Alternative C3 pathway activiation in pneumococcal glomerulonephritis.

Glomerulonephritis following pneumococcal infection has been observed, but possible immunopathologic mechanisms have not been adequately explored. Multiple serologic studies as well as light, immunofluorescence and electron microscopic evaluation of kidney biopsy tissue from a 4 year old girl with pneumococcal glomerulonephritis were performed. Clinical studies at the onset of the disease showed normal serum C3 and C4 levels (third and fourth components of complement) with progression to selective C3 hypocomplementemia from days 2 to 58. A serum factor capable of breaking down C3 in normal human serum was present during the period of maximum C3 hypocomplementemia. Renal glomerular histology revealed a mesangial proliferative glomerulonephritis. Glomerular bound C3 and type 14 pneumococcal antigen were associated with similar, but less extensive, deposits of properdin. Minimal immunoglobulin M (IgM) and C4 were seen, but immunoglobulin G (IgG) and fibrinogen were absent. Ultrastructurally, subepithelial "humps" and intramembranous electron dense deposits were noted. It is hypothesized that the pneumococcal polysaccharide can activate the alternate complement pathway and may be responsible for a limited course of glomerulonephritis.