[21] Roles for GM-CSF and MDC in airway injury are biologically plausible since GM-CSF is elaborated by macrophages causing Th-2 polarization during antigen presentation in asthma. [22] In addition, human bronchial epithelial cells produce GM-CSF in response to PM2.5.[23]–[25] MDC (CCL22) is elevated in models of tobacco-induced lung injury and may be responsible for recruiting inflammatory cells to the lung. [26]. This evidence concerns the gene CSF2 and asthma.