Aeromonas veronii, a newly discovered aquatic pathogen, is also crucial for food safety and public health [1,2,3]. A. veronii can currently be isolated from almost any environment, and its prevalence in food and the environment may contribute to human illness [4,5,6]. Many infection pathways exist in A. veronii, posing unique challenges for the associated study [7]. Increasing numbers of patients have been infected with A. veronii, which has shown increasing virulence and drug resistance year after year. An increasingly large number of countries are now specifying A. veronii as a critical indicator of water quality because of its dangers. Additionally, as a result of mixed infections caused by A. veronii and other pathogens, septicemia has been a major problem in recent years in the aquaculture industry, posing a biosafety threat to humans [8]. The wide array of virulence factors present in A. veronii, as well as the complexity and diversity of its pathogenic pathways, create particular difficulties in research related to it [9]. However, there is no effective vaccine against A. veronii to combat its devastating effect on various fish species [10].
Aeromonas veronii TH0426, a new strain of A. veronii, exhibits enhanced virulence and adhesion due to cadaverine reverse transporter (CadB protein) and maltoporin (LamB protein) [9, 11, 12]. The LamB protein is a member of the gram-negative bacteria’s outer membrane porin family, which also includes the Maltoporin protein. The function of LamB protein has been studied in bacteria, the majority of which is E. coli. Previous research has shown that LamB protein is a λ phage receptor protein that also controls the content of maltose in fish body cells by transporting maltose and maltodextrin [13, 14]. At the same time, related studies suggested that as the LamB protein is a typical porins, the adhesion and internalization of it are important steps for pathogens to infect epithelial cells [15]. Moreover, wet lab analysis proved that the lamB gene of A. veronii plays a crucial role in the pathogenesis in various fish species [11]. Similarly, cadaverine metabolism is related to only one transport system, the lysine cadaverine reverse transport system. Cadaverine reverse transporter gene encodes a protein with 12 transmembrane helices, similar in structure to PotE protein from E. coli. The protein transfers lysine into the fish cell from the outside and excretes cadaverine [16]. Related research has found a link between the pathogenicity of pathogenic bacteria and the cadaverine reverse transport pathway, which is dominated by the Cad gene [17, 18]. The cadaverine reverse transport system increases pathogenicity of pathogenic bacteria directly or indirectly by boosting the transcriptional expression of virulence factors and influencing bacterial biofilm production, in addition to assisting pathogens in overcoming host pressure [19]. Moreover, experiment revealed that the cadB gene encoding the highly expressed protein CadB in A. veronii TH0426 strain and presence of cadB significantly enhanced the biofilm formation ability of A. veronii [9]. Ultimately, all these mechanisms cause serious infection and mortality in numerous fish species including Nile tilapia, rainbow trout, catfish, Japanese flounder, and sea bass.
The prompt discovery of safe, efficient, uncomplicated, economical, dependable, and fast development of immune induces against the guided antigen is made possible by the in silico design of multiepitope vaccines against microbial pathogens. Epitope-based vaccines have been successfully created in the postgenomic period to stimulate responsiveness against some of the worst human viruses, including chikungunya, ebola, influenza, Nipah, MARS-CoV, rota, and zika [20,21,22,23,24,25]. Previously, the in silico technique in fish had not been developed due to a lack of understanding of the differences between major histocompatibility complexes (MHC class I and II) and human leukocyte antigen (HLA) [26], but recent research on fish species has generated data to enable in silico techniques [26,27,28,29]. Already an in silico technique was effective in predicting epitopes and multi-epitopes with significant responsiveness against Edwardsiella tarda, Flavobacterium columnarie, Vibrio harveyi, marine birnavirus, and Streptococcus agalactiae, harmful pathogens in fish, separately [30,31,32,33,34].
MHC Class I, Class IIA, and Class IIB genes have been isolated and characterized in a wide range of fish species since Hashimoto et al. (1990) reported the first MHC genes in carp [35], including zebrafish [36], turbot [37], red sea bream [38], tongue sole [39], and Nile tilapia [40]. Both MHC class I and class II molecules were originated in the experimental data of the cord and tilapia for starting immune responses against infections. In this regard, the peptide with excellent binding capacities to HLA-A*0201, HLA-B*3501, and HLA-B*3508 might be employed as efficient vaccinations against certain fish diseases [26, 29]. In orange-spotted grouper and pompano, certain MHC Class IIB alleles have recently been found to be linked to viral and bacterial infections [41, 42]. The MHC IIB allele DBB*1001 was found to be strongly related to resistance to Singapore grouper iridovirus in orange-spotted grouper [43]. The DAB*01 allele was linked to immunity to Photobacterium damselae in pompano, while the DAB*04, DAB*05, and DAB*10 alleles were linked to P. damselae sensitivity [42]. In Nile tilapia Oreochromis niloticus, genetic variation in the major histocompatibility complex (MHC) Class IIB was investigated, as well as the relationship between MHC IIB alleles and disease resistance [44]. Resistance to S. agalactiae was found to be significantly connected to the alleles DAB*0107, DAB*0201, and DAB*0302, whereas susceptibility to S. agalactiae was found to be substantially linked to the allele DAB*0701 [44]. It was reported twenty-five MHC IIA alleles in Tilapia, among which DAA*1101 was significantly associated with Tilapia [44]. In addition, The O. niloticus genome was shown to contain at least 28 class I genes or gene fragments [45]. In Osteichthyes, a gene lineage of MHC class II molecules and three MHC class I molecules have previously been found [27]. Within the class Osteichthyes, which includes both marine and freshwater fish species, bony fishes are one of the most varied groups of vertebrates. Previous study reported that non-polymorphic MHC II sublineage E genes, which have a poor expression in immune system tissues, can be found in primitive fishes such as paddlefish, sturgeons, and spotted gar, as well as cyprinids, Atlantic salmon, European bass, channel catfish, turbot, and rainbow trout [46]. In fish, three MHC II molecule sublineages (MHC II-A, -B, and -E) have been found. The presence of MHC I-U and -Z lineage molecules has been demonstrated in zebrafish, Atlantic salmon, medaka, Nile tilapia, three-spined stickleback, spotted green pufferfish, and Mexican tetra, among other fish species [46]. In contrast, all of the MHC I and MHC II molecule lines, each with a distinct number of genes, were discovered only in Atlantic salmon. Furthermore, genotyping of MHC gene polymorphisms using targeted next-generation sequencing (NGS) technologies has recently been established for humans and certain nonhuman animals, and most species, including fish and crabs, have numerous highly similar MHC loci [27, 46, 47].
It is expected that in the coming days, computer-assisted techniques will be increasingly successful in controlling fish diseases [48, 49]. The final vaccine design implemented in this study also included in silico cloning, which could be used for future wet-lab synthesis and animal model testing. Engaged together based on computer-assisted techniques, the main objective of this research was to identify multiepitope from the best antigenic protein to fight motile aeromonads disease in fish species caused by A. veronii. Therefore, this study will further help the entire aquaculture sector to use vaccines against diseases and this novel approach will help the researcher to design a fast and effective vaccine for emerging diseases in fish.