DNA vaccination against a fish rhabdovirus promotes an early chemokine-related recruitment of B cells to the muscle

Preparation of the monoclonal antibody against Nile tilapia Igλ and study on the Igλ+ B cell subset in Nile tilapia

Immunoglobulins (Igs) are important effector molecules that mediate humoral immunity. A typical Ig consists of two heavy and two light chains. In teleosts, three Ig heavy chain isotypes (Igμ, Igδ and Igτ) and three Ig light chain isotypes (Igκ, Igλ and Igσ) have been identified. Compared to the heavy chains, teleost Ig light chains have been poorly studied due to the lack of antibodies. In this study, a mouse anti-Nile tilapia Igλ monoclonal antibody (mAb) was prepared, which could specifically recognize Igλ in serum and Igλ+ B cells in tissues. Further, the composition of IgM+ and Igλ+ B cell subsets was analyzed using this antibody and a mouse anti-tilapia IgM heavy chain mAb. The ratio of IgM+Igλ+ B cells to total IgM+ B cells in head kidney and peripheral blood was about 30%, while that in spleen was about 50%; the ratio of IgM-Igλ+ B cells to total Igλ+ B cells in head kidney and peripheral blood was about 45%, while that in spleen was about 25%. The IgM-Igλ+ B cells was speculated to be IgT+ B cells. Finally, we detected an increase in the level of specific antibodies against the surface antigen-Sip of Streptococcus agalactiae in serum after S. agalactiae infection, indicating that mouse anti-tilapia Igλ mAb can be used to detect the antibody level after immunization of Nile tilapia, which lays a foundation for the evaluation of immunization effect of tilapia vaccine.

Comparative genomics studies on the stk gene family in vertebrates: From the bighead carp (Hypophthalmichthys nobilis) genome

The mammalian sterile 20-like (MST) family belongs to the serine/threonine protein kinase (STK) superfamily and participates in a variety of biological processes, such as cell apoptosis, polarity, migration, immune regulation, inflammatory responses, and cancer. In the economically important bighead carp (Hypophthalmichthys nobilis), the STK gene family and immune-related biological functions may be helpful in increasing its economic yield. However, the comprehensive role of STKs in the bighead carp remains unclear. In this study, the five stk sequences from the bighead carp were divided into two classes: stk3/4 and stk24/25/26. Gene structure and motif prediction analyses confirmed that stk is conserved in the bighead carp. Compared to 26 other vertebrate species, teleosts (including bighead carp) possess more stk members because of teleost-specific whole-genome duplication. Synteny analysis revealed that stk3, stk24, stk25, and stk26 have been relatively conserved in bighead carp during evolution. Meanwhile, stk4 was lost in most Cyprinid species, including bighead carp, during evolution. RNA-seq data revealed that STK expression was associated with various pathogens, and the expression of these STKs (Hnstk3, Hnstk24a, Hnstk24b, Hnstk25, and Hnstk26) was different in seven tissues of bighead carp. In addition, we showed that STK expression levels were dramatically altered in the head kidney and that stk24 was involved in defense against Aeromonas hydrophila. This study provides a molecular basis for the analysis of stk function in bighead carp, and can be used as a reference for further phylogenomics.

Oral vaccination for sustainable disease prevention in aquaculture-an encapsulation approach

The prevalence of infectious diseases in the aquaculture industry and a limited number of safe and effective oral vaccines has imposed a challenge not only for fish immunity but also a threat to human health. The availability of fish oral vaccines has expanded recently, but little is known about how well they work and how they affect the immune system. The unsatisfactory efficacy of existing oral vaccinations is partly attributable to the antigen degradation in the adverse gastrointestinal environment of fishes, the highly tolerogenic gut environment, and inferior vaccine formulation. To overcome such challenges in designing: an easier, cost-efficient, and effective vaccination method, several encapsulation methods are being adopted to safeguard antigens from the intestinal atmosphere for their immunogenic functions. Oral vaccination is easily degraded by gastric acids and enzymes before reaching the immunological site; however, this issue can be solved by encapsulating antigens in poly-biodegradable nanoparticles, transgenic designed bacteria, plant systems, and live feeds. To enhance the immunological impact, each antigen delivery method operates at a different level. Utilizing nanotechnology, it has been possible to regulate vaccination parameters, target particular cells, and lower the antigen dosage with potent nanomaterials such as chitosan, poly D,L-lactic-co-glycolic acid (PLGA) as vaccine carriers. Live feeds such as Artemia salina can be utilized as bio-carrier, owing to their appropriate size and non-filter feed system, through a process called bio-encapsulation. It ensures the protection of antigens over the fish intestine and ensures complete uptake by immune cells in the hindgut for increased immune response. This review comprises recent advances in oral vaccination in aquaculture in terms of an encapsulation approach that can aid in future research.

DNA vaccine dual-expressing viral hemorrhagic septicemia virus glycoprotein and C-C motif chemokine ligand 19 induces the expression of immune-related genes in zebrafish (Danio rerio)

Glycoprotein (G protein)-based DNA vaccines are effective in protecting aquaculture fish from rhabdoviruses but the degree of immune response they elicit depends on plasmid concentration and antigen cassette. Here, we developed a DNA vaccine using the viral hemorrhagic septicemia virus G (VG) gene and chemokine (C-C motif) ligand 19 (CCL19)a.2 regulated by the CMV promoter as the molecular adjuvant. After transfection of the prepared plasmid (pVG + CCL19) into epithelioma papulosum cyprini cells, mRNA expression was confirmed through quantitative real-time polymerase chain reaction. The vaccine was intramuscularly injected into zebrafish (Danio rerio), and 28 days after immunization, viral hemorrhagic septicemia virus (10⁵ TCID₅₀/10 µl/fish) was intraperitoneally injected. A survival rate of 68% was observed in the pVG + CCL19 group but this was not significantly different from the survival rate of fish treated with pVG alone, that is, without the adjuvant. However, the expression of interferon- and cytokine-related genes in the spleen and kidney tissues of zebrafish was significantly increased (p < 0.05) on days 1, 3, 7, and 14 after immunization. Thus, CCL19a.2 induced an initial immune response as a molecular adjuvant, which may provide initial protection against virus infection before vaccination-induced antibody formation. This study provides insights on the functions of CCL19a.2 adjuvant in DNA vaccines.

