Nitric Oxide Modulates Macrophage Responses to Mycobacterium tuberculosis Infection through Activation of HIF-1α and Repression of NF-κB

Abstract

IFN-γ is essential for control of Mycobacterium tuberculosis infection in vitro and in vivo. However, the mechanisms by which IFN-γ controls infection remain only partially understood. One of the crucial IFN-γ target genes required for control of M. tuberculosis is Inducible nitric oxide synthase (iNOS). While nitric oxide (NO) produced by iNOS is thought to have direct bactericidal activity against M. tuberculosis, the role of NO as a signaling molecule has been poorly characterized in the context M. tuberculosis infection. Here, we find that iNOS broadly regulates the macrophage transcriptome during M. tuberculosis infection, activating antimicrobial pathways while also limiting inflammatory cytokine production. The transcription factor Hypoxia inducible factor-1α (HIF-1α) was recently shown to be critical for IFN-γ mediated control of M. tuberculosis infection. We find that HIF-1α function requires NO production, and that HIF-1α and iNOS are linked by a positive feedback loop that amplifies macrophage activation. Furthermore, we find that NO inhibits NF-kB activity to prevent hyper-inflammatory responses. Thus, NO activates robust microbicidal programs while at the same time limiting damaging inflammation. IFN-γ signaling must carefully calibrate an effective immune response that does not cause excessive tissue damage, and this work identifies NO as a key player in establishing this balance during M. tuberculosis infection.

Myeloid HIF‐1α regulates pulmonary inflammation during experimental Mycobacterium tuberculosis infection

“…Considering the role of HIF‐1α −/− in the transcriptional control of the macrophages responses, we next investigated if the susceptibility of mHIF‐1α −/− mice was caused by an altered expression of protective molecules in the lungs. To this end, we measured the expression of IFN‐γ and antimycobacterial genes induced by IFN‐γ, including that for iNOS (encoded by Nos2 ) and that for the immunity‐related GTPase family M member 1 gene ( Irgm1 ) .…”

“…Recent studies suggest that HIF‐1α is a central regulator of the macrophage response to mycobacterial infections . One particular study showed that mice deficient in HIF‐1α in the myeloid lineage (mHIF‐1α −/− ) were acutely susceptible to aerosol Mtb Erdman challenge .…”

Hypoxia-inducible factor-1α regulation of myeloid cells

“…In addition, Anand and colleagues found that hypoxia or genetic overexpression of HIF-1α enhances phagocytic capacity of activated macrophages, partially by promoting the p38-MAP kinase signaling pathway [59]. More recently, Bravermand and Stanley showed that inducible nitric oxide synthase (iNOS) activates HIF-1α in a NO-dependent manner to form a positive feedback loop amplifying antimicrobial activity [60]. During infection with Mycobacterium tuberculosis , which establishes latency in macrophages, NO was previously thought to have bactericidal activity against the infection.…”

“…During infection with Mycobacterium tuberculosis , which establishes latency in macrophages, NO was previously thought to have bactericidal activity against the infection. Instead, they found that NO acts a signaling molecule downstream of IFN-γ to stabilize HIF-1α and promote M. tubercuolosis killing indirectly [60]. …”

“…As described above, NO rapidly stabilizes HIF-1α, which also increases NO production, creating a positive feedback loop. Thus, it is likely that HIF-1α activation continues to play an important role in inflammatory DC activity, even after the initial stimulus [60]. Corroborating this, Perrin-Cocon and colleagues recently found that inhibition of HIF-1α with echinomycin prevents secretion of proinflammatory cytokines by DCs [106].…”