Factors influencing the binding of large immune complexes to the primate erythrocyte CR1 receptor

This study was undertaken to ascertain the relationship between the size of immune complexes and their capacity to be bound by primate erythrocytes, and to examine the role of complement in the binding of very large immune complexes to erythrocytes. Preformed bovine serum albumin-rabbit anti-bovine serum albumin immune complexes were solubilized in a fivefold antigen excess, and the supernatant fluids were subjected to differential centrifugation to select immune complexes of varying size. The size profile of immune complexes was measured on isokinetic sucrose gradients. Immune complexes were incubated with erythrocytes, and unbound immune complexes were separated from erythrocyte-bound immune complexes by centrifugation on Percoll. The results indicate that large immune complexes were bound more efficiently by primate erythrocytes than were smaller immune complexes. Depletion of complement by a variety of procedures abrogated binding of immune complexes to erythrocytes. Only the erythrocytes of primates had the capacity to bind immune complexes. Baboon or rabbit sera supported immune complex binding to baboon or human erythrocytes. In contrast, human and guinea pig sera supported immune complex binding to human erythrocytes, but these sera failed to support immune complex binding to baboon erythrocytes. These data indicate that the immune complex size and the source of serum complement are important factors which influence the binding of immune complexes to primate erythrocytes.