CXCL9-11 chemokines and CXCR3 receptor in teleost fish species

The coordinated migration of immune cells from lymphoid organs to in or out of the bloodstream, and towards the site of infection or tissue damage is fundamental for an efficient innate and adaptive immune response. Interestingly, an essential part of this movement is mediated by chemoattractant cytokines called chemokines. Although the nature and function of chemokines and their receptors are well documented in mammals, much research is needed to accomplish a similar level of understanding of the role of chemokines in fish immunity. The first chemokine gene identified in teleosts (rainbow trout, Oncorhynchus mykiss) was CK1 in 1998. Since then, the identification of fish chemokine orthologue genes and characterization of their role has been more complex than expected, primarily because of the whole genome duplication processes occurring in fish, and because chemokines evolve faster than other immune genes. Some of the most studied chemokines are CXCL9, CXCL10, CXCL11, and the CXCR3 receptor, all involved in T cell migration and in the induction of the T helper 1 (Th1) immune response. Data from the zebrafish and rainbow trout CXCL9-11/CXCR3 axis suggest that these chemokines and the receptor arose early in evolution and must be present in most teleost fish. However, the pieces of knowledge also indicate that different numbers of gene copies can be present in different species, with distinct regulatory expression mechanisms and probably, also with different roles, as the differential expression in fish tissues suggest. Here, we revised the current knowledge of the CXCL9-11/CXCR3 axis in teleost fishes, identifying the gaps in knowledge, and raising some hypotheses for the role of CXCL9, CXCL10 CXCL11, and CXCR3 receptor axis in fish, which can encourage further studies in the field.

The Influence of Temperature on the Antiviral Response of mIgM+ B Lymphocytes Against Hirame Novirhabdovirus in Flounder (Paralichthys olivaceus)

Hirame novirhabdovirus (HIRRV) is an ongoing threat to the aquaculture industry. The water temperature for the onset of HIRRV is below 15°C, the peak is about 10°C, but no mortality is observed over 20°C. Previous studies found the positive signal of matrix protein of HIRRV (HIRRV-M) was detected in the peripheral blood leukocytes of viral-infected flounder. Flow cytometry and indirect immunofluorescence assay showed that HIRRV-M was detected in mIgM⁺ B lymphocytes in viral-infected flounder maintained at 10°C and 20°C, and 22% mIgM⁺ B lymphocytes are infected at 10°C while 13% are infected at 20°C, indicating that HIRRV could invade into mIgM⁺ B lymphocytes. Absolute quantitative RT-PCR showed that the viral copies in mIgM⁺ B lymphocytes were significantly increased at 24 h post infection (hpi) both at 10°C and 20°C, but the viral copies in 10°C infection group were significantly higher than that in 20°C infection group at 72 hpi and 96 hpi. Furthermore, the B lymphocytes were sorted from HIRRV-infected flounder maintained at 10°C and 20°C for RNA-seq. The results showed that the differentially expression genes in mIgM⁺ B lymphocyte of healthy flounder at 10°C and 20°C were mainly enriched in metabolic pathways. Lipid metabolism and Amino acid metabolism were enhanced at 10°C, while Glucose metabolism was enhanced at 20°C. In contrast, HIRRV infection at 10°C induced the up-regulation of the Complement and coagulation cascades, FcγR-mediated phagocytosis, Platelets activation, Leukocyte transendothelial migration and Natural killer cell mediated cytotoxicity pathways at 72 hpi. HIRRV infection at 20°C induced the up-regulation of the Antigen processing and presentation pathway at 72 hpi. Subsequently, the temporal expression patterns of 16 genes involved in Antigen processing and presentation pathway were investigated by qRT-PCR, and results showed that the pathway was significantly activated by HIRRV infection at 20°C but inhibited at 10°C. In conclusion, HIRRV could invade into mIgM⁺ B lymphocytes and elicit differential immune response under 10°C and 20°C, which provide a deep insight into the antiviral response in mIgM⁺ B lymphocytes.

Activation of the TCA Cycle to Provide Immune Protection in Zebrafish Immunized by High Magnesium-Prepared Vibrio alginolyticus Vaccine

Vaccines are safe and efficient in controlling bacterial diseases in the aquaculture industry and are in line with green farming. The present study develops a previously unreported approach to prepare a live-attenuated V. alginolyticus vaccine by culturing bacteria in a high concentration of magnesium to attenuate bacterial virulence. Furthermore, metabolomes of zebrafish immunized with the live-attenuated vaccines were compared with those of survival and dying zebrafish infected by V. alginolyticus. The enhanced TCA cycle and increased fumarate were identified as the most key metabolic pathways and the crucial biomarker of vaccine-mediated and survival fish, respectively. Exogenous fumarate promoted expression of il1β, il8, il21, nf-κb, and lysozyme in a dose-dependent manner. Among the five innate immune genes, the elevated il1β, il8, and lysozyme are overlapped in the vaccine-immunized zebrafish and the survival from the infection. These findings highlight a way in development of vaccines and exploration of the underlying mechanisms.

Molecular cloning and characterization of CD63 in common carp infected with koi herpesvirus

CD63 is a member of the four-transmembrane-domain protein superfamily and is the first characterized tetraspanin protein. In the present study, we cloned the common carp (Cyprinus Carpio) CD63 (ccCD63) sequence and found that the ccCD63 ORF contained 711 bp and encoded a protein of 236 amino acids. Homology analysis revealed that the complete ccCD63 sequence had 84.08% amino acid similarity to CD63 of Sinocyclocheilus anshuiensis. Subcellular localization analysis revealed that ccCD63 was localized in the cytoplasm. Quantitative real-time PCR (qRT-PCR) analysis indicated that ccCD63 was expressed in the gill, intestine, liver, spleen, brain and kidney, with higher expression in spleen and brain tissues than in the other examined tissues. After koi herpesvirus (KHV) infection, these tissues exhibited various expression levels of ccCD63. The expression level was the lowest in the liver and highest in the brain; the expression level in the brain was 8.7-fold higher than that in the liver. Furthermore, knockdown of ccCD63 promoted KHV infection. Moreover, ccCD63 was correlated with the regulation of RIG-I/MAVS/TRAF3/TBK1/IRF3 and may be involved in the antiviral response through the RIG-I viral recognition signalling pathway in a TRAF3/TBK1-dependent manner. Taken together, our results suggested that ccCD63 upregulated the interaction of KHV with the host immune system and suppressed the dissemination of KHV.

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.

