WASH orchestrates phagosomal maturation and bacterial killing in neutrophils through syntaxin 19-mediated lysosomal fusion

Neutrophil phagocytosis is crucial for combating infections, yet the actin-mediated regulation of vesicular trafficking during late phagosomal maturation remains obscure. Using neutrophils lacking the actin nucleation regulator Wiskott-Aldrich Syndrome Protein and Scar Homolog (WASH), we identify a mechanism linking actin remodeling, lysosomal fusion, and bacterial killing. Although WASH is dispensable for particle internalization, WASH deficiency impairs lysosome-phagosome fusion and intracellular bacterial killing. Lysosomes from WASH-deficient neutrophils show decreased association with actin comets and impaired dynamics. Rac1 activation rescues lysosomal trafficking but not phagosomal maturation. Mass spectrometry of phagolysosomes and TR-FRET identify atypical syntaxin 19 (STX19) as a WASH binding partner, and WASH recruits STX19 to phagosomes. Reconstituting WASH in primary neutrophils rescues phagocytosis, STX19 interference impairs phagosomal maturation, while STX19 overexpression enhances it. WASH is thus a central mediator of phagosomal maturation through STX19-mediated membrane fusion and actin-driven lysosomal dynamics, revealing how dysregulated trafficking contributes to immunodeficiency. How actin regulates late phagosomal maturation and microbial killing in neutrophils remains unclear. The authors here show that WASH drives actin-dependent lysosomal dynamics and STX19 recruitment to phagosomes, enabling lysosome fusion and bacterial killing.