Signaling through CD40 enhances cytotoxic T lymphocyte generation by CD8+ T cells from mice bearing large tumors.

Recent studies have demonstrated the importance of CD40/CD154 (CD40L) interactions for the generation of cell-mediated antitumor immune responses. Here we show that signaling via CD40 (through the use of the activating anti-CD40 mAb, IC10) can actually promote the in vitro generation of CTL activity by CD8+ splenic T cells from mice bearing a large MOPC-315 tumor. Anti-CD40 mAb had to be added at the initiation of the stimulation cultures of tumor-bearing splenic cells in order to realize fully its potentiating activity for cytotoxic T lymphocyte (CTL) generation, suggesting that signaling through CD40 is important at the inductive stage of antitumor cytotoxicity. Moreover, anti-CD40 mAb was found to enhance the expression of the B7-2 (CD86) and, to a lesser extent, the B7-1 (CD80) costimulatory molecules on B220+ cells (i.e., B cells), and B7-2 and, to a lesser extent, B7-1 molecules played an important role in the potentiating effect of anti-CD40 mAb for CTL generation by tumor-bearer splenic cells. Furthermore, B220+ cells were found to be essential for the potentiating effect of anti-CD40 mAb, as depletion of B220+ cells at the inductive stage completely abrogated the ability of anti-CD40 mAb to enhance CTL generation. Thus, signaling through CD40 enhances CTL generation by CD8+ T cells from tumor-bearing mice by a mechanism that involves the up-regulation of B7-2 and, to a lesser extent, B7-1 expression on B220+ cells.