The neutrophil behaviour termed swarming is the coordinated recruitment of neutrophils towards a site of challenge where they form clusters around the injury or infection. Neutrophils are deployed against Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), but whether they employ swarming to fight this infection remains unclear. The alarming rise in TB antimicrobial resistance urgently necessitates development of new treatments. In this thesis, I investigate neutrophil swarming in response to mycobacteria in an in vivo zebrafish model and assess whether this neutrophil behaviour can be modulated for therapeutic benefit.
Localised infection with Mycobacterium marinum (Mm), a close relative of human Mtb, in a muscle block was used to investigate and characterise neutrophil swarming in the zebrafish model. Neutrophil swarms initiated rapidly in response to live and heat-killed Mtb¸ and were stable for at least 3 hours on average. Swarms did not appear to increase reductions of bacterial burden over time.
In other experimental models, such as mice and cell cultures, leukotriene B4 (LTB4) has been established as a crucial mediator in the swarming response. Signalling of LTB4 and other potential swarming mediators was modulated to investigate involvement in neutrophil swarming to Mm. Genetic knockdown of leukotriene A4 hydrolase (LTA4H) reduced swarm duration and area, but not neutrophil numbers within the somite, and inhibition of various other targets had no effect. These data show the swarming response in zebrafish can be modulated.
Stabilisation of hypoxia-inducible factor (HIF)-1α can be induced to boost neutrophil killing of Mm and thereby reduce bacterial burden, but its involvement in swarming is unclear. The host-protective effect of Hif-1α stabilisation was detectable in localised Mm infection and increased swarming prevalence. This research shows Hif-1α may exert its protective effect partially through neutrophil swarming.
In conclusion, this marks the first in-depth investigation into neutrophil swarming to Mm infection and lays the foundation for its modulation as a potential therapeutic strategy.
