Novel development of cationic surfactant-based mucoadhesive nanovaccine for direct immersion vaccination against Francisella noatunensis subsp. orientalis in red tilapia (Oreochromis sp.)

Evaluation of immune responses and protection in Asian seabass (Lates calcarifer Bloch, 1790) against Vibrio vulnificus using immersion and oral nanoemulsion vaccines

Vaccination is an important strategy in aquaculture for enhancing disease resistance and ensuring the health and productivity of cultured fish. This study investigated the distinct systemic and mucosal immune responses elicited by immersion (ImV) and oral (OrV) nanoemulsion vaccines encapsulating Vibrio vulnificus in Asian seabass (Lates calcarifer). Two vaccination methods, ImV and OrV, were administered to separate groups of Asian seabass, followed by challenges with V. vulnificus via immersion and intraperitoneal (i.p.) injection administration. Immune responses were assessed by measuring specific IgM antibodies, lysozyme activity, bactericidal activity, and immune-related gene expression across multiple organs, including serum, head kidney, liver, gills, skin, and intestine. The results demonstrate that the OrV group had a significantly increased level of specific IgM antibody in their serum, head kidney and liver compared to the immersion vaccine group and control, with the OrV group maintaining higher antibody levels post-immersion and injection challenge. In mucosal tissues, the ImV group elicited stronger IgM responses in the gills and skin, whereas the OrV group enhanced antibody responses in the liver and intestine. Lysozyme and bactericidal activities varied by organ and vaccination route, indicating differential activation of innate immune defenses. Additionally, immune-related gene expression was upregulated in a tissue-specific manner, reflecting the targeted activation of adaptive and innate immune mechanisms by each vaccine type. Both vaccination strategies significantly improved survival and relative percent survival (RPS), with ImV offering stronger protection against immersion challenges, while OrV provided strong protection in systemic (injection) challenges. These findings suggest that ImV and OrV groups induced distinct immune responses in Asian seabass, with immersion vaccination primarily enhancing mucosal immunity at barrier surfaces and oral vaccination promoting systemic and intestinal mucosal immunity. The integration of both vaccination strategies could provide synergistic protection, enhancing vaccine efficacy against V. vulnificus and other pathogens, thereby optimizing disease prevention and fish health management in aquaculture settings.

Effectiveness of a new cationic lipid-based nanovaccine for enhancing immersion vaccination against Flavobacterium oreochromis in red tilapia (Oreochromis sp.)

Flavobacterium oreochromis (Fo) poses a significant threat to tilapia, leading to economic losses due to mortality. Immersion vaccines, while practical for small fish, often result in limited antigen absorption. This study aimed to develop a cationic lipid-based nanoparticle vaccine using cetyltrimethylammonium bromide (CTAB) and evaluate its efficacy against F. oreochromis in red tilapia. In the first trial, healthy red tilapia were immersion vaccinated for 30 min, with three groups included in the trial: control (non-vaccinated), formalin-killed sonicated cells (FK-SC), or cationic lipid-based nanoparticles (Fo-NV). The second trial followed the same design, with booster vaccinations (FK-SC-B, Fo-NV-B) administered 14 days after the first vaccination. Fish were challenged with virulent F. oreochromis at multiple time points up to 120 days post-vaccination (dpv) in the first trial and up to 180 dpv in the second, with survival recorded for 10 days post-challenge during each challenge. Specific IgM antibody levels were measured at various dpv intervals. The Fo-NV group, characterized by nanoscale size (179 nm) and positive charge (13 mV), showed enhanced stability and mucoadhesion compared to FK-SC. In the first trial, the Fo-NV group had significantly higher relative percentage survival (RPS) (83.3-63.3 %) compared to FK-SC (33.3-10.0 %) during the first three months. In the second trial, the Fo-NV-B group exhibited elevated IgM levels and higher RPS (81.8-58.5 %) compared to control groups over five months. In conclusion, a booster dose of Fo-NV improved vaccine efficacy, enhancing antigen delivery to mucosal surfaces and providing prolonged protection against F. oreochromis infection.

