Complement inhibitor S protein is associated with membranes of red blood cells from patients with paroxysmal nocturnal haemoglobinuria.

Abstract

S protein is a plasma glycoprotein (Mr = 78,000) which binds to nascent C5b-7 complexes upon complement activation in the fluid phase in whole serum. It thereby protects innocent bystander cells from complement mediated lysis. It is unknown whether S protein also functions as complement inhibitor on cell surfaces. We here report that S protein is recognized on red blood cells (RBC) from patients with paroxysmal nocturnal haemoglobinuria (PNH), but not on normal RBC. RBC from eight PNH patients showed 12-48% haemolysis subsequent to complement activation in the fluid phase, while normal RBC did not respond. Preincubation of the PNH cells with affinity-purified antibodies against human S protein resulted in a three- to five-fold increase of haemolysis, while preincubation of these cells with S protein decreased haemolysis by 40%. In contrast, haemolysis remained unaffected by other unrelated antibodies, i.e. IgG anti-Rh(D) and anti-A. If PNH RBC, normal RBC pretreated with 2-amino-ethylisouronium bromide (AET), or untreated normal RBC, respectively, were incubated with purified S protein in vitro, the uptake of antibodies against S protein was significantly enhanced with PNH and with AET-treated, but not with untreated normal RBC. Additionally, while normal RBC did not respond to reactive lysis initiated by purified C5b-6 and C7, PNH as well as AET-RBC showed significant haemolysis that could be inhibited by S protein in a dose-dependent fashion. These findings strengthen the assumption that the increased sensitivity of PNH cells towards reactive complement lysis is either due to the lack of an inhibitor of the terminal complement sequence and/or enhanced insertion of the membrane attack complex. These defects of PNH RBC may partly be overcome by the fluid phase complement inhibitor S protein which binds to PNH RBC and may thereby suppress homologous cytolysis.