Elevated plasma levels of cell-free mtDNA are associated with acute rejection following heart transplantation

Acute rejection (AR) following heart transplantation (HTx) is a common complication, especially in the early post-HTx period. Mitochondrial DNA (mtDNA), released into circulation from stressed mitochondria, mimics ongoing immune activation and facilitates the release of pro-inflammatory substances. Our study aimed to assess cell-free mtDNA levels to identify early indicators of acute rejection progression. The absolute concentration of cf-mtDNA (cp/μl) was measured in 77 adult patients using quantitative polymerase chain reaction. Blood samples (n = 300) were collected before their corresponding biopsy according to the timeline within the first year post-HTx. The median cf-mtDNA levels in samples with confirmed AR (n = 57) was higher compared to samples without diagnosed rejection (n = 210; Padj < 0.01). When acute cellular (ACR; n = 39) and antibody-mediated rejection (AMR; n = 18) were analyzed separately, only AMR demonstrated higher levels compared to samples without diagnosed rejection (Padj = 0.02). The highest cf-mtDNA levels were detected in samples collected during early post-HTx complications compared to samples without rejection and AR samples (for both Padj < 0.0001). Both ACR and AMR were observed throughout the one-year period, with the majority (3rd quartile) occurring during the first 200 days post-HTx. Post-HTx complications, such as graft dysfunction or acute kidney injury, were observed within the first 11 days, with the majority (71.4%) occurring within 5 days post-HTx. The presence of AR, and specifically AMR, is associated with elevated levels of cf-mtDNA. The increase in plasma cf-mtDNA levels strongly reflects the occurrence of early complications following HTx.