Binding Site of Human IgG Subclasses and Their Domains for Fc Receptors of Activated Murine T Cells

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Abstract
A subpopulation of activated murine T cells (25%) were found to bear Fc receptors for mouse, rabbit, and human IgG. Studies, by rosette inhibition and indirect immunofluorescence, of their specificity for human IgG subclasses have shown that these receptors bound nonaggregated IgG1, IgG2, and IgG3 in a comparable fashion, but did not bind IgG4. The binding site has been localized exclusively to the Fc region and the binding capacity of this fragment was not affected by mild reduction. Binding of the Fc fragment of IgG4 could not be demonstrated. Using isolated Cγ2 and Cγ3 (pFc′) domains from IgG1 and a Cγ3-derived fragment (Fc′), a major binding site was localized to the Cγ3 domain between residues Gln342 and His433. A 10-fold molar excess of Cγ3 dimer over native Fc was, however, required to show comparable activity. The monomeric Cγ2 domain was 10-fold less active than the Cγ3 fragment. Human urine β2-microglobulin was not cytophilic. These data are discussed and it is suggested that the Cγ2 and Cγ3 domains may contribute to the formation of a cooperative binding site through quaternary interdomain interactions. By redistribution experiments, it was demonstrated that the three human IgG subclasses shown to be cytophilic share common or linked receptors, which were not associated with the H-2 alloantigens.