Dendritic cells and resting B cells form clusters in vitro and in vivo: T cell independence, partial LFA-1 dependence, and regulation by cross-linking surface molecules.

Initiation of an Ab response requires interaction between dendritic cells (DC), T cells, and B cells in a T cell area. We demonstrate that rat DC and B cells form T cell-independent clusters in vitro and in vivo. In vitro clusters form within 1 h and dissociate within 24 to 48 h. Clustering is restricted to resting B cells, is energy, cytoskeleton, and protein kinase C dependent, and is inhibited by anti-LFA-1 but not anti-ICAM-1 mAbs. Spleen and lymph node B cells cluster more strongly than those from lymph or blood, suggesting up-regulation of adhesiveness during transendothelial migration. Bone marrow B cells do not form clusters. DC from spleen and lymph nodes show the most clustering, lymph-borne DC are intermediate, and DC from lamina propria, Peyer's patches, and those grown from bone marrow form the fewest clusters. Clustering is stimulated by cross-linking MHC class II (whole mAb or F(ab')2) on DC or B cells or Thy-1 on DC, but not MHC class I, CD45, or CD44. Stimulation by mAb is energy, cytoskeletal, and protein kinase C dependent, but is not inhibited by anti-LFA-1 mAbs, suggesting involvement of other, unidentified adhesion molecules. We suggest that interactions between DC and B cells will occur regularly during B cell recirculation. Cross-linking of MHC class II-peptide molecules on DC by specific T cells would increase binding avidity, causing retention of Ag-specific B cells on DC long enough for the B cells to process Ag, thereby facilitating cognate interactions between T and B cells.