Temporal effects of tumor necrosis factor-α on intracellular survival of Mycobacterium paratuberculosis

The causative agent in Johne's disease is Mycobacterium paratuberculosis, an intracellular pathogen which causes enteritis in ruminants. Little is known about interactions between the host cell (macrophage) and M. paratuberculosis; however, this bacterium is able to evade normal host immune defenses and cause a chronic infective state. In the present study, we evaluated whether activation of a murine macrophage cell line (J774.16) by pretreatment with recombinant murine tumor necrosis factor alpha (TNF) prior to infection with M. paratuberculosis would affect their ability to restrict growth and kill the ingested bacteria. A murine cell line was utilized owing to difficulty in obtaining bovine reagents and lack of a continuous bovine macrophage cell line for repeated experimentation. After 4 h of infection, numbers of viable bacteria in cell lysates were significantly lower for macrophages pretreated with 1000 IU TNF ml-1. The rate of bacterial growth as assessed by BACTEC radiometric culture system was also reduced at this time point. Upon further extension of the infection period to 72 h, we observed that moderate doses of TNF (10-1000 IU ml-1) significantly increased the number of viable M. paratuberculosis recovered whereas the highest dose of TNF (4000 IU ml-1) effectively reduced bacterial numbers. These data indicate that TNF can either enhance or reduce macrophage mycobactericidal and mycobacteriostatic activity depending upon both the level of TNF to which cells are exposed and the duration of infection.