Expression of biological effector fu molecules lacking the hinge region (complement/Fc receptors/protein A/conformation/segmental flexib

Several biological effector functions mediated by sites on the Fc region of human IgGI have been studied in two variant IgGlic monoclonal proteins (Dob and Lec) which contain deletions corresponding to the entire hinge region of the heavy chains. Neither Dob nor Lec protein in aggregated form was able to activate the classical complement pathway, and this was shown to be due to an inability to bind the first component of complement (C1). By rosette inhibition assays, Dob and Lec proteins were shown to have no measurable affinity for Fc receptors on human B cells or neutrophils. Dob and Lee proteins had a much reduced affinity for Fc receptors on the murine macrophage-like cell line P388D1 when compared to normal human IgGI. Furthermore, the hinge-deleted proteins were able to compete with murine IgG2b for P388D1 receptors but not with murine IgG2a. In con- trast, the binding of Dob and Lec proteins to protein A from Staph- ylococcus aureus was entirely normal. The functional conse- quences of the hinge deletion were parallel to those seen when normal IgGl was reduced and alkylated. It was concluded that the functional impotency of Dob and Lec proteins was related to the close association between the Fab and Fc regions in these mole- cules and the limited degree of segmental flexibility permitted in the absence of the hinge region. The data also suggest a major role for the Cy2 domain (C is the constant region) in mediating effector functions in normal IgGl.