Abstract

Piscirickettsia salmonis is an intracellular bacterium responsible for Salmonid Rickettsial Septicemia (SRS) in Atlantic salmon. This pathogen survives within macrophages, hindering immune clearance. In mammals, CD8+ T lymphocytes represent a crucial component of adaptive immunity, as they specifically recognize and eliminate cells infected by intracellular pathogens. Evidence indicates that an analogous mechanism may also operate in teleost fish. Here, the role of CD8+ T lymphocytes in the immune response against P. salmonis was investigated in Atlantic salmon (Salmo salar). Using autologous dorsal fin-derived target cells and leukocytes from infected donors, an ex vivo model to assess antigen-specific cytotoxicity was established. Fin-derived cells internalized P. salmonis and trafficked the bacteria to lysosomal compartments, supporting antigen processing and MHC-I presentation. Sensitized leukocytes exhibited robust antigen-specific lysis of infected target cells, whereas non-specific lysis was negligible. Depletion of CD8a+ cells from sensitized peripheral blood leukocytes resulted in a loss of cytotoxic activity. These CD8a+ cells expressed CD3 zeta and TCR beta transcripts, confirming the T-cell phenotype of the effector population. Together, these results provide the first functional evidence that Atlantic salmon mount CD8+ T cell-mediated cytotoxic responses against P. salmonis, highlighting cell-mediated immunity as a critical component of host defense, being a promising target for next-generation vaccines against SRS.