THE PROTOZOAN PARASITE PERKINSUS ATLANTICUS-ELICITS A UNIQUE DEFENSIVE RESPONSE IN THE CLAM TAPES-SEMIDECUSSATUS

The venerid clams, Tapes decussatus and T. semidecussatus, develop a s ingular defensive response to Perkinsus atlanticus infection. This rea ction involves the redifferentiation of recruited granulocytes and the expression de novo of the polypeptide p225. To determine whether the association of this defensive process with the natural parasitism by P . atlanticus is unique, the inflammatory response elicited by inoculat ions of bacteria, algae and non-viable P. atlanticus prezoosporangia i n the clam T. semidecussatus was shown. Inoculated areas were heavily infiltrated by granulocytes and delimited by myofibroblast-like cells and extracellular matrix. While bacteria and algae were phagocytosed b y the infiltrated granulocytes, pre-zoosporangia were not. After 40 da ys, neither cell redifferentiation nor the expression of p225 was obse rved. These findings indicate that both redifferentiation and p225 exp ression are specifically associated with P. atlanticus infection. Afte r 5-bromodeoxyuridine administration, only a few cells were labelled e ither in the inoculated zone or in the cellular reaction around P. atl anticus meronts. Significant differences between untreated and inocula ted groups were observed in the epicardic connective tissue soon after injection. These results suggest that this anatomical region could be the main site of haemocyte proliferation stimulated after inoculation of foreign bodies in T. semidecussatus.