Evaluation of a hydrogel platform for encapsulated multivalent Vibrio antigen delivery to enhance immune responses and disease protection against vibriosis in Asian seabass (Lates calcarifer)

This study reports the development and evaluation of a novel multivalent oral hydrogel vaccine designed to protect Asian seabass (Lates calcarifer) against vibriosis caused by multiple Vibrio species. The hydrogel formulation, composed of alginate and bentonite, was engineered to encapsulate three Vibrio pathogens (V. harveyi, V. vulnificus, and P. damsela), and subsequently freeze-dried to yield stable, dry hydrogel beads. Although the process achieved a relatively low yield (approximately 10 %), the dry beads maintained their structural integrity, retained antigenic components with 2.4 ± 1.6 × 107 DNA copies of total cell antigens/mg dry beads, and resisted acidic degradation, ensuring antigen preservation during simulated gastric exposure. Physicochemical characterization (FTIR) confirmed that the encapsulation process preserved the structural and functional properties of alginate, bentonite, and bacterial antigens without introducing new chemical bonds. Scanning electron microscopy (SEM) revealed a porous, sponge-like internal architecture that supported antigen entrapment and controlled release. Under simulated gastric conditions (pH 2.0), the hydrogel exhibited remarkable stability for up to 8 h, preventing antigen loss. Upon transitioning to intestinal conditions (pH 7.2), the matrix gradually disintegrated, releasing the antigens in a controlled manner for up to 8 h. Oral vaccination trials demonstrated enhanced immune responses in Asian seabass. Fish receiving the multivalent Vibrio antigen-containing hydrogel vaccine (Hg-mVibrioAg) displayed elevated serum-specific IgM titers against all three Vibrio species, with significantly higher and more sustained antibody levels following a 7-day and 14-day vaccination regimen. Increased lysozyme and bactericidal activity further supported improved innate defense mechanisms. Subsequent pathogen challenge tests confirmed that vaccinated fish, particularly those following the 7-day and 14-day regimens, exhibited significantly higher survival rates and robust protection. Concurrently, immune-related gene expression in the head kidney, peripheral blood leukocytes, gills, and intestines was upregulated, indicating a broad immune activation associated with specific IgM responses. No significant alterations in blood biochemistry or tissue histology were observed, highlighting the vaccine's biocompatibility. Additionally, these findings underscore the potential of this multivalent oral hydrogel vaccine as a promising, safe, and effective prophylactic strategy against Vibrio infections in Asian seabass aquaculture.

Intestinal mucosal transcriptomic responses of Asian seabass (Lates calcarifer) vaccinated with an oral hydrogel-encapsulated multivalent Vibrio antigen following Vibrio spp. infection

This study examined the intestinal mucosal immune responses elicited by an oral hydrogel-encapsulated multivalent Vibrio vaccine in Asian seabass (Lates calcarifer) to protect against vibriosis caused by Vibrio harveyi, V. vulnificus, and Photobacterium damselae (formerly Vibrio damsela). Both 7-day and 14-day oral vaccination regimens effectively enhanced innate and adaptive immune responses while supporting gut recovery post-infection. Transcriptomic analyses of intestines from fish that received consecutive 7- and 14-day vaccination regimens, followed by co-infection with multistrain Vibrio spp., revealed significant upregulation of innate and specific immune markers at week 8 post-vaccination. These responses were further bolstered by a strong adaptive immune activation, characterized by T-cell and B-cell receptor signaling as well as antibody production. In addition, the vaccine also demonstrated cross-protective immunomodulatory effects, evidenced by elevated interferon-related pathways (e.g., IFNAR2 and IFN-induced proteins), suggesting its potential to protect against co-infecting pathogens, a critical advantage in aquaculture systems facing diverse pathogen pressures. Beyond immune activation, the involvement proteins of TGF-β family members, including BMP3 and BMP4, highlights the vaccine's role in tissue repair and remodeling. These responses likely mitigate epithelial damage and preserve gut barrier integrity post-infection, showcasing the dual benefits of immunoprotection and post-infection recovery. The findings highlight the oral hydrogel-encapsulated multivalent Vibrio vaccine's ability to enhance immunity against specific bacterial pathogens while offering broader immunomodulatory and tissue-repair benefits. Its cross-protective and recovery-supporting properties make it a promising solution for sustainable aquaculture practices, effectively addressing pathogen control and boosting host resilience.

