VTN and systemic lupus erythematosus: We calculated the correlation between serum IFN activity and each principal component and found that serum IFN activity did not correlate with PC1 (r = 0.14, p = 0.373) but did correlate with PC2 (r = 0.56, p < 0.001), which consisted of constitutional, hematologic, mucocutaneous, musculoskeletal, anti-phospholipid antibodies, complement proteins, and SLE-specific antibodies domains.