CD4 and obstructive sleep apnea syndrome: Paradoxically, some studies suggest that monocytes from OSA patients exhibit an immunosuppressive phenotype, which includes increased expressions of programmed cell death (PD)-1 receptor and its ligand (PD-L1) [120], a co-inhibitory immune checkpoint to maintain the quiescence of autoreactive T cells [121], reduced CD4 CD8 T cell proliferation and CD8 cytotoxicity [120], and high levels of TGF-β which impairs NK cytotoxicity and maturation [122].