In vivo expansion of CD8⁺ T cells specific for CEA-Cap1 is associated in time with GVHD onset. PBMCs obtained at different time points before and after transplantation were stained with HLA-A^*0201/CEA-Cap1 pentamer as described in “Patients, materials, and methods.” The percent of CD8⁺ HLA-A^*0201/CEA pentamer⁺ cells within the CD8⁺ population is shown (black line). Arrows indicate time points in which immunosuppressive treatment was modified (CSA indicates cyclosporin A; BDP, beclomethasone diproprionate; and PDn, prednisone). The dashed line indicates background threshold of pentamer staining. Concomitant with pentamer staining, serum CEA levels were determined by carbonyl metallo-immunoassay (gray line). (A) Time course of patient no. 11. (B) Time course of patient no. 8.