Diel Variation in CC Chemokine Gene Expression in the Japanese Pufferfish Takifugu rubripes

CC chemokines are key molecules in the regulation of leukocyte trafficking to the site of injury, infection, or inflammation. In recent years, some mammalian chemokines have been shown to exhibit rhythmic expression, regulated by clock genes. However, the rhythmic expression of chemokines in teleost fish remains unknown. In the present study, the diel variation of teleost CC chemokine genes was investigated using the model fish, Fugu (Takifugu rubripes). Diel variation analysis revealed that clock (bmal1, clock1, per2, rorα, and rev-erbβ) and CC chemokine (ccl18l, ccl19, and ccl25l) genes show diel expression under 12:12 light-dark cycle (LD12:12) conditions. CC chemokine genes, which exhibit diel expression, contain RORE (ccl18l, ccl19, ccl25l) and/or E-box (ccl25l) motifs in their transcription regulatory region. Moreover, in vitro head kidney stimulation with lipopolysaccharide (LPS) at different zeitgeber times (ZT) under LD12:12 conditions affected the degree of ccl18l, ccl19, and ccl25l expression; high and low responsiveness to LPS stimulation at ZT12 and ZT0 (ccl25l), and ZT16 and ZT4 (ccl18l and ccl19), respectively, were observed. These results suggest that the expression of some fish CC chemokines is affected by the diel variation regulated by clock proteins, and that responsiveness against bacterial infection depends on the time zone.

IgM+ and IgT+ B Cell Traffic to the Heart during SAV Infection in Atlantic Salmon

B cells of teleost fish differentiate in the head kidney, and spleen, and either remain in the lymphatic organs or move to the blood and peripheral tissues. There is limited knowledge about piscine B cell traffic to sites of vaccination and infection and their functional roles at these sites. In this work, we examined the traffic of B cells in Atlantic salmon challenged with salmonid alphavirus (SAV). In situ hybridization (RNAScope) showed increased numbers of immunoglobin (Ig)M+ and IgT+ B cells in the heart in response to SAV challenge, with IgM+ B cells being most abundant. An increase in IgT+ B cells was also evident, indicating a role of IgT+ B cells in nonmucosal tissues and systemic viral infections. After infection, B cells were mainly found in the stratum spongiosum of the cardiac ventricle, colocalizing with virus-infected myocardial-like cells. From sequencing the variable region of IgM in the main target organ (heart) and comparing it with a major lymphatic organ (the spleen), co-occurrence in antibody repertoires indicated a transfer of B cells from the spleen to the heart, as well as earlier recruitment of B cells to the heart in vaccinated fish compared to those that were unvaccinated. Transcriptome analyses performed at 21 days post-challenge suggested higher expression of multiple mediators of inflammation and lymphocyte-specific genes in unvaccinated compared to vaccinated fish, in parallel with a massive suppression of genes involved in heart contraction, metabolism, and development of tissue. The adaptive responses to SAV in vaccinated salmon appeared to alleviate the disease. Altogether, these results suggest that migration of B cells from lymphatic organs to sites of infection is an important part of the adaptive immune response of Atlantic salmon to SAV.

Chilean aquaculture and the new challenges: Pathogens, immune response, vaccination and fish diversification

In Chile, the salmon and trout farmed fishing industries have rapidly grown during the last years, becoming one of the most important economic sources for the country. However, infectious diseases caused by bacteria, virus, mycoses and parasites, result in losses of up to 700 million dollars per year for the Chilean aquaculture production with the consequent increase of antibiotic and antiparasitic usage. After 30 years of its first appearance, the main salmon health problem is still the salmonid rickettsial septicaemia (SRS), which together with other disease outbreaks, reveal that vaccines do not provide acceptable levels of long-lasting immune protection in the field. On the other hand, due to the large dependence of the industry on salmonids production, the Chilean government promoted the Aquaculture diversification program by 2009, which includes new species such as Merluccius australis, Cilus gilberti and Genypterus chilensis, however, specific research regarding the immune system and vaccine development are issues that still need to be addressed and must be considered as important as the farm production technologies for new fish species. Based on the experience acquired from the salmonid fish farming, should be mandatory an effort to study the immune system of the new species to develop knowledge for vaccination approaches, aiming to protect these aquaculture species before diseases outbreaks may occur. This review focuses on the current status of the Chilean aquaculture industry, the challenges related to emerging and re-emerging microbial pathogens on salmonid fish farming, and the resulting needs in the development of immune protection by rational designed vaccines. We also discussed about what we have learn from 25 years of salmonid researches and what can be applied to the new Chilean farmed species on immunology and vaccinology.

The effects of CCL3, CCL4, CCL19 and CCL21 as molecular adjuvants on the immune response to VAA DNA vaccine in flounder (Paralichthys olivaceus)

The magnitude of the immune response induced by DNA vaccines depends on the amount and type of antigen-presenting cells attracted to the injection site. In our previous study, a DNA plasmid encoding the VAA gene of Vibrio anguillarum was constructed and shown to confer moderate protection against V. anguillarum challenge. To augment the protective efficacy of the VAA DNA vaccine and compare the adjuvant effects of CCL3, CCL4, CCL19 and CCL21, four bicistronic DNA plasmids containing the VAA gene of V. anguillarum together with the gene encoding the CCL3/CCL4/CCL19/CCL21 chemokines of flounder were successfully constructed and administered to fish, and the immune response of the animals and the enhancement of immunoprotection by the four chemokines were investigated. Vaccinated with pCCL3-VAA, pCCL4-VAA, pCCL19-VAA and pCCL21-VAA, flounder showed relative percent survivals of 62.16%, 83.78%, 78.38% and 72.97%, respectively, higher than the relative survival of flounder immunized with pVAA (40.54%). Compared with the pVAA group, the percentages of sIgM⁺, CD4-1⁺, and CD4-2⁺ lymphocytes and the levels of specific antibodies increased in pCCL3-VAA, pCCL4-VAA, pCCL19-VAA and pCCL21-VAA injection groups; CCL4 and CCL19 induced significantly higher levels of these parameters than CCL3 and CCL21 did. The amount of V. anguillarum in liver, spleen and kidney of pCCL3-VAA-, pCCL4-VAA-, pCCL19-VAA- and pCCL21-VAA-immunized flounder after V. anguillarum challenge was reduced compared to that in the pVAA group. Moreover, the co-expression of CCL3/CCL4/CCL19/CCL21 up-regulated immune-related gene expression associated with the local immune response. Our results indicate that CCL4 and CCL19 are promising adjuvants for use in VAA DNA vaccine against V. anguillarum.

