The absolute number of circulating Treg cells is reduced in difficult-to-treat RA patients and is ameliorated by low-dose IL-2 - Yan, Huanhuan; Zi, Xiaoyu; Yan, Huer; Zhang, Xiaoying; Bai, Jie; Gao, Chong; Li, Xiaofeng; Wang, Caihong (Frontiers in Immunology, 2025)