B7-Negative Versus B7-Positive P815 Tumor: Differential Requirements for Priming of an Antitumor Immune Response in Lymph Nodes

Efficient T cell activation requires two synergistic but distinct signals derived from antigenic peptides presented by the MHC and from costimulatory molecules, particularly those belonging to the B7 family. Lack of B7-CD28 interaction may cause unresponsiveness of T cells to subsequent exposure to Ag. Nevertheless, immunization by some B7^- tumors induces an antitumor immune response. We found that the immune response against two B7^- tumors, the mouse P815 mastocytoma and the E7C3 melanoma, requires host-derived B7, since blockage of the B7-CD28 interaction facilitates tumor growth and eliminates an antitumor response. B7 costimulation is provided in the regional, tumor-draining lymph nodes for the induction of a primary CTL response against both B7⁺ tumor and B7^- tumor. However, the induction of a CTL response to B7⁺ tumors and its clonal expansion may occur at tumor sites in addition to secondary lymphoid organs so as to generate more effective tumor immunity.