Decreased generation of anti-tumor immunity after intrasplenic immunization.

The localization of antigen and the nature of the host antigen-presenting cells (APC) that present it to T cells are two major determinants of antigen immunogenicity. While lymph nodes appear to be the major site for T cell priming, recently the spleen was shown to provide an optimal microenvironment for direct CD8+ cytotoxic T cell (CTL) priming by tumor cells even in the absence of known costimulatory molecules on tumor cells. We analyzed whether the splenic microenvironment would support T cell priming also when host APC are involved (cross-priming) which is probably the major pathway during the generation of anti-tumor immunity. We performed immunization/challenge experiments using different tumor cells (B7.1+, B7.1- and/or beta-gal+, beta-gal-) known to induce CTL to a variable extent either exclusively by cross-priming (B7-) or at least partially by direct priming (B7+). Our results demonstrate that tumor take in the spleen required much less cells than at a subcutaneous injection site. Additionally, intrasplenic immunization was invariably ineffective compared to subcutaneous immunization. We further showed that B cells were not responsible for the inefficient intrasplenic immunization. Therefore delivering the tumor cell antigens inside the spleen by intrasplenic immunization did not improve but rather decreased the efficacy of tumor cell vaccines.