Eosinophils infiltrating interleukin-5 gene-transfected tumors do not suppress tumor growth

Tumor-associated eosinophils have been observed in human tumors and in experimental tumor models, but their function is poorly understood. To study the role of eosinophils during tumor growth, the plasmacytoma J558L and the mammary adenocarcinoma TS/A were transfected with an expression vector encoding the murine gene for interleukin-5 (IL-5), a cytokine inducing proliferation and activation of eosinophils. Injection of parental cells, mock-transfectants and IL-5-producing cells into syngeneic mice showed that local IL-5 secretion induced rapid tumor infiltration by eosinophils, as evidenced by immunohistochemical staining, but nevertheless did not alter the tumor growth kinetics of IL-5 transfectants. Therefore, the mere presence of IL-5 and eosinophils was not sufficient to induce a protective host immune response.