Macrophages recognize additional abnormal cell characteristics such as elevated lateral mobility of PS11 or modifications of the glycocalyx.43 These interactions initiate signaling pathways that rearrange the actin cytoskeleton thereby enabling the engulfment of apoptotic cells.44 Finally, the rapid and effective removal of apoptotic cells by phagocytes is crucial for prevention of an undesirable inflammatory response and maintenance of an anti-inflammatory status during homeostasis (tolerogenic Ranolazine signals’), a term that has sometimes been called silent apoptosis to convey immune downregulation. bound by a large number of serum proteins and opsonins that facilitate efferocytosis. Under physiological conditions, externalized PS functions as a dominant and evolutionarily conserved immunosuppressive signal that promotes tolerance and prevents local and systemic immune activation. Pathologically, the innate immunosuppressive effect of externalized PS has been hijacked by numerous viruses, microorganisms, and parasites to facilitate infection, and in many cases, establish infection latency. PS is also profoundly dysregulated in the tumor microenvironment and antagonizes the development of tumor immunity. In this review, we discuss the biology of PS with respect to its role as a global immunosuppressive signal and how PS is exploited to drive diverse pathological processes such as infection and cancer. Finally, we outline the rationale that agents targeting PS could have significant value in cancer and infectious disease therapeutics. Facts PS externalization during apoptosis and cell stress are mediated by scramblases Xkr8 and TMEM16, respectively. Exposed PS is an evolutionarily conserved anti-inflammatory and immunosuppressive signal. Ranolazine An astonishing number of pathogens causing major infectious diseases utilize PS and apoptotic mimicry to evade host immune responses. PS signaling is highly dysregulated in the tumor microenvironment and autoimmune diseases. PS-targeting therapeutics (e.g., AnxA5, bavituximab) can stimulate immune activity. Open Questions Is PS dysregulation a universal mechanism of immune evasion for bacteria, viruses and protists? Should PS targeting be considered a global therapeutic option for infectious diseases? Should PS be considered a global checkpoint inhibitor for cancer? Are all PS signaling equally immunosuppressive? Are cofactors involved? Many critical biochemical pathways require the presence of specific phospholipid classes in the inner and outer leaflet of the plasma membrane. Virtually all eukaryotic cells have an asymmetric distribution of phospholipids across their bilayer membrane, where the choline-containing phospholipids, phosphatidylcholine (PC) and sphingomyelin are predominately maintained on the outer membrane leaflet, and the amino-phospholipids (phosphatidylserine (PS) and phosphatidylethanolamine (PE)) are predominately localized in the inner membrane leaflet.1 This asymmetry is actively maintained by the regulated activity of ATP-dependent lipid transporters. However, membrane asymmetry collapses under a variety of physiological and pathological conditions resulting in dramatic changes in the biochemical properties of the membrane. For example, the redistribution of PS to the external face of the plasma membrane flags cells for their recognition, phagocytosis,2 and ultimate degradation by phagocytes (efferocytosis). Moreover, the interaction between PS-expressing cells and immune cells elicits profound immunological consequences by triggering immunosuppressive pathways that prevent both local and systemic immune activation. Although these pathways are used by apoptotic cells to quell potential immune sequelae against self’, these same pathways are hijacked by pathogens and tumors to promote their sinister life-threatening expansion. Taken together, these observations suggest that PS functions as an upstream immune checkpoint that suppresses the development of immunity. This raises the possibility that PS blockade by the therapeutic administration of PS-targeting agents can restore pathogen and tumor immunity. PS Asymmetry in Biological Membranes PS, the most abundant negatively charged glycerophospholipid in eukaryotic membranes, is comprised of a glycerol backbone esterified at the gene) are able to survive when grown on high concentrations of ethanolamine,7 suggesting Ranolazine that PS is an essential membrane lipid in higher metazoans. Interestingly, genetic linkage analysis suggest that rare sporadic dominant gain-of-function mutations in PTDSS1 occur in patients with Lenz-Majewski syndrome, biochemically characterized by increased PS in their membranes, and phenotypically by multiple congenital abnormalities of generalized craniotubular hyperostosis.8 Open in a separate window Figure 1 Molecular structure of PS and major biosynthetic and degradative pathways: (a) PS is comprised of a glycerol backbone esterified at the viable cells that engage receptors in distinct ways. Indeed, recent studies examining the effects of ligand-density on the activation of AXL receptor tyrosine kinase (Axl; a Dnmt1 PS receptor) support this idea, in which it was concluded that the specific sensing of ligand spatial distribution is a critical feature for PS-dependent (Axl) receptor activation.30 Although the preceding sections have focused on the interplay.
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- This was considered and rejected as being cost-prohibitive