PDCD1 and infection: Firstly, the abnormally elevated expression of the T-cell exhaustion markers Cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and Programmed Cell Death Protein 1 (PD-1) suggests that a state of immunosuppression occurred in the microenvironment of the local infection site during NTM infection, where chronic infections such as M. abscessus might induce host T-cell depletion [34].