Advances in Vaccine Adjuvants for Teleost Fish: Implications for Aquatic Welfare and the Potential of Nanoparticle-Based Formulations

Vaccine adjuvants are crucial for reinforcing the immunogenicity of vaccines. Therefore, they are widely used in the aquaculture sector as vaccine components, facilitating the efficient prevention of infectious diseases and promoting sustainable teleost fish growth. Despite their benefits, there has been a growing concern about the potential adverse effects of vaccine adjuvants in teleost fish, connoting a valid impact on their overall health and welfare. Among the adjuvants used in aquaculture vaccinology, nanoparticle-based formulations have given rise to a promising new alternative to traditional options, such as oil-based emulsions and aluminum compounds, offering the benefit of minimizing relevant side effects. The aim of this paper was to review the current status of the adjuvants used in aquaculture, provide a description and an evaluation of their mode of action and side effects, and explore the potential use of nanoparticle formulations as adjuvants to improve the efficacy of aquaculture vaccines. By demonstrating and assessing the equilibrium between teleost fish welfare and immunological efficacy, this review presents a collective perspective that will assist in establishing a framework for the utilization of effective species-specific practices around adjuvant use in aquaculture, while also addressing the challenges of welfare-friendly immunization.

Emerging Cationic Nanovaccines

Cationic vaccines of nanometric sizes can directly perform the delivery of antigen(s) and immunomodulator(s) to dendritic cells in the lymph nodes. The positively charged nanovaccines are taken up by antigen-presenting cells (APCs) of the lymphatic system often originating the cellular immunological defense required to fight intracellular microbial infections and the proliferation of cancers. Cationic molecules imparting the positive charges to nanovaccines exhibit a dose-dependent toxicity which needs to be systematically addressed. Against the coronavirus, mRNA cationic nanovaccines evolved rapidly. Nowadays cationic nanovaccines have been formulated against several infections with the advantage of cationic compounds granting protection of nucleic acids in vivo against biodegradation by nucleases. Up to the threshold concentration of cationic molecules for nanovaccine delivery, cationic nanovaccines perform well eliciting the desired Th 1 improved immune response in the absence of cytotoxicity. A second strategy in the literature involves dilution of cationic components in biocompatible polymeric matrixes. Polymeric nanoparticles incorporating cationic molecules at reduced concentrations for the cationic component often result in an absence of toxic effects. The progress in vaccinology against cancer involves in situ designs for cationic nanovaccines. The lysis of transformed cancer cells releases several tumoral antigens, which in the presence of cationic nanoadjuvants can be systemically presented for the prevention of metastatic cancer. In addition, these local cationic nanovaccines allow immunotherapeutic tumor treatment.

Molecular structure and functional responses of IgM, IgT and IgD to Flavobacterium covae and Streptococcus iniae infection in Asian seabass (Lates calcarifer Bloch, 1790)

The Asian seabass (Lates calcarifer) faces significant disease threats, which are exacerbated by intensive farming practices and environmental changes. Therefore, understanding its immune system is crucial. The current study presents a comprehensive analysis of immune-related genes in Asian seabass peripheral blood leukocytes (PBLs) using Iso-seq technology, identifying 16 key pathways associated with 7857 immune-related genes, comprising 634 unique immune-related genes. The research marks the first comprehensive report on the entire immunoglobulin repertoire in Asian seabass, revealing specific characteristics of immunoglobulin heavy chain constant region transcripts, including IgM (Cμ, ighm), IgT (Cτ, ight), and IgD (Cδ, ighd). The study confirms the presence of membrane-bound form, ighmmb, ightmb, ighdmb of IgM, IgT and IgD and secreted form, ighmsc and ightsc of IgM and IgT, respectively, with similar structural patterns and conserved features in amino acids across immunoglobulin molecules, including cysteine residues crucial for structural integrity observed in other teleost species. In response to bacterial infections by Flavobacterium covae (formerly F. columnare genomovar II) and Streptococcus iniae, both secreted and membrane-bound forms of IgM (ighmmb and ighmsc) and IgT (ightmb and ightsc) show significant expression, indicating their roles in systemic and mucosal immunity. The expression of membrane-bound form IgD gene, ighdmb, predominantly exhibits targeted upregulation in PBLs, suggesting a regulatory role in B cell-mediated immunity. The findings underscore the dynamic and tissue-specific expression of immunoglobulin repertoires, ighmmb, ighmsc, ightmb, ightsc and ighdmb in Asian seabass, indicating a sophisticated immune response to bacterial pathogens. These findings have practical implications for fish aquaculture, and disease control strategies, serving as a valuable resource for advancing research in Asian seabass immunology.

