Enhanced binding of low-affinity antibodies interacting simultaneously with targeted cell surface molecules and Fc receptor

The possibility that different forms of class I molecules might be expressed on the cell surface of lymphocytes has been investigated periodically over the past several decades. A series of major histocompatibility complex (MHC) class I-specific monoclonal antibodies, including the commonly used antibodies 64-3-7 and 25-D1.16, bind B cells differentially, suggesting the existence of differentially expressed class I-associated cell surface determinants on B lymphocytes. However, the ability of antibodies to bind cells is determined by the sum of interactions between the antibodies and the molecules expressed on the cell surface. The interactions of class I-specific antibodies with B cells were dissected, revealing dual specificity of the antibodies for the targeted class I molecules, as well as to Fc receptors preferentially expressed by B cells. We demonstrate that antibodies simultaneously bind targeted class I molecules and Fc receptors expressed on the surface of B cells. Simultaneous binding to two cell surface structures significantly enhances the class I-specific binding pattern of certain antibodies by increasing their avidity, leading to apparent cell-specific differences in MHC expression patterns. We conclude that no differences in MHC structures need be postulated to account for the observed binding patterns.