Differential sensitivity to cadmium of immunomarkers measured in hemocyte subpopulations of zebra mussel Dreissena polymorpha.

Increasing discharge of industrial wastes into the environment results in pollution transfer towards hydrosystems. These activities release heavy metals such as cadmium, known as persistent pollutant that is accumulated by molluscs and exercise immunotoxicological effects. Among molluscs, the zebra mussel, Dreissena polymorpha constitutes a suitable support for freshwater ecotoxicological studies. In molluscs, homeostasis maintain is ensured in part by hemocytes that are composed of several cell populations involved in multiple physiological processes such as cell-mediated immune response or metal metabolism. Thus, hemocytes constitute a target of concern to study adverse effects of heavy metals. The objectives of this work were to determine whether immune-related endpoints assessed were of different sensitivity to cadmium and whether hemocyte functionalities were differentially affected depending on hemocyte subpopulation considered. Hemocytes were exposed ex vivo to concentrations of cadmium ranging from 10-6 M to 10-3 M for 21h prior flow cytometric analysis of cellular markers. Measured parameters (viability, phagocytosis, oxidative activity, lysosomal content) decreased in a dose-dependent manner with sensitivity differences depending on endpoint and cell type considered. Our results indicated that phagocytosis related endpoints were the most sensitive studied mechanisms to cadmium compared to other markers with EC50 of 3.71±0.53×10-4M for phagocytic activity and 2.79±0.19×10-4M considering mean number of beads per phagocytic cell. Lysosomal content of granulocytes was less affected compared to other cell types, indicating lower sensitivity to cadmium. This suggests that granulocyte population is greatly involved in metal metabolism. Mitochondrial activity was reduced only in blast-like hemocytes that are considered to be cell precursors. Impairment of these cell functionalities may potentially compromise functions ensured by differentiated cells. We concluded that analysis of hemocyte activities should be performed at sub-population scale for more accurate results in ecotoxicological studies.