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Fish lack immunoglobulin A, which suggests that they lack specialized mucosal antibodies. Sunyer and colleagues show that immunoglobulin T fulfills this mucosal antibody function and engenders protection against gut parasites. Teleost fish are the most primitive bony vertebrates that contain immunoglobulins. In contrast to mammals and birds, these species are devoid of immunoglobulin A (IgA) or a functional equivalent. This observation suggests that specialization of immunoglobulin isotypes into mucosal and systemic responses took place during tetrapod evolution. Challenging that paradigm, here we show that IgT, an immunoglobulin isotype of unknown function, acts like a mucosal antibody. We detected responses of rainbow trout IgT to an intestinal parasite only in the gut, whereas IgM responses were confined to the serum. IgT coated most intestinal bacteria. As IgT and IgA are phylogenetically distant immunoglobulins, their specialization into mucosal responses probably occurred independently by a process of convergent evolution.

Changes in the early-life microbiota of hosts might affect infectious disease risk throughout life, if such disruptions during formative times alter immune system development. Here, we test whether an early-life disruption of host-associated microbiota affects later-life resistance to infections by manipulating the microbiota of tadpoles and challenging them with parasitic gut worms as adults. We find that tadpole bacterial diversity is negatively correlated with parasite establishment in adult frogs: adult frogs that had reduced bacterial diversity as tadpoles have three times more worms than adults without their microbiota manipulated as tadpoles. In contrast, adult bacterial diversity during parasite exposure is not correlated with parasite establishment in adult frogs. Thus, in this experimental setup, an early-life disruption of the microbiota has lasting reductions on host resistance to infections, which is possibly mediated by its effects on immune system development. Our results support the idea that preventing early-life disruption of host-associated microbiota might confer protection against diseases later in life. Early-life microbiota alterations can affect infection susceptibility later in life, in animal models. Here, Knutie et al. show that manipulating the microbiota of tadpoles leads to increased susceptibility to parasitic infection in adult frogs, in the absence of substantial changes in the adults’ microbiota.

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.

Nile tilapia ( Oreochromis niloticus ) is valued in aquaculture because of its quick development and ability to thrive in various environments. Myxosporeans are among the fish parasites that affect fish productivity, as they impact fish growth and reproduction, resulting in large fish deaths in farms and hatcheries. This study has been focused on morpho-molecular identification for the myxosporean parasites infecting Nile tilapia from three governorates in Egypt and assessment of gene expression of different cytokines (Interleukin-1βeta ( IL-1β ), major histocompatibility complex class II ( MHC-II ), and clusters of differentiation 4 ( CD-4 ) and 8 ( CD-8 )) in tissues. Additionally, this work aimed to correlate the developed histopathological alterations and inflammatory reactions in gills with immunohistochemical expression of inducible nitric oxide synthase ( iNOS ) and tumor necrosis factor-alpha ( TNF-α ). Finally, the infected fish’s cortisol levels and blood glucose were assessed. Results of BLAST sequence analysis of the 18S rRNA for the collected protozoans confirmed Myxobolus agolus , M . brachysporus , M . tilapiae , and Henneguya species. The molecular characterization of the immunological status of gills revealed marked upregulation of different inflammatory cytokines in the gills of infected fish. There was a significantly increased serum cortisol and glucose level in infected fish compared with control, non-infected ones. Severe histopathological alterations were observed in the infected fish gills, associated with increased expression of iNOS and TNF-α and related to myxosporean infection. The present study provides new insights into oxidative stress biomarkers in Nile tilapia infected with Myxosporeans and elucidates the gill’s immune status changes as a portal of entry for protozoa that contribute to tissue damage.

