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 a nd 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 follo wing sensitization, we studied natural antibodies specific for Gal alpha 1- 3Gal in patients sensitized to that antigen as a result of perfusion of the ir blood through porcine livers for the treatment of hepatic failure. The n atural antibodies against Gal alpha 1-3Gal, which occur in all unsensitized individuals, were predominantly IgM and IgG2, with average functional Livi dities of 5 x 10(-9) and 2 x 10(-8) M, respectively. After sensitization, t he classes of anti-Gal alpha 1-3Gal included IgM, IgG2, and IgG1, and the a verage functional avidities of IgM and IgG were 3 x 10(-9) and 2 x 10(-9) M , respectively. The activation of com plement by anti-Gal alpha 1-3Gal per microgram of Ab, measured by the fixation of C3bi on porcine cells, increas ed after sensitization owing to changes in subclass and avidity. Deposition of C3bi correlated with the concentrations of IgG1 and IgM but not IgG2 ag ainst Gul alpha 1-3Gal. Consistent with this finding, purified IgG1, but no t IgG2 anti-Gal alpha 1-3Gal fixed complement on pol cine cells. These resu lts demonstrate that the properties of anticarbohydrate antibodies evolve a fter sensitization to increase complement fixation on potential targets. Th ese properties may result from the altered costimulation of the humoral res ponse to Gal alpha 1-3Gal due to sensitization.