Ontogeny of Stromal Organizer Cells during Lymph Node Development

The development of secondary lymphoid organs, such as lymph nodes (LNs), in the embryo results from the reciprocal action between lymphoid tissue inducer (LTi) cells and stromal cells. However, the initial events inducing LN anlagen formation before the LTi stromal cells cross-talk interactions take place are not fully elucidated. In this study, we show that the inguinal LN anlagen in mouse embryos developed from mesenchymal cells surrounding the lymph sacs, spherical structures of endothelial cells that bud from veins. Using inguinal and mesenteric LNs (mLNs), we provide evidence supporting a two-step maturation model for stromal cells: first, ICAM-1−VCAM-1− mesenchymal precursor cells become ICAM-1intVCAM-1int cells, in a process independent of LTi cells and lymphotoxin β receptor (LTβR) signaling. The second step involves the maturation of ICAM-1intVCAM-1int cells to ICAM-1highVCAM-1high mucosal addressin cell adhesion molecule-1+ organizer cells and depends on both LTi cells and LTβR. Addition of αLTβR agonist to LN organ cultures was sufficient to induce ICAM-1intVCAM-1int cells to mature. In LtβR−/− embryos, both inguinal and mLN stromal cells showed a block at the ICAM-1intVCAM-1int stage, and, contrary to inguinal LNs, mLNs persist longer and contained LTi cells, which correlated with the sustained gene expression of Il-7, Cxcl13, and, to a lesser degree, Ccl21. Taken together, these results highlight the importance of the signals and cellular interactions that induce the maturation of stromal cells and ultimately lead to the formation of lymphoid tissues.