Rainbow trout mount a robust specific immune response upon anal administration of thymus-independent antigens

Inflammatory responses in Atlantic lumpfish (Cyclopterus lumpus L.) after intraperitoneal injection of a vaccine against Aeromonas salmonicida and Vibrio salmonicida at different water temperatures

Studying inflammatory responses induced by vaccination can contribute to a more detailed understanding of underlying immune mechanisms in lumpfish (Cyclopterus lumpus). Tissue samples from lumpfish intraperitoneally immunized with a divalent oil-adjuvanted vaccine (Aeromonas salmonicida and Vibrio salmonicida) at water temperatures of 5, 10, and 15°C were collected at 630 day degrees and 18 weeks post injection. The relative amount of secretory and membrane-bound immunoglobulin M (IgM) gene transcripts in the head kidney was determined by qPCR. Vaccine-induced inflammatory lesions were assessed on histological sections of abdominal pancreatic/intestinal tissue from vaccinated fish in all three temperature groups. Inflammatory cells forming dense aggregations in lesions showed proliferative activity, many of which were identified as eosinophilic-granulocyte-like cells. IgM+ cells were scattered in inflammatory tissue dominated by connective tissue, showing no difference in numbers between lesions from fish vaccinated at 5, 10, and 15°C. Relative gene expression analysis of secretory and membrane-bound IgM revealed low overall expression in the head kidney of vaccinated fish at both 630 day-degrees and 18 weeks post injection. The results of this study indicate that the vaccine stimulated prolonged local inflammatory responses at the injection site, which were not influenced by temperature.

Molecular characterization of the B lymphocyte-induced maturation protein-1 (blimp1) gene of common carp (Cyprinus carpio) and its transcription repression involves recruitment of histone deacetylase HDAC3

Blimp1 is the master regulator of B cell terminal differentiation in mammals, it inhibits expression of many transcription factors including bcl6, which provides the basis for promoting further development of activated B lymphocytes into plasma cells. Blimp-1 is thought to act as a sequence-specific recruitment factor for chromatin-modifying enzymes including histone deacetylases (HDAC) and methyltransferases to repress target genes. The cDNA of Ccblimp1a (Cyprinus carpio) open reading frame is 2337 bp encoding a protein of 777 amino acids. CcBlimp1a contains a SET domain, two Proline Rich domains, and five ZnF_C2H2 domains. Blimp1 are conserved in vertebrate species. Ccblimp1a transcripts were detected in common carp larvae from 1 dpf (day post fertilization)to 31 dpf. Ccblimp1a expression was up-regulated in peripheral blood leukocytes (PBL) and spleen leukocytes (SPL) of common carp stimulated by intraperitoneal lipopolysaccharide (LPS) injection. Ccblimp1a expression in PBL and SPL of common carp was induced by TNP-LPS and TNP-KLH. The results indicated TNP-LPS induced a rapid response in PBL and TNP-KLH induced much stronger response in SPL and PBL. IHC results showed that CcBlimp1 positive cells were distributed in the head kidney, trunk kidney, liver, and gut. Immunofluorescence stain results showed that CcBlimp1 was expressed in IgM + lymphocytes. The subcellular localization of CcBlimp1 in the nuclei indicated CcBlimp1 may be involved in the differentiation of IgM + lymphocytes. Further study focusing on the function of CcBlimp1 transcriptional repression was performed using dual luciferase assay. The results showed that the transcription repression of CcBlimp1 on bcl6aa promoter was affected by the histone deacetylation inhibitor and was synergized with histone deacetylase 3 (HDAC3). The results of Co-IP in HEK293T and immunoprecipitation in SPL indicated that CcBlimp1 recruited HDAC3 and might be involved in the formation of complexes. These results suggest that CcBlimp1 is an important transcription factor in common carp lymphocytes. Histone deacetylation modification mediated by HDAC3 may have important roles in CcBlimp1 transcriptional repression during the differentiation of lymphocytes.