Rainbow Trout IgM+ B Cells Preferentially Respond to Thymus-Independent Antigens but Are Activated by CD40L

In the absence of class switch recombination and germinal centers, the mechanisms through which B cells from teleost fish mount extrafollicular immunoglobulin M (IgM) memory responses remains mostly unexplored. In this report, we demonstrate that teleost IgM⁺ B cells respond to CD40L, a thymus-dependent activation signal, similarly to mammalian B2 cells. However, when stimulated with different types of antigens, fish IgM⁺ B cells only reach a general activation state in response to antigens cataloged as thymus-independent 1 (TI-1) in mammals, as established through both functional assays and RNA sequencing. Interestingly, fish IgM⁺ B cells remained completely unresponsive to TI-2 antigens, suggesting that the engagement of innate receptors provided by TI-1 antigens is required for the activation of teleost B cells. Finally, a synergy between CD40L and TI-1 antigens was also demonstrated, further supporting that there is no clear dichotomy between thymus-dependent and TI responses in teleost fish.

Intramuscular vaccination of Atlantic lumpfish (Cyclopterus lumpus L.) induces inflammatory reactions and local immunoglobulin M production at the vaccine administration site

Atlantic lumpfish were vaccinated by intramuscular (im) or intraperitoneal (ip) injection with a multivalent oil-based vaccine, while control fish were injected with phosphate-buffered saline. Four lumpfish per group were sampled for skin/muscle and head kidney tissue at 0, 2, 7, 21 and 42 days post-immunization (dpi) for histopathology and immunohistochemistry (IHC). Gene expressions of secretory IgM, membrane-bound IgM, IgD, TCRα, CD3ε and MHC class IIβ were studied in tissues by using qPCR. Im. vaccinated fish showed vaccine-induced inflammation with formation of granulomas and increasing number of eosinophilic granulocyte-like cells over time. On IHC sections, we observed diffuse intercellular staining of secretory IgM at the injection site at 2 dpi, while IgM + cells appeared in small numbers at 21 and 42 dpi. Skin/muscle samples from im. vaccinated fish demonstrated an increase in gene expression of IgM mRNA (secretory and membrane-bound) at 21 and 42 dpi and small changes for other genes. Our results indicated that im. vaccination of lumpfish induced local IgM production at the vaccine injection site, with no apparent proliferation of IgM + cells. Eosinophilic granulocyte-like cells appeared shortly after im. injection and increased in numbers as the inflammation progressed.

Development of monoclonal antibody against IgM of large yellow croaker (Larimichthys crocea) and characterization of IgM+ B cells

In the present study, a monoclonal antibody (mAb) against large yellow croaker IgM was produced by immunizing mice with purified large yellow croaker serum IgM. Western blotting showed that this mAb could specifically react with the heavy chain of large yellow croaker serum IgM. Indirect immunofluorescence assay (IFA) analysis suggested that the resulting mouse anti-IgM mAb could recognize membrane-bound IgM (mIgM) molecules of large yellow croaker. This mouse anti-IgM mAb also can be used for sorting of large yellow croaker IgM⁺ B cells through the magnetic-activated cell sorting (MACS) method, which was further confirmed by RT-PCR analysis of specific marker genes for B cells. Flow cytometry analysis showed that the percentages of IgM⁺ B cells in head kidney, spleen and peripheral blood lymphocytes were 29.00 ± 1.58%, 33.00 ± 1.64%, and 16.50 ± 2.39%, respectively. Additionally, the phagocytosis rates of IgM⁺ B cells for 0.5 μm beads in head kidney, spleen and peripheral blood were calculated to be 7.56 ± 0.58%, 4.053 ± 0.62% and 23.17 ± 2.26%, respectively, while only 2.36 ± 0.23%, 1.16 ± 0.44% and 6.41 ± 0.45 of IgM⁺ B cells in these three tissues ingested 1 μm beads. Taken together, our data demonstrated that the mouse anti-IgM mAb produced in this study could be used as a tool to characterize IgM⁺ B cells and to study functions of IgM in large yellow croaker.

A DNA Vaccine Encoding the VAA Gene of Vibrio anguillarum Induces a Protective Immune Response in Flounder

Vibrio anguillarum is a pathogenic bacterium that infects flounder resulting in significant losses in the aquaculture industry. The VAA protein previously identified in flounder is associated with a role in immune protection within these fish. In the present study, a recombinant DNA plasmid encoding the VAA gene of V. anguillarum was constructed and its potential as a DNA vaccine, to prevent the infection of V. anguillarum in flounder fish, investigated. We verified the expression of the VAA protein both in vitro in cell lines and in vivo in flounder fish. The protective effects of pcDNA3.1-VAA (pVAA) were analyzed by determination of the percentage of sIgM⁺, CD4-1⁺, CD4-2⁺, CD8β⁺ lymphocytes, and the production of VAA-specific antibodies in flounder following their immunization with the DNA vaccine. Histopathological changes in immune related tissues, bacterial load, and relative percentage survival rates of flounder post-challenge with V. anguillarum, were all investigated to assess the efficacy of the pVAA DNA vaccine candidate. Fish intramuscularly immunized with pVAA showed a significant increase in CD4-1⁺, CD4-2⁺, and CD8β⁺ T lymphocytes at days 9, 11, and 14 post-vaccination, reaching peak T-cell levels at days 11 or 14 post-immunization. The percentage of sIgM⁺ lymphocytes reached peak levels at weeks 4–5 post-immunization. Specific anti-V. anguillarum or anti-rVAA antibodies were induced in inoculated fish at days 28–35 post-immunization. The liver of vaccinated flounder exhibited only slight histopathological changes compared with a significant pathology observed in control immunized fish. Additionally, a lower bacterial burden in the liver, spleen, and kidney were observed in pVAA protected fish in response to bacterial challenge, compared with pcDNA3.1 vector control injected fish. Moreover, the pVAA vaccine confers a relative percentage survival of 50.00% following V. anguillarum infection. In summary, this is the first study indicating an initial induction of the T lymphocyte response, followed by B lymphocyte induction of specific antibodies as a result of DNA immunization of flounder. This signifies the important potential of pVAA as a DNA vaccine candidate for the control of V. anguillarum infection.

