In the Eye of the Swarm: Unravelling Neutrophil Swarming Dynamics in Zebrafish

Neutrophil dysfunctions are a major cause of chronic inflammatory diseases like COPD, but also infectious diseases such as COVID-19. Understanding neutrophil behaviour and pathways can lead to new therapeutic approaches aiding to relieve the burden of neutrophil-associated illnesses. Neutrophil swarming is a relatively recently described, neutrophil specific behaviour. During swarming, neutrophils are recruited in a bi-phasic manner through initial recruitment triggers followed by autocrine signalling of leukotriene B4, ultimately stabilising into a cluster of neutrophils. Although pathways driving the formation of neutrophil swarms have been increasingly understood, some initiating triggers remain undescribed and pathways resulting in arrest or resolution of swarms are largely unknown. I hypothesise that neutrophil swarming is a modulatory process that plays an essential role in the timescale of the inflammatory response. I demonstrate the use of the zebrafish model of inflammation for observing endogenous neutrophil swarm dynamics in vivo. Utilising a multitude of newly developed, automated analyses, neutrophil swarming observed via time-lapse imaging was, characterised, quantified and compared. Pharmacological modulation with TLR agonists via immersion did not significantly influence neutrophil swarming. However, treatment with prolyl-hydroxylase inhibitors by immersion suggested the HIF-1α pathway to play a role in the swarming response, as a significant reduction in swarms was observed. Finally, through the use of an mfap4:mCherry/mpx:GFP reporter line, I was able to show how interplay between macrophages and neutrophils might change swarming dynamics in vivo. In conclusion, this research demonstrates the use of the zebrafish model of inflammation to show innovative approaches for in depth analysis of swarming dynamics, aiming to provide a deeper understanding of neutrophil swarming, through which novel therapeutic approaches could be identified for patients affected by chronic inflammatory diseases.