Signal 1 Facilitators and Signal 2 Facilitators, Their Immunomodulatory Compounds, Receptors They Act on and Principal Immune Responses They Elicit
11.2 AQUATIC ORGANISMS’ IMMUNE SYSTEM
170 Fish Vaccines other pathogenic diseases that contribute a lot to social, economic, and environmental sustainabil- ity. The first instance of vaccination was reported in the late 1940s, followed by many vaccines being developed, which significantly reduces the impact of diseases or prevents certain pathogenic diseases (Snieszko et al., 2019). In certain areas of the world, there has been a transition toward vaccination from antibiotics, which has led to annual vaccination of millions of fishes rather than negative implications of antibiotics. For example, in Norwegian salmon farming, there has been a considerable reduction in the use of antibiotics since the application of vaccines, and vaccines have become the most sustainable and cost-efficient method for preventing/controlling various diseases in aquaculture (Horzinek et al., 1997). Usually, a typical aquaculture vaccine consists of antigens, which stimulate adaptive or innate immune response of an aquatic organism in resistance to specific pathogens (bacteria or virus). During the past two decades, the research activities on fish immunol- ogy and fish vaccines have increased in a dramatic way. History, types, advancements, and admin- istration routes of aquaculture-based vaccines along with the prospects and challenges of vaccine development has been described in several researches and review articles by various researchers (Thompson & Roberts, 2016).
Recently, other than injection as a delivery method of vaccine (Plant & Lapatra, 2011), several alternative methods have been developed along with traditional and promising new age adjuvants based on their protective efficacies (Tafalla et al., 2014). In addition, several review articles have focused on present applications of vaccines in large-scale operational fish farms and future perspec- tives on diverse types of vaccines such as DNA, live attenuated, and inactivated vaccines, but still there is a void for the development of aquaculture vaccine technologies and therefore the vaccine development sector needs comprehensive additional information (Sommerset et al., 2005). The pre- ponderance of licensed vaccines is produced by means of conventional methods that are initiated by culturing target pathogenic organisms. A variety of severe fish diseases has been protected by the abovementioned array of vaccines; aquatic vaccinology can be understood by having a detailed knowledge in two branches of science: immunology and microbiology. The development of vaccines for animals and human use can be obtained by the knowledge advancement in protective antigens and molecular biology (Kim et al., 2016). According to Cimica and Galarza (2017), modern vaccin- ology targets specific pathogenic components and vaccines that are developed by these approaches, which might include recombinant DNA or subunit vaccines, whereas these vaccines contain novel antigens that are produced through several expression systems. Globally, the developed RNA particle vaccines induce higher level of immunity when compared to other vaccines (Frietze et al., 2016).
Even though these advancements are promising, in commercial aqua farming, the implementation/
application of mass vaccination is still somewhat limited than in laboratorial conditions due to prac- tical challenges in the aqua environment (Dhar & Allnut, 2011). In this chapter, we describe the application of various molecular approaches and the conventional aquaculture vaccines in large-scale aqua farming systems and challenges, and limitations in mass vaccination and future perspectives of aquaculture vaccine technology and its implication in the aqua environment.
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phagocytosis, nodulation of pathogens, cytotoxicity, hemolysis, and melanization of pathogens are all actions performed by circulating hemocytes. Hemocytes have two types of secretory granules:
big and tiny granules, which carry a variety of defensive molecules.
The morphology and staining features of crustacean hemocytes are used to classify them.
Generally, hemocytes are differentiated into three types: granular cells (GC), agranular hyaline cells (HC), and semigranular cells (SGC) (Figure 11.1). Hyaline cells have been studied in the
FIGURE 11.1 Defense mechanism in aquatic organisms. [Created by www.biorender.com.]
172 Fish Vaccines crayfish, the swimming crab, and the Penaeus monodon (Lin & Soderhall, 2011; van de Braak et al., 2002; Hammond & smith, 2002). Smaller globular cells with a bigger central or eccentric nucleus bounded by cytoplasmic granules are known as SGC. The number of tiny eosinophilic granules in SGC varies, although the granules exhibit weak staining characteristics. Granular cells are the biggest hemocytes with a bilobate nucleus, kidney-shaped, and contain differing stain properties for various eosinophilic secretory granules. As a result, certain HCs are eosinophilic, whereas oth- ers are partially basophilic and are stainless in some cases. In P. vannamei, however, five physi- ologically active and morphologically different hemocyte subpopulations were extracted using a unique iodixanol density gradient centrifugation approach that differed from the classic Percoll gradient procedure (Dantas-Lima et al., 2013). Hemocytes perform various immune activities and consist of different biochemical properties rather than just morphology. Because of the sensitivity and responsiveness of crustacean hemocytes, functional studies have been difficult, and there have been a number of improvements in methodology and upgrades for extensive investigations of the crustacean distinct hemocytes’ features.
