Reduced host cell-free DNA as a biomarker in latent tuberculosis infection among tuberculosis contacts

Circulating cell-free DNA (cfDNA), including mitochondrial cfDNA (mt-cfDNA) and nuclear cfDNA (nu-cfDNA), are potential biomarkers for infectious diseases. However, cfDNA variations in TB contacts with latent tuberculosis infection (LTBI) and their potential link to a predominance of M1 monocyte polarization in LTBI remain unexplored.

Contacts of TB patients were screened for LTBI using the Interferon gamma (IFN-γ) release assay. Blood cfDNA was extracted, and mt-cfDNA and nu-cfDNA copy numbers were quantified by qPCR. cfDNA levels in the supernatant of M1-polarized THP-1-derived macrophages were measured.

Levels of mt-cfDNA and nu-cfDNA were lower in the LTBI group (n = 76) than in the uninfected group (n = 58) (p = 0.012, and p < 0.001). The results were consistent in an age- and sex-matched analysis (n = 41 pairs). mt-cfDNA and nu-cfDNA levels were negatively associated with the TB-specific IFN-γ response (p = 0.009, p < 0.001). In the LTBI group, mt-cfDNA was negatively associated with the index case's bacterial burden (p = 0.045). In cell model, mt-cfDNA and nu-cfDNA levels in the supernatant from M1-polarized macrophage were lower than those from M2-polarized cells (p = 0.030, p = 0.045).

TB contacts with LTBI had lower cfDNA levels, which correlated with index case infectivity. Reduced cfDNA in M1-polarized macrophages warrants further investigation into the mechanisms of cfDNA reduction in LTBI.