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SUMMARYOne hundred piiticnls diagnosed with hypocomplementaemic MPGN (C3<40%) were studied lo detcrmincihc presence of C3 nephritic factor (C3NeF) and or C"4 nephritic facIor(C4NcF). Of those studied. 12 were C^NeF-positivc. nine were C4NeF-positive and KJ were positive for both C3NeFã nd C4NeF. In the 10 patienis bolh C3NeF-and C4NeF-posiiive, a marked decrease in C3 and C5 levels and a decrease in levels of laic components from C6 lo C9 were observed. This observation was in contrast to that seen in patients who were either C 3NeF-or C'4Ne!--pusitive. Patients positive for both C'3NcP and C'4NeF continued to exhibit hypocoinplctncnlaemia after therapy, linnninotUiorescent findings revealed heavy C.I immunoglobulin deposits in the II) palienls who were bolhC3NcF-andC4NeI"-positive. whereas no such deposits were found in those patients who were either C.I NeFor C4NeF-po,sitivc only. When ihose patients who were bolh C.lNeF-and C4NeF-positive were eompared with Ihose who were either C'3Nel -or C"4NcF-posilivc. nephritic syndrome and a poor prognosis vverc observed more IrequeniH, This study demonstrates a correlation between clinical outcome and hypocomplementaemic MPGN. Further investigations of MPGN as an atitoimmune disease are necessary.

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SUMMARYC3NeF is an autoantibody of C3 convertase (C3bBb) and is often detected in the serum of hypocomptcmentitemic MPGN patients. Serum samples from 104 non-hypocomplementaemic MPGN patients (C3NeF) were studied. C3NeF, whieh eannot activate the alternative pathway, was found in the sera of 6 patients. We examined the C3NeF in purified IgG from five of the nonhypocomplementaemic serum samples (non-hypo C3NcF) and four hypocomplementacmic scrum samples (hypocomplcmcntaemic C3NeF) to determine why C3NeF does not induce C3 splitting and hypocomplcmcntaemia. Purified IgG from non-hypo C3NeF stabilized EAC4b3bBb cells in a manner similar to IgG from hypocomplementaemie C3NeF in EDTA gelatin veronal buffer. However, the non-hypo C3NeF IgG did not stabilize C3 convertase (EAC4b3bBb eells) in the presence of control proteins (factors H and 1). whereas the hypocomplementaemie C3NeF IgG did. The C3NeF in the hypoeomplementaemic serum displayed two characteristics: (i) inhibition of intrinsic decay of Ce convertase (C3bBb); and (ii) inhibition of extrinsic decay by factors H and I. Although the C3NeF in the non-hypocomplementaemic sera did inhibit the intrinsic decay in a manner similar to the hypocomplementaemie C3NeF IgG. il did nol inhibit the extrinsic decay. Due lo the different characteristics of hypocomplementaemic C3NeF and non-hypo C3NeF in the serum samples, the non-hypo C3NeF did not activate C3. Therefore, we conclude that C3NeF exhibits a heterogeneity which is very important in relation to the pathogenesis of MPGN.

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