Smoking-related emphysema is a chronic inflammatory disease driven by the T.sub.H17 subset of helper T cells through molecular mechanisms that remain obscure. Here we explored the role of the microRNA miR-22 in emphysema. We found that miR-22 was upregulated in lung myeloid dendritic cells (mDCs) of smokers with emphysema and antigen-presenting cells (APCs) of mice exposed to smoke or nanoparticulate carbon black (nCB) through a mechanism that involved the transcription factor NF-κB. Mice deficient in miR-22, but not wild-type mice, showed attenuated T.sub.H17 responses and failed to develop emphysema after exposure to smoke or nCB. We further found that miR-22 controlled the activation of APCs and T.sub.H17 responses through the activation of AP-1 transcription factor complexes and the histone deacetylase HDAC4. Thus, miR-22 is a critical regulator of both emphysema and T.sub.H17 responses.