Nephritogenic lambda light chain dimer: a unique human miniautoantibody against complement factor H

A unique monoclonal Ig lambda light chain dimer (protein LOI) was isolated from the serum and urine of a patient with hypocomplementemic membranoproliferative glomerulonephritis. In vitro the lambda light chain dimer efficiently activated the alternative pathway of complement (AP). When added to normal human serum, LOI temporarily enhanced AP hemolytic activity, but during a prolonged incubation the hemolytic activity was depleted. Protein LOI was found to bind to factor H, the main regulator molecule of AP. By binding to the short consensus repeat domain 3 of factor H, the dimer LOI blocked one of three interaction sites between H and C3b and thus inhibited the activity of H and induced an uncontrolled activation of the AP. Structural analysis showed that LOI belonged to the Vlambda3a subgroup of lambda light chains. The variable (V) region of LOI was most closely related to the predicted product of the Vlambda3 germline gene Iglv3s2, although it contained several unique residues that in a tertiary homology model structure form an unusual ring of charged residues around a hydrophobic groove in the putative Ag binding site. This site fitted considerably well with a putative binding site in the molecular model of domain 3 of factor H containing a reciprocal ring of charged amino acids around a hydrophobic area. Apparently, functional blocking of factor H by the Ab fragment-like lambda light chain dimer had initiated the development of a severe form of membranoproliferative glomerulonephritis. Thus, the lambda light chain dimer LOI represents the first described pathogenic miniautoantibody in human disease.