DENDRITIC CELLS INTERACT DIRECTLY WITH NAIVE B-LYMPHOCYTES TO TRANSFER ANTIGEN AND INITIATE CLASS SWITCHING IN A PRIMARY T-DEPENDENT RESPONSE

Dendritic cells (DC) are thought to initiate Ab synthesis by activatio n of T cells, which then pro vide cytokine and cell-bound ''help'' to B cells. Here, we provide evidence that DC can capture and retain unpr ocessed Ag in vitro and in vivo, and can transfer this Ag to naive B c ells to initiate a specific Ab response. The response is skewed with 4 - to 13-fold higher titers of IgG than IgM, and the predominant subcla sses of Ab produced in naive animals are those associated with Th2-typ e responses. Ag retention and the skew in class switching is a physiol ogic phenomenon because DC loaded with Ag in vivo and isolated 24 h la ter initiated a class-switched, Ag-specific Ab response in naive anima ls. In vitro studies confirmed that DC provide naive B cells with sign als that are essential for the synthesis of class-switched Ab. Taken t ogether, these observations show that DC have an important role in the initiation of Ab synthesis by direct interaction with B cells.