The Red Blood Cell Surface Defines a Distinct Tumor DNA Compartment for Liquid Biopsy

Red blood cells (RBCs) modulate host inflammatory responses by scavenging mitochondrial and pathogen DNA. However, how RBC surface properties govern the capture of tumor-derived DNA remains poorly understood. We show that RBCs tether and retain tumor-derived DNA on their surface, forming a distinct circulating pool of RBC-bound tumor DNA (rtDNA). Desialylation increased DNA capture in a Toll-like receptor 9 (TLR9)-dependent manner, identifying RBC surface sialylation and TLR9 as regulators of DNA binding. In mice bearing lung cancer xenografts, rtDNA was detected in all animals and its abundance correlated with tumor burden, whereas tumor DNA was undetectable in matched plasma samples. In patients with lung cancer, the majority of detected mutant tumor DNA was associated with RBCs rather than plasma, and RBC-derived mutations were concordant with molecular alterations identified in matched tumor tissue. Collectively, these findings establish the RBC surface as a tunable site of tumor DNA capture and retention and challenge the plasma-centric paradigm of liquid biopsy by identifying a distinct cellular reservoir of tumor-derived genetic material.