Live attentuated bacteria seen as a promising weapon against SRS

The fight against Piscirickettsia salmonis, the bacterium that causes salmonid rickettsial septicaemia (SRS), may require a change in approach to how vaccines for salmon farming are evaluated and designed.

This challenge was addressed by Chilean researchers in a recently published review in which they raise the need to move towards strategies capable of more effectively stimulating cellular and trained immunity.

One of the authors is Dr Alexis Kalergis, a scientist at the Pontifical Catholic University of Chile, who participates in initiatives aimed at developing new vaccine platforms against P. salmonis. Among these is a project by the Technological Consortium for Preventive Immunity (CTIP), focused on developing recombinant live attenuated bacteria against SRS.

In conversation with Fish Farming Expert's Chilean sister site, Salmonexpert.cl, Kalergis explains why the intracellular biology of P. salmonis presents a particular challenge for conventional vaccines, what potential platforms such as lipid nanoparticles and recombinant bacteria have, and how trained immunity could open the door to broader protection against different pathogens.

What is the difference between a response based primarily on antibodies and a cellular immune response against P. salmonis?

The main difference between an antibody-based immune response and a cell-mediated immune response against P. salmonis lies in how these immune components access the pathogen. Because P. salmonis is an intracellular bacterium, its biological niche prevents direct contact with antibodies, which circulate in the extracellular environment. Therefore, a purely humoral response is ineffective. Cell-mediated immunity, on the other hand, plays a crucial role in the active elimination of cells infected by the bacteria and removes those that serve as reservoirs for the pathogen. CD4+ T lymphocytes secrete immunomodulatory factors called cytokines (IFN-γ, TNF-α) to activate the microbicidal capacity of macrophages, while CD8+ T lymphocytes and NK cells destroy infected cells using molecules called perforins and granzymes. This directly correlates with a lower bacterial load and increased fish survival.

The review suggests that stimulating cellular immunity (ICM) is necessary. What are currently the most promising strategies for achieving this in fish?

One of the most promising strategies, in our view, based on our vaccine studies and previous experience with immunogenic formulations to induce cellular immunity, is the use of live attenuated bacteria. Due to their intracellular nature and immunogenicity, these live attenuated bacteria act as a natural adjuvant. This platform potently activates cellular immunity, promoting responses with optimal cytotoxic profiles against intracellular pathogens. Studies conducted in species such as Japanese flounder and zebrafish have demonstrated the ability to confer protection against marine bacteria and other pathogens. Based on this, these live attenuated bacteria can be genetically modified to express heterologous antigens and act as a recombinant vaccine against other intracellular pathogens of interest.

On the other hand, the use of lipid nanoparticles (LNPs) has been explored as vehicles that allow the introduction of protein antigens or mRNA and DNA sequences of interest into the cytosol to subsequently promote their antigen presentation. In this context, bioinformatics plays a fundamental role in identifying which specific proteins or epitopes have a high affinity for fish antigen-presenting molecules, thus ensuring the effective activation of a cellular response against pathogens such as P. salmonis.The study analyzes technologies such as lipid nanoparticles (LNPs) and recombinant bacteria like BCG. What advantages do these platforms offer compared to conventional vaccine formulations?

Unlike conventional formulations, based primarily on inactivated or attenuated bacteria or recombinant proteins, which are insufficient on their own to generate a fully protective or long-lasting cellular response against intracellular pathogens, new platforms such as lipid nanoparticles act as advanced immunogenic delivery systems that encapsulate and protect both protein antigens and genetic material. This significantly increases the immunogenicity of recombinant proteins, which, currently formulated with adjuvants such as oils, have not achieved the desired effectiveness.

On the other hand, these live attenuated bacteria offer key biotechnological advantages, such as low manufacturing cost, high production efficiency, great stability, and the absence of a need for additional adjuvants, since they are inherently highly immunogenic. At the immunological level, they provide robust and prolonged protection that not only induces a potent cellular and humoral response but also activates trained immunity—that is, the prolonged activation of cells of the innate immune response—conferring cross-protection against other pathogens.

One of the central concepts of the study is “trained immunity.” How does this mechanism work in fish, and how does it differ from traditional adaptive immunological memory?

Trained immunity in fish corresponds to a phenomenon of immunological memory developed exclusively by cells of the innate immune system (such as macrophages, neutrophils, and NK cells) and is characterized by its ability to confer a cross-response to different pathogens. Unlike traditional adaptive memory, which depends on T and B lymphocytes and generates specific responses, this prolonged response is mediated by the innate immune system.

How long will it take for these strategies to become concrete tools against SRS and contribute to reducing antibiotic use?

For these strategies to translate into practical tools, it depends not only on investing in research for their experimental validation, but also on how the regulatory frameworks applicable to immunological products for fish evolve. Furthermore, our review article suggests that current vaccine efficacy assessments are often limited to antibody quantification and conventional survival assays, which are insufficient to more accurately predict their effectiveness against intracellular pathogens such as P. salmonis. Therefore, efforts should be made to incorporate the measurement of cellular immunity markers, as well as metabolic and epigenetic indicators of host immune cells linked to trained immunity, into efficacy assessments.

In our research group, we are already working on the development of recombinant live attenuated bacteria against P. salmonis within the framework of the Technological Consortium on Preventive Immunity (CTIP), an initiative led by BiotherVax at the Catholic University of Chile (UC) and also funded by CORFO (Chilean Economic Development Agency). The project also includes the participation of a significant group of Chilean salmon producers as collaborating entities, through Yelcho and Veterquímica. UC, for its part, participates as a co-executor of the project. The focus is on research, building upon local capabilities to combat this pathogen and improve production conditions and competitiveness in the region.

Led by Kalergis and Dr Pablo González, the project also aims to strengthen the training of new researchers and PhDs in the development of vaccines against salmonid pathogens, with the active participation of PhD candidate Esteban Valdebenito. The team is confident that this strategy will soon show the expected results in controlling SRS and making a concrete contribution to a more sustainable salmon farming industry by reducing antibiotic use.