Phosphatidylserine Exposure Controls Viral Innate Immune Responses by Microglia

Abstract

SUMMARY Microglia are the intrinsic immune sentinels of the central nervous system. Their activation restricts tissue injury and pathogen spread, but in some settings, including viral infection, this response can contribute to cell death and disease. Identifying mechanisms that control microglial responses is therefore an important objective. Using replication-incompetent adenovirus 5 (Ad5)-based vectors as a model, we investigated the mechanisms through which microglia recognize and respond to viral uptake. Transgenic, immunohistochemical, molecular-genetic, and fluorescence imaging approaches revealed that phosphatidylserine (PtdSer) exposure on the outer leaflet of transduced cells triggers their engulfment by microglia through TAM receptor-dependent mechanisms. We show that inhibition of phospholipid scramblase 1 (PLSCR1) activity reduces intracellular calcium dysregulation, prevents PtdSer externalization, and enables months-long protection of vector-transduced, transgene-expressing cells from microglial phagocytosis. Our study identifies PLSCR1 as a potent target through which the innate immune response to viral vectors, and potentially other stimuli, may be controlled.

Protein S and Gas6 induce efferocytosis of HIV-1-infected cells

“…Cells transduced with adenovirus vectors, which expose PtdSer due to the toxicity of viral transduction, were recently shown to be removed by efferocytosis by the interactions between PtdSer, protein S/Gas6, and TAM receptors, resulting in decreased transgene expression of adenoviral vectors (Tufail et al, 2017). Because adenoviruses require cell death for egress (Fields et al, 2013), it is of interest to determine whether protein S/Gas6-mediated efferocytosis can inhibit release of replication-competent adenovirus from infected cells.…”

“…Efferocytosis can also play a role in protecting the host from pathogens (Martin et al, 2014; Nainu et al, 2015; Tufail et al, 2017). When cells are infected with certain types of viruses, the cells undergo apoptosis to prevent pathogens from exploiting the host cell machinery.…”

Phosphatidylserine Is the Signal for TAM Receptors and Their Ligands

“…For example, when neurogenic cells in the adult SVZ are BrdU pulse-labeled during cell division, many more healthy BrdU-labeled neurons are present in the olfactory bulb (to which newborn SVZ cells migrate) one month after labeling in Axl −/− Mertk −/− mice than in wild-type [65]. More definitively, adenovirus-transduced astrocytes have been found to transiently externalize PtdSer, and over the course of several days after the transduction, to be phagocytically cleared by activated microglia [66]. This phagocytosis is both TAM- and PtdSer-dependent, since clearance does not occur in either Axl −/− Mertk −/− mutants or when PtdSer externalization is genetically inhibited [66].…”

“…More definitively, adenovirus-transduced astrocytes have been found to transiently externalize PtdSer, and over the course of several days after the transduction, to be phagocytically cleared by activated microglia [66]. This phagocytosis is both TAM- and PtdSer-dependent, since clearance does not occur in either Axl −/− Mertk −/− mutants or when PtdSer externalization is genetically inhibited [66]. Moreover, in these mutant settings, the non-eaten transduced cells persist as healthy functioning astrocytes for many months [66].…”

“…This phagocytosis is both TAM- and PtdSer-dependent, since clearance does not occur in either Axl −/− Mertk −/− mutants or when PtdSer externalization is genetically inhibited [66]. Moreover, in these mutant settings, the non-eaten transduced cells persist as healthy functioning astrocytes for many months [66]. These studies are consistent with: (a) earlier analyses of tissue recovery following focal brain ischemia, in which areas (brain volume) of neuronal atrophy are reduced, and recovery is enhanced, in Mertk −/− mice relative to wild-type [67]; and (b) even earlier work in C. elegans embryos, in which genetic inactivation of engulfment (phagocytosis) genes results in the long-term survival of a subset of cells that would otherwise die [68, 69].…”