Isolated granular haemocytes (blood cells) from the crayfish Pacifastacus leniusculus attached and spread in vitro on coverslips coated with a lysate of crayfish haemocytes. No cell adhesion activity was detected in crayfish plasma. The cell adhesion activity was only present in haemocyte lysates in which the prophenoloxidase (proPO) activating system (Söderhäll and Smith, 1986a, b) had been activated; either by lipopolysaccharide (LPS), the beta-1,3-glucan laminarin, or by preparing the lysate in 5 mM Ca2+. Both lysates of granular or of semigranular haemocytes could mediate adhesion. After A23187-induced exocytosis of the granular cells, cell adhesion activity could be generated in the secreted material if it was incubated with laminarin. The factor responsible for cell adhesion was isolated from an active haemocyte lysate and purified by ammonium sulfate precipitation, cation exchange chromatography and Con A-Sepharose; it had a molecular mass of approximately 76 kD on an SDS-polyacrylamide gel. An antibody to this 76-kD band inhibited cell adhesion. Ca2+ was necessary in the medium for the cells to adhere to the adhesion factor. With cyanide or azide, the cells attached but failed to spread. It is suggested that in vivo the cell adhesion factor is stored in the secretory granules of the semigranular and the granular cells in a putative inactive pro-form, which can be released during exocytosis and, in the presence of beta-1,3-glucans or LPS, be activated outside the cells to mediate cell attachment and spreading, processes of essential importance in arthropod host defense.

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