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

## TL;DR

Red blood cells capture bacterial DNA during sepsis, influencing inflammation and immune responses in both mice and humans.

## Contribution

This study reveals red blood cells as novel carriers of microbial DNA that modulate host immune responses during sepsis.

## Key findings

- Murine RBCs acquire microbial DNA in vitro, enhancing macrophage activation via TLR9.
- RBC-bound bacterial DNA is elevated in septic mice and correlates with plasma IL-6 levels.
- Human septic RBCs contain more bacterial DNA than healthy controls, with distinct microbial communities identified.

## Abstract

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.