Role of teleost B cells in viral immunity

Teleost fish possess all the necessary elements to mount an adaptive immune response. Despite this, the important physiological and structural differences between the mammalian and the teleost fish immune system, anticipate significant changes regarding how this response is coordinated and executed. B cells are key players in adaptive immune responses through the production of antibodies. However, recent studies performed in mammals and other species including fish point to many additional functions of B cells within both the adaptive and the innate immune system, in many occasions taking part in the crosstalk between these two arms of the immune response. Furthermore, it should be taken into account that fish B cells share many functional and phenotypical features with innate B cell populations from mammals, which will surely condition their response to antigens. Concerning viral infections, although most studies undertaken to date in fish have been focused on characterizing antibody production, some recent studies have demonstrated that fish B cells are able to interact with viruses at different levels. In this sense, in the current review, we have tried to provide an overview of what is currently known regarding the role of teleost B cells in antiviral immunity.

DNA vaccination for finfish aquaculture

In fish, DNA vaccines have been shown to give very high protection in experimental facilities against a number of viral diseases, particularly diseases caused by rhabdoviruses. However, their efficacy in generating protection against other families of fish viral pathogens is less clear. One DNA vaccine is currently in use commercially in fish farms in Canada and the commercialisation of another was authorised in Europe in 2017. The mechanism of action of DNA vaccines, including the role of the innate immune responses induced shortly after DNA vaccination in the activation of the adaptive immunity providing longer term specific protection, is still not fully understood. In Europe the procedure for the commercialisation of a veterinary DNA vaccine requires the resolution of certain concerns particularly about safety for the host vaccinated fish, the consumer and the environment. Relating to consumer acceptance and particularly environmental safety, a key question is whether a DNA vaccinated fish is considered a Genetically Modified Organism (GMO). In the present opinion paper these key aspects relating to the mechanisms of action, and to the development and the use of DNA vaccines in farmed fish are reviewed and discussed.

Regulation of IgM+ B Cell Activities by Rainbow Trout APRIL Reveals Specific Effects of This Cytokine in Lower Vertebrates

Tumor necrosis factor ligand superfamily members such as B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) have been identified in mammals as key regulators of B cell homeostasis and activation. However, the immune functions of APRIL are not as well defined as those of BAFF. Furthermore, while BAFF is present in all vertebrates, APRIL is missing in some animal groups, suggesting that BAFF has compensated the functions of APRIL in these species. In this context, we thought of great interest to explore the effects of APRIL on teleost B cells, given that APRIL appears for the first time in evolution in bony fish. Thus, in this study, we have performed an extensive analysis of the effect of APRIL on B cells using rainbow trout (Oncorhynchus mykiss) as a model species. Our results demonstrate that APRIL induces a specific proliferation of IgM⁺ B cells by itself and increases IgM secretion without promoting a terminal differentiation to plasma cells. APRIL also increased the levels of surface MHC II and augmented the capacity of these cells to process antigen, effects that were exclusively exerted on IgM⁺ B cells. Although our results point to a highly conserved role of APRIL on B cell homeostasis and activation throughout evolution, some specific differential effects have been observed in fish in comparison to the effects of APRIL previously described in mammals. Finally, the effects that APRIL induces on rainbow trout IgM⁺ B cells described in this paper have been compared with those previously reported in response to BAFF.

Dysregulation of B Cell Activity During Proliferative Kidney Disease in Rainbow Trout

Proliferative kidney disease (PKD) is a widespread disease caused by the endoparasite Tetracapsuloides bryosalmonae (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 (Oncorhynchus mykiss) naturally infected with T. bryosalmonae 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 T. bryosalmonae. 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.

Transcriptome analysis of plasmid-induced genes sheds light on the role of type I IFN as adjuvant in DNA vaccine against infectious salmon anemia virus

A previous study showed that a plasmid expressing IFNa (pIFNa) strongly enhanced protection and antibody production of a DNA vaccine against infectious salmon anemia virus (ISAV) in Atlantic salmon. The vaccine consisted of a plasmid (pHE) expressing the virus hemagglutinin-esterase as an antigen. To increase the understanding of the adjuvant effect of pIFNa, we here compared transcriptome responses in salmon muscle at the injection site at week 1 and 2 after injection of pIFNa, pHE, plasmid control (pcDNA3.3) and PBS, respectively. The results showed that the IFNa plasmid mediates an increase in gene transcripts of at least three major types in the muscle; typical IFN-I induced genes (ISGs), certain chemokines and markers of B- cells, T-cells and antigen-presenting cells. The latter suggests recruitment of cells to the plasmid injection site. Attraction of lymphocytes was likely caused by the induction of chemokines homologous to mammalian CCL5, CCL8, CCL19 and CXCL10. IFN may possibly also co-stimulate activation of lymphocytes as suggested by studies in mammals. A major finding was that both pcDNA3.3 and pHE caused responses similar to pIFNa, but at lower magnitude. Plasmid DNA may thus by itself have adjuvant activity as observed in mammalian models. Notably, pHE had a lower effect on many immune genes including ISGs and chemokines than pcDNA3.3, which suggests an inhibitory effect of HE expression on the immune genes. This hypothesis was supported by an Mx-reporter assay. The present study thus suggests that a main role for pIFNa as adjuvant in the DNA vaccine against ISAV may be to overcome the inhibitory effect of HE- expression on plasmid-induced ISGs and chemokines.

Intramuscular DNA Vaccination of Juvenile Carp against Spring Viremia of Carp Virus Induces Full Protection and Establishes a Virus-Specific B and T Cell Response

Although spring viremia of carp virus (SVCV) can cause high mortalities in common carp, a commercial vaccine is not available for worldwide use. Here, we report a DNA vaccine based on the expression of the SVCV glycoprotein (G) which, when injected in the muscle even at a single low dose of 0.1 µg DNA/g of fish, confers up to 100% protection against a subsequent bath challenge with SVCV. Importantly, to best validate vaccine efficacy, we also optimized a reliable bath challenge model closely mimicking a natural infection, based on a prolonged exposure of carp to SVCV at 15°C. Using this optimized bath challenge, we showed a strong age-dependent susceptibility of carp to SVCV, with high susceptibility at young age (3 months) and a full resistance at 9 months. We visualized local expression of the G protein and associated early inflammatory response by immunohistochemistry and described changes in the gene expression of pro-inflammatory cytokines, chemokines, and antiviral genes in the muscle of vaccinated fish. Adaptive immune responses were investigated by analyzing neutralizing titers against SVCV in the serum of vaccinated fish and the in vitro proliferation capacity of peripheral SVCV-specific T cells. We show significantly higher serum neutralizing titers and the presence of SVCV-specific T cells in the blood of vaccinated fish, which proliferated upon stimulation with SVCV. Altogether, this is the first study reporting on a protective DNA vaccine against SVCV in carp and the first to provide a detailed characterization of local innate as well as systemic adaptive immune responses elicited upon DNA vaccination that suggest a role not only of B cells but also of T cells in the protection conferred by the SVCV-G DNA vaccine.

