The role of neutrophils in mycobacterial infection in zebrafish

Abstract Tuberculosis (TB) is a chronic granulomatous disease caused by the pathogen Mycobacterium tuberculosis (Mtb). Antibiotic resistance has led to TB being one of the leading infectious causes of death worldwide. Mtb is notable for surviving within leukocytes in which it creates a permissive niche essential for its proliferation and dissemination. Previous research has demonstrated that stabilised hypoxia inducible factor 1-alpha (Hif-1α) reduced bacterial burden due to increased levels of potent antimicrobial nitric oxide (NO), predominantly found in neutrophils. However, the roles of the neutrophil in this protective effect are not fully elucidated. Here I used a zebrafish model of TB to investigate neutrophils and macrophages during both initial stages of infection and granuloma formation. I tested the hypothesis that Hif-1α stabilisation is protective for the host by maintaining high levels of antimicrobial neutrophil NO, better controlling bacteria. I used a natural fish pathogen, Mycobacterium marinum (Mm), to investigate the roles of neutrophils and macrophages by altering cell populations in vivo. I then modulated Hif signalling to assess the effects on infection outcome when macrophage and neutrophil populations were altered. Here, I demonstrated that Hif-1α stabilisation is able to help the host immune response con-trol infection, even when macrophages, a normally critical leukocyte in Mm control, are deplet-ed. Conversely, genetic depletion of neutrophils in Hif-1α stabilised zebrafish larvae abolished the host-protective effect. Blocking NO in this context abrogated the protective effect, suggest-ing that neutrophil NO is responsible for the improved infection control. My data also shown that Hif-1α stabilisation maintains high levels of NO and increases neutrophil internalisation of Mm within granulomas, even in macrophage-depleted fish. These findings add further mechanistic insights to evidence suggesting that targeting neutrophils and Hif-1α could be used as future therapeutic avenues, potentially bypassing the emerging problem of emerging antibiotic resistance.