L-methionine supplementation modulates IgM+ B cell responses in rainbow trout

The interest in dietary amino acids (AAs) as potential immunomodulators has been growing the recent years, since specific AAs are known to regulate key metabolic pathways of the immune response or increase the synthesis of some immune-related proteins. Methionine, tryptophan and lysine are among the ten essential AAs for fish, meaning that they cannot be produced endogenously and must be provided through the diet. To date, although dietary supplementation of fish with some of these AAs has been shown to have positive effects on some innate immune parameters and disease resistance, the effects that these AAs provoke on cells of the adaptive immune system remained unexplored. Hence, in the current study, we have investigated the effects of these three AAs on the functionality of rainbow trout (Oncorhynchus mykiss) IgM+ B cells. For this, splenic leukocytes were isolated from untreated adult rainbow trout and incubated in culture media additionally supplemented with different doses of methionine, tryptophan or lysine in the presence or absence of the model antigen TNP-LPS (2,4,6-trinitrophenyl hapten conjugated to lipopolysaccharide). The survival, IgM secreting capacity and proliferation of IgM+ B cells was then studied. In the case of methionine, the phagocytic capacity of IgM+ B cells was also determined. Our results demonstrate that methionine supplementation significantly increases the proliferative effects provoked by TNP-LPS and also up-regulates the number of cells secreting IgM, whereas tryptophan or lysine have either minor or even negative effects on rainbow trout IgM+ B cells. This increase in the number of IgM-secreting cells in response to methionine surplus was further verified in a feeding experiment, in which the beneficial effects of methionine on the specific response to anal immunization were also confirmed. The results presented demonstrate the beneficial effects of dietary supplementation with methionine on the adaptive immune responses of fish.

Regulatory roles of cytokines in T and B lymphocytes-mediated immunity in teleost fish

T and B lymphocytes (T and B cells) are immune effector cells that play critical roles in adaptive immunity and defend against external pathogens in most vertebrates, including teleost fish. In mammals, the development and immune response of T and B cells is associated with cytokines including chemokines, interferons, interleukins, lymphokines, and tumor necrosis factors during pathogenic invasion or immunization. Given that teleost fish have evolved a similar adaptive immune system to mammals with T and B cells bearing unique receptors (B-cell receptors (BCRs) and T-cell receptors (TCRs)) and that cytokines in general have been identified, whether the regulatory roles of cytokines in T and B cell-mediated immunity are evolutionarily conserved between mammalians and teleost fish is a fascinating question. Thus, the purpose of this review is to summarize the current knowledge of teleost cytokines and T and B cells as well as the regulatory roles of cytokines on these two types of lymphocytes. This may provide important information on the parallelisms and dissimilarities of the functions of cytokines in bony fish versus higher vertebrates, which may aid in the evaluation and development of adaptive immunity-based vaccines or immunostimulants.

Teleost fish IgM+ plasma-like cells possess IgM-secreting, phagocytic, and antigen-presenting capacities

