Complement Components and Complement Activation in Acute Poststreptococcal Glomerulonephritis

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Abstract
Serial samples from 40 patients with acute poststreptococcal glomerulonephritis (AGN) were studied. Early in the disease, all patients showed decreased C3, and C5 and/or properdin values were low in 91%. The concentrations of C1q, C1s, C2, C4 and factor B were largely normal or increased. Concluding from determinations of C-reactive protein, orosomucoid and α1-antitrypsin, complement component levels were influenced by an acute-phase reaction related to infection preceding AGN. Increased amounts in serum of α2-complexes composed of C1r, C1s and C1 inactivator proteins in 83% and moderately reduced C2 in 23% of the patients gave evidence of classical pathway activation during the early phase of AGN. Early in the disease, C3-cleaving activity in serum was found in 72%. The activity was heat-labile and produced C3 cleavage in serum chelated with Mg++ EGTA and in some cases also in the presence of EDTA. The findings suggested that complement activation early in AGN proceeds mainly by an alternative pathway mechanism. In 2 patients, activation of the classical pathway occurred fairly late in the disease and was then associated with the transient appearance of heat-stable C3-cleaving activity in serum. Serial measurements of C1q-binding substances were not clearly informative as to the role of circulating immune complexes in AGN.