A role for IL-5 in the induction of cytotoxic T lymphocytes in vivo

IL-5 is generally regarded as a Th2 cytokine involved in eosinophil maturation and function and in B cell growth and antibody production, but without any well-established effects on T cells. Early reports suggested that IL-5 could stimulate the production of cytotoxic T lymphocytes (CTL) in vitro, but no evidence has been obtained to date for such a role in studies with IL-deficient (IL-5(-/-)) mice. Here we demonstrate that when oxidized mannan MUC1 fusion protein (M-FP) is used as an antigen in mice, IL-5 is required for the optimal generation of the CTL response. IL-5 was as effective as IL-2 for the induction of CTL from spleen cells in vitro and both CD4⁺ and CD8⁺ T cells from M-FP-immunized animals could be shown to secrete IL-5 in culture. In IL-5(-/-) mice, CTLp frequency was greatly diminished resulting in the inability to reject MUC1⁺ tumors. Clearly, IL-5 is produced by functional T cells, especially the Tc1 type, after M-FP immunization and is required for an optimal CTL response to this antigen.