Interferon-gamma (IFN-γ) production by human T lymphocytes upon Legionella pneumophila stimulation in vitro

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Abstract
In vitro immune responses to Legionella pneumophila were investigated. When human peripheral blood lymphocytes (PBL) from healthy volunteers were stimulated with formalin‐killed L. pneumophila for 7 days in vitro, strong proliferative responses were observed. The responding cells were shown to be a CD4a T cell subset. It was also found that the CD4 T cells secreted significant amounts of IFN‐γ into the PBL culture supernatant. The production of IFN‐γ and IL‐4 by PBL was measured semiquantitatively by reverse transcriptase‐assisted polymerase chain reaction (RT‐PCR) methods. Formalin‐killed or live L. pneumophila‐ stimulated PBL expressed the mRNA for IFN‐γ but not the mRNA for IL‐4. The results suggest that the whole bacterium, as opposed to the supernatant, predominantly stimulates Th1 type helper T cells. The cloned T cells specific for L. pneumophila expressed the mRNA for IFN‐γ but not for IL‐4. In contrast to formalin‐killed or live L. pneumophila stimulation, when PBL were stimulated with the bacterial culture supernatant, the proliferating T cells produced the mRNA for IL‐4 as well as for IFN‐γ. A significant correlation between the proliferative response to formalin‐killed L. pneumophila and IFN‐γ release in culture was observed (r= 0.6932, P 0.001) in PBL from 30 healthy volunteers. From these in vitro studies, it is suggested that the whole L. pneumophila bacterium and their soluble antigens stimulate T cells in a manner which results in a different pattern of cytokine production.