Thy-1-mediated activation of rat basophilic leukemia cells does not require co-expression of the high-affinity IgE receptor

The glycosylphosphatidylinositol (GPI)-anchored protein Thy-1 is one of the most abundant molecules expressed on the surface of rat mast cells and rat basophilic leukemia cells, RBL-2H3. Antibody-mediated aggregation of Thy-1 induces in these cells release of secretory components; so does aggregation of the receptor with high affinity for IgE (Fc?RI). To examine whether there is any relationship between Thy-1- and Fc?RI-mediated activation, we have isolated from mutagenized RBL-2H3 cells a variant cell line deficient in the expression of surface Fc?RI, and analyzed its ability to be activated by an antibody to Thy-1. Northern and immuno-blot analyses revealed that the variant cells were deficient in the expression of a structural or a regulatory gene for Fc?RI γ subunit. The cells did not respond by release of secretagogues and protein-tyrosine phosphorylation to IgE and antigen and anti-Fc?RI monoclonal antibody (mAb) but their response to anti-Thy-1.1 mAb and calcium ionophore A23187 was retained. Transfection of the cloned Fc?RI γ subunit into the variant cells restored the surface expression of Fc?RI and responsiveness to both the antigen and anti-Fc?RI mAb but had no effect on responsiveness to anti-Thy-1 mAb. The combined data indicate that aggregation of surface Thy-1 glycoproteins activates a metabolic pathway which is independent of the presence of Fc?RI γ subunit and surface expression of Fc?RI.