Plasma cells are terminally differentiated antibody-secreting B lymphocytes that contribute to humoral immunity by producing large numbers of antibodies. Increasing evidence suggests that teleost fish B cells share certain characteristics with mammalian B1 B cells, including antibody-secreting, phagocytic, and antigen-presenting capacities. However, the difference between mature B cells and plasma cells remains unclear. In this study, we found that, based on their light-scattering characteristics, tilapia anterior kidney (AK) leukocytes can be categorized into two IgM⁺ B-cell subsets: the lymphoid (L) gate and granulocyte–monocyte/macrophage (G-M) subsets. G-M gate cells are more numerous than L-gate cells and have higher mean fluorescence, but lower forward scatter and side scatter. We analyzed the morphological and ultrastructural features of sorted IgM⁺ cells and found that L-gate IgM⁺ cells have a high nucleus–cytoplasm ratio and lymphocyte-like morphology, whereas G-M gate IgM⁺ cells have a small nucleus, more abundant endoplasmic reticulum, and a larger number of mitochondria, and have a plasma cell-like or macrophage-like morphology. To further characterize the cell types, we examined the specific patterns of expression of B-cell- and T-cell-related genes. We found that B-cell-specific genes were expressed by both L-gate and G-M gate IgM⁺ cells, and that G-M gate IgM⁺ cells secreted extremely high levels of IgM. However, T-cell-related genes were highly expressed only in L-gate IgM^– cells. These results suggest that G-M gate IgM⁺ cells are similar to plasma-like cells, with high antibody-secreting capacity. Given that G-M gate cells include the granulocyte, monocyte, and macrophage cell types, but not B cells, monocyte/macrophage markers were used to investigate the cell types further. A macrophage receptor with a collagenous structure was frequently observed, and macrophage-expressed gene-1 was highly expressed, in the G-M gate IgM⁺ cells. Phagocytic capacity, as determined by ingestion of beads or bacteria, was significantly higher in G-M gate IgM⁺ cells than in L-gate IgM⁺ cells, as was antigen-processing capacity. Our findings show that tilapia AK leukocytes can be divided into two IgM⁺ B-cell subsets and that G-M gate IgM⁺ cells resemble plasma-like cells, having high antibody-secreting, phagocytic, and antigen-presenting capacities. Thus, this study increases our understanding of the functions of teleost fish plasma-like cells.

Conserved Role of mTORC1 Signaling in B Cell Immunity in Teleost Fish

Mammalian studies have demonstrated that B cell immune responses are regulated by mechanistic target of rapamycin complex 1 (mTORC1) signaling. Teleost fish represent the oldest living bony vertebrates that contain bona fide B cells. So far, whether the regulatory mechanism of mTORC1 signaling in B cells occurred in teleost fish is still unknown. In this study, we developed a fish model by using rapamycin (RAPA) treatment to inhibit mTORC1 signaling and demonstrated the role of mTORC1 signaling in teleost B cells. In support, we found inhibition of mTORC1 signaling by RAPA decreased the phagocytic capacity, proliferation, and Ig production of B cells. Critically, Flavobacterium columnare induced specific IgM binding in serum, and these titers were significantly inhibited by RAPA treatment, thus decreasing Ab-mediated agglutination of F. columnare and significantly increasing the susceptibility of fish upon F. columnare reinfection. Collectively, our findings elucidated that the mTORC1 pathway is evolutionarily conserved in regulating B cell responses, thus providing a new point for understanding the B cells functions in teleost fish.

Functional and Phenotypic Characterization of B Cells in the Teleost Adipose Tissue

The immune response of the adipose tissue (AT) has been neglected in most animal models until investigations in human and mice linked obesity to chronic inflammation, highlighting the immune nature of this tissue. Despite this, in teleost fish, only a few studies have addressed the immune role of the AT. These studies have mostly focused on reporting transcriptional changes in the AT in response to diverse intraperitoneally delivered stimuli. Although the presence of B cells within the AT was also previously revealed, these cells have never been phenotypically or functionally characterized and this is what we have addressed in the current study. Initially, the B cell populations present in the rainbow trout (Oncorhynchus mykiss) AT were characterized in comparison to B cells from other sources. As occurs in other rainbow trout tissues, IgM⁺IgD⁺, IgM⁺IgD^- and IgD⁺IgM^- B cell subsets were identified in the AT. Interestingly, AT IgM⁺IgD^- B cells showed a transcriptional profile that agrees with that of cells that have committed to plasmablasts/plasma cells, being this profile much more pronounced towards a differentiation state than that of blood IgM⁺IgD^- B cells. Accordingly, the IgM-secreting capacity of AT B cells is significantly higher than that of blood B cells. Additionally, AT IgM⁺IgD⁺ B cells also showed specific phenotypic traits when compared to their counterparts in other tissues. Finally, we established how these B cell subsets responded when rainbow trout were intraperitoneally injected with a model antigen. Our results demonstrate that the AT hosts plasmablasts/plasma cells that secrete specific IgMs, as happens in the peritoneal cavity and systemic immune tissues. Although the presence of these antigen-specific IgM-secreting cells was more abundant in the peritoneal cavity, these specific differentiated B cells were detected in the AT for long time periods at levels similar to those of spleen and head kidney. Our results provide new evidence regarding the immune role of the teleost AT, indicating that it functions as a secondary lymphoid organ that promotes immunity to peritoneal antigens.

