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.
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