Abstract
Neutralization of pathogens by phagocytic immune cells requires the biogenesis of a compartmentalized hotspot of reactive species called the phagosome. One of these reactive species is hypochlorous acid (HOCl), produced by the enzyme myeloperoxidase (MPO) after the phagosome fuses with the lysosome. Mapping HOCl during phagosome maturation can report on pathogen killing and offer insights into regulation of MPO activity, mechanisms of resistance and host-pathogen interactions. However, this has been difficult because of a lack of a suitable method to chemically map a transient organelle with pH fluctuations like the phagosome. Here, we detail a protocol for quantifying HOCl dynamics in phagosomes using a fluorescent DNA-based reporter. Compared to traditional methods of visualizing HOCl or measuring MPO activity, this method offers sub-cellular spatial resolution and the capacity to assay HOCl production with single cell resolution.