Fish immunity and parasite infections: from innate immunity to immunoprophylactic prospects.

The increasing economic importance of fish parasitoses for aquaculture and fisheries has enhanced the interest in the defence mechanisms against these infections. Both innate and adaptive immune responses are mounted by fish to control parasite infections, and several mechanisms described for mammalian parasitoses have also been demonstrated in teleosts. Innate immune initiation relies on the recognition of pathogen-associated molecular patterns (PAMPs) by pathogen recognizing receptors (PRRs). A number of PRRs, mainly Toll-like receptors (TLRs), have been characterized in fish, and some molecules susceptible of functioning as PAMPs are known for some fish parasites. A lectin-carbohydrate interaction has also been described in some host fish-parasite systems, thus probably involving C-type lectin receptors. Inflammatory reactions involving cellular reactions, as phagocytosis and phagocyte activity (including oxidative mechanisms), as well as complement activity, are modulated by many fish parasites, including mainly ciliates, flagellates and myxozoans. Besides complement, a number of humoral immune factors (peroxidases, lysozyme, acute-phase proteins) are also implicated in the response to some parasites. Among adaptive responses, most data deal with the presence of B lymphocytes and the production of specific antibodies (Abs). Although an increasing number of T-cell markers have been described for teleosts, the specific characterization of those involved in their response is far from being obtained. Gene expression studies have demonstrated the involvement of other mediators of the innate and adaptive responses, i.e., cytokines [interleukins (IL-1, IL-8), tumor necrosis factor (TNF), interferon (IFN)], chemokines (CXC, CC), as well as several oxidative enzymes [inducible nitric oxide synthase (iNOS), cyclo-oxygenase 2 (COX-2)]. Information is scarcer for factors more directly linked to adaptive responses, such as major histocompatibility (MH) receptors, T cell receptors (TCRs) and IgM. Expression of some immune genes varied according to the phase of infection, and proinflammatory cytokines were mainly activated in the early stages. Gene expression was generally higher in the target tissues for some skin and gill parasites, as Ichthyophthirius multifiliis, Neoparamoeba spp. and Lepeophtheirus salmonis, thus confirming the relevance of mucosal immunity in these infections. The existence of protective responses has been demonstrated for several fish parasites, both in natural infections and in immunization studies. Most information on the mechanisms involved in protection deals with the production of specific Abs. Nevertheless, their levels are not always correlated to protection, and the precise involvement of immune mechanisms in the response is unknown in many cases. No commercial vaccine is currently available for piscine parasitoses, although experimental vaccines have been assayed against I. multifiliis, Cryptobia salmositica and scuticociliates. The known information points to the need for integrated studies of the mechanisms involved in protection, in order to choose the optimum antigen candidates, adjuvants and formulations.