Studies on the formation of a unique cellular intermediate (EAC14256) from EAC142 and C56.

The binding reaction of C56 to EAC142 was studied. The following was observed: (1) C56 can bind to EAC142, without participation of C3 or C7, to form EAC14256; (2) the efficiency of EAC14256 formation from EAC142 and C56 depends highly on the ionic strength of the buffer; (3) EAC14256, generated from EAC142 and C56, decays spontaneously by elution of C56 into the medium and thus the reaction between C56 and EAC142 is reversible; (4) the receptor on EAC142 for C56 is C2 or C42 on the cell surface, and (5) the binding of C56 to C2 or C42 on cells is independent of the usual (C3-dependent) complement cascade activation. These observations, together with the results of our previous report, indicate that the binding of C56, generated from the activation of C5 by C42 (without C3), to EAC142 may be the initial step of membrane attack complex formation in C3-independent immune hemolysis.