cTfh17.1 cells were the major subtype for IL-21 production in MS patients
As shown in Fig. 3d and e, the level of IL-21 secreting cTfh cells was significantly higher in MS patients. We then examined the secretion of IL-21 by different subsets of cTfh cells and compared between HC and MS patients. Although all the four subtypes of cTfh cells produce IL-21, cTfh17.1 cells were associated with the greatest production of IL-21 in both HC and MS (Fig. 4a,b). Interestingly, IL-21 secreting cTfh17.1 cells was significantly higher in MS patients (52.52±4.86% vs. 24.81±5.34%, p=0.002) than that in HC (Fig. 4a,b). However, other subsets of IL-21 secreting cTfh cells did not differ significantly between HC and MS. We also analyzed and compared IL-17 secreting cTfh17 cells in HC and MS patients, followed by a comparative analysis of IL-17 production by cTfh17 vs. non-follicular Th17 cells. We found no noticeable difference in IL-17 secreting cTfh17 cells between HC and MS (Fig. 4c and S3a). Moreover, IL-17 secreting cTfh17 versus non-follicular Th17 cells did not show any differences between HC and MS (Fig. 4d and S3a,b,e,f). However, a significant increase of CXCR3+CCR6+ cTfh17.1 cells in MS patients led us to investigate them further based on the cytokine types, especially IFN-γ and IL-17. Along with IL-21, cTfh17.1 cells also produced IFN-γ and IL-17 with different frequencies (Fig. S3c,d and Fig. 4e,f), although no significant difference was observed between HC and MS regarding IFN-γ and IL-17 secreting cTfh17.1 cells. Importantly, IL-21 was the major cytokine secreted by the cTfh17.1 subtype over others suggesting that cTh17.1 cells are the major secretor of IL-21 in MS patients.