Lympho-Hematopoietic Microenvironments and Fish Immune System

In the last 50 years information on the fish immune system has increased importantly, particularly that on species of marked commercial interest (i.e., salmonids, cods, catfish, sea breams), that occupy a key position in the vertebrate phylogenetical tree (i.e., Agnatha, Chondrichtyes, lungfish) or represent consolidated experimental models, such as zebrafish or medaka. However, most obtained information was based on genetic sequence analysis with little or no information on the cellular basis of the immune responses. Although jawed fish contain a thymus and lympho-hematopoietic organs equivalents to mammalian bone marrow, few studies have accounted for the presumptive relationships between the organization of these cell microenvironments and the known immune capabilities of the fish immune system. In the current review, we analyze this topic providing information on: (1) The origins of T and B lymphopoiesis in Agnatha and jawed fish; (2) the remarkable organization of the thymus of teleost fish; (3) the occurrence of numerous, apparently unrelated organs housing lympho-hematopoietic progenitors and, presumably, B lymphopoiesis; (4) the existence of fish immunological memory in the absence of germinal centers.

RNA-Seq of Single Fish Cells - Seeking Out the Leukocytes Mediating Immunity in Teleost Fishes

The immune system is a complex and sophisticated biological system, spanning multiple levels of complexity, from the molecular level to that of tissue. Our current understanding of its function and complexity, of the heterogeneity of leukocytes, is a result of decades of concentrated efforts to delineate cellular markers using conventional methods of antibody screening and antigen identification. In mammalian models, this led to in-depth understanding of individual leukocyte subsets, their phenotypes, and their roles in health and disease. The field was further propelled forward by the development of single-cell (sc) RNA-seq technologies, offering an even broader and more integrated view of how cells work together to generate a particular response. Consequently, the adoption of scRNA-seq revealed the unexpected plasticity and heterogeneity of leukocyte populations and shifted several long-standing paradigms of immunology. This review article highlights the unprecedented opportunities offered by scRNA-seq technology to unveil the individual contributions of leukocyte subsets and their crosstalk in generating the overall immune responses in bony fishes. Single-cell transcriptomics allow identifying unseen relationships, and formulating novel hypotheses tailored for teleost species, without the need to rely on the limited number of fish-specific antibodies and pre-selected markers. Several recent studies on single-cell transcriptomes of fish have already identified previously unnoticed expression signatures and provided astonishing insights into the diversity of teleost leukocytes and the evolution of vertebrate immunity. Without a doubt, scRNA-seq in tandem with bioinformatics tools and state-of-the-art methods, will facilitate studying the teleost immune system by not only defining key markers, but also teaching us about lymphoid tissue organization, development/differentiation, cell-cell interactions, antigen receptor repertoires, states of health and disease, all across time and space in fishes. These advances will invite more researchers to develop the tools necessary to explore the immunology of fishes, which remain non-conventional animal models from which we have much to learn.

CD38 Defines a Subset of B Cells in Rainbow Trout Kidney With High IgM Secreting Capacities

