Targeting of <i>Mycobacterium tuberculosis</i> Heparin-Binding Hemagglutinin to Mitochondria in Macrophages

다국어 초록 (Multilingual Abstract)

<▼1><P><I>Mycobacterium tuberculosis</I> heparin-binding hemagglutinin (HBHA), a virulence factor involved in extrapulmonary dissemination and a strong diagnostic antigen against tuberculosis, is both surface-associated and secreted. The role of HBHA in macrophages during <I>M. tuberculosis</I> infection, however, is less well known. Here, we show that recombinant HBHA produced by <I>Mycobacterium smegmatis</I> effectively induces apoptosis in murine macrophages. DNA fragmentation, nuclear condensation, caspase activation, and poly (ADP-ribose) polymerase cleavage were observed in apoptotic macrophages treated with HBHA. Enhanced reactive oxygen species (ROS) production and Bax activation were essential for HBHA-induced apoptosis, as evidenced by a restoration of the viability of macrophages pretreated with N-acetylcysteine, a potent ROS scavenger, or transfected with Bax siRNA. HBHA is targeted to the mitochondrial compartment of HBHA-treated and <I>M. tuberculosis</I>-infected macrophages. Dissipation of the mitochondrial transmembrane potential (ΔΨ<SUB>m</SUB>) and depletion of cytochrome <I>c</I> also occurred in both macrophages and isolated mitochondria treated with HBHA. Disruption of HBHA gene led to the restoration of ΔΨ<SUB>m</SUB> impairment in infected macrophages, resulting in reduced apoptosis. Taken together, our data suggest that HBHA may act as a strong pathogenic factor to cause apoptosis of professional phagocytes infected with <I>M. tuberculosis</I>.</P></▼1><▼2><P><B>Author Summary</B></P><P>Cell death is a common outcome during infection with a number of pathogenic microorganisms. Therefore, defining the factors responsible for killing of host cells is important to uncovering mechanisms of pathogenesis. World-wide, two billon people are latently infected with <I>Mycobacterium tuberculosis</I>, which is still killing 2–3 million people each year. Heparin-binding hemagglutinin (HBHA) protein of <I>M. tuberculosis</I> is known to interact specifically with non-phagocytic cells and to be involved in dissemination from lungs to other tissues. Nevertheless, the role of HBHA in phagocytic cells such as macrophages, which are the first cells of the immune system to encounter inhaled pathogens, has been unknown. In the present study, we suggest HBHA as a critical bacterial protein for macrophage cell death. After <I>M. tuberculosis</I> infection or HBHA treatment of macrophages, HBHA targeted to mitochondria and then caused mitochondrial damage and oxidative stress, which eventually lead to apoptosis. A mutant of <I>M. tuberculosis</I> lacking HBHA induced less apoptosis with moderated mitochondrial damage. These experiments provide a candidate virulence factor which may be a novel target for tuberculosis treatment.</P></▼2>