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
immune system. It is required for lymphopoiesis and for the development of peripheral lymph
nodes (LN). IL-7-/-mice have impaired B and T cell lymphopoiesis, decreased numbers of
peripheral B and T cells and are devoid of γδ T cells. IL-7 signals through a receptor composed
of the common γ (γc) and the IL-7Rα chain. The latter chain can also pair with the γc-like chain
called thymic stromal lymphopoietin receptor (TSLPR). Both form the receptor of the cytokine
called thymic stromal lymphopoietin (TSLP). .
Originally identified for its capacity to promote B cell development in vitro, TSLP was
later shown to induce dendritic cell maturation, to trigger allergic diseases and to drive TH2
differentiation. Several evidences suggested that TSLP might play a role in fetal B
lymphopoiesis and that fetal but not adult cells were TSLP-responsive. However, the function of
TSLP in hematopoiesis and in LN organogenesis in vivo remained elusive. In the work presented
in the first part of this thesis, I have characterized the function of TSLP in adult lymphopoiesis.
This study shows that TSLP transgene (Tg) expression restored all developing B cell
compartments in the bone marrow (BM), DN1 and DN2 thymocytes and thymic architecture,
and all peripheral B and αβ and γδ T cell compartments in IL-7-/- mice. The expression of the
TSLP Tg increased thymic and splenic cellularities. The analysis of the junctions of the
immunoglobulin heavy chain locus showed that the DNA of B cells from IL-7-/- TSLP Tg mice
contained N nucleotides, suggesting that adult hematopoietic progenitors are TSLP-responsive.
Moreover, BM chimera experiments showed that WT BM precursors differentiated towards Band
T-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
and the differentiation of DN1 and DN2 thymocytes as well as the differentiation of
uncommitted adult BM precursors towards the B and the T cell lineage in vitro. Hence, these
results altogether showed that TSLP has the capacity to promote long-term adult lymphopoiesis
in the absence of IL-7.
Lymph node (LN) development starts during fetal life and crucially relies on the
interaction between the hematopoietic lymphoid tissue inducer (LTi) cells and the mesenchymal
organizer cells. Both together cluster in a cellular aggregate called LN anlage. This LN anlage is
colonized by peripheral lymphocytes after birth, and gives rise to a mature LN organized into B
cell follicles and a T-cell zone. Mice deficient for IL-7 or for molecules of the IL-7 signaling
pathway lack several LN but the reasons underlying this defect are still not clear. As IL-7
regulates 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
colonization of the LN anlage by peripheral lymphocytes might prevent the maintenance of the
LN anlage. I show in the second part of this thesis, that TSLP overexpression increased LTi cell
number and restored LN development in IL-7-/- and RAG2-/- γc
-/- mice, suggesting that LTi cell
number is a critical parameter for LN organogenesis. The LN anlage of RAG2-/- γc
-/- TSLP Tg
mice were devoid of peripheral lymphocytes, ruling out that lymphocytes are required for LN
maintenance. Thus, the results shown here define organizer and LTi cells as the minimal cellular
requirement for LN development and suggest that the lack of LN in mice lacking molecules of
the IL-7 pathway is the result of suboptimal LTi cell number. This study further shows that
lymphocyte colonization is required for establishing a correct LN architecture and for the
differentiation of some mesenchymal populations within the LN microenvironment.
Overall, this study shows that TSLP can substitute IL-7 for murine lymphopoiesis and for
LN organogenesis and suggest that the impaired lymphopoiesis and LN organogenesis in IL-7-/-
mice is the consequence of limited availability of endogenous TSLP.