Interaction of anti-staphylococcal protein A antisera with Fc receptor-bearing human normal lymphocytes

Staphylococcal protein A (SpA) is known to bind the Fc region of IgG of most mammalians and to possess biologic activity both in vivo and in vitro, where it acts as a lymphocyte polyclonal mitogen. Its binding to the Fc gamma portion bears many features of the antibody-antigen interaction, such as the dissociation constant, lattice formation, and complement activation. Moreover, SpA seems to compete with membrane Fc receptors for IgG so that the possibility of an interaction with the same CH domain(s) of IgG can be considered. In the present study, evidence is given that anti-SpA antisera obtained from chickens and rabbits are able to inhibit EA rosette formation by normal human lymphocytes and that they are able to recognize, with immunofluorescent staining, a subpopulation of normal human peripheral blood lymphocytes (PBL) that closely resembles that of EA rosette-forming cells (RFC). Moreover, the depletion of EA RFC by means of a single gradient centrifugation is accomplished by the parallel depletion of PBL stainable by anti-SpA antisera. The relevance of these results in the hypothesis of a similarity between the combining sites of SpA and membrane Fc receptor(s) for IgG is discussed.