell-sufficient mice identified segmented filamentous bacteria (SFB) as capable of specifically inducing Th17 cells in the gut. SFB represent the first example of a commensal species that can skew the mucosal effector T cell balance and thus affect the immune fitness of the individual

Background. Identification of the Th17 T cell subset as important mediators of host defense and pathology prompted us to determine their susceptibility to human immunodeficiency virus (HIV) infection. Methods and results. We found that a sizeable portion of Th17 cells express HIV coreceptor CCR5 and produce very low levels of CCR5 ligands macrophage inflammatory protein (MIP)-1alpha and MIP-1beta. Accordingly, CCR5(+) Th17 cells were efficiently infected with CCR5-tropic HIV and were depleted during viral replication in vitro. Remarkably, HIV-infected individuals receiving treatment had significantly reduced Th17 cell counts, compared with HIV-uninfected subjects, regardless of viral load or CD4 cell count, whereas treatment-naive subjects had normal levels. However, there was a preferential reduction in CCR5(+) T cells that were also CCR6 positive, which is expressed on all Th17 cells, compared with CCR6(-)CCR5(+) cells, in both treated and untreated HIV-infected subjects. This observation suggests preferential targeting of CCR6(+)CCR5(+) Th17 cells by CCR5-tropic viruses in vivo. Th17 cell levels also inversely correlated with activated CD4(+) T cells in HIV-infected individuals who are receiving treatment. Conclusions. Our findings suggest a complex perturbation of Th17 subsets during the course of HIV disease potentially through both direct viral infection and virus indirect mechanisms, such as immune activation

The stability of a lineage program (cellular memory) is dependent on mechanisms that epigenetically maintain active or repressed states of gene expression (transcriptional memory). Although epigenetic silencing of genes has been clearly demonstrated from yeast to mammals, heritable maintenance of active transcription has been less clearly defined. To investigate the potential role of active transcriptional memory during lineage diversification, we employed targeted mutation of a positive-acting cis element in the Cd4 locus to determine the impact on CD4 expression and the differentiation of CD4(+) helper T cells in mice. We show that the proximal enhancer (E4(P)) of Cd4 is essential for CD4 expression in immature CD4(+)8(+) thymocytes. Furthermore, its loss resulted in reduced and unstable expression of CD4 in mature T cells. However, if the enhancer was deleted after cells had already committed to the helper T-cell lineage, CD4 expression remained high and was stable upon cell division. 'Active' histone modifications, once initiated by E4(P), were also propagated independently of the enhancer. Thus, E4(P) is responsible for establishing an epigenetically inherited active Cd4 locus in the helper T-cell lineage. To our knowledge, this is the first genetic demonstration of active transcriptional memory in mammalian cells

Commensal microbes can have a substantial impact on autoimmune disorders, but the underlying molecular and cellular mechanisms remain largely unexplored. We report that autoimmune arthritis was strongly attenuated in the K/BxN mouse model under germ-free (GF) conditions, accompanied by reductions in serum autoantibody titers, splenic autoantibody-secreting cells, germinal centers, and the splenic T helper 17 (Th17) cell population. Neutralization of interleukin-17 prevented arthritis development in specific-pathogen-free K/BxN mice resulting from a direct effect of this cytokine on B cells to inhibit germinal center formation. The systemic deficiencies of the GF animals reflected a loss of Th17 cells from the small intestinal lamina propria. Introduction of a single gut-residing species, segmented filamentous bacteria, into GF animals reinstated the lamina propria Th17 cell compartment and production of autoantibodies, and arthritis rapidly ensued. Thus, a single commensal microbe, via its ability to promote a specific Th cell subset, can drive an autoimmune disease

