FNDC5 and Joint swelling: Interestingly, several cytokines were associated with joint effusion in KOIP-4, including positive correlations with leptin (SC = 0.405, pv = 0.016 in the synovial fluid; SC = 0.344, pv = 0.043 in the plasma), synovial irisin (SC = 0.405, pv = 0.016), and omentin (SC = 0.363, pv = 0.041) and negative correlations with CPR (SC = − 0.590, pv = 0.0002 in the synovial fluid: SC = − 0.512, pv = 0.002 in the plasma) and plasma IL-6 (SC = − 0.368, pv = 0.030) and calprotectin (SC = − 0.336, pv = 0.049) (Additional file 1: Figs. S26–S33).