Complement-Induced Ultrastructural Membrane Lesions: Requirement for Terminal Components

Abstract

The step in the complement (C) sequence at which 8- to 11-nm ring-shaped lesions are formed on antibody-coated erythrocytes (EA) has remained controversial. Some workers have concluded that these lesions appear at the C5 step and are not ultrastructural correlates of lysis; others hold that these lesions are formed only after the action of C8 and C9 in association with lysis. We have re-examined this problem by using sheep EA and human sera genetically lacking C5, C6, C7, or C8. Electron micrographs of negatively stained membranes (× 220,000) were read in blind fashion and the results correlated with ¹²⁵I-C5 binding. Rare structures resembling C-induced ring lesions were found on EA exposed to C5-deficient (C5D), C6D, C7D and C8D sera or to heated normal serum, with no significant differences among these sera (lesion density 0 to 0.26/µm²). Fresh normal serum (NHS) produced 140 to 220 ring lesions/µm². C5 binding to EA in C8D serum was 60% of that observed in an NHS control; in C6D and C7D sera C5 binding was 4 to 11% of the normal value. Iodine treatment of sera (to enhance C5 uptake by C2 oxidation) increased C5 binding in C6D serum to 40 to 65% of that seen in native NHS; in iodine-treated C7D and C8D sera C5 binding was 250 and 440%, respectively, of the native NHS value. No increase in ring lesions was observed, however, except in the iodine-treated NHS. Thus, in whole serum, C5 binding is not sufficient to produce ultrastructural membrane rings in the absence of later-acting C components, at least through C8. The formation of ring lesions appears to have C requirements similar to those necessary for lysis.