Immune priming and haemolymph-mediated protection in Mytilus edulis against Vibrio splendidus

Improving disease resistance in farmed bivalves is essential for the long-term sustainability of aquaculture, yet practical immune-based approaches remain scarce. In a few species, immune priming has shown encouraging potential, where a first encounter with a pathogen enhances the host's ability to cope with later infections by improving recognition and clearance of encountered microbes. In this study, we report the first evidence of immune priming in the blue mussel Mytilus edulis, a species of major commercial value. We show that mussels previously exposed to Vibrio splendidus, whether live or heat-killed, display markedly higher survival rates during a subsequent bacterial challenge, with protection lasting for at least 21 days. Strikingly, adoptive transfer assays revealed that both cell-free plasma and haemocytes from primed mussels can convey protection to naïve individuals. Primed mussels also exhibited sustained increases in total haemocyte concentration, shifts in cell population profiles, altered phagocytic activity, enhanced lysosomal function, and a biphasic pattern of ROS production. The distinct cellular responses triggered by live and heat-killed bacteria suggest that different activation pathways are involved. Together, these results provide the first demonstration of transferable immune memory in M. edulis, involving both cellular reprogramming and humoral components. This work opens promising perspectives for developing immune-boosting strategies to strengthen disease resistance in mussel aquaculture.