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 epsilon RI). To examine whether there is any relationship between Thy-1- and Fc epsilon RI-mediated activation, we have isolated from mutagenized RBL-2H3 cells a variant cell line deficient in the expression of surface Fc epsilon 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 epsilon RI gamma subunit. The cells did not respond by release of secretagogues and protein-tyrosine phosphorylation to IgE and antigen and anti-Fc epsilon RI monoclonal antibody (mAb) but their response to anti-Thy-1.1 mAb and calcium ionophore A23187 was retained. Transfection of the cloned Fc epsilon RI gamma subunit into the variant cells restored the surface expression of Fc epsilon RI and responsiveness to both the antigen and anti-Fc epsilon 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 epsilon RI gamma subunit and surface expression of Fc epsilon RI.