Calprotectin (S100A8/A9) has the strongest association with ultrasound-detected synovitis and predicts response to biologic treatment: results from a longitudinal study of patients with established rheumatoid arthritis
In this 1-year follow-up study of 141 patients with RA starting bDMARDs, we found calprotectin to be more strongly associated with US and clinical scores of inflammation than ESR, CRP, S100A12, IL-6 and VEGF. In addition, a higher level of calprotectin was found in EULAR responders, and decrease of calprotectin during the first month predicted reduced PD sum scores at 3, 6 and 12 months.
The present study supports the previous finding of strong associations between calprotectin and US sum scores in a small longitudinal study [27] and two small cross-sectional studies [28, 29]. The association is likely due to calprotectin being found in leukocytes, and, with its impact on immune cells and endothelial cells in the joints, calprotectin may have the ability to reflect the US findings of inflammation. This is also supported by the finding of normal levels of calprotectin in patients with no sign of PD activity, which is in agreement with a recent study in which calprotectin could help identify PD synovitis in patients with DAS28???3.2 [29].
There are conflicting results on baseline calprotectin as a predictive marker of response to bDMARDs [13, 17, 43]. High levels of biomarkers at baseline imply a large potential for improvement but might not consistently predict response to treatment. Calprotectin showed a rapid decrease during bDMARD treatment, and ?calprotectin after 1 month predicted EULAR response. In addition, calprotectin and ESR were the only biomarkers where change during the first month predicted change in PD sum scores. Thus, calprotectin may have a potential for early prediction of response to bDMARD treatment. ?Calprotectin during the first month was more predictive of change in PD sum score than change in GS sum score. This might be explained by PD detecting arterioles dilated because of chemical and inflammatory mediators locally in the area of synovitis. PD activity is found to diminish rapidly after glucocorticoid injection in a joint, and this is thought to be caused by a reduction of inflammatory mediators causing the arterioles not to be dilated [44], even if they are still present in the synovium. Thus, on one hand, calprotectin may be associated with, and may predict, the PD activity because both calprotectin and PD are reflecting inflammation. The GS findings, on the other hand, represent synovium that may be more or less actively inflamed and may therefore not have the same association with inflammatory markers. As a parallel, in inflammatory bowel diseases, calprotectin levels (in faeces) have been shown to be associated with inflammation and found to be highly useful in the clinics by predicting clinical response and mucosal healing in patients treated with biologics [45].
Tocilizumab suppresses the conventional inflammatory markers. Only a small number of patients used tocilizumab in this study. However, calprotectin was shown to have stronger associations with the US scores than ESR and CRP in these patients. This is supported by a study of 33 patients with RA treated with tocilizumab in whom calprotectin had stronger correlations with composite scores (DAS28, Simplified Disease Activity Index and Clinical Disease Activity Index) and joint counts compared with CRP and ESR [46].
Prednisolone may decrease inflammatory markers, but use of prednisolone did not seem to suppress the levels of calprotectin, supporting the results of a previous study [11]. Thus, in patients on prednisolone, the inflammatory activity in established RA may be better reflected by calprotectin than by the commonly used ESR and CRP.
Associations of S100A12 with clinical variables and US scores were comparable with those of CRP, but lower than for calprotectin. The explanation for the two S100 proteins’ having different reflection of inflammation could in part be the differences in their distribution and amounts in leukocytes. PD may reflect the angiogenesis, and S100A12 is particularly found in perivascular neutrophils of RA synovia [47] and is able to activate endothelial cells [7]. We previously reported associations between S100A12 and PD in a small longitudinal study [16], and the results of the present study strengthen this preliminary finding by showing consistent associations at all time points.
IL-6 was a relatively strong marker of inflammation in this study, probably reflecting its key functions in RA [1, 20]. This is in agreement with previous studies [21, 30, 31]. However, IL-6 levels increase during tocilizumab treatment [48], which was also shown in this study (Additional file 1: Figure S1).
VEGF had the weakest associations with US and clinical variables in the present study. This angiogenic marker was expected to be associated with PD scores. However, there were low correlations between VEGF and PD sum scores. Previous studies have diverged in designs and results [32–38]. Only small groups of patients were included (15–29 patients) in most of the previous studies on VEGF, and different numbers of joints have been assessed (PD in 1–22 joints). In addition, all but one study had a cross-sectional design. Thus, the present large longitudinal study, in which we examined a high number of joints, supports previous studies indicating that VEGF is not a strong marker of active synovitis in patients with RA.
The present study has several strengths. A large number of patients with RA were included; they were assessed six times during 1 year after initiation of treatment with bDMARDs; and the comprehensive US examination was performed by one experienced sonographer. The study was carried out in a regular clinical setting, which may support the external validity of the findings. A limitation may be the relatively low disease activity in spite of including only patients starting a bDMARD, which could have had an impact on the responsiveness of the inflammatory markers. A high number of joints were examined by US, but the inclusion of even more joints might possibly have influenced the results. Weighting of joints in accordance with the Lansbury method did not increase the correlation between US scores and calprotectin. Whether another form of weighting of joints would increase the associations between calprotectin and US pathology remains to be explored.