Abstract
The introduction of retroviral vector producer cells (VPC) into tumors as a means of increasing transduction efficiency has recently been employed in human gene therapy trials. However, the fate of these xenogeneic 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 lines in serum from humans and various other species. VPC derived from the murine NIH-3T3 cell line, including PA317, Psi CRIP, and GP+E–86, were effectively killed in sera from Old World primates, including human and baboon. Conversely, the same murine cell lines survived exposure to sera from dog, rabbit, rat, and mouse. This pattern of serum killing parallels the occurrence of the anti-α-galactosyl natural antibody (Ab) found exclusively in Old World primates. The anti-α-galactosyl Ab targets the terminal glycosidic structure Galα1-3Galβ1-4GlcNAc-R (α-galactosyl epitope) found on the surface of mammalian cells, excluding Old World primates. All murine-derived VPC tested expressed high levels of the α-galactosyl epitope as determined by FACS analysis. VPC killing was complement-mediated, because preincubation of human serum with a functionally blocking anti-C5 mAb completely abolished cell lysis. Furthermore, addition of soluble galactose(α1-3)galactose (Galα1-3Gal) to human serum or down-regulation of the α-galactosyl epitope on the surface of VPC effectively reduced VPC killing, indicating that complement activation by these cells is primarily initiated by natural antibody recognition of the α-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 elimination of the α-galactosyl epitope or complement blockade may provide a strategy to prolong the survival of VPC and the particles that they produce in vivo.
Overview summary
Many of the current in vivo retrovirus-mediated gene transfer protocols for the treatment of tumors involve the introduction of vector producer cells (VPC) directly into the tumor site as a means of increasing transduction efficiencies. In the present study, we show that VPC are killed effectively in human serum via natural antibody (Ab)-mediated complement lysis. Furthermore, our data suggest that preventing natural Ab binding or complement assembly may prove useful in human gene transfer protocols involving the introduction of murine-derived VPC into areas where exposure to Ab and complement is unavoidable.
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