TEMPORAL EFFECTS OF TUMOR-NECROSIS-FACTOR-ALPHA ON INTRACELLULAR SURVIVAL OF MYCOBACTERIUM-PARATUBERCULOSIS

The causative agent in Johne's disease is Mycobacterium paratuberculos is, an intracellular pathogen which causes enteritis in ruminants. Lit tle 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 prese nt 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 a ffect their ability to restrict growth and kill the ingested bacteria. A murine cell line was utilized owing to difficulty in obtaining bovi ne reagents and lack of a continuous bovine macrophage cell line for r epeated experimentation. After 4 h of infection, numbers of viable bac teria in cell lysates were significantly lower for macrophages pretrea ted with 1000 IU TNF ml(-1). The rate of bacterial growth as assessed by BACTEC radiometric culture system was also reduced at this time poi nt. Upon further extension of the infection period to 72 h, we observe d that moderate doses of TNF (10-1000 IU ml(-1)) significantly increas ed the number of viable M. paratuberculosis recovered whereas the high est 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 infect ion.