Antioxidant and antibacterial efficiency of the ethanolic leaf extract of Kratom (Mitragyna speciosa (Korth.) Havil) and its effects on growth, health, and disease resistance against Edwardsiella tarda infection in Nile tilapia (Oreochromis niloticus)

The research examined the impact of an ethanolic extract from the leaves of Kratom (Mitragyna speciosa (Korth.) Havil.) on the growth, antioxidant capacity, immune-related gene expression, and resistance to disease caused by Edwardsiella tarda in Nile tilapia (Oreochromis niloticus). The findings revealed that the extract had the important phytochemical content in the extract included total phenolics content, total flavonoids content, vitamin C, and total antioxidant capacity and 5.42 % of the crude extract was mitragynine. The extract demonstrated antioxidant activity, as evidenced by its IC50 values against ABTS and DPPH radicals and its ferric reducing power in vitro. Moreover, the MIC-IC50 value of 0.625 mg/mL indicated that the growth of the bacteria was reduced by approximately 50 %, and the MBC was 2.50 mg/mL against E. tarda. Furthermore, the orally administered Kratom leaf extract to fingerling tilapia for 8 weeks exhibited a noticeable increase in oxidative stress, as demonstrated by the increase in MDA production in the 10 and 25 g/kg groups. It also exhibited an increase in acetylcholinesterase (AChE) activity in muscle tissue at the 50 g/kg group. However, when administered at a feeding rate of 5-10 g/kg feed, the extract showed an increase in the expression of immune-related genes (IL1, IL6, IL8, NF-kB, IFNγ, TNFα, Mx, CC-chemokine, CD4, TCRβ, MHC-IIβ, IgM, IgT, IgD) and enhanced resistance to E. tarda infection in fish. Conversely, administering the extract at 25-50 g/kg feed resulted in contrasting effects, suppressing and reducing the observed parameters. Nevertheless, feeding the extract at all concentrations for 8 weeks did not produce any changes in the histology or systemic functioning of the liver and intestines, as indicated by blood biochemistry. These findings suggest that the ethanolic leaf extract from Kratom has the potential to be used as a substitute for antibiotics in the management of bacterial infections in Nile tilapia culture, with a recommended dosage of 5-10 g/kg feed/day for a maximum of 8 weeks.

Systemic and mucosal immune responses in red tilapia (Oreochromis sp.) following immersion vaccination with a chitosan polymer-based nanovaccine against Aeromonas veronii

A mucoadhesive chitosan polymer-based nanoplatform has been increasingly recognized as an effective mucosal vaccine delivery system for fish. The present study aimed to investigate the effectiveness of immersion vaccination with a chitosan polymer-based nanovaccine to elicit an immune response in serum and mucus of red tilapia and evaluate its protective efficacy after immersion challenge with a heterogenous strain of Aeromonas veronii UDRT09. Six hundred red tilapia (22 ± 1.8 g) were randomly allocated into four experimental groups: control, empty-polymeric nanoparticle (PC), formalin-killed vaccine (FKV), and chitosan polymer-based nanovaccine (CS-NV) in triplicate. The specific IgM antibody levels and their bactericidal activity were assessed in serum and mucus for 28 days after immersion vaccination and followed by immersion challenge with A. veronii. The immersion vaccine was found to be safe for red tilapia, with no mortalities occurring during the vaccination procedure. The specific IgM antibody levels and bactericidal activity against A. veronii in both serum and mucus were significantly higher in red tilapia vaccinated with CS-NV compared to the FKV and control groups at all time points. Furthermore, the serum lysozyme activity, ACH50, and total Ig levels demonstrated a significant elevation in the groups vaccinated with CS-NV compared to the FKV and control groups. Importantly, the Relative Percentage Survival (RPS) value of the CS-NV group (71 %) was significantly higher than that of the FKV (15.12 %) and PC (2.33 %) groups, respectively. This indicates that the chitosan polymer-based nanovaccine platform is an effective delivery system for the immersion vaccination of tilapia.

