Main Components of Fish Immunity - 6. Immunoglobulins

 

6. Immunoglobulins

 

In bony fish and other jawed vertebrates, the adaptive immune system relies on B and T cells and on the tremendous variety and specificity of their antigen receptors, the immunoglobulins (IG) or antibodies, and the T cell receptors, respectively [165,166]. Immunoglobulins are the main constituents of the immune response against pathogens [30]. Among the mechanisms implying humoral immunity mediated by IGs are pathogen elimination via phagocytosis, toxin and virus neutralization, and complement cascade activation [167,168].

 

In bony fish, three isotypes of IGs are produced by B cells: IgD, IgM, and IgT/Z. Therefore, three major B cell lineages have been characterized in teleosts [169].

 

The predominant and most ancient immunoglobulin in fish is the IgM class tetramer, which contains eight antigenic combining sites [170]. Although IgM can be expressed at the surface of B cells, secreted tetrameric IgM is considered the most predominant IG in fish serum [166,171]. Many studies have reported that mucosal and serum noncovalently accompanies IgM monomers in diverse fish species [172,173]. In rainbow trout, a new teleost fish Ig was detected and was named IgT.

 

This new IG was identified as IgZ in zebrafish [174]. This IgT has been distinguished in most studied teleost fish, except for medaka, channel catfish, and turquoise killifish [175–177]. In rainbow trout, at least three subclasses of IgT are expressed. Mucosal and systemic lymphoid tissues have been reported to express the IgT1 subclass, while IgT2 has been documented to be mainly expressed in systemic lymphoid organs. Furthermore, IgT3 protein is reported in rainbow trout serum [178]. IgD is the second immunoglobulin isotype characterized in fish, specifically channel catfish. As a result of its sequence similarity with mammalian IgD, its immediate location under the IgM gene, and its expression on B cells, IgD has been suggested to be as old as IgM [168,179,180]. The secreted form of IgD has been interestingly identified in trout and catfish, with a different structure [175,181].

 

7. Cytokines

Cytokines play an essential role in the immune system through their specific receptors binding to the cellular membrane, providing a cascade-enhancing induction, stimulation, or suppression of nuclear cytokine-regulated genes. Several critical cytokines have been recognized in teleost fish, including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), transforming growth factor-β (TGF-β), interferon (IFN), and many chemokines [182–185]. Recently, numerous other cytokines have been identified in teleosts [186–189].

 

The interferons (IFNs) are the key cytokines present in all jawed vertebrates and are involved in antiviral immunity [190]. In vertebrate host cells, IFNs play a critical role in the defense mechanism against viral infection through IFNα/β secretion upon the recognition of viral nucleic acid [191]. These IFNs keep other cells away from viral infection via binding to different receptors, resulting in the induction of multiple genes, where some of these genes encode antiviral proteins [192,193].

 

The IFNs have been categorized into three broad groups, where type I and III are originally embroiled in antiviral defense, while type II IFN has a broader function and shares in cell-mediated immunity in response to a wide variety of pathogens during adaptive immune responses [190].

 

IL-1β and TNFα are considered the proximal cytokines that are first released and boost response through enhancing the secretion of distal cytokines such as IL-6 and IL-8.

 

As reported in zebrafish, IL-1β is produced as a precursor molecule that is cleaved to release a bioactive mature protein, which is linked to the activation of the inflammasome complex that is accountable for the secretion of IL-1β [198]. IL-1β is produced by various cell types following the stimulation of host pattern recognition receptors through PAMPs or DAMPs [199–201].

 

IL-1β improves vaccine efficacy as it boosts antibody production when administered with bacterial vaccines [202,203]. In primary leucocytes and macrophages, IL-1β action has been extensively analyzed, where induced expression of pro-inflammatory genes such as TNF-α, IL-1β, and IL-6 is evident [204–206].

 

Furthermore, IL-1β activates the expression of genes that are considered to be immune response suppressors [207]. IL-1β genes have revealed wide variations among fish species.

 

In carp, two IL-1β genes were found: IL-β1 and IL-1β2 [208]. Three functional IL-1β genes (IL-1β1–3), in addition to pseudogene (IL-1β4), have been identified in salmonids [204], while only a single type II IL-1β was detected in gadoids [209].

 

IL-18 is an essential cytokine responsible for the induction of IFN-γ production and promotion of TH1 immunity in vertebrates. It exerts an essential role in regulating inflammation within mucosal tissues [210].

 

IL-18 has been described in a few fish species including elephant shark, pufferfish, rainbow trout, and seabream [211–214]. IL-18 is fundamentally expressed in fish immune or non-immune tissues, unlike IL-1β, which is induced by inflammatory stimuli [214].