Molecular characterization and expression analysis of three subclasses of IgT in rainbow trout (Oncorhynchus mykiss)

As the teleost specific immunoglobulin, IgT plays important roles in systemic and mucosal immunity. In the current study, in rainbow trout, we have cloned the heavy chain (Igτ) genes of a secretory form of IgT2 as well as the membrane and secretory forms of a third IgT subclass, termed IgT3. Conserved cysteine and tryptophan residues that are crucial for the folding of the immunoglobulin domain as well as hydrophobic and hydrophilic residues within CART motif were identified in all IgT subclasses. Through analysis of the rainbow trout genome assembly, Igτ3 gene was found localized upstream of Igτ1 gene, while Igτ2 gene situated on another scaffold. At the transcriptional level, Igτ1 was mainly expressed in both systemic and mucosal lymphoid tissues, while Igτ2 was largely expressed in systemic lymphoid organs. After LPS and poly (I:C) treatment, Igτ1 and Igτ2 genes exhibited different expression profiles. Interestingly the transcriptional level of Igτ3 was negligible, although its protein product could be identified in trout serum. Importantly, a previously reported monoclonal antibody directed against trout IgT1 was able to recognize IgT2 and IgT3. These data demonstrate that there exist three subclasses of IgT in rainbow trout, and that their heavy chain genes display different expression patterns during stimulation. Overall, our data reflect the diversity and complexity of immunoglobulin in trout, thus provide a better understanding of the IgT system in the immune response of teleost fish.

Expansion and differentiation of IgM+ B cells in the rainbow trout peritoneal cavity in response to different antigens

To date, intraperitoneal (i.p.) injection seems to be the most effective vaccination route in aquaculture, as many i.p. administered fish vaccines are capable of conferring strong and long-lasting immune responses. Despite this, how peritoneal leukocytes are regulated upon antigen encounter has only been scarcely studied in fish. Although, in the past, myeloid cells were thought to be the main responders to peritoneal inflammation, a recent study revealed that IgM⁺ B cells are one of the main cell types in the teleost peritoneal cavity in response to pathogenic bacteria. Thus, in the current work, we have focused on establishing how IgM⁺ B cells are recruited into the peritoneum in rainbow trout (Oncorhynchus mykiss) comparing different antigens: Escherichia coli as a bacterial model, E. coli-derived lipopolysaccharide (LPS) or viral hemorrhagic septicemia virus (VHSV). In addition to studying their capacity to dominate the peritoneal cavity, we have established how these IgM⁺ B cells are regulated in response to the different antigens, determining their levels of IgM secretion, surface MHC II expression, cell size and phagocytic abilities. Our results reveal that IgM⁺ B cells are one of the main cell types amplified in the peritoneum in response to either bacterial or viral antigens and that these immunogenic stimulations provoke a differentiation of some of these cells towards plasmablasts/plasma cells whereas others seem to be implicated in antigen presentation. These findings contribute to a better understanding of the immune processes that regulate peritoneal inflammation in teleost fish.

Characterization of BAFF and APRIL subfamily receptors in rainbow trout (Oncorhynchus mykiss). Potential role of the BAFF / APRIL axis in the pathogenesis of proliferative kidney disease

Proliferative kidney disease (PKD) is a parasitic infection of salmonid fish characterized by hyper-secretion of immunoglobulins in response to the presence of the myxozoan parasite, Tetracapsuloides bryosalmonae. In this context, we hypothesized that the BAFF/APRIL axis, known to play a major role in B cell differentiation and survival in mammals, could be affected by the parasite and consequently be involved in the apparent shift in normal B cell activity. To regulate B cell activity, BAFF and APRIL bind to transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and B cell maturation antigen (BCMA), whereas BAFF also binds to BAFF receptor (BAFF-R). In teleost fish, although some BAFF and APRIL sequences have been reported, their receptors have not been identified. Thus, as a first step in the current work, we have identified homologues to mammalian TACI, BCMA and BAFF-R in rainbow trout (Oncorhynchus mykiss), that constitute the first report of BAFF and APRIL receptor sequences in fish. Subsequently we studied the transcriptional modulation of BAFF, APRIL, and the fish-specific related cytokine, BALM and their putative receptors in fish naturally exposed to T. bryosalmonae. Finally, to gain further insights on the functional role that these cytokines play during the course of PKD, we have studied their effect on the survival of kidney IgM⁺ B cells and on immunoglobulin transcription. Our results support the premise that the BAFF / APRIL axis could play an important role during PKD, which may open the possibility of new therapeutic treatments against the disease.

Rainbow trout CK9, a CCL25-like ancient chemokine that attracts and regulates B cells and macrophages, the main antigen presenting cells in fish

CK9 is a rainbow trout (Oncorhynchus mykiss) CC chemokine phylogenetically related to mammalian CCL25. Although CK9 is known to be transcriptionally regulated in response to inflammation particularly in mucosal tissues, its functionality has never been revealed. In the current work, we have demonstrated that CK9 is chemoattractant for antigen presenting cells (APCs) expressing major histocompatibility complex class II (MHC II) on the cell surface. Among these APCs, CK9 has a strong chemotactic capacity for both B cells (IgM+ and IgT+) and macrophages. Along with its chemotactic capacities, CK9 modulated the MHC II turnover of B lymphocytes and up-regulated the phagocytic capacity of both IgM+ cells and macrophages. Although CK9 had no lymphoproliferative effects, it increased the survival of IgT+ lymphocytes. Furthermore, we have established that the chemoattractant capacity of CK9 is strongly increased after pre-incubation of leukocytes with a T-independent antigen, whereas B cell receptor (BCR) cross-linking strongly abrogated their capacity to migrate to CK9, indicating that CK9 preferentially attracts B cells at the steady state or under BCR-independent stimulation. These results point to CK9 being a key regulator of B lymphocyte trafficking in rainbow trout, able to modulate innate functions of teleost B lymphocytes and macrophages.

Can VHS Virus Bypass the Protective Immunity Induced by DNA Vaccination in Rainbow Trout?

