A member of the dendritic cell family that enters B cell follicles and stimulates primary antibody responses identified by a mannose receptor fusion protein

Dendritic cells (DCs) are known to activate naive T cells to become effecti ve helper cells. In addition, recent evidence suggests that DCs may influen ce naive B cells during the initial priming of antibody responses. In this study, using three-color confocal microscopy and three-dimensional immunohi stograms, we have observed that in the first few days after a primary immun ization, cells with dendritic morphology progressively localize within prim ary B cell follicles. These cells were identified by their ability to bind a fusion protein consisting of the terminal cysteine-rich portion of the mo use mannose receptor and the Fc portion of human immunoglobulin (Ig)G1 (CR- Fc). In situ, these CR-Fc binding cells express major histocompatibility co mplex class II, sialoadhesin, and CD11c and are negative for other markers identifying the myeloid DC lineage, such as (CD11b), macrophages (F4/80), f ollicular DCs (FDC-M2), B cells (B220), and T cells (CD4). Using CR-Fc bind ing capacity and now cytometry, the cells were purified from the draining l ymph nodes of mice 24 h after immunization. When injected into naive mice, these cells were able to prime T cells as well as induce production of anti gen-specific IgM and IgG1. Furthermore, they produced significantly more of the lymphocyte chemoattractant, macrophage inflammatory protein (MIP)-1 al pha, than isolated interdigitating cells. Taken together, these results pro vide evidence that a subset of DCs enters primary follicles, armed with the capacity to attract and provide antigenic stimulation for T and B lymphocy tes.