Resting T cells enter cell cycle when challenged with anti-CD3 mAb and accessory cells that bear required Fc receptors (FcR). Presentation of anti-CD3 is thought to be a model for antigens presented by accessory cells to the TCR complex. We have obtained evidence that the number of anti-CD3 molecules that are associated with the accessory cell can be very small. We first noticed that thymic dendritic cells and cultured, but not freshly isolated, epidermal Langerhans cells (LC) were active accessory cells for responses to anti-CD3 mAb. DNA synthesis was abrogated by a mAb to the FcR but not by mAb to other molecules used in clonally specific antigen recognition, i.e., class I and II MHC products or CD4 and CD8. The requisite FcR could be identified on the LC but in small numbers. Freshly isolated LC had 20,000 FcR per cell, while the more active cultured LC had only 2,000 sites, using 125I-anti-FcR mAb in quantitative binding studies. Individual LC had similar levels of FcR, as evidenced with a sensitive FACS. FcR could not be detected on T cells or within the dendritic cell cytoplasm, at the start of or during the mitogenesis response. When the response was assessed at 30 h with single cell assays, at least 20 T cells became lymphoblasts per added LC, and at least 8 T cells were synthesizing DNA while in contact with the LC in discrete cell clusters. To the extent that anti-CD3 represents a polyclonal model for antigen presentation to specific T cell clones, these results suggest two conclusions. First, only 200-300 molecules of ligand on dendritic cells may be required to trigger a T cell. Second, the maturation of LC in culture entails "sensitizing" functions other than ligand presentation (anti-CD3 on FcR) to clonotypic T cell receptors.

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