Characterization of the function of thymic stromal lymphopoietin in lymphopoiesis and lymph node organogenesis

Interleukin (IL)-7 is a cytokine, which is crucial for the development of the murine \nimmune system. It is required for lymphopoiesis and for the development of peripheral lymph \nnodes (LN). IL-7-/-mice have impaired B and T cell lymphopoiesis, decreased numbers of \nperipheral B and T cells and are devoid of γδ T cells. IL-7 signals through a receptor composed \nof the common γ (γc) and the IL-7Rα chain. The latter chain can also pair with the γc-like chain \ncalled thymic stromal lymphopoietin receptor (TSLPR). Both form the receptor of the cytokine \ncalled thymic stromal lymphopoietin (TSLP). . \nOriginally identified for its capacity to promote B cell development in vitro, TSLP was \nlater shown to induce dendritic cell maturation, to trigger allergic diseases and to drive TH2 \ndifferentiation. Several evidences suggested that TSLP might play a role in fetal B \nlymphopoiesis and that fetal but not adult cells were TSLP-responsive. However, the function of \nTSLP in hematopoiesis and in LN organogenesis in vivo remained elusive. In the work presented \nin the first part of this thesis, I have characterized the function of TSLP in adult lymphopoiesis. \nThis study shows that TSLP transgene (Tg) expression restored all developing B cell \ncompartments in the bone marrow (BM), DN1 and DN2 thymocytes and thymic architecture, \nand all peripheral B and αβ and γδ T cell compartments in IL-7-/- mice. The expression of the \nTSLP Tg increased thymic and splenic cellularities. The analysis of the junctions of the \nimmunoglobulin heavy chain locus showed that the DNA of B cells from IL-7-/- TSLP Tg mice \ncontained N nucleotides, suggesting that adult hematopoietic progenitors are TSLP-responsive. \nMoreover, BM chimera experiments showed that WT BM precursors differentiated towards Band \nT-cell lineages in response to TSLP, further suggesting that adult hematopoietic cells are TSLP-responsive. In this line, we show that TSLP had the capacity to promote the proliferation \nand the differentiation of DN1 and DN2 thymocytes as well as the differentiation of \nuncommitted adult BM precursors towards the B and the T cell lineage in vitro. Hence, these \nresults altogether showed that TSLP has the capacity to promote long-term adult lymphopoiesis \nin the absence of IL-7. \nLymph node (LN) development starts during fetal life and crucially relies on the \ninteraction between the hematopoietic lymphoid tissue inducer (LTi) cells and the mesenchymal \norganizer cells. Both together cluster in a cellular aggregate called LN anlage. This LN anlage is \ncolonized by peripheral lymphocytes after birth, and gives rise to a mature LN organized into B \ncell follicles and a T-cell zone. Mice deficient for IL-7 or for molecules of the IL-7 signaling \npathway lack several LN but the reasons underlying this defect are still not clear. As IL-7 \nregulates the size of the LTi cell pool, a possibility is that LN development in IL-7-/- mice is impaired because of insufficient LTi cell number. Alternatively, it was proposed that the lack of \ncolonization of the LN anlage by peripheral lymphocytes might prevent the maintenance of the \nLN anlage. I show in the second part of this thesis, that TSLP overexpression increased LTi cell \nnumber and restored LN development in IL-7-/- and RAG2-/- γc \n-/- mice, suggesting that LTi cell \nnumber is a critical parameter for LN organogenesis. The LN anlage of RAG2-/- γc \n-/- TSLP Tg \nmice were devoid of peripheral lymphocytes, ruling out that lymphocytes are required for LN \nmaintenance. Thus, the results shown here define organizer and LTi cells as the minimal cellular \nrequirement for LN development and suggest that the lack of LN in mice lacking molecules of \nthe IL-7 pathway is the result of suboptimal LTi cell number. This study further shows that \nlymphocyte colonization is required for establishing a correct LN architecture and for the \ndifferentiation of some mesenchymal populations within the LN microenvironment. \nOverall, this study shows that TSLP can substitute IL-7 for murine lymphopoiesis and for \nLN organogenesis and suggest that the impaired lymphopoiesis and LN organogenesis in IL-7-/- \nmice is the consequence of limited availability of endogenous TSLP. \n