Fc receptors on lymphocytes and K cells

High resolution mapping of the binding site on human IgG1 for Fc gamma RI, Fc gamma RII, Fc gamma RIII, and FcRn and design of IgG1 variants with improved binding to the Fc gamma R

Immunoglobulin G (IgG) Fc receptors play a critical role in linking IgG antibody-mediated immune responses with cellular effector functions. A high resolution map of the binding site on human IgG1 for human Fc gamma RI, Fc gamma RIIA, Fc gamma RIIB, Fc gamma RIIIA, and FcRn receptors has been determined. A common set of IgG1 residues is involved in binding to all Fc gamma R; Fc gamma RII and Fc gamma RIII also utilize residues outside this common set. In addition to residues which, when altered, abrogated binding to one or more of the receptors, several residues were found that improved binding only to specific receptors or simultaneously improved binding to one type of receptor and reduced binding to another type. Select IgG1 variants with improved binding to Fc gamma RIIIA exhibited up to 100% enhancement in antibody-dependent cell cytotoxicity using human effector cells; these variants included changes at residues not found at the binding interface in the IgG/Fc gamma RIIIA co-crystal structure (Sondermann, P., Huber, R., Oosthuizen, V., and Jacob, U. (2000) Nature 406, 267-273). These engineered antibodies may have important implications for improving antibody therapeutic efficacy.

Inhibition of natural killer cell activity by antigen-antibody complexes

The natural killer (NK) cell activity of human peripheral blood mononuclear cells was found to be inhibited by precipitated tetanus toxoid anti-tetanus toxoid complexes (Te/aTe) as well as soluble Te/aTe. Preincubation of the immune complexes with protein A decreased the inhibition of NK cell activity. When mononuclear cells were preincubated with interferon (IF) or interleukin 2 (Il-2) before incubation with Te/aTe, the immune complex-induced inhibition was decreased, while IF or Il-2 added after incubation with the immune complexes had no effect. Using NK cell-enriched suspensions in a single cell agarose assay, the immune complexes were shown to inhibit NK cell activity by inhibiting the formation of effector/target cell conjugates.