LIPOPOLYSACCHARIDE-STIMULATED EXOCYTOSIS OF NONSELF RECOGNITION PROTEIN FROM INSECT HEMOCYTES DEPEND ON PROTEIN-TYROSINE PHOSPHORYLATION

Insect hemocytes (blood cells) synthesize the major nonself recognitio n 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 prohem ocytes. In plasmatocytes this protein appears to be localized both in vesicles and in the cell surface. The cell surface-associated 47 kDa p rotein,vas released from membrane fraction by 1 M NaCl, indicating tha t it is not tightly bound. Bacterial lipopolysaccharide (LPS) can func tion on isolated hemocytes from Ceratitis capitata larvae, inducing th eir 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 sec retion. The secretion of the 47 kDa protein was found to be LPS-depend ent, whereas its presence on plasmatocyte surface is LPS independent. LPS-stimulated exocytosis of the 47 kDa protein appears to be dependen t on protein tyrosine phosphorylation. We have now demonstrated that L PS increases tyrosine phosphorylation of 19 and 22 kDa polypeptides in C. capitata hemocytes. Inhibition of the LPS-induced tyrosine phospho rylation mediated by tyrosine kinase inhibitor, genistein, was accompa nied by the inhibition of the secretion of the 47 kDa protein. These r esults support the hypothesis that tyrosine protein phosphorylation is a signal reaction in hemocytes after LPS exposure. These LPS response s of insect plasmatocytes show strong similarities to mammalian macrop hages (S. Weinstein et al., a. Immunol. 151, 3829-3838 (1993)). In a m odel we propose that the LPS-independent cell surface-associated 47 kD a 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.