A similar pattern was observed for CTLA-4, the highest percentages of which were found on the surface of CD4+(Fig. 5G), CD8+ (Fig. 5H) T cells, and CD19 + B cells (Fig. 5I) in the SID EBV + and CLL EBV + groups, suggesting significant functional exhaustion of these cells as a result of chronic immune stimulation. Here, CTLA4 is linked to B-cell chronic lymphocytic leukemia.