Evidence for in vivo breakdown of beta-10-globulin in hypocomplementemic glomerulonephritis.

Evidence has been obtained for the presence in vivo of alpha(2D)-globulin, a breakdown product of serum beta(1C)-globulin, in patients with acute and persistent hypocomplementemic glomerulonephritis. The protein has been identified by immunoelectrophoretic analysis, and the amounts present have been determined by direct measurement of specific antigenic determinants present on alpha(2D). beta(1A)-Globulin, another breakdown product of beta(1C)-globulin, may also be present in vivo in severely hypocomplementemic patients, but its levels are much lower than those of alpha(2D)-globulin.Alpha(2D)-globulin has been identified by immunoelectrophoretic analysis of fresh EDTA plasma from patients with hypocomplementemic nephritis as an arc in the alpha(2) region that shows a reaction of identity with the arc representing alpha(2D)-globulin produced by aged normal serum. beta(1A)-Globulin was not seen in these patterns. Measurement of specific antigenic determinants has been carried out in both fresh EDTA plasma and aged serum. In the fresh plasma, the concentration of D antigen, found on both beta(1C)- and alpha(2D)-globulins, has been related to that of B antigen, found only on beta(1C) and taken as a measure of the concentration of this protein. In the hypocomplementemic patients, the concentration of D antigen, in comparison to that of B, was greater than in the normal subjects. Similarly, in aged serum, the level of alpha(2D) was greater than would be expected from the amount of beta(1C) that had been broken down in vitro, measured by the concentration of beta(1A). Calculations indicated that the in vivo alpha(2D) level in severely hypocomplementemic patients ranged from 7.5 to 18% of that which would be found in a pool of aged normal serum in which beta(1C) is completely broken down. The levels tended to be lower in less severely hypocomplementemic patients, and none could be detected in normal plasma. Only small quantities of A and D antigens are detectable in the urine of patients with hypocomplementemic nephritis. The rate of excretion is about equal to that of the normal subject. The study indicates that the low serum levels of beta(1C)-globulin that may be present over long periods in patients with persistent hypocomplementemic glomerulonephritis can be ascribed, in part, to in vivo breakdown of this protein as a result of reaction with immune complexes. The contribution of beta(1C) deposition on immune complexes and of diminished synthesis to the depressed serum levels cannot be assessed by the present study.