Studies on the role of SOCS3 and STAT3 in development and function of the immune system

Abstract: The immune system is composed of an innate and an adaptive response that protects us from the microbial attacks. The innate system provides a fast defense against infection. However, many microbes evade or overwhelm the innate host defenses and establish the infection in the host. In these cases, the innate immune system will be determinant in triggering the adaptive immune responses. The adaptive immunity consists of antibody-mediated immunity, and T cell-mediated immunity. T cells are the central object of study in this thesis. The thymus is a specialized primary lymphoid organ for T cell development from hematopoietic stem cells. Histologically, thymus can be divided into two parts, the central medulla and the peripheral cortex. Cells in thymus consist of developing T cells termed thymocytes and stromal cells including cortical epithelial cells (cTEC), medullary epithelial cells (mTEC), as well as dendritic cells (DC). The crosstalk between thymocytes, TECs and DCs that occurs via cell-to-cell contact, cytokines and growth factors is critical for T cell development. Several cytokines and growth factors activate the JAK/ STATs signaling pathway after binding to their specific receptors. The activated STATs transcription factors will later translocate to the nucleus and initiate the transcription of a variety of genes. The “Suppressor of Cytokine Signaling” (SOCS) is a family of proteins including SOCS1-7 and CISH that hamper the activation of different STAT transcription factors. STAT3 has been shown to be critical for the differentiation of many immune cells including those of different T helper subsets. STAT3 is involved in both inflammatory and anti-inflammatory responses. SOCS3 is a negative regulator of STAT3 when activated by some (but not all) cytokines. The role of SOCS3 in T cell development in the thymus is unknown. The purpose of this thesis is to study the role of SOCS3 in T cell development in thymus. The role ofSOCS3, CISH and STAT3 in control of M.tuberculosis infection was also addressed. By using an inducible SOCS3 conditional knockout mouse model, we demonstrated that SOCS3 was required for T cell development. The thymus from mice with SOCS3 deletion was atrophied, with a 90% decrease of thymus cellularity, and a high frequency of apoptosis on double positive (DP) thymocytes and a reduced differentiation of DN thymocytes to single positive (SP) cells. Experiments with bone marrow radiation chimeras indicated that SOCS3 in thymic stromal cells was involved in the T cells development. In addition, hematoxylin and eosin (HE) and immunohistochemistry staining showed that the mice with SOCS3 deletion displayed a thymus with disorganized cortex and medulla. However, SOCS3 did not alter the number of TEC but was necessary for the TEC maturation. A transcriptome analysis of TECs indicated that genes involved in apoptosis and cell survival, negative selection and lymphostromal interactions were regulated by SOCS3. Tuberculosis (TB) caused by infection with Mycobacterium tuberculosis is one of the top 10 causes of death worldwide. Approximately 90–95% of infected individuals successfully contain M. tuberculosis but 5–10% infected individuals will develop active TB. The mechanisms behind reactivation are still not well understood. SOCS3 in myeloid cells was shown to be critical for the control of infection with Mycobacterium tuberculosis. Moreover, single nucleotide polymorphisms (SNPs) of cish gene were associated with an increased risk of TB. Here we found mice deficient in STAT3 in myeloid cells had a decreased susceptibility to M.tuberculosis, with lower bacteria load in lungs and spleen, higher level of neutrophils infiltration and less area occupied by granuloma in lung. In STAT3 deficient mice, the levels of il-6, il-23, il-17 and il22 mRNA in lungs were increased during infection. STAT3 deficient bone marrow derived macrophages or dendritic cells expressed high levels of IL-6 and IL-23 at the protein and mRNA after M. tuberculosis infection. Furthermore, antigen-specific T cells released higher levels of IL-17 when co-cultured with M. tuberculosis infected APCs with STAT3 deficiency, indicating that STAT3 in APCs hampered Th17 differentiation and IL-17 secretion. The increased IL-17 levels resulted in an improved control of infection because neutralization of IL-17 receptor A in STAT3 deficient mice hampered bacterial control, as well as the neutrophil infiltration. Mice deficient in SOCS3 in myeloid cells or DCs were more susceptible to M. tuberculosis infection. The secretion of IL-17 by mycobacteria specific T cells was reduced when these cells were stimulated by mycobacteriainfected SOCS3 deficient APCs. Additionally, we showed that the SOCS family member CISH was induced during M.tuberculosis infection. We found that cish-/- mice had higher bacteria load in spleens and lungs up to 2.5 weeks after infection but not later compared to controls. This was associated to a decreased expression of transcripts of inos and tnf, which mediate defense against M.tuberculosis early but not late after infection. In addition, rag1-/- mice adaptively transferred with cish-/- or control T cells displayed similar protection to infection with M.tuberculosis. In conclusion, we demonstrated that SOCS3 expression in non-hematopoietic cells played a critical role for T cell development in the thymus and for maintenance of thymus architecture. On the other hand, STAT3 expression in myeloid cell hampers the control of M.tuberculosis infection by hampering Th17 stimulating cytokines secretion and the secretion of IL-17 by CD4+ T cells. In addition, we showed that CISH-mediated the control of M. tuberculosis in mice at an early stage ofinfection by the regulation ofthe innate immune responses.