Helper function of CD4⁺ lymphocytes in antiviral immunity in ginbuna crucian carp, Carassius auratus langsdorfii

Although many recent studies have suggested that CD4⁺ helper T cell (Th-cell) functions are well conserved among teleost fishes and mammals, there is little evidence that CD4⁺ Th-cells in fish are actually involved in both humoral and cell-mediated immunity during a secondary immune response. In the present study, adoptive transfer using clonal ginbuna crucian carp and crucian carp hematopoietic necrosis virus (CHNV) was used to investigate the functions of CD4⁺ cells during humoral and cell-mediated immunity. With regard to humoral immunity, transplanting CHNV-sensitized donor cells, containing CD4⁺ cells, into naive fish induced more rapid and stronger antibody production than by transplanting non-sensitized donor cells or sensitized donor cells lacking CD4⁺ cells. During cell-mediated immunity, no significant differences were found in recipients that received sensitized cells regardless of whether the donor cells contained CD4⁺ cells, although recipients that received both sensitized donor cells (with and without CD4⁺ cells) exhibited more efficient cell-mediated cytotoxicity than those that received non-sensitized donor cells. These findings suggest that inducing a secondary antibody response requires CD4⁺ cell help, and secondary cell-mediated immunity can be induced both by CD4⁺ cells and leukocytes other than CD4⁺ cells.