Increased levels of interleukin-16 in the airways of tobacco smokers: relationship with peripheral blood T lymphocytes

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Abstract
BACKGROUND The mechanisms behind the development of systemic immunomodulation among tobacco smokers are not fully understood, but several studies have indicated a role for CD8+ and/or CD4+ T cells. Interleukin (IL)-16, a cytokine released from inflammatory cells as well as bronchial epithelial cells, can recruit and activate CD4+ T cells. A study was undertaken to establish whether the IL-16 level is increased in the airways of tobacco smokers and to determine whether airway levels of IL-16 are related to the number and function of systemic T lymphocytes. METHODS Bronchoalveolar lavage (BAL) fluid was collected from eight never smokers and 18 tobacco smokers without clinical airway symptoms, and from 16 tobacco smokers with clinical airway symptoms. Interleukin-16 protein levels in BAL fluid were determined using enzyme-linked immunosorbent assay (ELISA). Peripheral blood was collected for determination of CD4+ T cell content using flow cytometry. The responsiveness of systemic lymphocytes in smokers was assessed by measuring the proliferative response of peripheral blood lymphocytes to the superantigen staphylococcus enterotoxin A (SEA). RESULTS The IL-16 protein level in the BAL fluid was significantly higher in tobacco smokers than in non-smokers. However, among tobacco smokers the IL-16 level was similar in asymptomatic smokers and in those with airway symptoms. The level of IL-16 in the BAL fluid of smokers correlated negatively with the percentage of CD4+ T cells and positively with superantigen stimulated lymphocyte proliferation in peripheral blood. CONCLUSIONS In tobacco smokers the airway IL-16 level is increased and it is possible that this increase in IL-16 influences systemic immunomodulation by altering the number and responsiveness of systemic T lymphocytes.