CD38 is a multifunctional molecule that functions both as a transmembrane signaling receptor and as an ectoenzyme with important roles in cell adhesion, calcium regulation and signal transduction. Within the B cell linage, CD38 is expressed in diverse murine B cell subsets, with highest levels in innate B cell subpopulations such as marginal zone (MZ) B cells or B1 cells. In humans, however, CD38 is transiently expressed on early lymphocyte precursors, is lost on mature B cells and is consistently expressed on terminally differentiated plasma cells. In the present work, we have identified two homologues of mammalian CD38 in rainbow trout (Oncorhynchus mykiss), designating them as CD38A and CD38B. Although constitutively transcribed throughout different tissues in homeostasis, both CD38A and CD38B mRNA levels were significantly up-regulated in head kidney (HK) in response to a viral infection. In this organ, after the generation of a specific monoclonal antibody (mAb) against CD38A, the presence of CD38A⁺ populations among IgM⁺ B cells and IgM^- leukocytes was investigated by flow cytometry. Interestingly, the percentage of IgM⁺CD38A⁺ B cells increased in response to an in vitro stimulation with inactivated Aeromonas salmonicida. Finally, we demonstrated that HK IgM⁺CD38A⁺ B cells had an increased IgM secreting capacity than that of cells lacking CD38A on the cell surface, also showing increased transcription levels of genes associated with B cell differentiation. This study strongly suggests a role for CD38 on the B cell differentiation process in teleosts, and provides us with novel tools to discern between B cell subsets in these species.

Sinomenine Hydrochloride Ameliorates Fish Foodborne Enteritis via α7nAchR-Mediated Anti-Inflammatory Effect Whilst Altering Microbiota Composition

Foodborne intestinal inflammation is a major health and welfare issue in aquaculture. To prevent enteritis, various additives have been incorporated into the fish diet. Considering anti-inflammatory immune regulation, an effective natural compound could potentially treat or prevent intestinal inflammation. Our previous study has revealed galantamine’s effect on soybean induced enteritis (SBMIE) and has highlighted the possible role of the cholinergic anti-inflammatory pathway in the fish gut. To further activate the intestinal cholinergic related anti-inflammatory function, α7nAchR signaling was considered. In this study, sinomenine, a typical agonist of α7nAChR in mammals, was tested to treat fish foodborne enteritis via its potential anti-inflammation effect using the zebrafish foodborne enteritis model. After sinomenine’s dietary inclusion, results suggested that there was an alleviation of intestinal inflammation at a pathological level. This outcome was demonstrated through the improved morphology of intestinal villi. At a molecular level, SN suppressed inflammatory cytokines’ expression (especially for tnf-α) and upregulated anti-inflammation-related functions (indicated by expression of il-10, il-22, and foxp3a). To systematically understand sinomenine’s intestinal effect on SBMIE, transcriptomic analysis was done on the SBMIE adult fish model. DEGs (sinomenine vs soybean meal groups) were enriched in GO terms related to the negative regulation of lymphocyte/leukocyte activation and alpha-beta T cell proliferation, as well as the regulation of lymphocyte migration. The KEGG pathways for glycolysis and insulin signaling indicated metabolic adjustments of α7nAchR mediated anti-inflammatory effect. To demonstrate the immune cells’ response, in the SBMIE larva model, inflammatory gatherings of neutrophils, macrophages, and lymphocytes caused by soybean meal could be relieved significantly with the inclusion of sinomenine. This was consistent within the sinomenine group as CD4⁺ or Foxp3⁺ lymphocytes were found with a higher proportion at the base of mucosal folds, which may suggest the Treg population. Echoing, the sinomenine group’s 16s sequencing result, there were fewer enteritis-related TM7, Sphingomonas and Shigella, but more Cetobacterium, which were related to glucose metabolism. Our findings indicate that sinomenine hydrochloride could be important in the prevention of fish foodborne enteritis at both immune and microbiota levels.

Mucosal immunoglobulins of teleost fish: A decade of advances

Immunoglobulins (Igs) are complex glycoproteins that play critical functions in innate and adaptive immunity of all jawed vertebrates. Given the unique characteristics of mucosal barriers, secretory Igs (sIgs) have specialized to maintain homeostasis and keep pathogens at bay at mucosal tissues from fish to mammals. In teleost fish, the three main IgH isotypes, IgM, IgD and IgT/Z can be found in different proportions at the mucosal secretions of the skin, gills, gut, nasal, buccal, and pharyngeal mucosae. Similar to the role of mammalian IgA, IgT plays a predominant role in fish mucosal immunity. Recent studies in IgT have illuminated the primordial role of sIgs in both microbiota homeostasis and pathogen control at mucosal sites. Ten years ago, IgT was discovered to be an immunoglobulin class specialized in mucosal immunity. Aiming at this 10-year anniversary, the goal of this review is to summarize the current status of the field of fish Igs since that discovery, while identifying knowledge gaps and future avenues that will move the field forward in both basic and applied science areas.