DNA vaccines encoding viral glycoproteins have been very successful for induction of protective immunity against diseases caused by rhabdoviruses in cultured fish species. However, the vaccine concept is based on a single viral gene and since RNA viruses are known to possess high variability and adaptation capacity, this work aimed at evaluating whether viral haemorrhagic septicaemia virus (VHSV), an RNA virus and member of Rhabdoviridae family, was able to evade the protective immune response induced by the DNA vaccination of rainbow trout. The experiments comprised repeated passages of a highly pathogenic VHSV isolate in a fish cell line in the presence of neutralizing fish serum (in vitro approach), and in rainbow trout immunized with the VHS DNA vaccine (in vivo approach). For the in vitro approach, the virus collected from the last passage (passaged virus) was as sensitive as the parental virus to serum neutralization, suggesting that the passaging did not promote the selection of virus populations able to bypass the neutralization by serum antibodies. Also, in the in vivo approach, where virus was passaged several times in vaccinated fish, no increased virulence nor increased persistence in vaccinated fish was observed in comparison with the parental virus. However, some of the vaccinated fish did get infected and could transmit the infection to naïve cohabitant fish. The results demonstrated that the DNA vaccine induced a robust protection, but also that the immunity was non-sterile. It is consequently important not to consider vaccinated fish as virus free in veterinary terms.

Comparative study of CXC chemokines modulation in brown trout (Salmo trutta) following infection with a bacterial or viral pathogen

Chemokine modulation in response to pathogens still needs to be fully characterised in fish, in view of the recently described novel chemokines present. This paper reports the first comparative study of CXC chemokine genes transcription in salmonids (brown trout), with a particular focus on the fish specific CXC chemokines (CXCL_F). Adopting new primer sets, optimised to specifically target mRNA, a RT-qPCR gene screening was carried out. Constitutive gene expression was assessed first in six tissues from SPF brown trout. Transcription modulation was next investigated in kidney and spleen during septicaemic infection induced by a RNA virus (Viral Haemorrhagic Septicaemia virus, genotype Ia) or by a Gram negative bacterium (Yersinia ruckeri, ser. O1/biot. 2). From each target organ specific pathogen burden, measured detecting VHSV-glycoprotein or Y. ruckeri 16S rRNA, and IFN-γ gene expression were analysed for their correlation to chemokine transcription. Both pathogens modulated CXC chemokine gene transcript levels, with marked up-regulation seen in some cases, and with both temporal and tissue specific effects apparent. For example, Y. ruckeri strongly induced chemokine transcription in spleen within 24h, whilst VHS generally induced the largest increases at 3d.p.i. in both tissues. This study gives clues to the role of the novel CXC chemokines, in comparison to the other known CXC chemokines in salmonids.

Teleost Chemokines and Their Receptors

Chemokines are a superfamily of cytokines that appeared about 650 million years ago, at the emergence of vertebrates, and are responsible for regulating cell migration under both inflammatory and physiological conditions. The first teleost chemokine gene was reported in rainbow trout in 1998. Since then, numerous chemokine genes have been identified in diverse fish species evidencing the great differences that exist among fish and mammalian chemokines, and within the different fish species, as a consequence of extensive intrachromosomal gene duplications and different infectious experiences. Subsequently, it has only been possible to establish clear homologies with mammalian chemokines in the case of some chemokines with well-conserved homeostatic roles, whereas the functionality of other chemokine genes will have to be independently addressed in each species. Despite this, functional studies have only been undertaken for a few of these chemokine genes. In this review, we describe the current state of knowledge of chemokine biology in teleost fish. We have mainly focused on those species for which more research efforts have been made in this subject, specifically zebrafish (Daniorerio), rainbow trout (Oncorhynchusmykiss) and catfish (Ictaluruspunctatus), outlining which genes have been identified thus far, highlighting the most important aspects of their expression regulation and addressing any known aspects of their biological role in immunity. Finally, we summarise what is known about the chemokine receptors in teleosts and provide some analysis using recently available data to help characterise them more clearly.

ORF75 of megalocytivirus RBIV-C1: A global transcription regulator and an effective vaccine candidate

Megalocytivirus, a DNA virus belonging to the Iridoviridae family, is a severe pathogen to a wide range of marine and freshwater fish. In this study, using turbot (Scophthalmus maximus) as a host model, we examined the immunoprotective property of one megalocytivirus gene, ORF75, in the form of DNA vaccine (named pORF75). Immunofluorescence microscopy and RT-PCR analysis showed that P444, the protein encoded by ORF75, was naturally produced in the tissues of turbot during megalocytivirus infection, and that the vaccine gene in pORF75 was expressed in fish cells transfected with pORF75 and in the tissues of turbot immunized with pORF75. Following vaccination of turbot with pORF75, a high level of survival (73%) was observed against a lethal megalocytivirus challenge. Consistently, viral replication in the vaccinated fish was significantly inhibited. Immune response analysis showed that pORF75-vaccinated fish (i) exhibited upregulated expression of the genes involved in innate and adaptive immunity, (ii) possessed specific memory immune cells that showed significant response to secondary antigen stimulation, and (iii) produced specific serum antibodies which, when co-introduced into turbot with megalocytivirus, blocked viral replication. Furthermore, whole-genome transcriptome analysis revealed that ORF75 knockdown altered the transcription of 43 viral genes. Taken together, these results indicate that ORF75 encoded a highly protective immunogen that is also a global transcription regulator of megalocytivirus.

Molecular characterization of CD9 and CD63, two tetraspanin family members expressed in trout B lymphocytes

Tetraspanins are a family of membrane-organizing proteins, characterized by the presence of four highly conserved transmembrane regions that mediate diverse physiological functions. In the current study, we have identified two novel tetraspanin members in rainbow trout (Oncorhynchus mykiss), homologs to mammalian CD9 and CD63. Both genes were expressed in muscle, skin, gills, hindgut, gonad, liver, spleen, head kidney, thymus and peripheral blood leukocytes. Throughout the early life cycle stages, CD9 mRNA levels significantly increased after first feeding, whereas CD63 transcription remained constant during all the developmental stages analyzed. In response to an experimental bath infection with viral hemorrhagic septicemia virus (VHSV), CD9 transcription was down-regulated in the gills, while CD63 mRNA levels were down-regulated in the head kidney. Instead, when the virus was intraperitoneally injected, the transcription of both genes was significantly up-regulated in peritoneal cells at several days post-infection. Additionally, both genes were transcriptionally up-regulated in the muscle of trout injected with a VHSV DNA vaccine. To gain insight on the relation of these tetraspanins with B cell activity we determined their constitutive expression in naive IgM(+) populations from different sources and observed that both molecules were being transcribed by IgM(+) cells in different tissues. Furthermore, CD9 transcription was significantly down-regulated in splenic IgM(+) cells in response to in vitro VHSV exposure. Our results provide insights on the potential role of these tetraspanins on teleost B cell and antiviral immunity.

