PDCD1 and B-cell chronic lymphocytic leukemia: (3) S-100 A9 has also been implicated in the increased expression of PD-1 and PD-L1 in MDSCs in CLL [69], and, when CLL was treated with hypomethylating agents, an increased expression of checkpoint proteins (PD-L1) in MDSCs was reported [117,118], with resistance attributed to a hypomethylating-resistant mechanism.