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The murine epidermis harbors two immune cell lineages, Langerhans cells (LCs) and γδ T cells known as dendritic epidermal T cells (DETCs). LCs develop from both early yolk sac (YS) progenitors and fetal liver monocytes before locally self-renewing in the adult. For DETCs, the mechanisms of homeostatic maintenance and their hematopoietic origin are largely unknown. Here, we exploited multicolor fate mapping systems to reveal that DETCs slowly turn over at steady state. Like for LCs, homeostatic maintenance of DETCs is achieved by clonal expansion of tissue-resident cells assembled in proliferative units. The same mechanism, albeit accelerated, facilitates DETC replenishment upon injury. Hematopoietic lineage tracing uncovered that DETCs are established independently of definitive hematopoietic stem cells and instead originate from YS hematopoiesis, again reminiscent of LCs. DETCs thus resemble LCs concerning their maintenance, replenishment mechanisms, and hematopoietic development, suggesting that the epidermal microenvironment exerts a lineage-independent influence on the initial seeding and homeostatic maintenance of its resident immune cells.

Follicular dendritic cells (FDCs) regulate B cell function and development of high affinity antibody responses but little is known about their biology. FDCs associate in intricate cellular networks within secondary lymphoid organs. In vitro and ex vivo methods, therefore, allow only limited understanding of the genuine immunobiology of FDCs in their native habitat. Herein, we used various multicolor fate mapping systems to investigate the ontogeny and dynamics of lymph node (LN) FDCs in situ. We show that LN FDC networks arise from the clonal expansion and differentiation of marginal reticular cells (MRCs), a population of lymphoid stromal cells lining the LN subcapsular sinus. We further demonstrate that during an immune response, FDCs accumulate in germinal centers and that neither the recruitment of circulating progenitors nor the division of local mature FDCs significantly contributes to this accumulation. Rather, we provide evidence that newly generated FDCs also arise from the proliferation and differentiation of MRCs, thus unraveling a critical function of this poorly defined stromal cell population.

Langerhans cells (LCs) constitute a network of immune sentinels in the skin epidermis that is seeded during embryogenesis. Whereas the development of LCs has been extensively studied, much less is known about the homeostatic renewal of adult LCs in “nonmanipulated” animals. Here, we present a new multicolor fluorescent fate mapping system and quantification approach to investigate adult LC homeostasis. This novel approach enables us to propose and provide evidence for a model in which the adult epidermal LC network is not formed by mature coequal LCs endowed with proliferative capabilities, but rather constituted by adjacent proliferative units composed of “dividing” LCs and their terminally differentiated daughter cells. Altogether, our results demonstrate the general utility of our novel fate-mapping system to follow cell population dynamics in vivo and to establish an alternative model for LC homeostasis.

Millions of lymphocytes enter and exit mammal lymph nodes (LNs) each day, accessing the parenchyma via high endothelial venules (HEVs) and egressing via lymphatics. Despite this high rate of cellular flux and the many entry and exit sites within a given LN, the number of lymphocytes present in a resting LN is extraordinary stable over time, raising the question of how this steady-state is maintained. Here we have examined the anatomic details of lymphocyte movement in HEVs, finding that HEVs create pockets within which lymphocytes reside for several minutes before entering the LN proper. The function of these pockets was revealed in experiments performed under conditions in which lymphocyte egress from the LN was compromised by any of several approaches. Under such conditions, the HEVs pockets behaved as “waiting areas” in which lymphocytes were held until space was made available to them for entry into the parenchyma. Thus, rather than being simple entry ports, HEVs act as gatekeepers able to stack, hold and grant lymphocytes access to LN parenchyma in proportion to the rate of lymphocyte egress from the LN, enabling the LN to maintain a constant steady-state cellularity while supporting the extensive cellular trafficking necessary for repertoire scanning.

Millions of lymphocytes enter and exit mammal lymph nodes (LNs) each day, accessing the parenchyma via high endothelial venules (HEVs) and egressing via lymphatics. Despite this high rate of cellular flux and the many entry and exit sites within a given LN, the number of lymphocytes present in a resting LN is extraordinary stable over time, raising the question of how this steady-state is maintained. Here we have examined the anatomic details of lymphocyte movement in HEVs, finding that HEVs create pockets within which lymphocytes reside for several minutes before entering the LN proper. The function of these pockets was revealed in experiments performed under conditions in which lymphocyte egress from the LN was compromised by any of several approaches. Under such conditions, the HEVs pockets behaved as “waiting areas” in which lymphocytes were held until space was made available to them for entry into the parenchyma. Thus, rather than being simple entry ports, HEVs act as gatekeepers able to stack, hold and grant lymphocytes access to LN parenchyma in proportion to the rate of lymphocyte egress from the LN, enabling the LN to maintain a constant steady-state cellularity while supporting the extensive cellular trafficking necessary for repertoire scanning.

In the bone marrow (BM), stromal cells constitute a supportive tissue indispensable for the generation of pro-B/pre-BI, pre-BII, and immature B lymphocytes. IL-7–producing stromal cells constitute a cellular niche for pro-B/pre-BI cells, but no specific stromal cell microenvironment was identified for pre-BII cells expressing a functional pre-B cell receptor (pre-BCR). However expression of the pre-BCR represents a crucial checkpoint during B-cell development. We recently demonstrated that the stromal cell derived-galectin1 (GAL1) is a ligand for the pre-BCR, involved in the proliferation and differentiation of normal mouse pre-BII cells. Here we show that nonhematopoietic osteoblasts and reticular cells in the BM express GAL1. We observed that pre-BII cells, unlike the other B-cell subsets, were specifically localized in close contact with GAL1+reticular cells. We also determined that IL-7+and GAL1+cells represent 2 distinct mesenchymal populations with different BM localization. These results demonstrate the existence of a pre-BII specific stromal cell niche and indicate that early B cells move from IL-7+to GAL1+supportive BM niches during their development.

In the bone marrow (BM), stromal cells constitute a supportive tissue indispensable for the generation of pro-B/pre-BI, pre-BII, and immature B lymphocytes. IL-7–producing stromal cells constitute a cellular niche for pro-B/pre-BI cells, but no specific stromal cell microenvironment was identified for pre-BII cells expressing a functional pre-B cell receptor (pre-BCR). However expression of the pre-BCR represents a crucial checkpoint during B-cell development. We recently demonstrated that the stromal cell derived-galectin1 (GAL1) is a ligand for the pre-BCR, involved in the proliferation and differentiation of normal mouse pre-BII cells. Here we show that nonhematopoietic osteoblasts and reticular cells in the BM express GAL1. We observed that pre-BII cells, unlike the other B-cell subsets, were specifically localized in close contact with GAL1+ reticular cells. We also determined that IL-7+ and GAL1+ cells represent 2 distinct mesenchymal populations with different BM localization. These results demonstrate the existence of a pre-BII specific stromal cell niche and indicate that early B cells move from IL-7+ to GAL1+ supportive BM niches during their development.