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

摘要

Interleukin (IL)-7 is a cytokine, which is crucial for the development of the murineudimmune system. It is required for lymphopoiesis and for the development of peripheral lymphudnodes (LN). IL-7-/-mice have impaired B and T cell lymphopoiesis, decreased numbers ofudperipheral B and T cells and are devoid of γδ T cells. IL-7 signals through a receptor composedudof the common γ (γc) and the IL-7Rα chain. The latter chain can also pair with the γc-like chainudcalled thymic stromal lymphopoietin receptor (TSLPR). Both form the receptor of the cytokineudcalled thymic stromal lymphopoietin (TSLP). .udOriginally identified for its capacity to promote B cell development in vitro, TSLP wasudlater shown to induce dendritic cell maturation, to trigger allergic diseases and to drive TH2uddifferentiation. Several evidences suggested that TSLP might play a role in fetal Budlymphopoiesis and that fetal but not adult cells were TSLP-responsive. However, the function ofudTSLP in hematopoiesis and in LN organogenesis in vivo remained elusive. In the work presentedudin the first part of this thesis, I have characterized the function of TSLP in adult lymphopoiesis.udThis study shows that TSLP transgene (Tg) expression restored all developing B celludcompartments in the bone marrow (BM), DN1 and DN2 thymocytes and thymic architecture,udand all peripheral B and αβ and γδ T cell compartments in IL-7-/- mice. The expression of theudTSLP Tg increased thymic and splenic cellularities. The analysis of the junctions of theudimmunoglobulin heavy chain locus showed that the DNA of B cells from IL-7-/- TSLP Tg miceudcontained N nucleotides, suggesting that adult hematopoietic progenitors are TSLP-responsive.udMoreover, BM chimera experiments showed that WT BM precursors differentiated towards BandudT-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 proliferationudand the differentiation of DN1 and DN2 thymocytes as well as the differentiation ofuduncommitted adult BM precursors towards the B and the T cell lineage in vitro. Hence, theseudresults altogether showed that TSLP has the capacity to promote long-term adult lymphopoiesisudin the absence of IL-7.udLymph node (LN) development starts during fetal life and crucially relies on theudinteraction between the hematopoietic lymphoid tissue inducer (LTi) cells and the mesenchymaludorganizer cells. Both together cluster in a cellular aggregate called LN anlage. This LN anlage isudcolonized by peripheral lymphocytes after birth, and gives rise to a mature LN organized into Budcell follicles and a T-cell zone. Mice deficient for IL-7 or for molecules of the IL-7 signalingudpathway lack several LN but the reasons underlying this defect are still not clear. As IL-7udregulates 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 ofudcolonization of the LN anlage by peripheral lymphocytes might prevent the maintenance of theudLN anlage. I show in the second part of this thesis, that TSLP overexpression increased LTi celludnumber and restored LN development in IL-7-/- and RAG2-/- γcud-/- mice, suggesting that LTi celludnumber is a critical parameter for LN organogenesis. The LN anlage of RAG2-/- γcud-/- TSLP Tgudmice were devoid of peripheral lymphocytes, ruling out that lymphocytes are required for LNudmaintenance. Thus, the results shown here define organizer and LTi cells as the minimal cellularudrequirement for LN development and suggest that the lack of LN in mice lacking molecules ofudthe IL-7 pathway is the result of suboptimal LTi cell number. This study further shows thatudlymphocyte colonization is required for establishing a correct LN architecture and for theuddifferentiation of some mesenchymal populations within the LN microenvironment.udOverall, this study shows that TSLP can substitute IL-7 for murine lymphopoiesis and forudLN organogenesis and suggest that the impaired lymphopoiesis and LN organogenesis in IL-7-/-udmice is the consequence of limited availability of endogenous TSLP.ud