The Role of PIKfyve in the Neutrophil Innate Immune Response

Abstract

Phosphoinositides are lipid signals that help control leukocyte function including phagocytosis, phagosome maturation and chemotaxis. For example, phosphatidylinositol‐3,5‐bisphosphate (PtdIns(3,5)P2), a lysosomal phosphoinositide, was recently shown to control phagosome maturation in macrophages. Here, we investigated the role of PtdIns(3,5)P2 in neutrophils. Neutrophils are the most abundant leukocyte and the first leukocyte involved in cell‐mediated immunity because of their proficiency in chemotaxis. Once at a site of infection, neutrophils release bactericidal molecules by degranulation, production of reactive oxygen species and phagocytosis of pathogens. Here, we pre‐treated neutrophils with apilimod, an inhibitor of PIKfyve, the kinase that synthesizes PtdIns(3,5)P2 and examined the effects on neutrophils. Overall, we observed that PIKfyve loss of function was highly detrimental to neutrophils. First, apilimod‐treated neutrophils led to lysosome‐vacuoation but not granule‐vacuolation. Second, PIKfyve inhibited cells showed reduced cell velocity and directionality towards an fMLP‐gradient. Third, apilimod treated cells showed an ~80% decrease in its phagocytosis of bacteria and a 75% decrease in phagosomes stained for lysosome markers. Lastly, neutrophils treated with apilimod showed a ~50% decrease in the production of superoxide, a reactive oxygen species produced by respiratory burst to kill bacteria. Currently, we are testing whether PIKfyve activity is necessary for activation of the Rac GTPase, which is essential for the above neutrophil functions. Indeed, preliminary results show that PIKfyve inhibited cells have reduced levels of Rac‐GTP bound.