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Background Interleukin 16 (IL-16) can be detected by ELISA in pleural effusion (PE) and its concentration is higher than in serum.This study investigated the cellular sources of IL-16 in PE.Methods The samples of PE were collected from 34 patients who were newly diagnosed having PE in the pleural cavity.We performed cell culture to purify the pleural mesothelial cells (PMC),Wright staining to count the purity and immunocytochemical stain to identify the cultured cells.The intracellular IL-16 expression was detected by flow cytometry (FCM).The different cells in PE were first separated by magnetic cell sorting (MCAS) then the separated cells were cultured in RPMI1640 with 10％ fetal calf serum (FCS).We extracted the supernatant and detected IL-16 concentration by ELISA.The IL-16 protein was detected by immunohistochemistry and double immunofluorescence staining.Results The percentages of cells which secreted IL-16 were:CD3+CD8 cells ((74.27±15.56)％,n=34); CD3+CD8+ cells ((69.86±18.55)％,n=34); CD19+ cells ((45.30±18.77)％,n=15); CD14+ cells ((16.91±16.69)％,n=15); and PMC ((2.05±1.85)％,n=7).The concentrations of IL-16 in the supernatant from cultured cells were:CD4+ cells ((102.50±42.51) ng/L,n=5); CD8+ cells ((92.58±18.34) ng/L,n=5); CD19+ cells ((79.85±5.62) ng/L,n=5); CD14+ cells ((58.51±25.38) ng/L,n=5); and PMC ((18.14±8.37) ng/L,n=5).In lymphocytes,monocytes/macrophages and PMC,we could observe the cells that expressed IL-16 protein.In paraffin-embedded sections,we also could observe by immunohistochemistry the CD4+IL-16+ cells,CD8+IL-16+ cells,CD19+IL-16+ cells,and CD14+IL-16+ cells.Conclusions IL-16 in PE is mainly secreted by T lymphocytes,including CD3+CD8- cells and CD3+CD8+celIs.CD19+ cells and CD14+ cells can also secrete IL-16,but the percentage of PMC that can secrete IL-16 is very low.