Parasitic infections, such as those caused by the myxozoans Myxobolus cerebralis and Tetracapsuloides bryosalmonae, pose major threats to wild and farmed salmonids due to severe tissue damage and impairment of the host immune system. While individual infections have been studied, limited information is available on the host response during co-infection. This study investigated the transcriptomic immune response of rainbow trout (Oncorhynchus mykiss) during single and sequential co-infections with M. cerebralis and T. bryosalmonae using RNA-seq. Trout were exposed to single infections (Mc or Tb) followed by co-infections (Mc+ or Tb+). Fish were sampled at 31 days post-single infection (1 day post-co-infection). RNA from gill and caudal fin (portal of parasite entry) was sequenced, followed by differentially expressed genes (DEGs) identification and GO and KEGG enrichment. In the caudal fin, Mc+ (1 day after co-infection with T. bryosalomne) fish showed mild immune activation with C4B upregulation, while Tb+ fish exhibited a stronger response involving IFI44, ISG15, RSAD2, and TLR7 signaling. In gills, Mc+ fish showed moderate cytokine-related gene upregulation, while Tb+ (1 day after co-infection with M. cerebralis) fish displayed increased expression of humoral response genes (C3, immunoglobulin pathways) but suppression of genes involved in B cell development. These results indicate that the order of infection shapes the outcome of the host immune response, offering candidate targets at the host–pathogen interface.

Transgenerational immune priming (TGIP) is a phenomenon by which an initial exposure to a pathogen, here Tetracapsuloides bryosalmonae (causative agent of Proliferative Kidney Disease), stimulates the subsequent immune response to the same or a different pathogen in future generations. The impact of rearing conditions in previous generations, regardless of their exposure to the pathogen, on the immune cell composition and immune response in subsequent generations has not yet been investigated in the brown trout- T. bryosalmonae system. In the present work, we performed flow cytometry to analyze immune cell populations of brown trout. Parental generations (F0) differed in rearing conditions and exposure to the parasite. The study evaluated the baseline frequencies of IgM + B cells, myeloid cells, and CD8 + T cells in the offspring (F1) young-of-the-year brown trout. Afterwards, F1 fish were experimentally infected with T. bryosalmonae spores and monitored during eight weeks post-infection. The kidney was identified as an immune-cell niche dominated by myeloid cells, which represent approximately two-thirds of the total immune cell population, along with a substantial proportion of IgM + B cells. CD8 + T cells constitute only a minor fraction within this niche. As measured by flow cytometry, the immune-cell frequencies of offspring were largely unaffected by the parental rearing background (F0) and infection history. Parental history had no influence on the outcome of experimental infection. In T. bryosalmonae exposed animals, parasite concentration increased significantly over time. Moreover, a proportional increase in IgM + B cells and a proportional decrease in myeloid cells over time was observed. However, the increase in IgM + B cells was also detected in control animals. In conclusion, this study presents the first analysis of immune cell composition in F1 brown trout derived from parents reared under three distinct environmental conditions with varying parasite exposures. Our flow cytometry results highlight the need for alternative approaches to investigate transgenerational immune priming (TGIP) in brown trout.

Immersion and intraperitoneal injection are the two most common methods used for the vaccination of fish. Because both methods require that fish are handled and thereby stressed, oral administration of vaccines as feed supplements is desirable. In addition, in terms of revaccination (boosting) of adult fish held in net pens, oral administration of vaccines is probably the only feasible method to obtain proper protection against diseases over long periods of time. Oral vaccination is considered a suitable method for mass immunization of large and stress-sensitive fish populations. Moreover, oral vaccines may preferably induce mucosal immunity, which is especially important to fish. Experimental oral vaccine formulations include both non-encapsulated and encapsulated antigens, viruses and bacteria. To develop an effective oral vaccine, the desired antigens must be protected against the harsh environments in the stomach and gut so they can remain intact when they reach the lower gut/intestine where they normally are absorbed and transported to immune cells. The most commonly used encapsulation method is the use of alginate microspheres that can effectively deliver vaccines to the intestine without degradation. Other encapsulation methods include chitosan encapsulation, poly D,L-lactide-co-glycolic acid and liposome encapsulation. Only a few commercial oral vaccines are available on the market, including those against infectious pancreatic necrosis virus (IPNV), Spring viremia carp virus (SVCV), infectious salmon anaemia virus (ISAV) and Piscirickettsia salmonis. This review highlights recent developments of oral vaccination in teleost fish.

