Interferon-gamma (IFN-γ) production by human T lymphocytes uponLegionella pneumophilastimulationin vitro

SUMMARY 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.