RAPID ACTIVATION OF THE COMPLEMENT-SYSTEM BY CUPROPHANE DEPENDS ON COMPLEMENT COMPONENT C4

Rapid activation of the complement system by cuprophane depends on com plement component C4. Hemodialysis with cuprophane dialyzer membranes promotes rapid activation of the complement system, which is thought t o be mediated by the alternative pathway. Complete hereditary deficien cy of complement C4, a classical pathway component, in two hemodialysi s patients provided the opportunity to investigate a possible role of the classical pathway. In two hemodialysis patients with both C4 isoty pes, C4A and C4B, and in one patient with C4B deficiency complement ac tivation occurred immediately after the onset of hemodialysis, with pe ak levels of C3a and terminal complement complex (TCC) after ten to fi fteen minutes. In patients with complete C4 deficiency, C3a and TCC re mained unchanged for fifteen minutes and increased thereafter, reachin g the highest level after thirty minutes. The leukocyte nadir was also delayed from fifteen to thirty minutes. In vitro incubation of normal , C4A- or C4B-deficient serum with cuprophane caused complement activa tion after fifteen minutes. In contrast, no activation was observed in sera of four C4-deficient patients. The addition of normal serum or p urified human C4 restored the capacity for rapid complement activation . In one patient with severe immunoglobulin deficiency C3a and TCC lev els increased only moderately after 25 minutes of cuprophane dialysis. This patient's serum also exhibited delayed complement activation in vitro, which was normalized after pretreatment of cuprophane with immu noglobulins. Preincubation of normal serum with MgEGTA, a blocker of t he classical pathway, inhibited rapid complement activation through cu prophane. As basal levels of C4a are markedly increased in hemodialysi s patients (3450 +/- 850 ng/ml) compared to healthy controls (224 +/- 81 ng/ml), no further elevation of C4a was detectable during cuprophan e hemodialysis. Incubation of normal serum with cuprophane, however, c aused a slight increase in C4a after five minutes. These results indic ate that the initial deposition of complement C3b on the cuprophane me mbrane, necessary for activation of the amplification loop of the alte rnative pathway, is mediated by the classical pathway C3-convertase C4 b2a. We propose an extended concept of complement activation through c uprophane, which is based on four steps: (rt) binding of anti-polysacc haride antibodies, (b) classical pathway activation, (c) alternative p athway activation and (d) terminal pathway activation.