Patients with bronchiolitis obliterans syndrome (BOS) during chronic GVHD (cGVHD) also show an increased population of circulating GZMK+CCR5+CD8+T cells (Figure 5), which are responsible for increased expression of fibrosis essential proteins, such as collagen type 1 alpha 1 chain (COL1A1) and fibronectin (FN1) in their fibroblasts [307]. Here, FN1 is linked to chronic graft versus host disease.