A contractile activity that closes phagosomes in macrophages

Studies of Fc-mediated phagocytosis by mouse macrophages identified a contr actile activity at the distal margins of forming phagosomes. Time-lapse vid eo microscopic analysis of macrophages containing rhodamine-labeled actin a nd fluorescein dextran showed that actin was concentrated at the distal mar gins of closing phagosomes. Phagocytosis-related contractile activities wer e observed when one IgG-opsonized erythrocyte was engaged by two macrophage s, Both cells extended pseudopodia until they met midway around the erythro cyte, It was then constricted and pulled into two phagosomes, which remaine d interconnected by a string of erythrocyte membrane, Butanedione monoxime, an uncompetitive inhibitor of class II and perhaps other myosins, and wort mannin and LY294002, inhibitors of phosphoinositide 3-kinase, prevented the constrictions without inhibiting the initial pseudopod extension. Immunofl uorescence microscopy showed the presence of myosins IC, II, V and IXb in p hagosomes. Of these, only myosin IC was concentrated around the strings con necting shared erythrocytes, suggesting that myosin IC mediates the purse-s tring-like contraction that closes phagosomes, The sequential processes of pseudopod extension and contraction can explain how macropinosomes and spac ious phagosomes form without guidance from a particle surface.