Key Points• Rigidity of an opsonized red cell that contacts a macrophage is found to hyperactivate myosin-II and thus overpowers CD47's self-signaling. • Red cell shape modulates CD47's signaling of self and highlights biophysical contributions to phagocytosis.A macrophage engulfs another cell or foreign particle in an adhesive process that often activates myosin-II, unless the macrophage also engages "marker of self" CD47 that inhibits myosin. For many cell types, adhesion-induced activation of myosin-II is maximized by adhesion to a rigid rather than a flexible substrate. Here we demonstrate that rigidity of a phagocytosed cell also hyperactivates myosin-II, which locally overwhelms selfsignaling at a phagocytic synapse. Cell stiffness is one among many factors including shape that changes in erythropoiesis, in senescence and in diseases ranging from inherited anemias and malaria to cancer. Controlled stiffening of normal human red blood cells (RBCs) in different shapes does not compromise CD47's interaction with the macrophage self-recognition receptor signal regulatory protein alpha (SIRPA). Uptake of antibodyopsonized RBCs is always fastest with rigid RBC discocytes, which also show that maximal active myosin-II at the synapse can dominate self-signaling by CD47. Rigid but rounded RBC stomatocytes signal self better than rigid RBC discocytes, highlighting the effects of shape on CD47 inhibition. Physical properties of phagocytic targets thus regulate self signaling, as is relevant to erythropoiesis, to clearance of rigid RBCs after blood storage, clearance of rigid pathological cells such as thalassemic or sickle cells, and even to interactions of soft/stiff cancer cells with macrophages. (Blood. 2015;125(3):542-552)

Domain formation in binary mixtures of cholesterol and a single phospholipid has been studied for a long time but the nature of these domains is still a matter of some debate. One interpretation of these domains is that they arise from the coexistence of liquid-ordered and liquid-disordered phases within the membranes. Here, we study the effect of adhesive surfaces and environments on the proposed phase behavior theoretically. The adhesion or partial support of the membranes leads to two or several membrane segments that exhibit different molecular interactions with their environments and, thus, have different molecular affinities to these environments. We show that the affinity contrasts between different environments strongly affect the phase behavior of the membranes. First, the affinity contrasts confine the domain formation spatially to single membrane segments. Second, these contrasts lead to a partitioning of the coexistence region in the composition-temperature plane into distinct coexistence regions for the different membrane segments. Third, the range of membrane compositions, for which one can observe domain formation in one of the membrane segments, is always reduced compared to the composition range of the free membrane. Furthermore, small affinity contrasts represent singular perturbations in the sense that the phase diagrams change in a discontinuous manner as one varies the affinity contrast from small positive to small negative values. All of these results are quite general because they follow from thermodynamic stability alone and apply to any two-component membrane with fluid-fluid coexistence. In addition to these generic features, we also provide a computational scheme, by which one can explicitly calculate the partitioning of the coexistence regions for a specific binary mixture, and illustrate this scheme for binary cholesterol-DPPC membranes. Both the generic and the specific features of domain formation as predicted by our theory are accessible to experimental studies.

The interplay of adhesion and phase separation is studied theoretically for two-component membranes that can phase separate into two fluid phases such as liquid-ordered and liquid-disordered phases. Many adhesion geometries provide two different environments for these membranes and then partition the membranes into two segments that differ in their composition. Examples are provided by adhering vesicles, by hole- or pore-spanning membranes, and by membranes supported by chemically patterned surfaces. Generalizing a lattice model for binary mixtures to these adhesion geometries, we show that the phase behavior of the adhering membranes depends, apart from composition and temperature, on two additional parameters, the area fraction of one membrane segment and the affinity contrast between the two segments. For the generic case of non-vanishing affinity contrast, the adhering membranes undergo two distinct phase transitions and the phase diagrams in the composition/temperature plane have a generic topology that consists of two two-phase coexistence regions separated by an intermediate one-phase region. As a consequence, phase separation and domain formation is predicted to occur separately in each of the two membrane segments but not in both segments simultaneously. Furthermore, adhesion is also predicted to suppress the phase separation process for certain regions of the phase diagrams. These generic features of the adhesion-induced phase behavior are accessible to experiment.

No abstract