 

A recent study has suggested that IL-18 is likely developed in an evolutionary-independent event to IL-1β [215].

 

TNFα is a member of the TNF superfamily, which primarily comprises type II transmembrane proteins, which can be cleaved to release a soluble cytokine. TNF-α presence has been reported in teleosts [216–218]. In carp, many TNF-α copies have been detected [219]. In catfish cell lines, TNF-α expression has been observed in T cells and macrophages; however, there are no expressions in B cells or fibroblasts [220]. In salmonids, four copies of TNF-α have been reported [205,221].

 

Several fish studies have suggested that TNF-α and -β are essential macrophage activators. Previous studies in some fish species, including goldfish, rainbow trout, sea bream, turbot, and catfish, have revealed that TNF induces macrophage activation, resulting in increased respiratory activity, phagocytosis, and nitric oxide production [222,223].

 

IL-6 was first discovered in teleost fish [187,224]. It is a member of the IL-6 family, and is considered a pivotal acute phase response cytokine. Moreover, it is implicated in the differentiation of B cells into antibody-secreting cells [225].

 

It is highly expressed in the kidney and spleen, and during caudal fin regeneration following amputation [226]. IL-6 has been used as a vaccine adjuvant, to achieve higher serum antibody levels for protection [227].

 

In salmonids, two IL-6 genes have been detected (IL-6A and IL-6B) [227,228].

 

IL-11 is one of the other members of the IL-6 cytokine family related to inflammatory responses. It was first detected in teleosts in the bacterial-challenged fish [229].

 

Two IL-11 types (IL-11a, IL-11b) have been reported in teleosts, where IL-11a is highly expressed in the intestine and gills of trout [229,230].

 

TGF-β is one of the TGF-β protein family. It plays a fundamental role in various functions, including cell proliferation, differentiation, and regulation. In fish, three TGF-β proteins have been detected (TGF-β1, TGF-β2, TGF-β3) [231].

 

TGF-β1 is the main immune active isoform, and two TGF-β1 paralogues (TGF-β1a and TGF-β1b) have been found in zebrafish [232].

 

In carp, TGF-β1 is highly expressed in the spleen [233], and it can induce expressions of pro inflammatory cytokine and IL-10 [234]. In rainbow trout, three paralogues of TGF-β1 have been reported [235], with expression highest in lymphoid tissues. The highest levels of TGF-β1 expression have been found in macrophages and the gills of seabream [236].

 

 

8. Immune Cells—Nerve Interaction(보류*)

 

*NEUROIMMUNE INTERACTIONS: FROM THE BRAIN TO THE IMMUNE SYSTEM AND VICE VERSA 선행중

 

To monitor infections and inflammation and to take appropriate action, the neurological and immune systems work in concert as an integrated physiological system. The nervous system, including the brain and peripheral divisions, can stimulate or inhibit a variety of immune system functions, including those of the innate and adaptive immune systems.

 

It has been established that the anti-bacterial peptides (AMPs), which are signaling molecules involved in the antimicrobial immunity, are synthesized by the enzymatic processing of neuropeptide precursors such as proenkephalin A (PEA) and chromogranin B [237,238].

 

It has been shown that some peptides involved in neuronal or neuroendocrine signaling also have potent antibacterial activities. Additionally, they are widely distributed across immunological, endocrine, neuroendocrine, and neuronal cells [239]. The microbiota–gut–brain axis (MGB axis) is based on the reciprocal interactions between the CNS and gut microbiota that involve endocrine, immunological, and neurological pathways [240].

 

