Salmonid Antibacterial Immunity: An Aquaculture Perspective

Abstract

The aquaculture industry is continuously threatened by infectious diseases, including those of bacterial origin. Regardless of the disease burden, aquaculture is already the main method for producing fish protein, having displaced capture fisheries. One attractive sector within this industry is the culture of salmonids, which are (a) uniquely under pressure due to overfishing and (b) the most valuable finfish per unit of weight. There are still knowledge gaps in the understanding of fish immunity, leading to vaccines that are not as effective as in terrestrial species, thus a common method to combat bacterial disease outbreaks is the use of antibiotics. Though effective, this method increases both the prevalence and risk of generating antibiotic-resistant bacteria. To facilitate vaccine design and/or alternative treatment efforts, a deeper understanding of the teleost immune system is essential. This review highlights the current state of teleost antibacterial immunity in the context of salmonid aquaculture. Additionally, the success of current techniques/methods used to combat bacterial diseases in salmonid aquaculture will be addressed. Filling the immunology knowledge gaps highlighted here will assist in reducing aquaculture losses in the future.

Keywords: adjuvants; aquaculture; bacterial pathogenesis; bacterial pathogens; comparative immunology; immunological memory; salmonids; teleost; vaccines.

Figure 1

Schematic depiction of NADPH oxidase activation to enable the respiratory burst activity of phagocytes. The NADPH oxidase enzyme consists of six subunits. (A) When the phagocyte is in an inactive state, two of the subunits (gp91^phox and p22^phox) are transmembrane components, while the remaining four components are cytosolic (p40^phox, p47^phox, p67^phox and Rac2). (B) Upon the phagocyte being activated by external stimuli, the four cytosolic components complex with gp91^phox and p22^phox to form the active NADPH oxidase. When in the active form, the now functional enzyme can convert molecular oxygen into superoxide anions to aid in the degradation or killing of that which was phagocytosed.

Figure 2

Structure of the MH class II molecule. The structure and function of the teleostean MH class II dimer is very similar to mammalian MHC class II. The molecule is a dimer, consisting of an alpha chain and beta chain, both of which have transmembrane regions. As outlined in red, both the α1 and β1 domains of this molecule have a hypervariable region that is part of the peptide binding groove. It is this region that is capable of binding to compatible antigens and presenting it to T cells.