Generally, HCs are responsible for phagocytosis in crustaceans, whereas SGCs are responsible for melanization, encapsulation, and coagulation, as well as phagocytosis in certain species (e.g., Macrobrachium rosenbergii and Penaeus japonicas). The prophenoloxidase (proPO) activation mechanism, cytotoxicity, melanization, and the secretion of antimicrobial peptides are all carried out by GCs containing multiple, large eosinophilic granules. Hemocytes are activated and typically localized in the area of damage or at the surfaces of invading microorganisms during melanization;
in order to kill or immobilize invaders, toxic phenol or melanin intermediates are created during the melanization process. According to Liu et al. (2007), melanization and phenoloxidase activity are important in establishing protective immunity in freshwater crayfish against severe pathogens. Since then, RNA-interference suppression of crayfish prophenoloxidase has resulted in greater bacterial growth and mortality, as well as reduced phagocytosis. With the existence of pathogen-associated molecular patterns (PAMPs), such as Gram-negative bacteria’s lipopolysaccharides (LPS), Gram- positive bacteria’s lipoteichoic acid (LTA), and peptidoglycans (PGN), nucleic acids derived from various microbial pathogens or β-glucans of fungal cells, GC, SGCs, and hemocytes endure hasty degranulation reactions (Amparyup et al., 2012). PAMPs are recognized by pathogen recognition receptors (PRRs) on hemocytes, which then in circulation, release a multitude of effective immune effector molecules near the PAMPs. Antimicrobial peptides (AMPs), lipases, proteases, reactive oxygen species, such as O2 and H2O2, cell adhesion molecules, cytokine-like molecules and cyto- kines, lysozyme, nitrogen intermediates, complement components (only C1q is known), and clotting proteins (CPs) are all examples of effector molecules (Wang & Wang, 2013). The hemocytes’ proPO system is also triggered when PAMPs are recognized. Invertebrates have the most conserved innate immune mechanism that is known as ProPO activation. Pathogens get melanized, and leads to the production of cytokine-like factors, antimicrobial peptides, and other compounds that are consid- ered to assist in iron sequestration and opsonization (Tassanakajon et al., 2017).
The clotting system is a critical component and one of the frontline defenses against invading pathogens in the humoral system. It serves an essential function in limiting hemolymph loss during wound healing and injury when the crustacean exoskeleton barrier is damaged. Shrimp and other crustaceans appear to use the basic coagulation pathway, which involves a calcium-dependent trans- glutaminase catalyzing the cross-linking of aggregates of CPs (TGase). Many shrimps have been shown to have Tgase, which is a conserved protein that is involved in coagulation as a defensive mechanism in several invertebrates. Aside from acute immunological synapse, hemocytes are key sources of antimicrobial peptides, lectins, proteinase inhibitors, and opsonins such as peroxinectin, a cell adhesion protein. Even though functional studies of hemocytes of crustaceans have been car- ried out during the 1980s, they have been difficult due to the mechanisms such as sensitivity and reactivity of hemocytes, and there have been many technical breakthroughs over these years that have enabled comprehensive studies and elucidation of immune defense mechanism of hemocytes.
The gene silencing/RNA-interference (RNAi) approach (Shockey et al., 2009) was established for
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studying the involvement of several proteins in hemocyte immune processes such as phagocytosis, virus defense, coagulation, and proPO (Visetnan et al., 2015). Clark and Greenwood attempted to use high-throughput next-generation sequencing technology for immune gene annotation of crusta- cean in the transcriptome of the American lobster (Homarus americanus). Larger invading micro- organisms that are too large to be removed by phagocytosis are destroyed by means of nodulation.
Multiple layers of hemocytes confine the microbes, and surround them, eventually proPO-mediated melanization end up killing them (Im et al., 2016). By detecting PAMPs and producing effector molecules, multiple conserved signaling pathways are engaged in the frontline of immune defense system to resist pathogen infections. The JAK/STAT, IMD, and toll signaling pathways, which in vertebrates govern the release of pro-inflammatory chemokines, interferons, AMPs, and cytokines, and are thought to exist in shrimp, are likely directly implicated, albeit little is known about their function or activation (Sun et al., 2017).