In the MGB axis, lymphocytes stimulate the gut lumen and internally release cytokines, which are then activated by gut peptides released by enteroendocrine cells from sensory nerve terminals, such as those of the vagus nerve. To create the link between the immune system and the central nervous system (CNS), which results in tissue homeostasis, communication pathways based on neurotransmitters, neuropeptides, cytokines, hormones, and growth factors are required [241]. The innervation of lymphoid tissue has been investigated in the Coho salmon (Oncorhynchus kisutch) spleen, where nerve fibers are accompanied by the vascular supply and MMCs. Additionally, adrenergic receptors are among the neurotransmitter receptors that are expressed by immune cells. Adrenergic receptors have so far been sequenced in the leukocytes of zebrafish, trout, catfish, and goldfish. [242]. The activation of the sympathetic nervous system can modulate the production and circulation of leucocytes, through activation of hematopoietic stem cells and NK cells, upregulation of myelopoiesis, and production of monocytes and neutrophils by the spleen [243]. Gut neuropeptides, such as gut antimicrobial neuropeptides, constitute a complex network between the neurological and immunological systems, where they play a crucial modulatory role [244]. In response to various stimuli, both enteroendocrine cells and enteric neurons release gut neuropeptides. Most neuropeptides created by neurons during an immune response have neuroendocrine properties that can affect both the stomach and the brain [245]. The role of the mast cells, mucous cells, and neuroepithelial endocrine cells (NECs) of teleosts’ gills in innate and adaptative immunity has been examined in several studies. These substances, such as immunoglobulins (Igs), antibody-secreting cells, and piscidins, are produced by these molecules or cells through immune-related pathways [246,247]. Piscidin 1, an antimicrobial peptide, and enkephalin, were found to be co-localized in fish gill NECs and the neurological system of the gill filaments, suggesting a potential role in the bidirectional relationship between endocrine, immune cells, and their interaction with the filament nervous system [111,248]. Investigations have been conducted into the possibility of tight communication between immune cells and afferent nerve terminals in the gill. The eosinophils in the gill lamellae of Boleophthalmus are immunostained with antibodies against Pis 1 and 5HT (serotonin), and they come into contact with neurons that are also immunolabeled with antibodies against acetylated tubulin [111]. Many neuroactive substances and AMPs, including 5-HT, piscidin 1, nitric oxide (NO), tyrosine hydroxylase (TH), and the nicotinic acetylcholine receptor, are found in the terminals of afferent nerves. These substances are released by mast cells and eosinophils. Additionally, it has been discovered that eosinophils and NECs include neuropeptide receptors, such as the GABA B R1 receptor, which suggests that the immune and neurological systems interact closely [249]. During lung infections, neuro-immune interactions also take place at the surface of the lung air barrier. Bacterial pathogens are killed by the neuropeptide calcitonin-gene related peptide (CGRP). In addition to sensory nerves, the pulmonary neuroepithelial endocrine cells (PNECs) also express CGRP. PNECs have been identified to be co-localized with innate lymphoid cells (ILC2) [250]. Cytokines or other mediator molecules stimulated by cytokines would be accessible to the fish brain. However, the preoptic region of the carp brain contains the cytokine receptor IL-1RI mRNA [251]. Prostaglandins, which are smaller, neuroactive, and lipophilic, can be the mediators [252]. There is evidence that immune mechanisms that signal to the brain in fish are similar to those in mammals. It has been suggested that the vagus nerve could act as a channel for cytokines to reach the brain [252]. However, because stresses can increase peripheral cytokines by stimulating pathways such as the noradrenergic pathway, these, in turn, may cause a rise in brain IL-1 via one of the immune to brain pathways. The role of cytokines in the activation of the HPI and sympatho-chromaffin (SC) axis and the central regulation of the stress reaction in fish is not well understood because there have been few studies that have demonstrated the signaling pathways from peripheral blood molecules to the brain. Adrenaline has been demonstrated to modulate cytokines in fish [253], which can cause the signaling pathways to the brain to activate. Pro-inflammatory cytokines in the brain decrease the activity of neurotransmitters such as noradrenaline, serotonin, and dopamine, which will have a substantial impact on how behavioral patterns are modulated [254]. To further integrate peripheral pro-inflammatory and antigenic signals with physiological and behavioral response, cytokines also activate a number of other signaling pathways in the brain. Under stressful conditions, all these processes will cooperate to combine immune cells’ necessary protective response with the physiological and behavioral response. Finally, other forms of indirect control, such as those connected to the relationship between the gut and the brain, can also exist in fish. It has been demonstrated that information is transmitted between the two organs via chemical signals in both fish and mammals [255]. As a result, when the neuro-immuno-endocrine relationship is effectively initiated, other processes such as reproduction or growth will also be impacted. The Sockeye salmon (Oncorhynchus nerka) provides a clear case study, as degeneration of various glands and organs has been linked to extremely high steroid levels and the loss of immunocompetence, particularly innate responses, during the migratory period [256]. In order to gather energy that ensures the fish can spawn, cortisol and other reproductive steroids redirect energy from numerous biological processes, including immunological functions. Sex steroids have also been found to modulate the cortisol production in the internal tissues of salmonids [257].

 

9. Conclusions

The immune system of the fish comprises innate and adaptive immune responses. Among these, the innate immune response is essential for combating pathogens and for providing resistance to diseases because of delayed adaptive immune response. Three major parameters are responsible for the innate immune response: physical, humoral, and cellular. Furthermore, there is a complicated cytokine network in fish for the regulation and activation of their immune system, and for producing appropriate protective responses to various pathogens. The data described here will facilitate better understanding, determination, and modulation of the protective immunity pathways for improving fish health. Furthermore, this knowledge on fish immunology will facilitate the development of vaccines designed to target immune mechanisms responsible for disease protection.