Vaccines and crustacean aquaculture—A mechanistic exploration

One of the dogmas of comparative (evolutionary) immunology for the last few decades has been that all invertebrates lack any form of immunological memory similar to that found in jawed vertebrates. Vaccinating invertebrates, such as shrimp, should therefore be an ineffective management strategy resulting in no more than short-lived, non-specific immune stimulation. Recent studies in crustaceans and insects, however, suggest a form of immune memory in these animals described variously as 'immune priming', 'specific immune priming' or 'line specific immune memory'. Workers have injected various arthropod species with inactivated pathogens and then later challenged them with a dose of the same pathogen capable of killing the host, reporting enhanced survival or increased reproductive fecundity compared with animals vaccinated with an unrelated organism. Interestingly, in some studies this enhanced immunity is trans-generational (i.e. passed on to progeny). This brief review focusses on a mechanistic exploration of 'specific immune priming' with particular reference to shrimp culture. Humoral antimicrobial factors, such as antimicrobial peptides and lysozyme, do not possess the required specificity to explain this phenomenon. Instead, recent studies have demonstrated elevated phagocytic activity after "vaccination" that does not result from general (non-specific) stimulation in phagocytosis. This enhanced phagocytic activity is likely linked to specific recognition of determinants on the outside of microbes by the haemocytes of these "vaccinated" animals. In some cases, at least, the mechanism for recognising these determinants appears to rely upon an invertebrate homologue of the Down syndrome cellular adhesion molecule (Dscam) and the ability of the Dscam gene to produce variants of this molecule with specific binding capabilities. Pioneering studies have demonstrated that Dscam is intricately involved in phagocytosis in insects and able to produce pathogen-specific splice form variants after immune challenge or infection. This would enable an animal to tailor an immune response specifically to a pathogen. It should be noted, however, that the phenomenon of immune priming is certainly not universal, possibly because of the much lower numbers of different immune receptors that arthropods are able to produce compared with jawed vertebrates. The review concludes that immune priming in invertebrates should be re-evaluated on an animal to animal and pathogen to pathogen basis. It also notes that whilst there is evidence that vaccinating shrimp against viral and bacterial infections shows promise, the practicality of such processes and their benefit to shrimp requires further evaluation.