Immune priming in shellfish: A review and an updating mechanistic insight focused on cellular and humoral responses

Immune priming has now been demonstrated as a form of immune memory in a wide range of invertebrate species that lack acquired immune systems. Owing to the specific activation of innate immunity upon reinfection, immune priming has important implications for vaccinating economically valuable shellfish to effectively prevent infectious diseases. To gain a clearer understanding of the underlying mechanisms, extensive research has recently been concentrated on priming responses and better evidence-based mechanistic explanations appear as a prerequisite for the development of prophylactic strategies. This review specially focused on cellular and humoral responses to examine the evidence and inquire into the possible mechanisms involved in broad-spectrum or specific immune priming with particular reference to shellfish, with the aims of providing an overview of the current status of research and helping us better understand the existing knowledge. Current data suggest that both humoral and cellular immune factors are to some extent involved in priming protection, but it is far more likely that cellular immunity is more important in terms of the clearance of pathogens and survival. Antimicrobial peptides and other humoral factors seem to be unable to generate priming specificity, but the only selectivity. Alternatively, the elevated phagocytosis of hemocytes could be linked to the specific recognition of antigens in primed hosts. The molecular mechanism appears to rely upon certain receptors/effectors diversified via alternative splicing that could produce splice variants with specific binding capabilities. Our review concludes that whilst immune priming justifies the theoretical feasibility of ‘vaccination’ for bacterial and viral disease control, the priming effect and its impact on immune modulation remain to be further clarified. The practicality of vaccine strategy, particularly the benefit and efficiency, also needs reevaluations in the future. •Current evidence and potential underlying mechanisms of immune priming were examined.•Priming-induced phagocytic cells are linked to the efficient pathogen clearance.•Proliferation of hemocytes could be responsible for the enhanced phagocytosis.•Specific priming may be attributed to the receptor diversity via alternative splicing.