Differences in activation of human and guinea pig complement by retroviruses.

C type murine leukemia viruses (retroviruses) have been shown previously to possess a receptor for human C1 that activated human but not guinea pig complement. In the present study we provide evidence that the viral receptor also binds guinea pig C1 but that such binding does not lead to activation. However, incorporation of human C1s into guinea pig C1 to form a C1 hybrid results in activation of that hybrid and in viral lysis. In contrast, incorporation of guinea pig C1s into human C1 abolishes activation by the virus. These results demonstrate that C1s governs the activation of C1 of the viral receptor.