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

Abstract

(2008). Inhibition of "self " engulfment through deactivation of myosin-II at the phagocytic synapse between human cells. Retrieved from http://repository.upenn.edu/cbe_papers/108Inhibition of "self " engulfment through deactivation of myosin-II at the phagocytic synapse between human cells AbstractPhagocytosis 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, Factin, and the major motor isoform, nonmuscle myosin-IIA. When CD47 is functional, the macrophage counter-receptor and phosphatase-activator SIRPα 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-SIRPα 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. Comments

Self inhibition of phagocytosis: The affinity of ‘marker of self’ CD47 for SIRPα dictates potency of inhibition but only at low expression levels

“…RBC from mice as well as sheep respectively show weak to no binding to this recombinant hSIRPα when compared to human RBCs (Figure 1A). Based on the lack of sheep RBC interactions with soluble hSIRPα, human macrophages would be expected to phagocytose IgG-opsonized sheep RBC – which is indeed commonly reported in studies of Fcγ receptor mediated phagocytosis [18[19[20]. RBC membranes are nonetheless complex, with many additional differences between species: for example, CD47 on human RBCs is less mobile than on mouse RBCs in part due to human CD47's association with the species-specific Rh complex [24].…”

“…Based on this divergence, human macrophages might be expected to engulf opsonized sheep RBCs – an expectation borne out by many prior studies using sheep-RBCs as targets for human macrophages in phagocytosis (e.g. [18[19[20]). In addition to the recognition of SIRPα and CD47 between species, there exists considerable polymorphism in SIRPα between mouse strains [21] and between humans [2]; indeed, from a very limited but worldwide study of 37 human haplotypes, 10 variants of SIRPα's ~100 amino acid N-terminal domain were reported.…”

The CD47‐SIRPα signaling axis as an innate immune checkpoint in cancer

“…The involvement of SIRPα and that of CD47‐SIRPα interactions in RBC phagocytosis and clearance in vivo was confirmed using mice with a mutant SIRPα receptor lacking the cytoplasmic tail . Further mechanistic studies implicated that binding of CD47 to SIRPα leads to an RBC phagocytosis‐associated deactivation of myosin IIA, which is an important contributor of phagocytosis because of its participation in macrophage actino‐myosin contraction . CD47‐SIRPα interactions also suppress the clearance by macrophages of antibody‐ or complement‐ opsonized erythrocytes, through Fcγ‐ or complement‐ receptors on macrophages, and this could be relevant in the context of autoimmune hemolytic anemia (AIHA) …”

Correlating macrophage morphology and cytokine production resulting from biomaterial contact

“…Recently, we showed that the polymerized state of actin (i.e., actin elongation) in adherent macrophages correlated with the biocompatibility of surfaces 38. Specifically, macrophages showed a significantly reduced affinity for polymeric surfaces modified with recombinant CD47, a ubiquitously expressed transmembrane protein that reduces the polymerized state of actin through signaling mechanisms mediated by its cognate receptor, Signal Regulatory Protein alpha, or SIRPα 50. Second, 3D quantitative analysis of cell morphology indicates that macrophage surface area and volume as well as spreading behavior depend on surface type.…”