Monocyte recruitment to sites of inflammation is regulated by members of the chemokine family of chemotactic cytokines. However, the mechanisms that govern the migration of monocytes from bone marrow to blood and from blood to inflamed tissues are not well understood. Here we report that CC chemokine receptor 2 (CCR2) is highly expressed on a subpopulation of blood monocytes whose numbers are markedly decreased in CCR2 -/-mice. In bone marrow, however, CCR2 -/-mice had an increased number of monocytes, suggesting that CCR2 is critical for monocyte egress. Intravenous infusion of ex vivo-labeled WT or CCR2 -/-bone marrow into WT recipient mice demonstrated that CCR2 is necessary for efficient monocyte recruitment from the blood to inflamed tissue. Analysis of mice lacking monocyte chemoattractant protein-1 (MCP-1), MCP-3, MCP-5, or MCP-2 plus MCP-5 revealed that MCP-3 and MCP-1 are the CCR2 agonists most critical for the maintenance of normal blood monocyte counts. These findings provide evidence that CCR2 and MCP-3/MCP-1 are critical for monocyte mobilization and suggest new roles for monocyte chemoattractants in leukocyte homeostasis.

Langerhans cells (LCs) are bone marrow (BM)-derived epidermal dendritic cells (DCs) that represent a critical immunologic barrier to the external environment, but little is known about their life cycle. Here, we show that in lethally irradiated mice that had received BM transplants, LCs of host origin remained for at least 18 months, whereas DCs in other organs were almost completely replaced by donor cells within 2 months. In parabiotic mice with separate organs, but a shared blood circulation, there was no mixing of LCs. However, in skin exposed to ultraviolet light, LCs rapidly disappeared and were replaced by circulating LC precursors within 2 weeks. The recruitment of new LCs was dependent on their expression of the CCR2 chemokine receptor and on the secretion of CCR2-binding chemokines by inflamed skin. These data indicate that under steady-state conditions, LCs are maintained locally, but inflammatory changes in the skin result in their replacement by blood-borne LC progenitors.Langerhans cells (LCs) are members of a family of highly specialized antigen-presenting cells called dendritic cells (DCs) 1-3 . They are typically localized in the basal and suprabasal layers of the epidermis and represent the principal hematopoietic barrier to the external environment. LCs are well equipped to ingest foreign antigens that breach the skin. Upon activation, LCs increase their expression of major histocompatibility complex (MHC) class II and costimulatory molecules and migrate to the T cell areas of regional lymph nodes (LNs) where they initiate a systemic immune response by presenting processed antigens to T cells 1,4 .Given the importance of LCs in skin immunity, the mobilization of LCs to regional lymph nodes as well as the recruitment of LC precursors from the circulation into the skin must be tightly regulated events. Although we are beginning to understand the mechanisms that regulate the migration of LCs from the epidermis to regional LNs during inflammatory

Studying the influence of chemokine receptors (CCRs) on monocyte fate may reveal information about which subpopulations of monocytes convert to dendritic cells (DCs) and the migration pathways that they use. First, we examined whether prominent CCRs on different monocyte subsets, CCR2 or CX3CR1, mediated migration events upstream of the accumulation of monocyte-derived DCs in lymph nodes (LNs). Monocytes were labeled and traced by uptake of latex microspheres in skin. Unexpectedly, neither CCR2 nor CX3CR1 were required. However, absence of CCR2 led to an increased labeling of the minor Gr-1int monocyte population, and the number of latex+ DCs that emigrated to LNs was correspondingly increased. Characterization of Gr-1int monocytes revealed that they selectively expressed CCR7 and CCR8 mRNA in blood. CCR7 and CCR8 pathways were used by monocyte-derived DCs during mobilization from skin to LNs. The role of CCR8 in emigration from tissues also applied to human monocyte-derived cells in a model of transendothelial trafficking. Collectively, the data suggest that Gr-1int monocytes may be most disposed to become a lymphatic-migrating DCs. When these monocyte-derived DCs exit skin to emigrate to LNs, they use not only CCR7 but also CCR8, which was not previously recognized to participate in migration to LNs.

Although the protective cellular immune response to Mycobacterium tuberculosis requires recruitment of macrophages and T lymphocytes to the site of infection, the signals that regulate this trafficking have not been defined. We investigated the role of C-C chemokine receptor 2 (CCR2)-dependent cell recruitment in the protective response to M. tuberculosis. CCR2 ؊/؊ mice died early after infection and had 100-fold more bacteria in their lungs than did CCR2 ؉/؉ mice. CCR2 ؊/؊ mice exhibited an early defect in macrophage recruitment to the lung and a later defect in recruitment of dendritic cells and T cells to the lung. CCR2 ؊/؊ mice also had fewer macrophages and dendritic cells recruited to the mediastinal lymph node (MLN) after infection. T cell migration through the MLN was similar in CCR2 ؊/؊ and CCR2 ؉/؉ mice. However, T cell priming was delayed in the MLNs of the CCR2 ؊/؊ mice, and fewer CD4 ؉ and CD8 ؉ T cells primed to produce IFN-␥ accumulated in the lungs of the CCR2 ؊/؊ mice. These data demonstrate that cellular responses mediated by activation of CCR2 are essential in the initial immune response and control of infection with M. tuberculosis.