TREATMENT WITH OKADAIC ACID REVEALS STRONG THREONINE PHOSPHORYLATION OF CD18 AFTER ACTIVATION OF CD11 CD18 LEUKOCYTE INTEGRINS WITH PHORBOLESTERS OR CD3 ANTIBODIES/

The CD11/CD18 leukocyte integrins comprise three heterodimers involved in leukocyte adhesion. CD11/CD18 avidity may be regulated intracellul arly, and the CD18 polypeptide has previously been shown to become pho sphorylated in leukocytes after phorbol ester stimulation. The importa nce of phosphorylation in the regulation of CD11/CD18 avidity has, how ever, remained unclear. We have now activated T cells using phorbol es ters, CD3, and CD44 Abs. Both phorbol ester and CD3 treatment activate d protein kinase C. CD18 was shown to become more stably phosphorylate d after phorbol ester treatment and more transiently so after CD3 stim ulation. The phosphorylation was strongly augmented by okadaic acid, a serine/threonine phosphatase inhibitor. While phorbol ester treatment caused phosphorylation mainly on serine, in okadaic acid-pretreated c ells, both phorbol ester treatment as well as CD3 stimulation revealed strong threonine phosphorylation. Since earlier mutational studies ha ve demonstrated the functional importance of cytoplasmic threonine res idues in CD18, the threonine phosphorylation reported here indicates t he role of threonine phosphorylation in the regulation of CD11/CD18 av idity.