Detection of C3 Nephritic Factor by Hemolytic Assay

C3 nephritic Factor (C3NeF) is autoantibody that binds neoepitopes of the C3 convertase C3bBb, resulting in a stabilization of the enzyme. First functional characterizations of C3NeF were performed by hemolytic assays using preactivated sheep erythrocytes (bearing C3b). Sheep erythrocytes are beforehand sensitized with an anti-sheep red blood cell stroma antibody produced in rabbit (hemolysin). Sensitized sheep erythrocytes will initiate cascade complement activation via the classic pathway, followed by alternative pathway amplification loop, resulting in C3b covalent binding to cell surface. Sheep erythrocytes bearing C3b permit the alternative pathway exploration, in particular decay of alternative pathway C3 convertase.

Management of membranoproliferative glomerulonephritis type II with plasmapheresis

Membranoproliferative glomerulonephritis type II (MPGN II) is a rare kidney disease identified microscopically by electron-dense deposits surrounded by complement component C3 in glomerular basement membranes. MPGN II usually leads to renal failure, and patients with MPGN II experience a high rate of recurrence following renal transplantation. No treatment modalities have been proven successful if recurrence does occur. The sera of most patients with MPGN II contain complement C3 nephritic factor (C3NF), an IgG autoantibody directed against C3 convertase (C3bBb) that results in constitutive breakdown of C3. C3NF may be important in the pathogenesis of the disease. Since C3NF is IgG, we predicted that C3NF could be removed from the serum through plasmapheresis. We describe the use of long-term plasmapheresis to maintain good renal function in a 15-year-old girl with rapidly progressive recurrent MPGN II. After 73 plasmapheresis procedures over 63 weeks, her serum creatinine remained stable, and her creatinine clearance trended upward. Serial biopsies of the transplanted kidney demonstrated persistent MPGN II but no development of tubular atrophy. During the course of therapy, serum C3NF activity decreased; furthermore, C3NF activity was detected in the removed plasma. We have shown that plasmapheresis is a safe and effective method for delaying the onset of chronic renal failure in recurrent MPGN II. The efficacy may be due to the removal of serum C3NF.

Sifting the causes of microscopic hematuria

Among the goals of the primary care workup are detection of serious disease and determination of whether referral--such as to a urologist or nephrologist--is indicated. Diagnosis is facilitated when hematuria is accompanied by other findings; isolated hematuria presents a more complex challenge. Measurement of serum complement levels may be helpful in narrowing the differential.

Evidence that production of autoantibody to the alternative pathway C3 convertase is a normal physiologic event

The origin of autoantibody production was studied with the use of antibody to the alternative pathway C3 convertase (C3 nephritic factor (C3NeF), as a model. Pokeweed mitogen stimulation of peripheral mononuclear cells from newborn infants, normal adults, and patients with membranoproliferative glomerulonephritis indicated that the ability to make C3NeF is apparently present in everyone from the time of birth. In addition, C3NeF appeared to express a single or very limited idiotope (21/21 isolates). The data also suggest that the elaboration of C3NeF may approximate an antibody response after immunization. Thus the C3NeF fraction of the total IgG or IgM produced in culture by pokeweed mitogen-stimulated mononuclear cells from normal neonates and adults, as well as from patients, was in the range of the production of specific antibody. Further, both IgG and IgM C3NeF produced by cells from these normal individuals, including newborn infants, had an affinity for antigen (10(8) to 10(9) L/mol) that was also in the range of specific antibody. Most of the autoantibody molecules (5/7) from serum were IgG3; two B cell clones producing C3NeF were CD5-negative. These experiments indicate that unmutated germline genes are used in the production of C3NeF and that a limited spectrum of antiidiotypic antibodies regulate its production.

A properdin dependent nephritic factor slowly activating C3, C5, and C9 in membranoproliferative glomerulonephritis, types I and III

The IgG fraction of serum from patients with membranoproliferative glomerulonephritis (MPGN) types I and III was found to contain a nephritic factor (NFI/III) which differed from that usually present in MPGN type II and partial lipodystrophy (NFII) in that it converts C5 and C9 as well as C3, is dependent on properdin for its activity, and requires an incubation period of several hours rather than 30 min for its demonstration. C3 conversion occurred in the absence of an intact classical pathway. This nephritic factor was found in patients with reduced serum levels of terminal components and its activity, like that of the nephritic factor in MPGN type II, correlated with the serum C3 level indicating that these nephritic factors play a large role in producing hypocomplementemia. Although potentially nephritogenic because of its ability to activate the terminal pathway, the presence of this nephritic factor did not clearly correlate with clinical course.

Mesangial proliferative glomerulonephritis with unusual intramembranous granular dense deposits

We have studied two patients with histories of upper respiratory tract infection. Hematuria and proteinuria were the presenting renal symptoms in one patients and an acute nephritic syndrome in the other. Serological findings disclosed depression of total hemolytic complement activity with low levels of C3 and the presence of C3Nef activity. Light microscopy showed diffuse mesangial cell proliferation. By immunofluorescence, diffuse deposits of C3 were found in the glomeruli. Ultrastructural studies revealed segmental thickening of the glomerular basement membrane due to the deposition of granular electron-dense deposits in a laminar pattern. We suggest that our cases may represent a variant of hypocomplementemic glomerulonephritis or perhaps the early stages of dense deposit disease.

Identification of nephritic factor as an immunoglobulin

C3 nephritic factor (C3NeF) activity in sera from three patients with mesangiocapillary glomerulonephritis, one of whom had partial lipodystrophy, was found on chromatography to be associated with fractions containing IgG and no other detectable proteins. Immunoadsorption of IgG from these fractions with a highly purified anti-IgG removed the C3NeF, and the IgG, eluted after combination with the anti-IgG, retained C3NeF activity. In each case the isolated IgG with C3NeF activity was found to contain more than one subclass of IgG and both kappa and lambda chains, indicating that the immunoglobulin comprising C3NeF in these patients is heterogeneous and not monoclonal. The identification of C3NeF as an immunoglobulin suggests that it may be an autoantibody against antigenic determinants of complement components present in the C3 convertase of the alternative pathway.