Increased Immunogenicity of Tumor Vaccines Complexed with Anti-Gal Studies in Knockout Mice for α1,3Galactosyltransferase

A major prerequisite for the success of tumor vaccines is their effective uptake by antigen-presenting cells (APCs) and transport of these APCs to the draining lymph nodes, where the processed and presented tumor-associated antigens activate tumor-specific naive T cells. We previously suggested that the immunogenicity of autologous tumor vaccines in humans may be augmented by engineering vaccinating tumor cell membranes to express α-galactosyl (α-gal) epitopes ( i.e. , Galα1,3Galβ1,4GlcNAc-R). Subsequent in situ binding of natural anti-Gal IgG molecules to these epitopes would result in the formation of immune complexes that target tumor vaccines for uptake by APCs, via the interaction of the Fc portion of anti-Gal with Fcγ receptors on APCs. This hypothesis was tested in a unique experimental animal model of knockout mice for α1,3galactosyltransferase (α1,3GT) and the mouse melanoma B16-BL6 (referred to here as BL6). Like humans, these mice lack α-gal epitopes and produce anti-Gal. BL6 melanoma cells are highly tumorigenic, and like human tumor cells, they lack α-gal epitopes. Expression of α-gal epitopes on these melanoma cells was achieved by stable transfection with α1,3GT cDNA. The transfected melanoma cells (termed BL6αGT) express ≈2 × 106 α-gal epitopes per cell and readily form immune complexes with anti-Gal. Vaccination of the mice with 2 × 106 irradiated melanoma cells that express α-gal epitopes, followed by challenge with 0.5 × 106 live parental melanoma cells, resulted in protection for at least 2 months ( i.e ., no tumor growth) in one-third of the mice, whereas all mice immunized with irradiated parental melanoma cells developed tumors 21–26 days post-challenge. The proportion of protected mice doubled when the mice were immunized twice with irradiated melanoma cells expressing α-gal epitopes and challenged with 0.2 × 106 live BL6 cells. Histological studies on the developing tumors in challenged mice that were immunized with melanoma cells expressing α-gal epitopes demonstrated extensive infiltration of T lymphocytes and macrophages, whereas no mononuclear cell infiltrates were observed in tumors of mice immunized with parental tumor cells. Overall, these studies imply that immunization of α1,3GT knockout mice with BL6 melanoma cells that express α-gal epitopes elicits, in a proportion of the population, protective immune response against the same tumor lacking such epitopes. These studies further suggest that similar immunization of cancer patients with autologous tumor vaccines that are engineered to express α-gal epitopes may increase the immune response to autologous tumor-associated antigens and, thus, may elicit immune-mediated destruction of metastatic cells expressing these antigens.