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Am J Respir Crit Care Med. 2018 Feb 15;197(4):470-480. doi: 10.1164/rccm.201706-1161OC.

Red Blood Cells Homeostatically Bind Mitochondrial DNA through TLR9 to Maintain Quiescence and to Prevent Lung Injury.

Author information

1
1 Pulmonary, Allergy and Critical Care Division and.
2
2 Department of Paediatrics, University of Toronto, Toronto, Ontario, Canada; and.
3
3 Division of Hematology and.
4
5 Penn Center for Pulmonary Biology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
5
4 Division of Neonatology, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania.

Abstract

RATIONALE:

Potentially hazardous CpG-containing cell-free mitochondrial DNA (cf-mtDNA) is routinely released into the circulation and is associated with morbidity and mortality in critically ill patients. How the body avoids inappropriate innate immune activation by cf-mtDNA remains unknown. Because red blood cells (RBCs) modulate innate immune responses by scavenging chemokines, we hypothesized that RBCs may attenuate CpG-induced lung inflammation through direct scavenging of CpG-containing DNA.

OBJECTIVES:

To determine the mechanisms of CpG-DNA binding to RBCs and the effects of RBC-mediated DNA scavenging on lung inflammation.

METHODS:

mtDNA on murine RBCs was measured under basal conditions and after systemic inflammation. mtDNA content on human RBCs from healthy control subjects and trauma patients was measured. Toll-like receptor 9 (TLR9) expression on RBCs and TLR9-dependent binding of CpG-DNA to RBCs were determined. A murine model of RBC transfusion after CpG-DNA-induced lung injury was used to investigate the role of RBC-mediated DNA scavenging in mitigating lung injury in vivo.

MEASUREMENTS AND MAIN RESULTS:

Under basal conditions, RBCs bind CpG-DNA. The plasma-to-RBC mtDNA ratio is low in naive mice and in healthy volunteers but increases after systemic inflammation, demonstrating that the majority of cf-mtDNA is RBC-bound under homeostatic conditions and that the unbound fraction increases during inflammation. RBCs express TLR9 and bind CpG-DNA through TLR9. Loss of TLR9-dependent RBC-mediated CpG-DNA scavenging increased lung injury in vivo.

CONCLUSIONS:

RBCs homeostatically bind mtDNA, and RBC-mediated DNA scavenging is essential in mitigating lung injury after CpG-DNA. Our data suggest a role for RBCs in regulating lung inflammation during disease states where cf-mtDNA is elevated, such as sepsis and trauma.

KEYWORDS:

CpG-DNA; RBC; Toll-like receptor 9; mitochondrial DNA

PMID:
29053005
PMCID:
PMC5821907
DOI:
10.1164/rccm.201706-1161OC
[Indexed for MEDLINE]
Free PMC Article

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