Local expression of IL-4 by gene-modified tumor cells increases their immunogenicity by inducing an inflammatory response that is dominated by eosinophils. Eosinophils have been implicated as antitumor effector cells because the application of a granulocyte-depleting Ab inhibited rejection of IL-4 transfected tumors. This Ab did not discriminate between eosinophils and neutrophils and, therefore, this experiment could not exclude neutrophils as primary effector cells, whereas eosinophils were innocent bystander cells in IL-4 transfected tumors. We analyzed tumor growth suppression and granulocyte infiltration in IL-5-deficient (IL-5(-/-)) mice that had a deficiency of eosinophils, using two tumor lines (B16-F10 and MCA205) transfected to secrete IL-4. IL-4-expressing tumors were at least as efficiently rejected in IL-5(-/-) mice as in wild-type mice, despite an almost complete absence of tumor-infiltrating eosinophils. However, neutrophils were present in undiminished amounts and their depletion partially restored tumor growth. Furthermore, the growth of IL-5-secreting tumors was not impaired in either wild-type or IL-5(-/-) mice, even though it induced eosinophilia in both mouse strains. These findings demonstrate that eosinophils can be induced in IL-5(-/-) mice by exogenous IL-5 and argue against a compensatory effect of neutrophils in the absence of eosinophils. We conclude that 1) infiltration of IL-4 transfected tumors by eosinophils is completely IL-5 dependent, 2) eosinophils have no tumoricidal activity, and 3) neutrophils are responsible, at least in part, for tumor suppression.
