Tyrosine kinase-mediated cell signalling in the activation of Mytilus hemocytes:: possible role of STAT-like proteins

In bivalve molluscs, cell-mediated immunity is carried out by circulating hemocytes, resembling the monocyte/macrophage lineage of vertebrates, that can kill the microbes through phagocytosis and various cytotoxic reactions. Previous data demonstrated that activation of MAPKs (Mitogen Activated Protein Kinases) is involved in the response of mussel hemocytes (Mytilus galloprovincialis Lam.) to bacterial challenge. In this work, the possibility that modulation of components of tyrosine kinase-mediated cell signalling may participate in the activation of mussel hemocytes was investigated. Cell pre-treatment with the macrophage activator IFNgamma significantly increased the bactericidal activity of mussel hemocytes towards E coli. Human recombinant IFNgamma stimulated tyrosine phosphorylation of different members of STAT-like proteins (Signal Transducers and Activators of Transcription), as evaluated by Western blotting of hemocyte protein extracts with specific anti-phospho-STAT antibodies. A similar increase in phosphorylation of immunoreactive STATs was observed in hemocytes incubated with E. coli, this indicating that tyrosine phosphorylation of STAT-like members represents a physiological step in hemocyte activation. IFNgamma lead to persistent phosphorylation of immunoreactive STAT1, a transcription factor that plays a critical role in innate immunity towards Gram negative bacteria in mammalian systems; moreover, hemocyte pretreatment with IFNgamma significantly increased bacteria-induced STAT1 phosphorylation, whereas IFNalpha did not. IFNgamma also transiently affected the phosphorylation state of different MAPKs. The extent and time course of MAPK phosphorylation induced by IFNgamma were distinct from those elicited by either IFNalpha or bacterial challenge. Overall, the results indicate that the hemocyte function can be modulated by heterologous cytokines and bacterial signals that act in concert through tyrosine kinase-mediated transduction pathways converging on STAT- and MAPK-like members. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.