Cytosolic Phospholipase A₂ Translocates to Forming Phagosomes during Phagocytosis of Zymosan in Macrophages

Resident tissue macrophages mediate early innate immune responses to microbial infection. Cytosolic phospholipase A₂α (cPLA ₂α) is activated in macrophages during phagocytosis of non-opsonized yeast (zymosan) triggering arachidonic acid release and eicosanoid production. cPLA₂α translocates from cytosol to membrane in response to intracellular calcium concentration ([Ca ²⁺]_i) increases. Enhanced green fluorescent protein (EGFP)-cPLA₂α translocated to forming phagosomes, surrounding the zymosan particle by 5 min and completely overlapping with early endosome (Rab5) and plasma membrane (F4/80) markers but only partially overlapping with resident endoplasmic reticulum proteins (GRP78 and cyclooxygenase 2). EGFP-cPLA₂α also localized to membrane ruffles during phagocytosis. Zymosan induced an initial high amplitude calcium transient that preceded particle uptake followed by a low amplitude sustained calcium increase. Both phases were required for optimal phagocytosis. Extracellular calcium chelation prevented only the sustained phase but allowed a limited number of phagocytic events, which were accompanied by translocation of cPLA ₂α to the phagosome although [Ca²⁺]_i, remained at resting levels. The results demonstrate that cPLA ₂α targets the phagosome membrane, which may serve as a source of arachidonic acid for eicosanoid production.