A Soluble PrP^C Derivative and Membrane-Anchored PrP^C in Extracellular Vesicles Attenuate Innate Immunity by Engaging...

Nonpathogenic cellular prion protein (PrP^C) demonstrates anti-inflammatory activity; however, the responsible mechanisms are incompletely defined. PrP^C exists as a GPI-anchored membrane protein in diverse cells; however, PrP^C may be released from cells by ADAM proteases or when packaged into extracellular vesicles (EVs). In this study, we show that a soluble derivative of PrP^C (S-PrP) counteracts inflammatory responses triggered by pattern recognition receptors in macrophages, including TLR2, TLR4, TLR7, TLR9, NOD1, and NOD2. S-PrP also significantly attenuates the toxicity of LPS in mice. The response of macrophages to S-PrP is mediated by a receptor assembly that includes the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1). PrP^C was identified in EVs isolated from human plasma. These EVs replicated the activity of S-PrP, inhibiting cytokine expression and IκBα phosphorylation in LPS-treated macrophages. The effects of plasma EVs on LPS-treated macrophages were blocked by PrP^C-specific Ab, by antagonists of LRP1 and the NMDA-R, by deleting Lrp1 in macrophages, and by inhibiting Src family kinases. Phosphatidylinositol-specific phospholipase C dissociated the LPS-regulatory activity from EVs, rendering the EVs inactive as LPS inhibitors. The LPS-regulatory activity that was lost from phosphatidylinositol-specific phospholipase C-treated EVs was recovered in solution. Collectively, these results demonstrate that GPI-anchored PrP^C is the essential EV component required for the observed immune regulatory activity of human plasma EVs. S-PrP and EV-associated PrP^C regulate innate immunity by engaging the NMDA-R/LRP1 receptor system in macrophages. The scope of pattern recognition receptors antagonized by S-PrP suggests that released forms of PrP^C may have broad anti-inflammatory activity.