IL-5 promotes myeloid suppressor cell recruitment to malignant pleural effusion (MPE). CD11b+GR-1+ myeloid suppressor cells in (A) blood and (B) MPEs obtained from wild-type (il5+/+), IL-5 knock-out (il5−/−), and phosphate-buffered saline–, rmIL-5–, IgG-, and TRFK5-treated il5+/+ C57BL/6 mice (A: n = 8, 8, 5, 3, 5, and 4, respectively; B: n = 8, 3, 5, 3, 5, and 4, respectively) 14 days after intrapleural delivery of LLC cells. IL-5 deficiency decreases, exogenous IL-5 enhances, and IL-5 neutralization inhibits recruitment of these cells in response to MPE. Images show representative flow cytometry results of blood (left; numbers, percentage of cells in quadrant) and MPE (right; numbers, percentage of cells in quadrant) from il5+/+ and il5−/− mice. Columns = mean; bars = SE. *P < 0.05 and **P < 0.01, respectively, for comparison with appropriate control. Ig = immunoglobulin; LLC = Lewis lung cancer; MPE = malignant pleural effusion; PBS = phosphate-buffered saline; rm = recombinant mouse; TRFK5 = anti–IL-5 neutralizing antibody.