Evaluation of Myeloperoxidase as Target for Host-Directed Therapy in Tuberculosis In Vivo.

Due to the rise of tuberculosis cases infected with multi and extensively drug-resistant Mycobacterium tuberculosis strains and the emergence of isolates resistant to antibiotics newly in clinical use, host-directed therapies targeting pathogenesis-associated immune pathways adjunct to antibiotics may ameliorate disease and bacterial clearance. Active tuberculosis is characterized by neutrophil-mediated lung pathology and tissue destruction. Previously, we showed that preventing M. tuberculosis induced necrosis in human neutrophils by inhibition of myeloperoxidase (MPO) promoted default apoptosis and subsequent control of mycobacteria by macrophages taking up the mycobacteria-infected neutrophils. To translate our findings in an in vivo model, we tested the MPO inhibitor 4-aminobenzoic acid hydrazide (ABAH) in C3HeB/FeJ mice, which are highly susceptible to M. tuberculosis infection manifesting in neutrophil-associated necrotic granulomas. MPO inhibition alone or as co-treatment with isoniazid, a first-line antibiotic in tuberculosis treatment, did not result in reduced bacterial burden, improved pathology, or altered infiltrating immune cell compositions. MPO inhibition failed to prevent M. tuberculosis induced neutrophil necrosis in C3Heb/FeJ mice in vivo as well as in murine neutrophils in vitro. In contrast to human neutrophils, murine neutrophils do not respond to M. tuberculosis infection in an MPO-dependent manner. Thus, the murine C3HeB/FeJ model does not fully resemble the pathomechanisms in active human tuberculosis. Consequently, murine infection models of tuberculosis are not necessarily adequate to evaluate host-directed therapies targeting neutrophils in vivo.

Survival of hypoxia-induced dormancy is not a common feature of all strains of the Mycobacterium tuberculosis complex.

While persistence in a dormant state is crucial for the life cycle of Mycobacterium tuberculosis, no investigation regarding dormancy survival of different strains across different lineages was performed so far. We analyzed responses to oxygen starvation and recovery in terms of growth, metabolism, and transcription. All different strains belonging to the Euro-American lineage (L4) showed similar survival and resuscitation characteristics. Different clinical isolates from the Beijing (L2), East African-Indian (L3), and Delhi/Central Asian (L1) lineage did not survive oxygen starvation. We show that dormancy survival is lineage-dependent. Recovery from O2 starvation was only observed in strains belonging to the Euro-American (L4) lineage but not in strains belonging to different lineages (L1, L2, L3). Thus, resuscitation from dormancy after oxygen starvation is not a general feature of all M. tuberculosis strains as thought before. Our findings are of key importance to understand infection dynamics of non-Euro-American vs Euro-American strains and to develop drugs targeting the dormant state.