Defense Responses of Hemocytes Withdrawn from Crassostrea virginica Infected with Perkinsus marinus

Abstract

Hatchery-produced, unparasitized oysters ( Crassostrea virginica) were transplanted to high and low salinity sites in the Chesapeake Bay in May 1992. These locations included Mobjack Bay, Virginia (MB, ∼20%), the Choptank River, Maryland (CR, ∼14%), and the Wye River, Maryland (WR, ∼8%); the salinities subsequently varied with rainfall in the region of each site. The protozoan parasite Perkinsus marinas rapidly infected the oysters at each site. Oysters were periodically sampled and individually assayed for intensity of P. marinus infection, total circulating hemocyte count (THC) and zymosan-iuduced chemiluminescence (CL) activity of the hemocytes. The CL assay measures the production of reactive oxygen intermediates (ROIs) thought to mediate antimicrobial activity of the hemocytes. The MB oysters were 100% infected and showed a high intensity of infection by October, 1992; during the following year, high levels of infection were reached rapidly and the population was completely decimated by mid-July, 1993. Disease intensity and progression were slower in oysters from the low salinity sites (CR and WR). The THC and zymosan-stimulated CL responses of hemocytes from moderately or heavily infected oysters were significantly higher than those of uninfected or lightly infected oysters. The increased number of hemocytes from oysters with advanced infections probably represents a general protective response to the presence of infectious agents in tbe hemolymph. Hemocytes withdrawn from oysters with heavy P. marinus infections have markedly increased CL activity after phagocytosis of zymosan particles. Statistical analysis of the data showed that this activated condition was strongly associated with the level of P. marinus infection of the host, but not influenced by the ambient salinity of the oysters' habitat. However, phagocytosis of P. marinus merozoites by hemocytes fails to trigger CL, regardless of the infection intensity of the oysters from which the hemocytes were withdrawn. These data suggest that ingestion of microorganisms may nonspecifically enhance hemocytic defense responsiveness, as measured by their increased production of ROIs upon subsequent phagocytic stimulation by zymosan. However, not all microbes can initiate responses from these activated hemocytes; this may partially explain the virtual inability of C. virginica hemocytes to control P. marinus infections by intracellular cidal mechanisms.