Differential complement activation by bovine IgG2 allotypes

Immunoglobulin allotypes and complement (C) are known to be related to susceptibility to infection. Because bovine IgG2 is important in resistance to pyogenic infections and because its two allotypes, IgG2(a) and IgG2(b), differ in sequence in the CH1, hinge, CH2, and CH3 regions, we tested the ability of these allotypes to initiate the bovine C cascade. Bovine IgG2(a) and IgG2(b) were standardized according to specific anti guinea pig red blood cell (GPRBC) ELISA activity using anti IgG2 reagents shown essentially unbiased for allotype. Complement activating activity of the allotypes was quantitated in a GPRBC lysis assay. With this system, IgG2(b) consistently had more than twice the activity in bovine C mediated lysis as compared with IgG2(a). The fact that both EDTA and EGTA/Mg almost completely inhibited C mediated lysis of GPRBCs indicated that lysis was due to the classical pathway. Since antibody usually activates C by the classical pathway, this supports the supposition that activation was by the IgG2-GPRBC complexes. Flexibility analyses showed that IgG2(b) had a more rigid hinge than IgG2(a), perhaps partially explaining the greater efficiency of IgG2(b) in C activation. Other mechanisms may include differences in glycosylation and in the amino acid at position 332. The difference in ability to activate C may mean that animals of the IgG2(a) allotype could be more susceptible to infection with extracellular pyogenic pathogens which are killed by C or by phagocytes after opsonization with IgG2 and C. (C) 1999 Elsevier Science B.V. All rights reserved.