Induction of complement attack on human cells by Gal(alpha 1,3)Gal xenoantigen expression as a gene therapy approach to cancer

Galactose(alpha 1,3)galactose on the surface of cells of non-primate organs is the major xenoantigen responsible for hyperacute rejection in xenotrans plantation. The antigen is synthesised by (alpha 1,3)galactosyl transferase . Humans lack this enzyme and their serum contains high levels of pre-exist ing natural antibody which recognises the structure and activates complemen t. We have evaluated in vitro the potential for delivery of this enzyme to sensitise human cells to complement attack as a gene therapy approach to ca ncer. Retrovirus-mediated delivery of (alpha 1,3)galactosyl transferase res ulted in high level expression which led to serum-mediated lysis of five hu man cell targets, including endothelial and primary melanoma cells. Lysis w as specific for those cells expressing the antigen in a mixed cell populati on. The mechanism of cell lysis mimicked that involved in hyperacute reject ion: activation of the classical complement pathway by natural antibody spe cific for galactose(alpha 1,3)galactose. The degree of lysis was determined by both the level of specific antibody and the expression of glycophosphat idylinositol-linked complement regulatory proteins. We conclude that expres sion of (alpha 1,3)galactosyl transferase is a promising new therapeutic ap proach for cancer gene therapy, avoiding toxicity problems associated with application of prodrugs and with the potential to elicit further immunologi cal responses.