Enhancement of tumor vaccine immunogenicity by the natural anti-gal antibody

Enhancing the immunogenicity of tumor vaccines is an important step for the stimulation of an antitumor immune response. This can be achieved by targeting tumor vaccines for uptake by host antigen-presenting cells (APCs). APCs are capable of processing and presenting tumor-associated antigens (TAAs) to T-cells in the context of the appropriate MHC molecule and with costimulation resulting in effective T-cell activation. One way to increase the uptake of tumor vaccines by APCs is to complex vaccines with IgG antibodies such that the resulting immune complexes are internalized by APCs through an Fc[gamma] receptor-mediated mechanism. In this study, the natural anti-Gal antibody was investigated as a potential opsonizing antibody for targeting tumor vaccines to APCs. Anti-Gal constitutes 1% of circulating IgC in humans and specifically interacts with a carbohydrate epitope, the [alpha]-gal epitope, which is not normally present on human cells. This epitope is abundantly produced in nonprimate mammals by the glycosylation enzyme [alpha]1,3galactosyltransferase ([alpha]1,3GT). Synthesis of [alpha]-gal epitopes on human tumor cells and cell membranes from primary tumor homogenates was demonstrated using recombinant [alpha]1,3GT in vitro. The newly synthesized [alpha]-gal epitopes were accessible for anti-Gal binding and this interaction facilitated the internalization of tumor cells by human macrophages in vitro. Finally, the efficacy of [alpha]-gal epitope-expressing tumor vaccines was assessed using [alpha]1,3GT knockout mice which lack [alpha]-gal epitope expression and produce anti-Gal in amounts similar to humans. B16-BL6 melanoma cells, which normally lack [alpha]-gal epitopes, were stably transfected with [alpha]1,3GT cDNA to express [alpha]-gal epitopes (i.e., BL6[alpha]GT). KO mice vaccinated with irradiated BL6[alpha]GT cells were significantly protected against a challenge with live wildtype BL6 cells, whereas all KO mice vaccinated with irradiated wildtype BL6 cells developed tumors. Histological analysis of the tumors developing in BL6[alpha]GT-vaccinated mice revealed infiltrating lymphocytes and macrophages surrounding the tumors, whereas no cellular infiltrate was observed in tumors from BL6-vaccinated mice. These findings suggest that the in situ complexing of anti-Gal with [alpha]-gal epitope-expressing tumor cells administered as vaccines can augment the immunogenicity of such vaccines and thereby elicit a protective immune response against tumor challenge. The implications of these findings indicate a potential universal use of such vaccines for all cancer patients.