The key role of interleukin (IL)-23 in the pathogenesis of autoimmune and chronic inflammatory disorders is supported by the identification of IL-23 receptor (IL-23R) susceptibility alleles associated with inflammatory bowel disease, psoriasis and ankylosing spondylitis. IL-23-driven inflammation has primarily been linked to the actions of T-helper type 17 (TH17) cells. Somewhat overlooked, IL-23 also has inflammatory effects on innate immune cells and can drive T-cell-independent colitis. However, the downstream cellular and molecular pathways involved in this innate intestinal inflammatory response are poorly characterized. Here we show that bacteria-driven innate colitis is associated with an increased production of IL-17 and interferon-gamma in the colon. Stimulation of colonic leukocytes with IL-23 induced the production of IL-17 and interferon-gamma exclusively by innate lymphoid cells expressing Thy1, stem cell antigen 1 (SCA-1), retinoic-acid-related orphan receptor (ROR)-gammat and IL-23R, and these cells markedly accumulated in the inflamed colon. IL-23-responsive innate intestinal cells are also a feature of T-cell-dependent models of colitis. The transcription factor ROR-gammat, which controls IL-23R expression, has a functional role, because Rag-/-Rorc-/- mice failed to develop innate colitis. Last, depletion of Thy1+ innate lymphoid cells completely abrogated acute and chronic innate colitis. These results identify a previously unrecognized IL-23-responsive innate lymphoid population that mediates intestinal immune pathology and may therefore represent a target in inflammatory bowel disease

The RUNX1/AML1 gene is the most frequently mutated gene in human leukemia. Conditional deletion of Runx1 in adult mice results in an increase of hematopoietic stem cells (HSCs), which serve as target cells for leukemia; however, Runx1(-/-) mice do not develop spontaneous leukemia. Here we show that maintenance of Runx1(-/-) HSCs is compromised, progressively resulting in HSC exhaustion. In leukemia development, the stem cell exhaustion was rescued by additional genetic changes. Retroviral insertional mutagenesis revealed Evi5 activation as a cooperating genetic alteration and EVI5 overexpression indeed prevented Runx1(-/-) HSC exhaustion in mice. Moreover, EVI5 was frequently overexpressed in human RUNX1-related leukemias. These results provide insights into the mechanism for maintenance of pre-leukemic stem cells and may provide a novel direction for therapeutic applications

Signaling through the T cell antigen receptor (TCR) is important for the homeostasis of naive and memory CD4(+) T cells. The significance of TCR signaling in regulatory T (Treg) cells has not been systematically addressed. Using an Ox40-cre allele that is prominently expressed in Treg cells, and a conditional null allele of the gene encoding p56(Lck), we have examined the importance of TCR signaling in Treg cells. Inactivation of p56(Lck) resulted in abnormal Treg homeostasis characterized by impaired turnover, preferential redistribution to the lymph nodes, loss of suppressive function, and striking changes in gene expression. Abnormal Treg cell homeostasis and function did not reflect the involvement of p56(Lck) in CD4 function because these effects were not observed when CD4 expression was inactivated by Ox40-cre.The results make clear multiple aspects of Treg cell homeostasis and phenotype that are dependent on a sustained capacity to signal through the TCR

Upon encountering antigen in the context of antigen presenting cells, naive CD4(+) T cells undergo differentiation into effector T helper (Th) cells, which can secrete high levels of cytokines and other immunomodulators to mediate host defense and tissue inflammation. During the past three years, the immunology field has witnessed an explosion of research advances in the biology of Th17 cells, the most recently described subset of T helper cells, which play crucial roles in host immunity and inflammation. Here we review emerging data on transcriptional regulatory networks that govern the differentiation program of Th17 cells, and focus on how the orphan nuclear receptor RORgammat coordinates this process in concert with diverse cytokine-induced transcription factors

NKp46+CD3- natural killer lymphocytes isolated from blood, lymphoid organs, lung, liver and uterus can produce granule-dependent cytotoxicity and interferon-gamma. Here we identify in dermis, gut lamina propria and cryptopatches distinct populations of NKp46+CD3- cells with a diminished capacity to degranulate and produce interferon-gamma. In the gut, expression of the transcription factor RORgammat, which is involved in the development of lymphoid tissue-inducer cells, defined a previously unknown subset of NKp46+CD3- lymphocytes. Unlike RORgammat- lamina propria and dermis natural killer cells, gut RORgammat+NKp46+ cells produced interleukin 22. Our data show that lymphoid tissue-inducer cells and natural killer cells shared unanticipated similarities and emphasize the heterogeneity of NKp46+CD3- cells in innate immunity, lymphoid organization and local tissue repair