The alpha-gal
epitope (Galalpha1-3Galbeta1-(3)4GlcNAc-R) is abundantly synthesized on
glycolipids and
glycoproteins of non-primate mammals and New World monkeys by the glycosylation
enzyme alpha1,3galactosyltransferase (alpha1,3GT). In humans, apes and Old World monkeys, this
epitope is absent because the alpha1,3GT gene was inactivated in ancestral Old World primates. Instead, humans, apes and Old World monkeys produce the anti-Gal antibody, which specifically interacts with alpha-gal
epitopes and which constitutes approximately 1% of circulating
immunoglobulins. Anti-Gal has functioned as an immunological barrier, preventing the
transplantation of pig organs into humans, because anti-Gal binds to the alpha-gal
epitopes expressed on pig cells. The recent generation of alpha1,3GT knockout pigs that lack alpha-gal
epitopes has resulted in the elimination of this immunological barrier. Anti-Gal can be exploited for clinical use in
cancer immunotherapy by targeting autologous tumour
vaccines to APC, thereby increasing their immunogenicity. Autologous intact tumour cells from haematological
malignancies, or autologous tumour cell membranes from solid tumours are processed to express alpha-gal
epitopes by incubation with
neuraminidase, recombinant alpha1,3GT and with
uridine diphosphate galactose. Subsequent immunization with such autologous tumour
vaccines results in in vivo opsonization by anti-Gal
IgG binding to these alpha-gal
epitopes. The interaction of the Fc portion of the
vaccine-bound anti-Gal with Fcgamma receptors of APC induces effective uptake of the vaccinating tumour cell membranes by the APC, followed by effective transport of the vaccinating tumour membranes to the regional lymph nodes, and processing and presentation of the tumour-associated
antigen (TAA)
peptides. Activation of tumour-specific T cells within the lymph nodes by autologous TAA
peptides may elicit an immune response that in some patients will be potent enough to eradicate the
residual tumour cells that remain after completion of standard
therapy. A similar expression of alpha-gal
epitopes can be achieved by transduction of tumour cells with an adenovirus vector (or other vectors) containing the alpha1,3GT gene, thus enabling anti-Gal-mediated targeting of the vaccinating transduced cells to APC. Intratumoral delivery of the alpha1,3GT gene by various vectors results in the expression of alpha-gal
epitopes. Such expression of the xenograft
carbohydrate phenotype is likely to induce anti-Gal-mediated destruction of the tumour lesion, similar to rejection of xenografts by this antibody. Opsonization of the destroyed tumour cell membranes by anti-Gal
IgG further targets them to APC, thus converting the tumour lesion, treated by the alpha1,3GT gene, into an in situ autologous tumour
vaccine.