2.1 Introduction
Fish are a wide and heterogeneous group of vertebrates that comprise about 40,000 species, divided into three classes: Agnatha (jawless fish represented by hagfish and lampreys), Chondrichthyes (cartilaginous fish comprising sharks, rays, and skates), and Osteichthyes (bony fish).
Fish heterogeneity is based on many aspects of their biology and habitats. There are fish species adapted to the most diverse aquatic environments with significant differences in morphology, size, physiology, and behavior.
These differences evolved in parallel with the fact that fish have undergone a second whole-genome duplication event (2R), following the ancient genome duplication that occurred in early vertebrates (1R), and a further duplication in the teleostean lineage (3R), all of these leading to the subsequent duplication or deletion of various genome parts (Figure 2.1) (Danzmann et al., 2008; Meyer and Van de Peer, 2005; Ravi and Venkatesh, 2008).
One of the first consequences of these genomic rearrangements during the evolution of the fish families is that some immune molecular families have expanded tremendously in some fish species, providing important differences with mammals and opening an exciting field to understand the functional effects of this molecular diversification. In vertebrates, immunity was classically divided into two components: the innate immune response and the adaptive or acquired immune response. Innate immunity is the first line of defense against infection and includes both physical barriers as well as humoral and cellular responses.
The adaptive immune response also has humoral and cellular players and is characterized by a specific antigen recognition that drives a stronger and faster secondary pathogen-specific immune response. In light of the discoveries from the past 20 years, an inseparable interrelationship among innate and adaptive components of the immune response has proven more complicated than previously thought, and many of the cells/molecules assigned to either the innate or acquired systems have specific roles in both of them.
This is why, in this chapter, we will not make this distinction, but describe each immune cell type or factor indicating their role in either innate or acquired responses. In a general view, the encounter with a pathogenic organism via mucosal tissues, such as gills, skin, or gut, is primary blocked or limited by physical barriers such as the mucus, the scales, and the epithelium.
The mucosal layer interferes with the pathogen not only by trapping it, but also through the action of a variety of antimicrobial factors present in the mucus like lectins, lysozymes, pentraxins, complement proteins, antibacterial peptides, and immunoglobulins, which are destined to directly eliminate the infectious agent (Alexander and Ingram, 1992b; Ellis, 2001).
If the pathogen succeeds in penetrating the epithelium, it then encounters the complete cellular and humoral machinery of the immune system, triggered first by cell types bearing invariable receptors called pattern recognition receptors, which are able to recognize common conserved molecular characteristics of many microbial agents.
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Simultaneously, the primary responses of antigen-specific lymphocytes bearing variable receptors that are able to specifically recognize molecules distinctive of a pathogen will also be activated, setting the basis for further secondary responses (Figure 2.2).
Figure 2.1 Schematic representation of the living-fish phylogeny (Near et al., 2012).
Two rounds of whole-genome duplication (WGD) occurred early in the history of vertebrate evolution. WGD event in the ancestral vertebrate lineage duplicated the ancestral possibly cephalochordata-like genome to two (1R), and then to four genomes after a second WGD (2R) before the divergence of fish and tetrapods. Recent findings on lamprey genome sequentiation suggest that this second WGD occurred before the divergence of Agnatha and Gnathostomata (Smith et al., 2013). In the last 10 years evidence has accumulated for a ray-finned- (actinopterygii) specific genome duplication (3R) about 350 million years ago, leading initially to eight copies of the ancestral deuterostome genome. Most duplicated genes were secondarily lost or evolved new functions (Danzmann et al., 2008; Meyer and Van de Peer, 2005; Ravi and Venkatesh, 2008). Whether this third WGD occurred in the whole actinopterygian group or in part of the divergent lineages, in example salmonids and cyprinids, needs further investigation.
Figure 2.2 General mechanisms of the fish immune response.
The encounter with a pathogenic organism via mucosal tissues, such as gills, skin, or gut, is primarily blocked or limited by physical barriers such as the mucus, the scales, and the epithelium. The mucus contains different humoral components with antimicrobial activity such as complement factors, lysozyme or Igs. In case the pathogen succeeds to penetrate the epithelium, it encounters the innate cellular machinery, triggered in a first step by those cell types bearing invariable receptors called pattern recognition receptors (PRRs), able to recognize common conserved molecules (PAMPs) characteristic of many microbial agents. The uptake of the antigen leads, on one hand, to the release of cytokine mediators and attractants for different cell types to unleash the inflammatory process and, on the other hand, to the antigenic presentation through the MHC in the lymphoid tissues for the activation of the primary responses of antigen-specific lymphocytes bearing variable receptors able to specifically recognize molecules distinctive of the pathogen, setting the bases for further secondary responses and memory.