ObjectiveComplement component C4 copy number variations are associated with various inflammatory diseases. This study was undertaken to investigate whether copy number variations of C4 are also involved in the pathogenesis of Behcet's disease (BD). MethodsGene expression was examined by enzyme-linked immunosorbent assay (ELISA) or real-time polymerase chain reaction (PCR). Copy number variations of C4 isotypes (C4A and C4B) were detected by real-time PCR in 905 patients with BD, 205 patients with ankylosing spondylitis (AS) and acute anterior uveitis, and 1,238 controls. The activation of CD4+ T cells was analyzed by flow cytometry, and cytokine production was detected by ELISA. ResultsProtein expression of total C4 in serum was significantly increased in patients with active BD compared with those with inactive BD or controls (Bonferroni corrected P [P-corr] = 1.64 x 10(-4) and P-corr = 0.037, respectively), but not in patients with AS and acute anterior uveitis. Copy number variation analysis identified a significantly increased frequency of more than 2 copies of C4A in BD patients (P = 1.65 x 10(-7), odds ratio [OR] 2.84). HLA-B51, which is located on the same chromosome as C4, showed a strong association with BD in the Han Chinese population (P = 8.90 x 10(-65), OR 5.05), but logistic regression showed that C4A copy number variation was an independent risk factor for BD. A significantly increased expression of C4A was observed in the high copy number groups (>2 copies or 2 copies) versus the low copy number group (P-corr = 0.019 and P-corr = 0.044, respectively). Increased production of interleukin-6 (IL-6) was also observed in the high C4A copy number group (P-corr = 0.037). No effect of C4 copy number variation on the expression of T cell activation markers was detected. ConclusionOur findings indicate that a high copy number of C4A confers risk for BD by modulating the expression of C4A and enhancing IL-6 production.