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.