About this Research Topic
Fish vaccination is an important disease control strategy that has contributed to the reduction of disease outbreaks and antibiotic use in aquaculture in the past decades. However, most of the vaccines in use to date correspond to vaccines against bacterial pathogens that are delivered via intraperitoneal injection. On the other hand, there are still a large number of pathogens for which effective vaccines are not available, mostly against intracellular pathogens and parasites. Important emerging infectious diseases, such as piscine reovirus and piscine myocarditis virus, are currently the focus of vaccine development efforts using recombinant technologies based on heterologous vectors for the expression of vaccine antigens. The vast losses caused by intracellular bacterial pathogens, such as Edwardsiella tarda or Piscirikettsia salmonis, call for a shift from the traditional inactivated whole vaccines (IWC) that best target extracellular pathogens towards vaccines that should evoke cell-mediated immune responses to ‘kill’ and eliminate infected cells in vaccinated fish. Finally, the unique biology of other fish species that have long fresh- and seawater production cycles may require primer-boost vaccination regimes in order to maintain protective immunity for longer time periods. Altogether, these examples illustrate that there are still a great number of challenges in fish vaccinology.
Research on fish vaccines has improved our understanding of the mechanisms of antigen uptake, processing and presentation to cells of the adaptive immune system. In this sense, for example, the characterization of fish T-cell receptor (TCR) chains that bind to MHC ligands on antigen presenting cells (APCs) suggests that the mechanism of activation of naïve CD8 T-cells is conserved across the vertebrate taxa. Moreover, ‘killing’ of infected cells by cytotoxic T-lymphocytes (CTLs) has been demonstrated in vaccinated fish. Recent studies on the kinetics of various transcription factors and cytokines that specify the differentiation of naïve CD4 cells into various T-helper (Th) subtypes has shown that cytokine signatures expressed by APCs are predictive of humoral and CMI polarization. Overall, these advances in the field of fish immunology mean that we are better positioned to design new fish vaccines and to evaluate their efficacy.
While a good understanding of the expression patterns of signature cytokines and immunomodulatory factors during immunization has been achieved thus far, less is known whether so-called immunostimulants (e.g. adjuvants) can be utilized to skew vaccine-induced responses into desired directions. Similarly, immunostimulants may also (without vaccine antigens) induce certain cytokine expression patterns that may further condition a posterior antigen-induced response.Finally, the compartmentalization of fish immunoglobulins has also raised great interest in the field over the last decade, as IgT has been postulated as a gate keeper at portals of pathogen entry through mucosal surfaces while IgM responses seemed mostly centered on preventing pathogen dissemination and disease establishment in vaccinated fish at a systemic level.
Taken together, the aforementioned considerations warrant collaborative efforts between studies focused on fish immunology (host immune responses) and those focused on vaccinology (antigens plus adjuvant/immunostimulant formulations) with the goal to define the immunological mechanisms through which vaccines confer protection in fish. As an initiative to foster this collaborative research, in this Research Topic, we welcome the the submission of articles that cover the following sub-topics:
1. Design and testing of different immunostimulants and vaccine formulations in fish.
2. Application of various recombinant technologies such as DNA and subunit vaccines (use of plants, microalgae, viral and bacterial vector systems for expression vaccine antigens).
3. Application of various attenuation strategies to develop live vaccines.
4. Prime-boost vaccination strategies and their protective role in vaccinated fish.
5. Mechanisms of molecular adjuvants, immunostimulants (β-glucan, Poly(I:C), LPS, etc.) and feed formulation in enhancing defense mechanisms and protective immunity in fish.
6. Studies on mechanisms of antigen uptake, processing and presentation in various fish species.
7. Studies on protective mechanisms of cell-mediated and humoral immunity in fish.
8. Characterization and profiling immune response genes by gene expression, cloning and omics technologies (e.g. RNA-seq).
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