PDCD1 and myeloid sarcoma: Furthermore, we find that Treg cells isolated from MS patients express high levels of PD-1, confirming the notion that in these individuals the immune system is chronically activated in waves (such as those which occur during a chronic viral infection) and thus regulatory T cells become functionally exhausted, and as the disease proceeds immunoregulation may become compromised also due to upregulation of inhibitor molecules such as PD-1.