IL21 and chronic obstructive pulmonary disease: Our study has shown strong correlations between the relative content of circulating CD4+CD282+ cells and serum levels of Th17 cytokines (IL-6, IL-17A, and IL-21) in patients with COPD GOLD I. It has been reported that the stimulation of CD4+ cells by TLR2 ligands leads to the activation of key transcription factors of Th17 cells (retinoic-acid-receptor-related orphan nuclear receptor γt(RORγt), RORα, RORC) and the production of effector Th17 cytokines (IL-6, IL-17A, IL-17F, IL-22, and IL-21) [33–36, 53].