Two distinct Fc receptors for IgG on human peripheral T lymphocytes.

The proportion of human peripheral T lymphocytes forming rosettes with IgG-coated ox erythrocytes (ORBC) is increased after controlled hypotonic treatment. This increment may be as high as 40% of total T cells, depending on the lymphocyte donor. Such treatment is shown not to result in selective cell loss. Induced rosetting is mediated by a receptor specific for the Fc portion of human IgG (Fc gamma R). Inhibition of induced Fc gamma R activity is equally well accomplished by monomeric and by aggregated IgG of defined size. This is in contrast to the Fc gamma R detected before hypotonic treatment, which is not significantly inhibited by monomeric IgG. Capping studies established the structural independence of these two types of Fc gamma R in the lymphocyte membrane by virtue of selective cross-linking of either receptor while leaving the respective counterpart unaffected. The biochemical basis of the hypotonic effect is not yet resolved. However, the data presented suggest that hypotonicity results in removal of Fc gamma R-bound cytophilic IgG. Operationally, we propose the term induced Fc gamma R (Fc gamma R-I) for the here-described new type of receptor with high affinity for monomeric IgG.Fc gamma R that are directly assayable without hypotonic induction and not inhibited by monomeric IgG are termed free Fc gamma R (Fc gamma R-F).