CD45 regulates Src family member kinase activity associated with macrophage integrin-mediated adhesion

Abstract 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 adhesion. In lymphocytes, CD45 is a leukocyte-specific transmembrane protein tyrosine phosphatase that activates Src family kinases associated with B-cell and T-cell antigen receptor signaling by constitutive dephosphorylation of the inhibitory carboxy-terminal tyrosine phosphorylation site. Here, we show that CD45 is also important in downregulating the kinase activity of Src family members during integrin-mediated adhesion in macrophages. Results: We found that CD45 colocalized with β 2 integrin and the Src family kinase p53/56 lyn to adhesion sites in bone marrow-derived macrophages. Macrophages from CD45 −/− mice were unable to maintain integrin-mediated adhesion. In adherent macrophages, absence of CD45 led to the hyperphosphorylation and hyperactivation 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 assays. Furthermore, coexpression of CD45 with p59 hck or p56 lyn containing a tyrosine to phenylalanine mutation at the carboxy-terminal negative regulatory site resulted in decreased tyrosine phosphorylation of the Src family member kinases due to dephosphorylation of the potentiating tyrosine phosphorylation site within the kinase domain. Conclusion: Using primary bone marrow macrophages, these studies demonstrate that CD45 regulates Src family kinases and is required to maintain macrophage adhesion. CD45 decreases Src family kinase activity by dephosphorylating the tyrosine residue located within the kinase domain.