Syntaxin-2 inhibits phagocytic uptake but promotes phagolysosome formation in macrophages

Abstract

Phagocytosis engulfs receptor-bound particles by structuring phagocytic cups that expand to form phagosome vesicles. Phagosomes fuse with endosomes and lysosomes to gradually mature into acidic, hydrolase-enriched phagolysosomes for content degradation. While an essential cellular process for eliminating unwanted substances to secure host defense and organismal homeostasis, defective or uncontrolled phagocytosis can be detrimental. Here, we report syntaxin-2 (Stx2), a poorly characterized SNARE in phagocytes, defines the course of macrophage phagocytosis by coordinating cell surface receptor density, phagocytic cup expansion, and phagosome maturation. Stx2 is expressed on the plasma membrane, phagosomes, early endosomes, and to some extent on recycling endosomes, late endosomes and VAMP4-positive compartments. Stx2 knockdown (Stx2-KD) augments engagement and uptake of IgG-opsonized particles by enhanced surface expression of Fc receptors (FcR) and efficient expansion of phagocytic cups. These events are driven by increased recycling of FcR, and enhanced delivery of early endosomes and VAMP4-positive post-Golgi compartments to phagocytic cups. Interestingly, Stx2-KD macrophages exhibit reduced secretion of pro-cathepsins with concomitant increase in lysosome content. However, Stx2 depletion prevents phagosome coalescence with late endosomes and lysosomes to impair maturation by depleted acquisition of cathepsins and reduced acidification. Consequently, macrophages depleted of Stx2 manifest uncontrolled uptake of IgG-opsonized Escherichia coli and impaired digestion resulting in increased bacterial load. Collectively, we uncovered a previously unknown role of Stx2 as a critical balancer of phagocytic uptake and phagolysosomal clearance in macrophages, suggesting that it could be an attractive target for modulation of phagocytosis plasticity and to control aberrant phagocytosis.