Novel Protein Adjuvant Activating Innate IFN-γ and IL-18 Expression and Inducing Rejection of Implanted Colorectal Cancer Following Immunotherapy Using This Adjuvant in Mice

Our recent study demonstrated that immunizations with nonreplicable MC38 colorectal cancer cells plus a novel recombinant protein adjuvant, the amino-terminus region of dense granule protein 6 (rGRA6Nt) of Toxoplasma gondii (a protozoan parasite), effectively activate the cancer cell-specific CD8⁺ T cytotoxic cells and inhibit the growth of implanted tumors of the identical cancer cells after its challenge implantation. In the present study, we first examined whether rGRA6Nt activates mRNA expression for IFN-g, IL-12, IL-15, and IL-18, which are known to assist an activation of the CD8⁺ T cells, in innate immune cells. Following an intraperitoneal injection of rGRA6Nt (40 mg) into SCID mice deficient in both T and B cells, markedly increased levels of mRNA for only IFN-g and IL-18 were detected in their peritoneal exudate innate immune cells. When C57BL/6 mice were immunized with nonreplicable MC38 colorectal cancer cells plus rGRA6Nt adjuvant twice and challenged with replication-capable cancer cells of the identical colorectal cancer, more than one fifth (22.2%, 6/27) of the immunized mice rejected the growth of the implanted tumors, whereas none (0/27, P<0.05) of unimmunized control mice rejected the implanted tumors. These results indicate that rGRA6Nt protein adjuvant has a unique capability to selectively activate expression of IFN-g and IL-18 in innate immune cells, and that immunizations with nonreplicable cancer cells in combination with this protein adjuvant can induce a protection to reject the growth of the identical tumor cells after its challenge implantation in a significant portion of the immunized mice.