Described in the present study is a major component of the cell wall of 2 of the most pathogenic parasites of molluscs, Perkinsus atlanticus and P. marinus. The component is a high molecular weight protein (233 kDa), which we have named PWP-1 (for Perkinsus wall protein-1). Western blots, using a polyclonal serum generated against purified PWP-1 from P. atlanticus, revealed that this protein is expressed by all walled developmental stages of this protozoon. By means of immunogold electron microscopy, labelling for PWP-1 was strong and specifically associated with the cell wall. The label density and distribution pattern was quite different between trophozoites and prezoosporangia. With regard to the structural organization of this protein, PWP-1 is disulphide-linked to other cell wall components and released from the cell wall only following treatment with a sulphydryl agent. We also report that PWP-1 is a trypsin-resistant protein, both in its native and heat-denatured conformation. In addition, results from the N-terminal microsequence of this protein allow us to define PWP-1 as a novel cell wall protein. Overall, our findings strongly suggest that PWP-1 plays a key role in the organization of the cell wall of these protozoa, promoting their survival.

The venerid clams, Tapes decussatus and T. semidecussatus, develop a singular defensive response to Perkinsus atlanticus infection. This reaction 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 inoculations of bacteria, algae and non-viable P. atlanticus pre-zoosporangia in 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 by the infiltrated granulocytes, pre-zoosporangia were not. After 40 days, neither cell redifferentiation nor the expression of p225 was observed. These findings indicate that both redifferentiation and p225 expression are specifically associated with P. atlanticus infection. After 5-bromodeoxyuridine administration, only a few cells were labelled either in the inoculated zone or in the cellular reaction around P. atlanticus meronts. Significant differences between untreated and inoculated 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.