Abstract / Summary
Background/Objectives: To compare the positivity rates and serum levels of antibodies against histone H3 lysine 9 (anti-H3K9) and its lactylated form (anti-H3K9la) among patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), or Sjögren disease (SjD) and controls. Methods: Serum anti-H3K9 and anti-H3K9la levels were measured by enzyme-linked immunosorbent assay from 93 patients with RA, 40 with SLE, 40 with SjD, and 40 controls. Antibody findings were compared across study groups and according to rheumatoid factor immunoglobulin M (RF-IgM) and anti-citrullinated protein antibody (ACPA) status in RA. Results: Anti-H3K9 positivity was 28.0% in RA, 40.0% in SLE, 0% in SjD, and 2.5% in controls. Anti-H3K9la positivity was 22.6% in RA and 20.0% in SLE; no SjD patients or controls were positive. Patients with SjD had lower levels of both antibodies than controls. No statistically significant differences were detected according to RF-IgM or ACPA status, although the seronegative subgroups were small. In exploratory multivariable models, TNF inhibitor use was associated with higher levels of both antibodies. Adding either antibody to ACPA and RF-IgM increased sensitivity at the expense of specificity. Conclusions: Anti-H3K9 and anti-H3K9la antibodies were detected more frequently in RA and SLE than in controls, whereas patients with SjD had lower serum levels of both antibodies. No statistically significant differences in antibody positivity or levels were detected according to RF-IgM or ACPA status in RA, although the small seronegative subgroups limited the precision of these comparisons. Their potential value as adjunctive diagnostic biomarkers requires confirmation in larger, independent cohorts with formal diagnostic-performance analyses.