Apoptosis is an evolutionarily conserved and tightly regulated cell death modality. It serves important roles in physiology by sculpting complex tissues during embryogenesis and by removing effete cells that have reached advanced age or whose genomes have been irreparably damaged. Apoptosis culminates in the rapid and decisive removal of cell corpses by efferocytosis, a term used to distinguish the engulfment of apoptotic cells from other phagocytic processes. Over the past decades, the molecular and cell biological events associated with efferocytosis have been rigorously studied, and many eat-me signals and receptors have been identified. The externalization of phosphatidylserine (PS) is arguably the most emblematic eat-me signal that is in turn 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.

The filoviruses Ebola Zaire virus and Marburg virus are believed to infect target cells through endocytic vesicles, but the details of this pathway are unknown. We used a pseudotyping strategy to investigate the cell biology of filovirus entry. We observed that specific inhibitors of the caveola system, including cholesterolsequestering drugs and phorbol esters, inhibited the entry of filovirus pseudotypes into human cells. We also measured slower cell entry kinetics for both filovirus pseudotypes than for pseudotypes of vesicular stomatitis virus (VSV), which has been recognized to exploit the clathrin-mediated entry pathway. Finally, visualization by immunofluorescence and confocal microscopy revealed that the filovirus pseudotypes colocalized with the caveola protein marker caveolin-1 but that VSV pseudotypes did not. Collectively, these results provide evidence suggesting that filoviruses use caveolae to gain entry into cells.

Human infections by Marburg (MBG) and Ebola (EBO) viruses result in lethal hemorrhagic fever. To identify cellular entry factors employed by MBG virus, noninfectible cells transduced with an expression library were challenged with a selectable pseudotype virus packaged by MBG glycoproteins (GP). A cDNA encoding the folate receptor-alpha (FR-alpha) was recovered from cells exhibiting reconstitution of viral entry. A FR-alpha cDNA was recovered in a similar strategy employing EBO pseudotypes. FR-alpha expression in Jurkat cells facilitated MBG or EBO entry, and FR-blocking reagents inhibited infection by MBG or EBO. Finally, FR-alpha bound cells expressing MBG or EBO GP and mediated syncytia formation triggered by MBG GP. Thus, FR-alpha is a significant cofactor for cellular entry for MBG and EBO viruses.

CD14 is a 55-kDa glycoprotein which binds lipopolysaccharide (LPS) and enables LPS-dependent responses in a variety of cells. Recent limited proteolysis studies have implicated a region in CD14 between amino acids 57 and 64 as being involved in LPS interaction. To specifically assess the importance of this region with respect to LPS binding, we constructed a mutant sCD14 (sCD14 delta 57-64) lacking amino acids 57-64. sCD14 delta 57-64 was isolated from the serum-free conditioned medium of this cell line, and, in all assays, the purified protein failed to recognize LPS or enable LPS-dependent responses in cells. We also demonstrated that the region between amino acids 57 and 64 is required for binding of a neutralizing CD14 mAb, MEM-18. Native polyacrylamide gel electrophoresis assays were used to demonstrate that MEM-18 and LPS compete for the same binding site on CD14. These data strongly suggest that the region spanning amino acids 57-64 binds LPS and that formation of sCD14.LPS complex is required in order for sCD14-mediated responses to occur.

The frequency of moderate to severe adverse reactions associated with smallpox vaccines currently stockpiled in the US, and the continued threat of bioterrorism have prompted the development of effective vaccines with improved safety profiles. LC16m8, an attenuated, replicating smallpox vaccine derived from the Lister strain of vaccinia, is currently licensed in Japan where it was safely used in over 50,000 children in the 1970s. It has been shown to have markedly less neurotoxicity than unattenuated vaccines in nonclinical studies. LC16m8 is immunogenic after a single dose, and recent studies in two different animal models have demonstrated protective efficacy equivalent to that of the only FDA-licensed smallpox vaccine. This article reviews the history and available scientific literature regarding LC16m8 and provides comparisons to other smallpox vaccines.