Foreign antigenic peptides delivered to the tumor as targets of cytotoxic T cells.
Cytotoxic T cells (CTL) are readily activated by immunogenic peptides and they exert potent anti-tumor activity if the same peptides are displayed on class I major histocompatibility complex (MHC) of the tumor cells. A handful of tumor-associated antigens have been identified and many of them are weak antigens. As an alternative strategy, strongly antigenic foreign peptides are delivered to the tumor, marking them for CTL recognition. To establish the principle of this new strategy, in vitro and in vivo tumor destruction was tested with BALB/c CTL to L(d)-associated beta-galactosidase (beta-gal) peptide p876. In vitro, anti-p876 CTL destroyed tumor cells in a single-cell suspension or in 3-D tumor boluses when exogenous p876 was added. Exogenous IL-2 was required to sustain CTL activity for complete destruction of tumor boluses. In vivo, BALB/c mice were immunized with p876 and a CD4 activating Pan DR reactive epitope (PADRE). PADRE, which binds to several different MHC class II antigen and activates CD4 T cells, induced delayed-type hypersensitivity and stimulated T cell proliferation. Immunized mice were injected with tumor cells loaded with p876 and mixed with PADRE. Starting from the day after tumor injection, mice received five rounds of peptide injection at the tumor sites and all tumors were rejected. Injection with saline had no effect. Injection with PADRE had minor anti-tumor activity. Immunization and treatment with p876 alone was not protective. Therefore, by delivering CD4 and CD8 reactive foreign peptides to the tumor, peptide-specific T cells rejected the tumors as demonstrated by the in vitro and in vivo tests.