KLRK1 and bronchiolitis obliterans syndrome: Mucosal-associated invariant T cell expression of immune inhibitory markers programmed cell death protein 1 (PD-1) and NKG2A, and costimulatory marker NKG2D, was assessed in the blood of 11 healthy donors, 23 patients with BO, and 58 patients with OAC, and in the oesophageal tissue of 22 BO and 39 OAC patients (Figure 3).