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10 of 11 patients who were diagnosed by kidney biopsy as atypical postinfectious glomerulonephritis (APGN) had abnormalities in alternative pathway of complement. APGN is characterized clinically by persistent hematuria and proteinuria and histologically by diffuse proliferative glomerulonephritis, C3 with/without immunoglobulin staining and subepithelial humps.The authors suggested that when patients with some degree of alternative pathway, whether mutation of genes responsible for complement-regulating protein or presence of antibodies, develop acute postinfectious glomerulonephritis (GN), they would not recover in a short period of time similar to ones with no such abnormalities. Some patients may not recover at all and have persistent hematuria and proteinuria.
The defective alternative pathway in these patients were unmasked by the inflammatory process brought about by acute postinfectious GN in which the complement system is massively activated. The abnormal hyperactivation of the pathway is not brought under control easily in these patients.
C3 glomerulonephritis (C3GN) is also a disease caused by defective alternative pathway, but it is not infectious-related and has somewhat different histologic and immunologic findings. Atypical hemolytic uremia syndrome (aHUS) is similar to APGN in term of both infectious-related cause and underlying disease mechanism, but no complement product accumulation is found in aHUS.
Renal pathologists are likely to see APGN because persistent urine abnormalities are the main reason patients with acute postinfectious GN get biopsy. Investigation for defective alternative pathway should be done in these patients. As in this series, many patients may have APGN without history of acute postinfectious GN.
