EFFECTS OF MYCOBACTERIA ON REGULATION OF APOPTOSIS IN MONONUCLEAR PHAGOCYTES

Since apoptosis is observed in tuberculous granulomata, we investigate d the molecular mechanisms underlying the apoptotic pathway in an in v itro model of mycobacterial infection of mononuclear phagocytes. We po stulated that Mycobacterium tuberculosis could trigger the apoptotic p athway in macrophages, resulting in death of the microorganism by modu lating the expression of bcl-2, bax, bcl-x(L), and bcl-x(s). We found that the mRNA of bcl-2, an inhibitor of apoptosis, was downregulated i n peripheral blood monocytes (PBM) between 2 and 6 h following infecti on with M. bovis BCG or induction with heat-killed M. tuberculosis H37 Ra, Western analysis showed a downregulation of the Bcl-2 protein, wit h a half-life of 24 h, At the same time points, there was no change in the expression of Bax or Bcl-x(s), inducers of apoptosis, but Bcl-x, another inhibitor of apoptosis, was minimally upregulated by BCG, To d etermine if apoptosis could be a mechanism for growth inhibition in vi vo, we obtained alveolar macrophages by bronchoalveolar lavage from in volved sites in patients with active pulmonary tuberculosis, Using the TUNEL (terminal deoxynucleotidyltransferase mediated nick end labelin g) technique, we observed significantly more apoptosis in involved seg ments of five tuberculosis patients (14.8 +/- 1.9%) than in those of n ormal controls (<1%, P = 0.02) or in uninvolved segments (4.3 +/- 0.9% , P < 0.05), We conclude that apoptosis of mononuclear phagocytes indu ced by M tuberculosis occurs in vivo and that in an in vitro model of mycobacterial infection, apoptosis may be mediated by downregulation o f Bcl-2.