Immunochemical properties of anti-Gal alpha 1-3Gal antibodies after sensitization with xenogeneic tissues.

In antigen-driven immune responses to proteins, antibodies of low avidity and limited complement fixing capacity undergo affinity maturation to yield antibodies of higher avidity which fix complement to a greater extent. The products of antigen-driven responses to carbohydrates are less defined. To investigate the evolution of natural antibodies against carbohydrates following sensitization, we studied natural antibodies specific for Gal alpha 1-3Gal in patients sensitized to that antigen as a result of perfusion of their blood through porcine livers for the treatment of hepatic failure. The natural antibodies against Gal alpha 1-3Gal, which occur in all unsensitized individuals, were predominantly IgM and IgG2, with average functional avidities of 5 x 10(-9) and 2 x 10(-8) M, respectively. After sensitization, the classes of anti-Gal alpha 1-3Gal included IgM, IgG2, and IgG1, and the average functional avidities of IgM and IgG were 3 x 10(-9) and 2 x 10(-9) M, respectively. The activation of complement by anti-Gal alpha 1-3Gal per microgram of Ab, measured by the fixation of C3bi on porcine cells, increased after sensitization owing to changes in subclass and avidity. Deposition of C3bi correlated with the concentrations of IgG1 and IgM but not IgG2 against Gal alpha 1-3Gal. Consistent with this finding, purified IgG1, but not IgG2, anti-Gal alpha 1-3Gal fixed complement on porcine cells. These results demonstrate that the properties of anticarbohydrate antibodies evolve after sensitization to increase complement fixation on potential targets. These properties may result from the altered costimulation of the humoral response to Gal alpha 1-3Gal due to sensitization.