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J Cell Biol. 2008 Mar 10;180(5):989-1003. doi: 10.1083/jcb.200708043.

Inhibition of "self" engulfment through deactivation of myosin-II at the phagocytic synapse between human cells.

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Biophysical Engineering Laboratory, University of Pennsylvania, Philadelphia, PA 19104, USA.


Phagocytosis of foreign cells or particles by macrophages is a rapid process that is inefficient when faced with "self" cells that display CD47-although signaling mechanisms in self-recognition have remained largely unknown. With human macrophages, we show the phagocytic synapse at cell contacts involves a basal level of actin-driven phagocytosis that, in the absence of species-specific CD47 signaling, is made more efficient by phospho-activated myosin. We use "foreign" sheep red blood cells (RBCs) together with CD47-blocked, antibody-opsonized human RBCs in order to visualize synaptic accumulation of phosphotyrosine, paxillin, F-actin, and the major motor isoform, nonmuscle myosin-IIA. When CD47 is functional, the macrophage counter-receptor and phosphatase-activator SIRPalpha localizes to the synapse, suppressing accumulation of phosphotyrosine and myosin without affecting F-actin. On both RBCs and microbeads, human CD47 potently inhibits phagocytosis as does direct inhibition of myosin. CD47-SIRPalpha interaction initiates a dephosphorylation cascade directed in part at phosphotyrosine in myosin. A point mutation turns off this motor's contribution to phagocytosis, suggesting that self-recognition inhibits contractile engulfment.

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