Protection of retroviral vector particles in human blood through complement inhibition

The rapid inactivation of murine-derived retroviral vectors in human or nonhuman primate sera is largely attributed to the activity of complement mediated through the classical pathway. In this study, we have further investigated the relationship between the human complement cascade and retrovirus inactivation. Preincubation in normal human serum effectively inactivated LXSN retroviral vector particles, whereas the vector maintained the ability to transduce cells following incubation in sera deficient in either the C1, C2, C3, C5, C6, C8, or C9 human complement proteins. Preincubation of serum with monoclonal antibodies (mAbs) that functionally block specific complement components, including C5, C6, C8, and C9, successfully protected the LXSN vector from complement-mediated inactivation. Treatment of serum with cobra venom factor, which consumes terminal complement, also effectively protected the vector from inactivation. LXSN vector survival in serum corresponded inversely to the level of complement activity following treatment of serum with anti-C5 mAb as assessed in an erythrocyte hemolytic assay. Additionally, pretreatment of human whole blood with anti-C5 mAb effectively inhibited inactivation of the LXSN vector. Taken together, these data demonstrate that formation of the membrane attack complex (MAC, C5b-9) is required for the inactivation of the murine-based LXSN retroviral vector in human blood and that this process can be abrogated with the use of soluble complement inhibitors.