RETROVIRAL VECTOR PRODUCER CELL-KILLING IN HUMAN SERUM IS MEDIATED BYNATURAL ANTIBODY AND COMPLEMENT - STRATEGIES FOR EVADING THE HUMORAL IMMUNE-RESPONSE

The introduction of retroviral vector producer cells (VPC) into tumors as a means of increasing transduction efficiency has recently been em ployed in human gene therapy. trials. However, the fate of these xenog eneic cells in humans is not well understood. In the present study, we used an in vitro model to examine the survival of commonly used VPC l ines in serum from humans and various other species. VPC derived from the murine NIH-3T3 cell line, including PA317, Psi GRIP, and GP+E-86, were effectively killed in sera from Old World primates, including hum an and baboon. Conversely, the same murine cell lines survived exposur e to sera from dog, rabbit, rat, and mouse. This pattern of serum kill ing parallels the occurrence of the anti-alpha-galactosyl natural anti body (Ab) found exclusively in Old World primates. The anti-alpha-gala ctosyl Ab targets the terminal glycosidic structure Gal alpha 1-3Gal b eta 1-4GlcNAc-R (alpha-galactosyl epitope) found on the surface of mam malian cells, excluding Old World primates. All murine-derived VPC tes ted expressed high levels of the alpha-galactosyl epitope as determine d by FAGS analysis. VPC killing was complement-mediated, because prein cubation of human serum with a functionally blocking anti-C5 mAb compl etely abolished cell lysis. Furthermore, addition of soluble galactose (alpha 1-3)galactose (Gal alpha 1-3Gal) to human serum or down-regulat ion of the alpha-galactosyl epitope on the surface of VPC effectively reduced VPC killing, indicating that complement activation by these ce lls is primarily initiated by natural antibody recognition of the alph a-galactosyl epitope. Finally, VPC incubated with human serum for 8 hr in the presence of complement inhibition continued to produce viable retroviral particles, thus demonstrating a correlation between VPC and particle survival. Taken together, these data suggest that eliminatio n of the alpha-galactosyl epitope or complement blockade may provide a strategy to prolong the survival of VPC and the particles that they p roduce in vivo.