Development of Immersion and Oral Bivalent Nanovaccines for Streptococcosis and Columnaris Disease Prevention in Fry and Fingerling Asian Seabass (Lates calcarifer) Nursery Farms

In the present study, chitosan-based bivalent nanovaccines of S. iniae and F. covae were administered by immersion vaccination at 30 and 40 days after hatching (DAH), and the third vaccination was orally administered by feeding at 50 DAH. ELISA revealed that the levels of total IgM and specific IgM to S. iniae and F. covae were significantly elevated in all vaccinated groups at 10, 20, and 30 days after vaccination (DAV). A qRT-PCR analysis of immune-related genes revealed significantly higher IgT expression in the vaccinated groups compared to the control group, as revealed by 44-100-fold changes in the vaccinated groups compared to the control (p < 0.001) at every tested time point after vaccination. All vaccinated groups expressed IgM, MHCIIα, and TCRα at significantly higher levels than the control group at 10 and/or 20 DAV (p < 0.05). In the S. iniae challenge tests, the survival of vaccinated groups ranged from 62.15 ± 2.11 to 75.70 ± 3.36%, which significantly differed from that of the control group (44.44 ± 1.92%). Similarly, all vaccinated groups showed higher survival rates of 68.89 ± 3.85 to 77.78 ± 5.09% during F. covae challenge than the control groups (50.00 ± 3.33%) (p < 0.05).

Addressing Nanovaccine Strategies for Tilapia

Tilapia is the world's most extensively farmed species after carp. It is an attractive species for aquaculture as it grows quickly, reaching harvest size within six to seven months of production, and provides an important source of food and revenue for many low-income families, especially in low- to middle-income countries. The expansion of tilapia aquaculture has resulted in an intensification of farming systems, and this has been associated with increased disease outbreaks caused by various pathogens, mostly bacterial and viral agents. Vaccination is routinely used to control disease in higher-value finfish species, such as Atlantic salmon. At the same time, many tilapia farmers are often unwilling to vaccinate their fish by injection once the fish have been moved to their grow-out site. Alternative vaccination strategies are needed to help tilapia farmers accept and use vaccines. There is increasing interest in nanoparticle-based vaccines as alternative methods for delivering vaccines to fish, especially for oral and immersion administration. They can potentially improve vaccine efficacy through the controlled release of antigens, protecting antigens from premature proteolytic degradation in the gastric tract, and facilitating antigen uptake and processing by antigen-presenting cells. They can also allow targeted delivery of the vaccine at mucosal sites. This review provides a brief overview of the bacterial and viral diseases affecting tilapia aquaculture and vaccine strategies for farmed tilapia. It focuses on the use of nanovaccines to improve the acceptance and uptake of vaccines by tilapia farmers.

Development of a bivalent mucoadhesive nanovaccine to prevent francisellosis and columnaris diseases in Nile tilapia (Oreochromis niloticus)

The occurrence of francisellosis caused by Francisella orientalis sp. nov. (Fo) and columnaris disease caused by Flavobacterium oreochromis (For) is negatively impacting Nile tilapia (Oreochromis niloticus) production, especially when high stocking densities are used. A new and innovative bivalent mucoadhesive nanovaccine was developed in this study for immersion vaccination of tilapia against francisellosis and columnaris disease. It was shown to have the potential to improve both innate and adaptive immunity in vaccinated Nile tilapia. It increased innate immune parameters, such as lysozyme activity, bactericidal activity, phagocytosis, phagocytic index, and total serum IgM antibody levels. Additionally, the vaccine was effective in elevating specific adaptive immune responses, including IgM antibody levels against Fo and For vaccine antigens and upregulating immune-related genes IgM, IgT, CD4⁺, MHCIIα, and TCRβ in the head kidney, spleen, peripheral blood leukocytes, and gills of vaccinated fish. Furthermore, fish vaccinated with the mucoadhesive nanovaccine showed higher survival rates and relative percent survival after being challenged with either single or combined infections of Fo and For. This vaccine is anticipated to be beneficial for large-scale immersion vaccination of tilapia and may be a strategy for shortening vaccination times and increasing immune protection against francisellosis and columnaris diseases in tilapia aquaculture.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.