Human red blood cells express the RNA sensor TLR7.

Red blood cells (RBCs) express the nucleic acid-binding toll-like receptor 9 (TLR9) and bind CpG-containing DNA. However, whether human RBCs express other nucleic acid-binding TLRs is unknown. Here we show that human RBCs express the RNA sensor TLR7. TLR7 is present on the red cell membrane and is associated with the RBC membrane protein Band 3. In patients with SARS-CoV2-associated sepsis, TLR7-Band 3 interactions in the RBC membrane are increased when compared with healthy controls. In vitro, RBCs bind synthetic ssRNA and RNA from ssRNA viruses. Thus, RBCs may serve as a previously unrecognized sink for exogenous RNA, expanding the repertoire of non-gas exchanging functions performed by RBCs.

Red Blood Cell DNA Capture and Delivery Drives Host Responses During Polymicrobial Sepsis.

Red blood cells (RBCs), traditionally recognized for their role in transporting oxygen, play a pivotal role in the body's immune response by expressing TLR9 and scavenging excess host cell-free DNA. DNA capture by RBCs leads to accelerated RBC clearance and triggers inflammation. Whether RBCs can also acquire microbial DNA during infections is unknown. Murine RBCs acquire microbial DNA in vitro and bacterial-DNA-induced macrophage activation was augmented by WT but not TLR9-deleted RBCs. In a mouse model of polymicrobial sepsis, RBC-bound bacterial DNA was elevated in WT but not in erythroid TLR9-deleted mice. Plasma cytokine analysis revealed distinct sepsis endotypes, characterized by persistent hypothermia and hyperinflammation in the most severely affected subjects. RBC-TLR9 deletion attenuated plasma and tissue IL-6 production in the most severe endotype. Parallel findings in human subjects confirmed that RBCs from septic patients harbored more bacterial DNA compared to healthy individuals. Further analysis through 16S sequencing of RBC-bound DNA illustrated distinct microbial communities, with RBC-bound DNA composition correlating with plasma IL-6 in patients with sepsis. Collectively, these findings unveil RBCs as overlooked reservoirs and couriers of microbial DNA, capable of influencing host inflammatory responses in sepsis.

Platelet factor 4 limits neutrophil extracellular trap- and cell-free DNA-induced thrombogenicity and endothelial injury.

Plasma cell-free DNA (cfDNA), a marker of disease severity in sepsis, is a recognized driver of thromboinflammation and a potential therapeutic target. In sepsis, plasma cfDNA is mostly derived from neutrophil extracellular trap (NET) degradation. Proposed NET-directed therapeutic strategies include preventing NET formation or accelerating NET degradation. However, NET digestion liberates pathogens and releases cfDNA that promote thrombosis and endothelial cell injury. We propose an alternative strategy of cfDNA and NET stabilization with chemokine platelet factor 4 (PF4, CXCL4). We previously showed that human PF4 (hPF4) enhances NET-mediated microbial entrapment. We now show that hPF4 interferes with thrombogenicity of cfDNA and NETs by preventing their cleavage to short-fragment and single-stranded cfDNA that more effectively activates the contact pathway of coagulation. In vitro, hPF4 also inhibits cfDNA-induced endothelial tissue factor surface expression and von Willebrand factor release. In vivo, hPF4 expression reduced plasma thrombin-antithrombin (TAT) levels in animals infused with exogenous cfDNA. Following lipopolysaccharide challenge, Cxcl4-/- mice had significant elevation in plasma TAT, cfDNA, and cystatin C levels, effects prevented by hPF4 infusion. These results show that hPF4 interacts with cfDNA and NETs to limit thrombosis and endothelial injury, an observation of potential clinical benefit in the treatment of sepsis.

Neutrophil extracellular trap stabilization by platelet factor 4 reduces thrombogenicity and endothelial cell injury.

Neutrophil extracellular traps (NETs) are abundant in sepsis, and proposed NET-directed therapies in sepsis prevent their formation or accelerate degradation. Yet NETs are important for microbial entrapment, as NET digestion liberates pathogens and NET degradation products (NDPs) that deleteriously promote thrombosis and endothelial cell injury. We proposed an alternative strategy of NET-stabilization with the chemokine, platelet factor 4 (PF4, CXCL4), which we have shown enhances NET-mediated microbial entrapment. We now show that NET compaction by PF4 reduces their thrombogenicity. In vitro, we quantified plasma thrombin and fibrin generation by intact or degraded NETs and cell-free (cf) DNA fragments, and found that digested NETs and short DNA fragments were more thrombogenic than intact NETs and high molecular weight genomic DNA, respectively. PF4 reduced the thrombogenicity of digested NETs and DNA by interfering, in part, with contact pathway activation. In endothelial cell culture studies, short DNA fragments promoted von Willebrand factor release and tissue factor expression via a toll-like receptor 9-dependent mechanism. PF4 blocked these effects. Cxcl4-/- mice infused with cfDNA exhibited higher plasma thrombin anti-thrombin (TAT) levels compared to wild-type controls. Following challenge with bacterial lipopolysaccharide, Cxcl4-/- mice had similar elevations in plasma TAT and cfDNA, effects prevented by PF4 infusion. Thus, NET-stabilization by PF4 prevents the release of short fragments of cfDNA, limiting the activation of the contact coagulation pathway and reducing endothelial injury. These results support our hypothesis that NET-stabilization reduces pathologic sequelae in sepsis, an observation of potential clinical benefit.

Bat Red Blood Cells Express Nucleic Acid-Sensing Receptors and Bind RNA and DNA.

RBCs demonstrate immunomodulatory capabilities through the expression of nucleic acid sensors. However, little is known about bat RBCs, and no studies have examined the immune function of bat erythrocytes. In this study, we show that bat RBCs express the nucleic acid-sensing TLRs TLR7 and TLR9 and bind the nucleic acid ligands, ssRNA, and CpG DNA. Collectively, these data suggest that, like human RBCs, bat erythrocytes possess immune function and may be reservoirs for nucleic acids. These findings provide unique insight into bat immunity and may uncover potential mechanisms by which virulent pathogens of humans are concealed in bats.