Marked inhibition of the intracellular multiplication of Legionella pneumophila in monocytes isolated from carriers of human T lymphotrophic virus type I.

Intracellular growth patterns of Legionella pneumophila were examined in monocytes obtained from carriers of human T-lymphotropic virus type I (HTLV-1) and controls who were HTLV-1 seronegative. All subjects were seronegative for antibodies against L. pneumophila. Bacterial growth was determined 0, 1, 2, and/or 3 days after infecting peripheral blood mononuclear cells (PBMCs) with the bacteria. The intracellular growth of L. pneumophila was markedly inhibited in HTLV-1 carriers compared with normal controls. When the lymphocytes were depleted from the HTLV-1 carrier PBMC cultures before infection, this inhibition was abolished. Inhibition reappeared, however, when the 72-h culture supernatants of PBMCs from HTLV-1 carriers were added to the lymphocyte-depleted cultures. Culture supernatants of infected and uninfected PBMCs from HTLV-1 carriers exhibited markedly increased levels of interferon-gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) compared with the HTLV-1 seronegative controls. In the HTLV-1 carriers, IFN-gamma was produced by the CD4+ lymphocytes, whereas TNF-alpha was secreted by the monocytes. Addition of anti-IFN-gamma or anti-TNF-alpha antibodies to the HTLV-1 carrier PBMC cultures diminished the inhibition of intracellular growth of L. pneumophila. Results suggest that the monocytes are activated in HTLV-1 carriers. These findings may explain why an opportunistic infection by certain intracellular pathogens is rarely seen among HTLV-1 carriers.