CROSS-LINKING OF THY-1 GLYCOPROTEINS OR HIGH-AFFINITY IGE RECEPTORS INDUCES MAST-CELL ACTIVATION VIA DIFFERENT MECHANISMS

Rat peritoneal and pleural mast cells and rat basophilic leukaemia cel ls, RBL-2H3, have been previously shown to be activated by Thy-1-speci fic monoclonal antibodies (mAb). In the present study we investigated the mechanism of Thy-1-mediated activation and compared it with activa tion induced by cross-linking of the high-affinity IgE receptor. Bindi ng of an IgG Thy-1.1-specific mAb, MRCOX7 (OX7), to RBL-2H3 cells and mast.cells, and activation of RBL-2H3 by the OX7 were abrogated by pre treatment of the cells with phosphatidyl inositol-specific phospholipa se C (PI-PLC). The F(ab')2 fragment of OX7, in contrast to the Fab' fr agment, induced cell activation as well as intact OX7 mAb. Cells sensi tized with IgE exhibited an increased responsiveness to anti-Thy-1 ant ibodies suggesting formation of functional complexes of IgE receptor/I gE/Thy-1/anti-Thy-1. Pretreatment of RBL-2H3 cells with cholera toxin potentiated activation induced by IgE + antigen (Ag) and IgE + OX7, bu t had no effect on activation induced by OX7 antibody alone. Similarly , dexamethasone had no effect on OX7-induced activation but inhibited IgE + Ag- and IgE + OX7-induced activation. Analysis of phosphotyrosin e-containing proteins in RBL-2H3 cell lysates revealed that IgE + Ag a nd IgE + OX7 induced a marked increase in tyrosine phosphorylation of several proteins that were not tyrosine phosphorylated in cells expose d to OX7 mAb alone. Similar results were obtained when RBL-2H3-derived cells, expressing transfected mouse Thy-1.2, were activated with Thy- 1.2-specific IgM antibody. The combined data suggest that Thy-1-specif ic antibodies activate cells by a mechanism that is different from act ivation induced by cross-linking of high-affinity IgE receptor.