CCR6 and viral infectious disease: On the other hand, in a viral infection model, miR-17–92 acts as a critical regulator of Tfh cell differentiation by restraining the expression of genes “inappropriate” to this cell subset (28); in an airway inflammation model in vivo, miR-18a specifically targets three key transcription factors in the Th17 gene-expression program small mother against decapentaplegic 4 (SMAD4), hypoxia inducible factor 1α (HIF1α), and retinoid-related orphan receptor α (RORα), and blocks the differentiation of tissue Th17 cells expressing C-C chemokine receptor 6 (CCR6), RORγt, and IL-17A (29).