Hypoxia Inducible Factor Signaling Modulates Susceptibility to Mycobacterial Infection via a Nitric Oxide Dependent Mechanism

Tuberculosis is a current major world-health problem, exacerbated by the causative pathogen, Mycobacterium tuberculosis (Mtb), becoming increasingly resistant to conventional antibiotic treatment. Mtb is able to counteract the bactericidal mechanisms of leukocytes to survive intracellularly and develop a niche permissive for proliferation and dissemination. Understanding of the pathogenesis of mycobacterial infections such as tuberculosis (TB) remains limited, especially for early infection and for reactivation of latent infection. Signaling via hypoxia inducible factor alpha (HIF-alpha) transcription factors has previously been implicated in leukocyte activation and host defence. We have previously shown that hypoxic signaling via stabilization of Hif-1 alpha prolongs the functionality of leukocytes in the innate immune response to injury. We sought to manipulate Hif-alpha signaling in a well-established Mycobacterium marinum (Mm) zebrafish model of TB to investigate effects on the host's ability to combat mycobacterial infection. Stabilization of host Hif-1 alpha, both pharmacologically and genetically, at early stages of Mm infection was able to reduce the bacterial burden of infected larvae. Increasing Hif-1 alpha signaling enhanced levels of reactive nitrogen species (RNS) in neutrophils prior to infection and was able to reduce larval mycobacterial burden. Conversely, decreasing Hif-2 alpha signaling enhanced RNS levels and reduced bacterial burden, demonstrating that Hif-1 alpha and Hif-2 alpha have opposing effects on host susceptibility to mycobacterial infection. The antimicrobial effect of Hif-1 alpha stabilization, and Hif-2 alpha reduction, were demonstrated to be dependent on inducible nitric oxide synthase (iNOS) signaling at early stages of infection. Our findings indicate that induction of leukocyte iNOS by stabilizing Hif-1 alpha, or reducing Hif-2 alpha, aids the host during early stages of Mm infection. Stabilization of Hif-1 alpha therefore represents a potential target for therapeutic intervention against tuberculosis.