Phagocytosis of Yeast by Coelomocytes of the Sipunculan Worm Themiste petricola: Opsonization by Plasma Components

Abstract

The phagocytic activity of hemocytes from the Sipunculan worm Themiste petricola was analyzed in a quantitative in vitro assay using heat killed Saccharomyces cerevisiae (HKSC) as target cells. Coelomocytes of T. petricola were capable of phagocytosing HKSC yeast in the absence of coelomic plasma but the phagocytosis level was enhanced when complete plasma was present in the culture medium. This phagocytosis promoting activity was completely eliminated by preabsorption of plasma with HKSC. The opsonizing activity of plasma was also observed when HKSC were preadsorbed in plasma. HKSC absorbed plasma and complete plasma were studied by SDS-PAGE and immunoblotting. Several plasma proteins with molecular weights (MW) within a range between 15 and 135 kDa were separated by SDS-PAGE. Some of the proteins present in the plasma (MW 23, 30, 60, 68, 98, and 102 kDa) were removed as a consequence of HKSC absorption. Plasma proteins preadsorbed on HKSC were dissociated by SDS detergent and analyzed by SDS-PAGE and immunoblotting. These proteins (MW 30, 60, 68, 98, and 102 kDa) are considered potential recognition molecules responsible for the opsonizing activity found in coelomic plasma of T. petricola . From results of this study it is concluded that recognition of HKSC by coe1omocytes of T. petricola can be performed by either of two mechanisms: by opsonization of yeast cells with plasma proteins which are subsequently recognized by phagocytic cells or by an opsonin independent mechanism through which coelomocytes are able to phagocytose HKSC in the absence of plasma.