Innate cell-mediated cytotoxicity of CD8⁺ T cells against the protozoan parasite Ichthyophthirius multifiliis in the ginbuna crucian carp, Carassius auratus langsdorfii

Cytotoxic T cells are known to have the ability to kill microbe-infected host cells, which makes them essential in the adaptive immunity processes of various vertebrates. In this study, we demonstrated innate cell-mediated cytotoxicity of CD8⁺ T cells against protozoan parasites found in the ginbuna crucian carp. When isolated effector cells such as CD8⁺, CD4⁺ (CD4-1⁺), or CD8⁻ CD4⁻ (double-negative, DN), from naïve ginbuna crucian carp were co-incubated with target parasites (Ichthyophthirius multifiliis), CD8⁺ cells from the kidney and gill showed the highest cytotoxic activity. On the other hand, DN cells, which include macrophages and CD4⁻ CD8⁻ lymphocytes, showed the lowest cytotoxic activity against I. multifiliis. Additionally, the cytotoxic activity of CD8⁺ cells was found to significantly decrease in the presence of a membrane separating the effector cells from I. multifiliis. Furthermore, the serine protease inhibitor 3,4-dichloroisocoumarin and perforin inhibitor concanamycin A significantly inhibited the cytotoxic activity of CD8⁺ cells. These results demonstrate that CD8⁺ T cells of ginbuna crucian carp can kill extracellular parasites in a contact-dependent manner via serine proteases and perforin. Therefore, we conclude that CD8⁺ T cells play an essential role in anti-parasite innate immunity of teleost fish.