SEQUENTIAL ACTIVATION OF ALVEOLAR MACROPHAGES BY IFN-GAMMA AND LPS ISREQUIRED FOR ENHANCED GROWTH-INHIBITION OF VIRULENT MYCOBACTERIUM-BOVIS BUT NOT M-BOVIS BCG

Alveolar macrophages (AM) form the first line of defence against most respiratory pathogens and, unlike tissue macrophages, are constantly e xposed to a wide variety of antigenic stimuli. In this study we invest igated the in vitro effects of IFN-gamma and LPS on growth of virulent Mycobacterium bovis and M. bovis bacille Calmette-Guerin (BCG) in bov ine AM. Bovine AM were purified from bronchial lavage fluid and cultur ed in serum-free medium. Pretreatment of bovine AM with IFN-gamma resu lted in growth inhibition of M. bovis BCG but only partially inhibited growth of virulent M. bovis. Enhanced inhibition of virulent M. bovis by bovine AM required sequential stimulation with IFN-gamma and LPS a nd was associated with increased induction of nitric oxide (NO) and IL -12 mRNA. Growth inhibition of M. bovis was not affected by treatment of macrophages with the L-arginine analogue, N-G-monomethyl-L-arginine although this treatment decreased NO production. These results sugges t that a second activation signal in the form of TNF-alpha or LPS may be required to induce bacteriostasis of virulent M. bovis by bovine AM in vivo, The ability of bovine AM to respond to activation stimuli in vitro suggests that these cells may play an important role in prevent ing establishment of intracellular bacterial infections in the lung.