Immunity and parasites have been linked to the success of invasive species. Especially lower parasite burden in invasive populations has been suggested to enable a general downregulation of immune investment (Enemy Release and Evolution of Increased Competitive Ability Hypotheses). Simultaneously, keeping high immune competence towards potentially newly acquired parasites in the invasive range is essential to allow population growth. To investigate the variation of immune effectors of invasive species, we compared the mean and variance of multiple immune effectors in the context of parasite prevalence in an invasive and a native Egyptian goose ( Alopochen aegyptiacus ) population. Three of ten immune effectors measured showed higher variance in the invasive population. Mean levels were higher in the invasive population for three effectors but lower for eosinophil granulocytes. Parasite prevalence depended on the parasite taxa investigated. We suggest that variation of specific immune effectors, which may be important for invasion success, may lead to higher variance and enable invasive species to reduce the overall physiological cost of immunity while maintaining the ability to efficiently defend against novel parasites encountered.

Proliferative kidney disease (PKD) is a widespread disease caused by the endoparasite (Myxozoa: Malacosporea). Clinical disease, provoked by the proliferation of extrasporogonic parasite stages, is characterized by a chronic kidney pathology with underlying transcriptional changes indicative of altered B cell responses and dysregulated T-helper cell-like activities. Despite the relevance of PKD to European and North American salmonid aquaculture, no studies, to date, have focused on further characterizing the B cell response during the course of this disease. Thus, in this work, we have studied the behavior of diverse B cell populations in rainbow trout ( ) naturally infected with at different stages of preclinical and clinical disease. Our results show a clear upregulation of all trout immunoglobulins (Igs) (IgM, IgD, and IgT) demonstrated by immunohistochemistry and Western blot analysis, suggesting the alteration of diverse B cell populations that coexist in the infected kidney. Substantial changes in IgM, IgD, and IgT repertoires were also identified throughout the course of the disease further pointing to the involvement of the three Igs in PKD through what appear to be independently regulated mechanisms. Thus, our results provide strong evidence of the involvement of IgD in the humoral response to a specific pathogen for the first time in teleosts. Nevertheless, it was IgT, a fish-specific Ig isotype thought to be specialized in mucosal immunity, which seemed to play a prevailing role in the kidney response to . We found that IgT was the main Ig coating extrasporogonic parasite stages, IgT B cells were the main B cell subset that proliferated in the kidney with increasing kidney pathology, and IgT was the Ig for which more significant changes in repertoire were detected. Hence, although our results demonstrate a profound dysregulation of different B cell subsets during PKD, they point to a major involvement of IgT in the immune response to the parasite. These results provide further insights into the pathology of PKD that may facilitate the future development of control strategies.

In male vertebrates, two conflicting paradigms—the energetic costs of high dominance rank and the chronic stress of low rank—have been proposed to explain patterns of immune function and parasitism. To date, neither paradigm has provided a complete explanation for status-related differences in male health. Here, we applied meta-analyses to test for correlations between male social status, immune responses and parasitism. We used an ecoimmunological framework, which proposes that males should re-allocate investment in different immune components depending on the costs of dominance or subordination. Spanning 297 analyses, from 77 studies on several vertebrate taxa, we found that most immune responses were similar between subordinate and dominant males, and neither dominant nor subordinate males consistently invested in predictable immune components. However, subordinate males displayed significantly lower delayed-type hypersensitivity and higher levels of some inflammatory cytokines than dominant males, while dominant males exhibited relatively lower immunoglobulin responses than subordinate males. Despite few differences in immunity, dominant males exhibited consistently higher parasitism than subordinate males, including protozoan blood parasites, ectoparasites and gastrointestinal helminths. We discuss our results in the context of the costs of dominance and subordination and advocate future work that measures both parasitism and immune responses in wild systems.