Local immune responses to two stages of Ichthyophthirius multifiliis in ginbuna crucian carp.

Ichthyophthirius multifiliis is a ciliated protozoan parasite and is known to infect many freshwater teleosts. Characterizing the immune system in epithelial tissues, where the parasites penetrate and settle, is key to understanding host-parasite interactions. This study examined local immune responses in vivo to the infective stage (theront and trophont) of the parasites using intra-fin administration, which has been developed to analyze in vivo immune responses using fish fin. CD8α+ and CD4+ T-cell compositions were increased significantly in the fin cavity injected with theront or trophont antigens. The expression of GATA-3 and T-bet mRNA, which regulate differentiation of helper T-cells, was upregulated significantly in leukocytes from the trophont antigen-injected site. In contrast, the percentages of macrophages and neutrophils, which are innate immunity components, were decreased significantly in the injection sites. These results suggest that I. multifiliis antigens inhibit the migration of macrophages and neutrophils, and T-cells are the first responders to I. multifiliis. Thus, to better understand the interaction of host immunity and I. multifiliis, further studies should focus on exploring the inhibitory factors from I. multifiliis or examining innate functions of teleost T-cells.

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.