Induction of an antitumour adaptive immune response elicited by tumour cells expressing de novo B7-1 mainly depends on the anatomical site of their delivery: the dose applied regulates the expansion of the response

De novo expression of costimulatory molecules in tumours generally increases their immuno- genicity, but does not always induce a protective response against the parental tumour. This issue was addressed in the mouse Sp6 hybridoma model, comparing different immunization routes (subcutaneous, intraperitoneal and intravenous) and doses (05 106 and 5 106 cells) of Sp6 cells expressing de novo B7-1 (Sp6/B7). The results can be summarized as follows. First, de novo expression of B7-1 rendered Sp6 immunogenic, as it signi®cantly reduced the tumour incidence to 15% with all delivery routes and doses tested, whereas wild-type Sp6 was invariably tumorigenic (100% tumour incidence). Second, long-lasting protection against wild-type Sp6 was mainly achieved when immunization with Sp6/B7 was subcutaneous: a dose of 05 106 Sp6/B7 cells elicited protection that was con®ned to sites in the same anatomical quarter as the immunizing injection. Repeated injections of the same dose extended protec- tion against wild-type Sp6 to other anatomical districts, as well as a single injection of a 10-fold higher dose (5 106 cells). Finally, Sp6-speci®c cytotoxic T-lymphocyte activity was detected in draining lymph nodes, and the splenic expansion of Sp6-speci®c cytotoxic T-lymphocyte precursors quantitatively correlated with the dose of antigen. We conclude that activation of a protective immune response against Sp6 depends on the local environment where the immunogenic form of the `whole tumour cell antigen' is delivered. The antigen dose regulates the anatomical extent of the protective response.