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ABSTRACT. Hemolytic complement was found to be absent in the serum of an 11-yr-old girl (R.N.) with meningococcal septicemia. C1, C4, an C2 were slightly decreased, C3 was absent, C5-C9 within the normal range. B levels immunochemically and electrophoretic mobility of B were normal. C3d was >1000% of a pooled EDTA-plasma standard indicating hypercatabolism of C3. On incubation of the patient's serum with normal human serum activation of C3 occurred even in the presence of 0.04 M EDTA. The amount of C3b generated was, however, greater without any chelating agent or in Mg-EGTA. On gel filtration of the serum two protein containing peaks were found to be responsible for activation of C3: the IgG containing peak was able to activate C3 in normal human serum without chelating agents and in Mg-EGTA but not in the presence of EDTA. The IgM-containing peak activated the third component of complement even in the presence of EDTA. The factor responsible for this phenomenon was termed C3 converting factor (C3 CoF). The IgG fraction of the patients serum caused activation of C3 in Mg-EGTA. However, in the presence of EDTA no activation of C3 could be induced even if physiological concentrations of the patients IgG were added to normal human EDTA-plasma. Thus the activity of the patient's IgG did not differ from typical C3 nephritic factor. The decay of C2 in EAC42 intermediates in the presence of the patient's IgG was uninfluenced indicating that it did not carry autoantibody activity against the classical pathway convertase C4b,2a, an activity recently termed NFc. As the ability of the patient's serum to activate C3 in the presence of excess EDTA was still unexplained further analyses were performed. C3 CoF was found to be a β 2-euglobulin and could be separated from IgG by anion exchange chromatography following euglobulin precipitation. C3 conversion with this material was independent from Ca2+and Mg2+. It was partly heat resistant at 56° C and could not be inhibited by soy bean trypsin inhibitor at 5 mg/ml. In contrast, DFP at 5 mM concentration lead to approximately 50% inhibition indicating that C3 CoF might be a serine protease. Treatment of patient's EDTA plasma with anti-C3 or anti-B totally eliminated C3 CoF activity while antialbumin, anti-C2, anti-C4, or nonimmune rabbit-IgG did not. Precipitation with anti-IgG partly eliminated C3 CoF but absorption of serum with an anti-IgG affinity column totally eliminated C3 CoF; these observations suggest that C3 CoF may be a C3 NeF-stabilized alternative pathway C3 convertase.

Salmonella typhimurium C5 is under normal conditions (physiological saline containing 0.002 M Mg2+) resistant to the action of antibody and complement (C). It becomes sensitive, however, when suspended in tris(hydroxymethyl)-aminomethane buffer (Reynolds and Pruul, 1971; Reynolds and Rowley, 1969). The interaction of complement components with this strain sensitized with specific antibody has been studied to identify the intermediate step at which inhibition occurs. The components C1 yields C2 react normally, as has been shown by lysis of complement-treated cells incubated with complement in ethylenediaminetetraacetic acid. Also, the reaction of C3 can be demonstrated by positive immune adherence and agglutination with anti-C3. The complement-treated cells do not, however, react with rabbit C6 to 9 or rabbit serum lacking C6 in tris(hydroxymethyl)aminomethan buffer. We conclude from these date that C5 can not react effectively under normal conditions. In contrast, if bacteria-antibody complexes are pretreated with rabbit serum lacking C6 in tris(hydroxymethyl)aminomethane buffer, they are readily lysed by incubation with C6 to 9. Thus, C5 can react with the bacterial surface in tris(hydroxymethyl)-aminomethane buffer.

The reactivity of histocompatibility antigens with serum antibodies and complement was studied in a short term model of allotransplantation. Lewis and DA rat kidneys were perfused with cytotoxic antisera and tissue sections analyzed for adsorption of fluorescent antibodies against γ-globulin and complement (ß1C protein). Both, the presumed cytotoxic antibody and C were localized at similar sites in the endothelial lining and in the walls of the arterial vessels and of the larger veins. The pattern of immune globulin deposition was distinct from that observed in glomerulonephritis. It may thus become helpful in the differential diagnosis between graft rejection and glomerulonephritis. At the sites of the deposits, the endothelial cells appeared enlarged and their nuclei protruded into the lumen. The findings are in line with the hypothesis that an antibody-mediated local activation of serum complement may be involved in the destruction of allografted tissue.

RECENTLY, Fischer and Haupt have shown1–3that the activation of complement in human, guinea pig, rabbit, and pig serum is paralleled by an increase in lytic activity due to an increase of lysolecithin. In these experiments complement was activated by the addition of soluble antigen–antibody-complexes or by the addition of heat-aggregated human γ-globulin. Complement activity was determined using sensitized sheep erythrocytes, while the lytic activity of lysolecithin was determined in lipid extracts of the same sera by the method of Schubothe and Fok4(see also legend to Table 1).