EFFICIENT DIRECT PRIMING OF TUMOR-SPECIFIC CYTOTOXIC T-LYMPHOCYTE IN-VIVO BY AN ENGINEERED APC

Although numerous studies have documented a role for B7-1. (CD80) in t he induction of antitumor CTL immunity, it is presently unclear to wha t extent expression of this costimulatory molecule truly endows tumors with significant in vivo APC (antigen-presenting cell) capacity, Rece nt studies have, in fact, demonstrated that cross-priming, rather than direct priming, may constitute the major mechanism of CTL induction b y B7-1 expressing tumors. We have, therefore, investigated the require ments for antigen density and costimulatory molecules in direct CTL pr iming with a prototype cell-based vaccine that uses a signal sequence- containing minigene to direct expression of a tumor-specific CTL epito pe to the endoplasmic reticulum, This design limits sources of antigen available to professional.SPC in the host and, thereby, the contribut ion of cross-priming. Induction of antitumor CTL immunity by our proto type APC was shown to solely involve direct priming, independent of ho st APC, NK1.1(+) cells, and CD4(+) cell help. CTL induction through th is mechanism required the engineered APC to express the B7-1 molecule as well as a sufficiently high density of peptide/MHC complexes at its surface. Our data, in contrast to previous studies using modified tum or cells, clearly define the antigenic and costimulatory requirements for a suitably engineered ''artificial'' APC to directly prime peptide -specific CTL in vivo, and demonstrate that the signal sequence minige ne approach allows the engineering of highly effective and well-define d cellular vaccines for activation of CTL against epitopes of choice.