Infection-induced inflammatory reactions involve a strong increase in dendritic cells (DCs) at the infection site and draining lymph nodes (dLNs). Whether inflammatory DCs are recruited to these locations or differentiate locally, and what their functional relevance is, remain unclear. Here we showed that during Leishmania infection, monocytes were recruited to the dermis and differentiated into "dermal monocyte-derived DCs," which subsequently migrated into the dLNs. In addition, monocyte recruitment to the dLNs resulted in the differentiation into "LN monocyte-derived DCs." Analysis of the kinetics of monocyte differentiation into DCs, susceptibility to infection, IL-12 production, and L. major-specific T cell stimulation potential suggest that dermal monocyte-derived DCs controlled the induction of protective T helper 1 responses against Leishmania. Thus, the demonstration of monocyte differentiation potential into DCs during in vivo infection and of local DC differentiation in inflammatory foci suggests that de novo formed monocyte-derived DCs are essential in T cell immunity against pathogens.

SummaryA new procedure for rapid isolation of dendritic cells (DC) was devised, involving collagenase digestion of tissues, dissociation of lymphoid-DC complexes, selection of light-density cells, then depletion of lymphocytes and other non-DC by treatment with a mixture of lineage-specific monoclonal antibodies (mAbs) and removal with antiimmunoglobulin-coupled magnetic beads. This enriched population (~80% DC) was further purified when required by fluorescence-activated cell sorting for cells expressing high levels of class II major histocompatibility complex (MHC). The isolated DC were characterized by immunofluorescent staining using a panel of 30 mAbs. Thymic DC were surface positive for a number of markers characteristic of T cells, but they were distinct from T-lineage cells in expressing high levels of class II MHC, in lacking expression of the T cell receptor (TCK)-CD3 complex, and having TCK/3 and 3" genes in germline state. Splenic DC shared many markers with thymic DC, but were negative for most T ceU markers, with the exception of CD8. A substantial proportion of DC from both thymus and spleen expressed CD8 at high levels, comparable with that on T cells. This appeared to be authentic CD8, and was produced by the DC themselves, since they contained CD8ce mRNA. Thymic DC presented both the CD8 c~ and/3 chains on the cell surface (Ly-2+3+), although the ce chain was in excess; the splenic DC expressed only the CD8 ol chain (Ly-2 + 3-). h is suggested that the expression of CD8 could endow certain antigen-presenting DC with a veto function. D endritic cells (DC), 1 first described by Steinman et al.(1, 2), are a minor population of irregularly shaped cells in lymphoid organs, distinguishable from both lymphocytes and macrophages. DC constitutively express high levels of both class I and class II MHC antigens (1, 2). They are highly efficient at presentation of antigen and stimulation of T lymphocytes (1-7). Those within the thymus are believed to effect deletion of developing T cells with self-reacting potential (8, 9). The surface antigenic pattern of thymic DC has been shown to differ somewhat from that of splenic DC (10-12), but it is not dear whether this implies the cells have a different origin, or a different function, or whether they simply represent different development states of the same functional lineage. There is evidence that it is the developmental state of the T cell that determines whether the T cell-DC interaction leads to T cell proliferation or to T cell death (13). As part of a study of the interaction between developing T cells in the thymus and thymic stromal elements (14), we have isolated thymic DC and compared them with those isolated from the spleen. The usual procedure for isolating DC involves enrichment in a low buoyant density fraction by centrifugation, followed by selection as cells that show an initial adherence but then release from the vessel surface when cultured overnight (10).We have used such a procedure to isolate DC from mouse thymus (15). However, we were con...

Dendritic cells, a minor cell population in lymphoid tissues, are specialized for presentation of antigenic peptides to T lymphocytes. Thymic dendritic cells are involved in the deletion of self-reactive T lymphocytes. Although all dendritic cells are ultimately of bone-marrow origin, it has not been clear whether thymic dendritic cells are produced in the adult thymus from a precursor cell or whether they migrate there preformed from the periphery. Recently we isolated from adult mouse thymus a population of early T precursors that could still form B lymphocytes, but not erythroid or myeloid cells, when transferred intravenously. Here we show that these thymic lymphoid precursor cells, as well as bone-marrow haematopoietic stem cells, are able to form both dendritic cells and T-cell progeny when transferred into an irradiated thymus. Such linked development may ensure that developing T cells are negatively selected predominantly by self antigens presented on newly formed thymic dendritic cells.