Differential Binding of IgG Subclasses to Enzyme‐treated Human Lymphocytes

Human erythrocytes coated with each of the four human IgG subclasses were used to detect Fc gamma receptors on human peripheral blood lymphocytes (HPBL). Rosette formation was obtained only with erythrocytes coated with IgG3 (EA gamma 3). Neuraminidase treatment of HPBL induced rosette formation with EA gamma 1, EA gamma 2, and EA gamma 4 complexes. Pronase treatment also induced rosette formation to a lesser extent, but abolished EA gamma 3 rosetting. Trypsin treatment enhanced EA gamma 3 rosette formation. These phenomena occurred on both 'T' and 'non-T' lymphocytes. Erythrocytes coated with small quantities of IgG 3 did not form rosettes with HPBL. Neuraminidase treatment enhanced their binding, whereas pronase did not. These two phenomena occurred only on non-T lymphocytes. Rosette formation with EA gamma 1 was also obtained with lymphocytes stimulated in vitro with mitogens. After 2 days of culture, stimulated lymphocytes expressed receptors that were able to bind both EA gamma 1 and EA gamma 3 complexes. These results suggest the existence of cryptic receptors for IgG1, IgG2, and IgG4 that could be disclosed by neuraminidase and pronase treatment and exposed on stimulated lymphocytes. A hypothesis of one or several Fc gamma receptors is suggested.