Involvement of two microRNAs in the early immune response to DNA vaccination against a fish rhabdovirus

Mechanisms that account for the high protective efficacy in teleost fish of a DNA vaccine expressing the glycoprotein (G) of Viral hemorrhagic septicemia virus (VHSV) are thought to involve early innate immune responses mediated by interferons (IFNs). Microribonucleic acids (miRNAs) are a diverse class of small (18-22 nucleotides) endogenous RNAs that potently mediate post-transcriptional silencing of a wide range of genes and are emerging as critical regulators of cellular processes, including immune responses. We have recently reported that miR-462 and miR-731 were strongly induced in rainbow trout infected with VHSV. In this study, we analyzed the expression of these miRNAs in fish following administration of the DNA vaccine and their potential functions. Quantitative RT-PCR analysis revealed the increased levels of miR-462, and miR-731 in the skeletal muscle tissue at the site of vaccine administration and in the liver of vaccinated fish relative to empty plasmid backbone-injected controls. The increased expression of these miRNAs in the skeletal muscle correlated with the increased levels of the type I interferon (IFN)-inducible gene Mx, type I IFN and IFN-γ genes at the vaccination site. Intramuscular injection of fish with either type I IFN or IFN-γ plasmid construct resulted in the upregulation of miR-462 and miR-731 at the site of injection, suggesting that the induction of these miRNAs is elicited by IFNs. To analyze the function of miR-462 and miR-731, specific silencing of these miRNAs using anti-miRNA oligonucleotides was conducted in poly I:C-treated rainbow trout fingerlings. Following VHSV challenge, anti-miRNA-injected fish had faster development of disease and higher mortalities than control fish, indicating that miR-462/731 may be involved in IFN-mediated protection conferred by poly I:C.

A Review of Intra- and Extracellular Antigen Delivery Systems for Virus Vaccines of Finfish

Vaccine efficacy in aquaculture has for a long time depended on evaluating relative percent survival and antibody responses after vaccination. However, current advances in vaccine immunology show that the route in which antigens are delivered into cells is deterministic of the type of adaptive immune response evoked by vaccination. Antigens delivered by the intracellular route induce MHC-I restricted CD8+ responses while antigens presented through the extracellular route activate MHC-II restricted CD4+ responses implying that the route of antigen delivery is a conduit to induction of B- or T-cell immune responses. In finfish, different antigen delivery systems have been explored that include live, DNA, inactivated whole virus, fusion protein, virus-like particles, and subunit vaccines although mechanisms linking these delivery systems to protective immunity have not been studied in detail. Hence, in this review we provide a synopsis of different strategies used to administer viral antigens via the intra- or extracellular compartments. Further, we highlight the differences in immune responses induced by antigens processed by the endogenous route compared to exogenously processed antigens. Overall, we anticipate that the synopsis put together in this review will shed insights into limitations and successes of the current vaccination strategies used in finfish vaccinology.

P247 and p523: two in vivo-expressed megalocytivirus proteins that induce protective immunity and are essential to viral infection

Megalocytivirus is a DNA virus with a broad host range among teleost fish. Although the complete genome sequences of a number of megalocytivirus isolates have been reported, the functions of most of the genes of this virus are unknown. In this study, we selected two megalocytivirus immunogens, P247 and P523, which were expressed during host infection and, when in the form of DNA vaccines (pCN247 and pCN523 respectively), elicited strong protectivity against lethal megalocytivirus challenge in a turbot (Scophthalmus maximus) model. Compared to control fish, fish vaccinated with pCN247 and pCN523 exhibited drastically reduced viral loads in tissues and high levels of survival rates. Immune response analysis showed that pCN247 and pCN523 (i) induced production of specific serum antibodies, (ii) caused generation of cytotoxic immune cells and specific memory immune cells that responded to secondary antigen stimulation, and (iii) upregulated the expression of genes involved in innate and adaptive immunity. To examine the potential role of P247 and P523 in viral infection, the expression of P247 and P523 was knocked down by siRNA. Subsequent in vivo infection study showed that P247 and P523 knockdown significantly impaired viral replication. Furthermore, whole-genome transcriptome analysis revealed that P247 and P523 knockdown altered the expression profiles of 26 and 41 viral genes, respectively, putatively participating in diverse aspects of viral infection. Taken together, these results indicate that P247 and P523 induce protective immunity in teleost and play fundamental roles essential to viral replication. These observations provide the first evidence that suggests a likely link between the protectivity of viral immunogens and their biological significance in viral replication.

Early immune responses in rainbow trout liver upon viral hemorrhagic septicemia virus (VHSV) infection

Among the essential metabolic functions of the liver, in mammals, a role as mediator of systemic and local innate immunity has also been reported. Although the presence of an important leukocyte population in mammalian liver is well documented, the characterization of leukocyte populations in the teleost liver has been only scarcely addressed. In the current work, we have confirmed the presence of IgM⁺, IgD⁺, IgT⁺, CD8α⁺, CD3⁺ cells, and cells expressing major histocompatibility complex (MHC-II) in rainbow trout (Oncorhynchus mykiss) liver by flow cytometry and/or immunohistochemistry analysis. Additionally, the effect of viral hemorrhagic septicemia virus (VHSV) on the liver immune response was assessed. First, we studied the effect of viral intraperitoneal injection on the transcription of a wide selection of immune genes at days 1, 2 and 5 post-infection. These included a group of leukocyte markers genes, pattern recognition receptors (PRRs), chemokines, chemokine receptor genes, and other genes involved in the early immune response and in acute phase reaction. Our results indicate that T lymphocytes play a key role in the initial response to VHSV in the liver, since CD3, CD8, CD4, perforin, Mx and interferon (IFN) transcription levels were up-regulated in response to VHSV. Consequently, flow cytometry analysis of CD8α⁺ cells in liver and spleen at day 5 post-infection revealed a decrease in the number of CD8α⁺ cells in the spleen and an increased population in the liver. No differences were found however in the percentages of B lymphocyte (IgM⁺ or IgD⁺) populations. In addition, a strong up-regulation in the transcription levels of several PRRs and chemokines was observed from the second day of infection, indicating an important role of these factors in the response of the liver to viral infections.