Hypothetical model of CCL15 role in chronic lung GVHD. Vascular smooth muscle cells release CCL15 into the blood vessel lumen, where it binds to CCR1 on macrophages. The macrophages then infiltrate the lung tissue where they release TGF-β and induce collagen deposition around the bronchioles. TGF, transforming growth factor. Professional illustration by Somersault18:24.