Surface modulation of classical pathway activation: C2 and C3 convertase formation and regulation on sheep, guinea...

We examined the formation of the early classical complement (C) pathway enzymes on sheep (Es), guinea pig (Egp), and human (Eh) erythrocytes (E). Each species' E were sensitized with sufficient IgM or IgG anti-E Ab to establish equal numbers of C1-fixing sites on all E. After sensitization with 100 C1-fixing sites of Ab and excess C1, uptake of C4 was equivalent on all three cell types, judged by anti-C4 binding (for guinea pig C4) or by direct uptake of radiolabeled protein (for human C4). With equal numbers of cell-bound C1 and C4, however, there were marked differences in C2 convertase activity on Es, Egp, and Eh. Sheep EAC14 utilized C2 at least 20 times faster than Egp and Eh bearing the same number of C1 and C4 molecules. C3 cleavage was even further depressed on Egp and Eh, and was not changed by the substitution of oxyC2 for normal human C2. In whole guinea pig serum (GPS), 300 times more C1-fixing sites were required on Egp than on Es to achieve similar amounts of lysis; however, equivalent C3 uptake on Egp and Es was associated with equal extents of lysis, demonstrating that GPS lysis of these cells was regulated by early steps in classical pathway (CP) activation. Incubation of E bearing radiolabeled C4b with Factor I demonstrated that C4b on Egp was highly resistant to cleavage compared to the same protein bound to Es or Eh. Studies with partially purified C3 convertase decay-accelerating factors from Eh stroma demonstrated that these membrane proteins could not account for the observed surface regulation of CP activity because these proteins do not affect the rate of C2 cleavage by EAC14. We conclude that E surface molecules have an important role in modulation of CP activation. This surface-associated CP regulation occurs at the level of cell-bound C4b.