CD45 REGULATES SRC FAMILY MEMBER KINASE-ACTIVITY ASSOCIATED WITH MACROPHAGE INTEGRIN-MEDIATED ADHESION

Background: Adhesion of leukocytes to the extracellular matrix and to other cells is mediated by members of the integrin family of adhesion molecules. Src family kinases are activated upon integrin-mediated adh esion. In lymphocytes, CD45 is a leukocyte-specific transmembrane prot ein tyrosine phosphatase that activates Src family kinases associated with B-cell and T-cell antigen receptor signaling by constitutive deph osphorylation of the inhibitory carboxy-terminal tyrosine phosphorylat ion site. Here, we show that CD45 is also important in downregulating the kinase activity of Src family members during integrin-mediated adh esion in macrophages. Results: We found that CD45 colocalized with bet a 2 integrin and the Src family kinase p53/56(lyn) to adhesion sites i n bone marrow-derived macrophages. Macrophages from CD45(-/-) mice wer e unable to maintain integrin-mediated adhesion. In adherent macrophag es, absence of CD45 led to the hyperphosphorylation and hyperactivatio n of p56/59(hck) and p53/56(lyn), but not of p58(c-fgr). CD45 directly inactivated p59(hck) but not p56(lck) in transient transfection assay s. Furthermore, coexpression of CD45 with p59(hck) or p56(lyn) contain ing a tyrosine to phenylalanine mutation at the carboxy-terminal negat ive regulatory site resulted in decreased tyrosine phosphorylation of the Src family member kinases due to dephosphorylation of the potentia ting tyrosine phosphorylation site within the kinase domain. Conclusio n: Using primary bone marrow macrophages, these studies demonstrate th at CD45 regulates Src family kinases and is required to maintain macro phage adhesion. CD45 decreases Src family kinase activity by dephospho rylating the tyrosine residue located within the kinase domain.