The neutrophil
enzyme myeloperoxidase (MPO) is a major
enzyme made by neutrophils to generate antimicrobial and
immunomodulatory compounds, notably
hypochlorous acid (HOCl), amplifying their capacity for destroying pathogens and regulating
inflammation. Despite its roles in innate immunity, the importance of MPO in preventing
infection is unclear, as individuals with
MPO deficiency are asymptomatic with the exception of an increased risk of
candidiasis. Dysregulation of MPO activity is also linked with inflammatory conditions such as
atherosclerosis, emphasising a need to understand the roles of the
enzyme in greater detail. Consequently, new tools for investigating granular dynamics in vivo can provide useful insights into how MPO localises within neutrophils, aiding understanding of its role in preventing and exacerbating disease. The zebrafish is a powerful model for investigating the immune system in vivo, as it is genetically tractable, and optically transparent. To visualise MPO activity within zebrafish neutrophils, we created a genetic construct that expresses human MPO as a fusion
protein with a C-terminal fluorescent tag, driven by the neutrophil-specific promoter lyz. After introducing the construct into the zebrafish genome by Tol2 transgenesis, we established the Tg(lyz:Hsa.MPO-mEmerald,cmlc2:EGFP)sh496 line, and confirmed transgene expression in zebrafish neutrophils. We observed localisation of MPO-mEmerald within a subcellular location resembling neutrophil granules, mirroring MPO in human neutrophils. In Spotless (mpxNL144) larvae-which express a non-functional zebrafish
myeloperoxidase-the MPO-mEmerald transgene does not disrupt neutrophil migration to sites of
infection or
inflammation, suggesting that it is a suitable line for the study of neutrophil granule function. We present a new transgenic line that can be used to investigate neutrophil granule dynamics in vivo without disrupting neutrophil behaviour, with potential applications in studying processing and maturation of MPO during development.