IMMOBILIZED ANTIBODIES TO CD45 INDUCE RAPID MORPHOLOGIC CHANGES AND INCREASED TYROSINE PHOSPHORYLATION OF P56(LCK)-ASSOCIATED PROTEINS IN T-CELLS

CD45 is a transmembrane protein tyrosine phosphatase required for sign al transduction through the Ag receptor complexes of T and B lymphocyt es. Herein, we demonstrate that immobilized mAbs to the external domai n of CD45 induce rapid and dramatic morphologic changes in a variety o f T cell lines, including CD8(+) cytotoxic clones. CD45-induced morpho logic changes can be inhibited by the cytoskeletal inhibitors cytochal asin D and and by the protein tyrosine kinase inhibitor herbimycin A. Consistent with the requirement for tyrosine kinase activity, tyrosine phosphorylation of proteins at about 60 to 75 kDa and 115 to 130 kDa is increased upon engagement with immobilized anti-CD45 mAb with kinet ics paralleling the observed changes in morphology. The phosphorylatio n of these proteins is inhibited by tyrosine kinase inhibitors at conc entrations that also inhibit changes in morphology. The phosphoprotein s induced when cells are added to immobilized anti-CD45 are co-immunop recipitated with p56(lck), suggesting that this tyrosine kinase might play a role in the phosphorylation of these proteins. Consistent with this, there is no increase in the phosphorylation of these proteins in p56(lck)-deficient CTLL-2 cells in response to immobilized anti-CD45 mAb. An important role for p56(lck) in the morphologic pathway is furt her supported by the observation that p56(lck)-deficient human J.CAM1. 6 cells, in contrast to the parental Jurkat line, cannot be induced to undergo morphologic changes. Taken together, these results suggest a possible role for CD45 in coordinating a cytoskeletal remodeling casca de that may be important in cell activation.