The Use of Chitosan-Coated Membrane Vesicles for Immunization Against Salmonid Rickettsial Septicemia in an Adult Zebrafish Model

The introduction of fish vaccination has had a tremendous impact on the aquaculture industry by providing an important measurement in regard to disease control. Infectious diseases caused by intracellular pathogens do, however, remain an unsolved problem for the industry. This is in many cases directly connected to the inability of vaccines to evoke a cellular immunity needed for long-term protection. Thus, there is a need for new and improved vaccines and adjuvants able to induce a strong humoral and cellular immune response. We have previously shown that membrane vesicles (MVs) from the intracellular fish pathogen Piscirickettsia salmonis are able to induce a protective response in adult zebrafish, but the incorporation of an adjuvant has not been evaluated. In this study, we report the use of chitosan as an adjuvant in combination with the P. salmonis-derived MVs for improved immunization against P. salmonis. Both free chitosan and chitosan-coated MVs (cMVs) were injected into adult zebrafish and their efficacy evaluated. The cMVs provided a significant protection (p<0.05), while a small but nonsignificant reduction in mortalities was registered for fish injected with free chitosan. Both free chitosan and the cMVs were shown to induce an increased immune gene expression of CD 4, CD 8, MHC I, Mpeg1.1, TNF, IL-1, IL-10, and IL-6, but to a higher degree in the cMV group. Taken together, the results indicate a potential use of chitosan-coated MVs for vaccination, and that zebrafish is a promising model for aquaculture-relevant studies.