Transepithelial migration of mucosal hemocytes in Crassostrea virginica and potential role in Perkinsus marinus...

We have recently described the presence of hemocytes associated with mucus covering the pallial organs (mantle, gills, and body wall) 3 of the eastern oyster Crassostrea virginica. These hemocytes, hereby designated "pallial hemocytes" share common general characteristics with circulating hemocytes but also display significant differences particularly in their cell surface epitopes. The specific location of pallial hemocytes as peripheral cells exposed directly to the marine environment confers them a putative sentinel role. The purpose of this study was to gain a better understanding of the source of these pallial hemocytes by evaluating possible exchanges between circulatory and pallial hemocyte populations and whether these exchanges are regulated by pathogen exposure. Bi-directional transepithelial migrations of hemocytes between pallial surfaces and the circulatory system were monitored using standard cell tracking approaches after staining with the vital fluorescent dye carboxyfluorescein diacetate succinimidyl ester (CFSE) in conjunction with fluorescent microscopy and flow cytometry. Results showed bi-directional migration of hemocytes between both compartments and suggest that hemocyte migration from the pallial mucus layer to the circulatory system may occur at a greater rate compared to migration from the circulatory system to the pallial mucus layer, further supporting the role of pallial hemocytes as sentinel cells. Subsequently, the effect of the obligate parasite Perkinsus marinus and the opportunistic pathogen Vibrio alginolyticus on transepithelial migration of oyster hemocytes was investigated. Results showed an increase in hemocyte migration in response to P. marinus exposure. Furthermore, P. marinus cells were acquired by pallial hemocytes before being visible in underlying tissues and the circulatory system suggesting that this parasite could use pallial hemocytes as a vehicle facilitating its access to oyster tissues. These results are discussed in light of new evidence highlighting the role of oyster pallial organs as a portal for the initiation of P. marinus infections in oysters.