From ancient cold-blooded fishes to mammals, all vertebrates are protected by adaptive immunity, and retain immunological memory. Although immunologists can demonstrate these phenomena in all fish, the responding cells remain elusive, without the tools to study them nor markers to define them. Fundamentally, we posited that it is longevity that defines a memory cell, like how it is antibody production that defines a plasma cell. We infected the common carp with Sphaerospora molnari, a cnidarian parasite which causes seasonal outbreaks to which no vaccine is available. B cells proliferated and expressed gene signatures of differentiation. Despite a half-year gap between EdU labeling and sampling, IgM+ B cells retained the thymidine analogue, suggesting that these are at least six-month-old resting memory cells stemming from proliferating precursors. Additionally, we identified a lymphoid organ-resident population of plasma cells by the exceptional levels of IgM they express. Thus, we demonstrate that a teleost fish produces the lymphocytes key to vaccination success and long-term disease protection, supporting the idea that immunological memory is observable and universal across vertebrates.

• Keywords
• antibody, antibody-secreting cell (ASC), humoral memory, immunoglobulin, myxozoa,
• MeSH
• B-Lymphocytes immunology   MeSH
• Cell Differentiation * immunology   MeSH
• Immunoglobulin M * immunology   MeSH
• Immunologic Memory * MeSH
• Carps * immunology   parasitology   MeSH
• Fish Diseases immunology   parasitology   MeSH
• Memory B Cells immunology   MeSH
• Plasma Cells * immunology   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Immunoglobulin M * MeSH

Myxozoans are a diverse group of microscopic cnidarian parasites and some representatives are associated with important diseases in fish, in both marine and freshwater aquaculture systems. Research on myxozoans has been largely hampered by the inability to isolate myxozoan parasites from their host tissues. In this study, we developed and optimized a method to isolate the myxozoan proliferative stages of different size and cellularity from fish blood, using DEAE-cellulose ion exchange chromatography. We optimized several parameters and obtained 99-100% parasite purity, as well as high survival and infectivity. Using polyclonal pan-carp blood cell-specific antibodies, we further developed a rapid cytometric assay for quantification of the proliferative stages, not only in highly concentrated DEAE-C isolates but also in dilute conditions in full blood. Early developmental stages of myxozoans are key to parasite proliferation, establishment, and pathology in their hosts. The isolation of these stages not only opens new possibilities for in vivo and in vitro studies, but also for obtaining purified DNA and protein extracts for downstream analyses. Hence, we provide a long-desired tool that will advance the functional research into the mechanisms of host exploitation and immune stimulation/evasion in this group, which could contribute greatly to the development of therapeutic strategies against myxozoans.

• Keywords
• Sphaerospora, anti-carp antibody, blood stages, cell separation, common carp, cytometry, diethylaminoethyl (DEAE) cellulose, parasite, teleost,
• MeSH
• Genomics MeSH
• Carps * MeSH
• Myxozoa * genetics   MeSH
• Fish Diseases * MeSH
• Antibodies MeSH
• Aquaculture MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Antibodies MeSH

Myxozoans are microscopic, metazoan, obligate parasites, belonging to the phylum Cnidaria. In contrast to the free-living lifestyle of most members of this taxon, myxozoans have complex life cycles alternating between vertebrate and invertebrate hosts. Vertebrate hosts are primarily fish, although they are also reported from amphibians, reptiles, trematodes, mollusks, birds and mammals. Invertebrate hosts include annelids and bryozoans. Most myxozoans are not overtly pathogenic to fish hosts, but some are responsible for severe economic losses in fisheries and aquaculture. In both scenarios, the interaction between the parasite and the host immune system is key to explain such different outcomes of this relationship. Innate immune responses contribute to the resistance of certain fish strains and species, and the absence or low levels of some innate and regulatory factors explain the high pathogenicity of some infections. In many cases, immune evasion explains the absence of a host response and allows the parasite to proliferate covertly during the first stages of the infection. In some infections, the lack of an appropriate regulatory response results in an excessive inflammatory response, causing immunopathological consequences that are worse than inflicted by the parasite itself. This review will update the available information about the immune responses against Myxozoa, with special focus on T and B lymphocyte and immunoglobulin responses, how these immune effectors are modulated by different biotic and abiotic factors, and on the mechanisms of immune evasion targeting specific immune effectors. The current and future design of control strategies for myxozoan diseases is based on understanding this myxozoan-fish interaction, and immune-based strategies such as improvement of innate and specific factors through diets and additives, host genetic selection, passive immunization and vaccination, are starting to be considered.

• Keywords
• B lymphocytes, RNAseq, T lymphocytes, adaptive immunity, immune evasion, immunoglobulin, parasite, teleost,
• MeSH
• Adaptive Immunity * MeSH
• Antiparasitic Agents pharmacology   MeSH
• B-Lymphocytes immunology   metabolism   parasitology   MeSH
• Immune Evasion MeSH
• Immunoglobulins immunology   metabolism   MeSH
• Host-Parasite Interactions MeSH
• Myxozoa drug effects   immunology   pathogenicity   MeSH
• Fish Diseases immunology   metabolism   parasitology   prevention & control   MeSH
• Parasitic Diseases, Animal immunology   metabolism   parasitology   prevention & control   MeSH
• Immunity, Innate * MeSH
• Fishes immunology   metabolism   parasitology   MeSH
• T-Lymphocytes immunology   metabolism   parasitology   MeSH
• Vaccines pharmacology   MeSH
• Aquaculture MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Research Support, U.S. Gov't, Non-P.H.S. MeSH
• Names of Substances
• Antiparasitic Agents MeSH
• Immunoglobulins MeSH
• Vaccines MeSH