Oral Probiotic Vaccine Expressing Koi Herpesvirus (KHV) ORF81 Protein Delivered by Chitosan-Alginate Capsules Is a Promising Strategy for Mass Oral Vaccination of Carps against KHV Infection

Koi herpesvirus (KHV) is highly contagious and lethal to cyprinid fish, causing significant economic losses to the carp aquaculture industry, particularly to koi carp breeders. Vaccines delivered through intramuscular needle injection or gene gun are not suitable for mass vaccination of carp. So, the development of cost-effective oral vaccines that are easily applicable at a farm level is highly desirable. In this study, we utilized chitosan-alginate capsules as an oral delivery system for a live probiotic (Lactobacillus rhamnosus) vaccine, pYG-KHV-ORF81/LR CIQ249, expressing KHV ORF81 protein. The tolerance of the encapsulated recombinant Lactobacillus to various digestive environments and the ability of the probiotic strain to colonize the intestine of carp was tested. The immunogenicity and the protective efficacy of the encapsulated probiotic vaccine was evaluated by determining IgM levels, lymphocyte proliferation, expression of immune-related genes, and viral challenge to vaccinated fish. It was clear that the chitosan-alginate capsules protected the probiotic vaccine effectively against extreme digestive environments, and a significant level (P < 0.01) of antigen-specific IgM with KHV-neutralizing activity was detected, which provided a protection rate of ca. 85% for koi carp against KHV challenge. The strategy of using chitosan-alginate capsules to deliver probiotic vaccines is easily applicable for mass oral vaccination of fish.IMPORTANCE An oral probiotic vaccine, pYG-KHV-ORF81/LR CIQ249, encapsulated by chitosan-alginate capsules as an oral delivery system was developed for koi carp against koi herpesvirus (KHV) infection. This encapsulated probiotic vaccine can be protected from various digestive environments and maintain effectively high viability, showing a good tolerance to digestive environments. This encapsulated probiotic vaccine has a good immunogenicity in koi carp via oral vaccination, and a significant level of antigen-specific IgM was effectively induced after oral vaccination, displaying effective KHV-neutralizing activity. This encapsulated probiotic vaccine can provide effective protection for koi carp against KHV challenge, which is handling-stress free for the fish, cost effective, and suitable for the mass oral vaccination of koi carp at a farm level, suggesting a promising vaccine strategy for fish.

Systemic and Mucosal B and T Cell Responses Upon Mucosal Vaccination of Teleost Fish

The development of mucosal vaccines against pathogens is currently a highly explored area of research in both humans and animals. This is due to the fact that mucosal vaccines have the potential to best elicit protective responses at these mucosal surfaces, which represent the frontline of host defense, thus blocking the pathogen at its initial replication sites. However, in order to provide an efficient long-lasting protection, these mucosal vaccines have to be capable of eliciting an adequate systemic immune response in addition to local responses. In aquaculture, the need for mucosal vaccines has further practical implications, as these vaccines would avoid the individual manipulation of fish out of the water, being beneficial from both an economic and animal welfare point of view. However, how B and T cells are organized in teleost fish within these mucosal sites and how they respond to mucosally delivered antigens varies greatly when compared to mammals. For this reason, it is important to establish which mucosally delivered antigens have the capacity to induce strong and long-lasting B and T cell responses. Hence, in this review, we have summarized what is currently known regarding the adaptive immune mechanisms that are induced both locally and systemically in fish after mucosal immunization through different routes of administration including oral and nasal vaccination, anal intubation and immersion vaccination. Finally, based on the data presented, we discuss how mucosal vaccination strategies could be improved to reach significant protection levels in these species.

