Generierung von Lymphknoten-Fett-Pad Chimären für die Studie der Lymphknoten Stromal Cell Origin

Lymph nodes (LNs) are key organs of the immune system situated at strategic sites in the body, along the lymphatic vasculature network. They enable filtration of antigens and pathogens and provide a site for antigen presentation to lymphocytes and induction of adaptive immune responses. The stroma, which forms the basic structure of the LN and orchestrates the movement of the different hematopoietic participants of the adaptive immune response, is central to the function of these organs. Different populations of stromal cells supply essential and specific cues for the movements, localization, survival, proliferation and maturation of the hematopoietic component of the immune system^1-3. Adult LN stromal cells fall in three categories: the blood endothelial cells, the lymphatic endothelial cells and fibroblasts. These three categories encompass heterogeneous populations. The fibroblastic populations contain fibroblastic reticular cells (FRC), follicular dendritic cells (FDC), marginal reticular cells (MRC), while the fibroblasts forming the capsule, the medulla and other cells which are not yet identified^1-5. The origins and the mechanisms governing the maturation of the different LN stromal cell populations are unclear and the absence of specific markers allowing the fate mapping of specific LN stromal cell populations rends their study particularly difficult. However, a full understanding of the ontogeny of LN stromal cells is necessary to the comprehension of adaptive immune responses, the mechanisms contributing to tolerance and is at the basis of the development of artificial LNs.

So far, the study of LN stromal cell origin and development has been mostly limited to the direct assessment of LN development in embryos and newborns in wild type mice and different knockout mouse strains^6-9. These approaches are limited by the embryonic and perinatal lethality of some of the mouse strains carrying deletions in genes important for LN development. Moreover, some of the genes essential for lymphoid tissue development are also involved in a wide range of biological process as it is the case for RANK^10-11 or NF-κB2¹². To address these issues, isolation and transplantation of embryonic LNs under the kidney capsule of a host mouse have been performed^12-13. This technique allows, for example, the transfer of genetically modified embryonic LNs in a wild type environment to assess organ development and the recruitment and organization of host cells. However, the growth of embryonic LN grafted under the kidney capsule of an adult host is impaired, thus limiting the use of this technique.

LNs and fat deposits are anatomically closely associated and they develop simultaneously during embryogenesis. We recently demonstrated that the association LN/adipose tissue plays a crucial role in the provision of stromal cell progenitors for the LNs. In particular, signaling through the LTβR controls the fate of adipocyte precursor cells by blocking adipogenesis and instead promoting maturation towards a LN stromal cell phenotype¹⁴. Here we describe a method based on generation of LN-fat pad chimeras allowing the tracing of adipose tissue derived cells in the developing LN. This method will be useful to determine the contribution of adipose tissues to different LN stromal cell populations and combined with the use of tissues from genetically modified mouse strains, will allow a better understanding of the mechanisms controlling the differentiation of the different LN stromal cell subsets.