CXCL8 and juvenile idiopathic arthritis: The most prominent truncated proteoform in synovial fluid from JIA patients was the most active CXCL8 protein CXCL8(9-77) [20.4 ± 6.5% (mean ± SEM)], followed by CXCL8(6-77) [9.3 ± 3.4% (mean ± SEM)], CXCL8(2-77) [8.6 ± 5.0% (mean ± SEM)], CXCL8(8-77) [7.0 ± 4.3% (mean ± SEM)] and CXCL8(7-77) [3.0 ± 1.6% (mean ± SEM)] (Figure 4C; Supplementary Table 1).