Insights Into the Evolution of the prdm1/Blimp1 Gene Family in Teleost Fish

In mammals, Blimp1 (B lymphocyte-induced maturation protein 1) encoded by the prdm1 gene and its homolog Hobit (homolog of Blimp1 in T cells) encoded by znf683, represent key transcriptional factors that control the development and differentiation of both B and T cells. Despite their essential role in the regulation of acquired immunity, this gene family has been largely unexplored in teleosts to date. Until now, one prdm1 gene has been identified in most teleost species, whereas a znf683 homolog has not yet been reported in any of these species. Focusing our analysis on rainbow trout (Oncorhynchus mykiss), an in silico identification and characterization of prdm1-like genes has been undertaken, confirming that prdm1 and znf683 evolved from a common ancestor gene, acquiring three gene copies after the teleost-specific whole genome duplication event (WGD) and six genes after the salmonid-specific WGD. Additional transcriptional studies to study how each of these genes are regulated in homeostasis, in response to a viral infection or in B cells in different differentiation stages, provide novel insights as to how this gene family evolved and how their encoded products might be implicated in the lymphocyte differentiation process in teleosts.

Immunoglobulins, Mucosal Immunity and Vaccination in Teleost Fish

Due to direct contact with aquatic environment, mucosal surfaces of teleost fish are continuously exposed to a vast number of pathogens and also inhabited by high densities of commensal microbiota. The B cells and immunoglobulins within the teleost mucosa-associated lymphoid tissues (MALTs) play key roles in local mucosal adaptive immune responses. So far, three Ig isotypes (i.e., IgM, IgD, and IgT/Z) have been identified from the genomic sequences of different teleost fish species. Moreover, teleost Igs have been reported to elicit mammalian-like mucosal immune response in six MALTs: gut-associated lymphoid tissue (GALT), skin-associated lymphoid tissue (SALT), gill-associated lymphoid tissue (GIALT), nasal-associated lymphoid tissue (NALT), and the recently discovered buccal and pharyngeal MALTs. Critically, analogous to mammalian IgA, teleost IgT represents the most ancient Ab class specialized in mucosal immunity and plays indispensable roles in the clearance of mucosal pathogens and the maintenance of microbiota homeostasis. Given these, this review summarizes the current findings on teleost Igs, MALTs, and their immune responses to pathogenic infection, vaccination and commensal microbiota, with the purpose of facilitating future evaluation and rational design of fish vaccines.

Application of Outer Membrane Protein-Based Vaccines Against Major Bacterial Fish Pathogens in India

Aquaculture is one of the fastest-growing food-producing sectors in the world. However, its growth is hampered by various disease problems due to infectious microorganisms, including Gram-negative bacteria in finfish aquaculture. Disease control in aquaculture by use of antibiotics is not recommended as it leads to antibiotic residues in the final product, selection, and spread of antibiotic resistance in the environment. Therefore, focus is on disease prevention by vaccination. All Gram-negative bacteria possess surface-associated outer membrane proteins (OMPs), some of which have long been recognized as potential vaccine candidates. OMPs are essential for maintaining the integrity and selective permeability of the bacterial membrane and play a key role in adaptive responses of bacteria such as solute and ion uptake, iron acquisition, antimicrobial resistance, serum resistance, and bile salt resistance and some adhesins have virulence attributes. Antigenic diversity among bacterial strains even within the same bacterial species has constrained vaccine developments, but OMPs that are conserved across serotypes could be used as potential candidates in vaccine development, and several studies have demonstrated their efficacy and potential as vaccine candidates. In this review, we will look into the application of OMPs for the design of vaccines based on recombinant proteins, subunit vaccines, chimeric proteins, and DNA vaccines as new-generation vaccine candidates for major bacterial pathogens of fish for sustainable aquaculture.