Extrafollicular activation of lymph node B cells by antigen-bearing dendritic cells.

In contrast to naïve T cells that recognize short antigen-derived peptides displayed by specialized antigen-presenting cells, immunoglobulin receptors of B lymphocytes primarily recognize intact proteins. How and where within a lymph node such unprocessed antigens become available for naïve B cell recognition is not clear. We used two-photon intravital imaging to show that, after exiting high-endothelial venules and before entry into lymph node follicles, B cells survey locally concentrated dendritic cells. Engagement of the B cell receptor by the dendritic cell (DC)-associated antigen leads to lymphocyte calcium signaling, migration arrest, antigen acquisition, and extrafollicular accumulation. These findings suggest a possible role for antigen-specific B-DC interactions in promoting T cell-dependent antibody responses in vivo.