Host immunity increases Mycobacterium tuberculosis reliance on cytochrome bd oxidase

Author summary Tuberculosis, caused by Mycobacterium tuberculosis (Mtb) is a serious global health problem that is responsible for over one million deaths annually, more than any other single infectious agent. In the host, Mtb can adapt to a wide variety of immunological and environmental pressures which is integral to its success as a pathogen. Accordingly, the respiratory capacity of Mtb is flexible. The electron transport chain of Mtb has two terminal oxidases, the cytochrome bc(1)/aa(3) super complex and cytochrome bd, that contribute to the proton motive force and subsequent production of energy in the form of ATP. The bc(1)/aa(3) super complex is required for optimal growth during infection but the role of cytochrome bd is unclear. Here we report that the cytochrome bd oxidase is required for resisting the adaptive immune response, in particular, acidification of the phagosome induced by lymphocyte-derived IFN gamma. We found that the cytochrome bd oxidase is specifically required under acidic conditions, where the bc(1)/aa(3) complex is preferentially inhibited. Additionally, we show that acidic conditions increased the potency of Q203, a cytochrome bc(1)/aa(3) inhibitor and candidate tuberculosis therapy. This work defines a new link between the host immune response and the respiratory requirements of Mtb that affects the potency of a potential new therapeutic. In order to sustain a persistent infection, Mycobacterium tuberculosis (Mtb) must adapt to a changing environment that is shaped by the developing immune response. This necessity to adapt is evident in the flexibility of many aspects of Mtb metabolism, including a respiratory chain that consists of two distinct terminal cytochrome oxidase complexes. Under the conditions tested thus far, the bc(1)/aa(3) complex appears to play a dominant role, while the alternative bd oxidase is largely redundant. However, presence of two terminal oxidases in this obligate pathogen implies that respiratory requirements might change during infection. We report that the cytochrome bd oxidase is specifically required for resisting the adaptive immune response. While the bd oxidase was dispensable for growth in resting macrophages and the establishment of infection in mice, this complex was necessary for optimal fitness after the initiation of adaptive immunity. This requirement was dependent on lymphocyte-derived interferon gamma (IFN gamma), but did not involve nitrogen and oxygen radicals that are known to inhibit respiration in other contexts. Instead, we found that Delta cydA mutants were hypersusceptible to the low pH encountered in IFN gamma-activated macrophages. Unlike wild type Mtb, cytochrome bd-deficient bacteria were unable to sustain a maximal oxygen consumption rate (OCR) at low pH, indicating that the remaining cytochrome bc(1)/aa(3) complex is preferentially inhibited under acidic conditions. Consistent with this model, the potency of the cytochrome bc(1)/aa(3) inhibitor, Q203, is dramatically enhanced at low pH. This work identifies a critical interaction between host immunity and pathogen respiration that influences both the progression of the infection and the efficacy of potential new TB drugs.

Automated summary

Tuberculosis, caused by Mycobacterium tuberculosis (Mtb), is a major global health problem resulting in over one million deaths annually. Mtb's ability to adapt to various immunological and environmental pressures, including through flexible respiratory capacity, is crucial for its pathogenic success. This study highlights the importance of cytochrome bd oxidase in Mtb's resistance to the adaptive immune response, particularly in acidic conditions induced by IFN gamma, and shows its impact on the efficacy of a potential new tuberculosis therapy.