In Thailand, Chitralada 3 tilapia is a strain of tilapia developed from GIFT (Genetic Improvement of Farmed Tilapia), a Philippine ICLARM unit's fifth generation (tilapia GIFT has traditional Thai Chitralada species mixed with it). "Chitralada 3" has been consistently bred because breeding via group selection method was claimed to have achieved success most recently in 2007. With a high yield, excellent survival rate, and good growth, "Chitralada 3" is distinguished by having small, thick, dense, and highly meaty heads [1].
One of the most harmful bacterial diseases for tilapia is Streptococcosis, caused by a group B Streptococcus agalactiae that can lead to a mortality rate reaching up to 90% [2, 3]. The bacteria generate the streptolysin S and O for blood hydrolysis and adhesion of cell surfaces, protect lysozyme, and replicate in serum or blood into target organs [4, 5, 6]. An infected fish exhibits external signs through abnormal behavior (swirling behavior, lethargy, bent bodies, and disorientation) and eye lesions (endophthalmia or exophthalmia), abscesses, skin hemorrhages around the mouth or at the base of the fin, and ascites [7]. The internal signs include septicemia, hemorrhages, and inflammation in the liver, spleen, kidney, heart, brain, eye, intestinal tract, and peritoneum. Adhesions to the peritoneal cavity occur in severe infections, resulting in high mortality and severe economic damage. The epidemiology of this disease is caused by stress conditions (high water temperature, suboptimal oxygen, and overcrowding), horizontal transmission from feces, bacteria via lesions, and weakfish [8]. Accordingly, several antibiotic products and probiotics have been used to control this disease, risking beneficial bacteria equivalence often administered through food and environmental conditions. More environmentally friendly methods must be developed to relieve the problem, and the S. agalactiae vaccine is an exciting option for amendments to this prevalent pandemic [9].
While S. agalactiae attacks the tilapia by leading to immune response as a pathogen exposure response, two different parts exist in the immune system: innate and acquired immunity with cell-mediated and humoral responses. The innate immune system is like a physical barrier (skin mucous) to first prevent the entry of microorganisms and chemicals (cytokines, agglutinins, precipitins, and interferons). Then it provides a generalized response (inflammation, opsonization, and phagocytosis). The macrophages and nonspecific cytotoxic cells are the effectors of the system. In contrast, acquired immunity (AC) is a system of specific antigen molecule responses, including lymphocytes and B-cells, ultimately producing immunoglobulin or antibodies. T-cells and regulator cells are the effectors of AC immunity [10].
Immunoglobulin M (IgM), a classical antibody isotype found in most vertebrates, plays a critical role in the host immune response, performing a variety of functions such as neutralizing specific antigens and activating the complement system [11], agglutination, binding of mannose binding lectin [12], and mediating cellular cytotoxicity [13]. As the main antibody generated in the immunological response to antigens and the first antibody isotype to emerge during ontogeny.
IgM is classified as the primordial immunoglobulin of the adaptive immune response and is found in monomeric and tetrameric forms in circulating blood [14]. IgM can exist in 2 forms, sIgM and membrane-bound (mIgM), which are generated via alternative RNA splicing of the primary transcript of the µ gene [15]. sIgM consists of the variable region and 4 constant domains in the heavy chain, whereas mIgM contains variable region, 3 constant domains and 2 additional transmembrane domains (TM1 and TM2) and acts as a B cell receptor for initial antigen binding [16]. Together with innate immunity factors, it offers the initial line of defense against microbial infection. Until date, the immunoglobulin heavy (IGH) chain gene complex has encoded three primary types of immunoglobulins (Igs) in teleost: IgM [17], IgD [18], and IgT/IgZ. The significant up-regulation of IgM expression upon bacterial challenge in Nile tilapia [13].
Although S. agalactiae vaccines are widely applied in tilapia culture. These vaccines include whole-cell inactivated, live attenuated, recombinant, or DNA vaccines; each type has different preparation and bacteria specificity [19, 20]. However, formalin-killed vaccine (FKC) is a whole-cell inactivated vaccine successfully developed for tilapia and comprises the whole cell and a subunit of dead bacterial cells [21]. The formalin-killed vaccine has been widely demonstrated in tilapia when intraperitoneally injected and has resulted in highly efficacious performance with a relative percent survival (RPS). The FKC vaccine involves adaptive immunity to release immunoglobulin M (IgM) for primary defense with immunological memory. Therefore, the S. agalactiae vaccination is an extensively recognized process to prevent streptococcosis outbreaks and reduce fish mortality in the tilapia industry [22, 23, 24].
This research aimed to compare the immunoglobulin gene expression immune responses of tilapia infected with different S. agalactiae (serotype Ia) concentrations. We also preliminarily investigated the FKC vaccine efficacy as an alternative prevention approach in tilapia against S. agalactiae.