Rationale: IL-5 is a T helper 2 cytokine important in the trafficking and survival of eosinophils. Because eosinophils can be found in malignant pleural effusions (MPE) from mice and humans, we asked whether IL-5 is involved in the pathogenesis of MPE.

Objectives: To determine the role of IL-5 in MPE formation.

Methods: The effects of IL-5 on experimental MPE induced in C57BL/6 mice by intrapleural injection of syngeneic lung (Lewis lung cancer [LLC]) or colon (MC38) adenocarcinoma cells were determined using wild-type (il5^+/+) and IL-5–deficient (il5^−/−) mice, exogenous administration of recombinant mouse (rm) IL-5, and in vivo antibody-mediated neutralization of endogenous IL-5. The direct effects of rmIL-5 on LLC cell proliferation and gene expression in vitro were determined by substrate reduction and microarray.

Measurements and Main Results: Eosinophils and IL-5 were present in human and mouse MPE, but the cytokine was not detected in mouse (LLC) or human (A549) lung and mouse colon (MC38) adenocarcinoma-conditioned medium, suggesting production by host cells in MPE. Compared with il5^+/+ mice, il5^−/− mice showed markedly diminished MPE formation in response to both LLC and MC38 cells. Exogenous IL-5 promoted MPE formation in il5^+/+ and il5^−/− mice, whereas anti–IL-5 antibody treatment limited experimental MPE in il5^+/+ mice. Exogenous IL-5 had no effects on LLC cell proliferation and gene expression; however, IL-5 was found to be responsible for recruitment of eosinophils and tumor-promoting myeloid suppressor cells to MPE in vivo.

Conclusions: Host-derived IL-5 promotes experimental MPE and may be involved in the pathogenesis of human MPE.