Lipopolysaccharide-stimulated exocytosis of nonself recognition protein from insect hemocytes depend on protein tyrosine phosphorylation

Insect hemocytes (blood cells) synthesize the major nonself recognition protein (47 kDa) during 3rd instar larvae (V.J. Marmaras, S. Tsakas, Dev. Biol. 129, 294-303 (1988)). In this study we show the presence of the 47 kDa protein in plasmatocytes (main hemocyte type) and prohemocytes. In plasmatocytes this protein appears to be localized both in vesicles and in the cell surface. The cell surface-associated 47 kDa protein was released from membrane fraction by 1 M NaCl, indicating that it is not tightly bound. Bacterial lipopolysaccharide (LPS) can function on isolated hemocytes from Ceratitis capitata larvae, inducing their spreading and degranulation. During degranulation (exocytosis) the plasmatocytes release the 47 kDa protein, among others. This protein could not be normally traced in serum, nor is it released by basal secretion. The secretion of the 47 kDa protein was found to be LPS-dependent, whereas its presence on plasmatocyte surface is LPS independent. LPS-stimulated exocytosis of the 47 kDa protein appears to be dependent on protein tyrosine phosphorylation. We have now demonstrated that LPS increases tyrosine phosphorylation of 19 and 22 kDa polypeptides in C. capitata hemocytes. Inhibition of the LPS-induced tyrosine phosphorylation mediated by tyrosine kinase inhibitor, genistein, was accompanied by the inhibition of the secretion of the 47 kDa protein. These results support the hypothesis that tyrosine protein phosphorylation is a signal reaction in hemocytes after LPS exposure. These LPS responses of insect plasmatocytes show strong similarities to mammalian macrophages (S. Weinstein et al., J. Immunol. 151, 3829-3838 (1993)). In a model we propose that the LPS-independent cell surface-associated 47 kDa protein is responsible for the phagocytosis and for the formation of nodules and capsules, whereas the LPS-dependent secreting counterpart is responsible for the extracellular killing of bacteria.