PDCD1 and neoplasm: Both hypoxia and adenosine limit T cell function, and indeed Sitkovsky’s group has elegantly shown that correcting these by treating tumor-bearing mice with respiratory hyperoxia enhances T cell cytotoxicity and cytokine release, improves tumor regression and the efficacy of immunotherapies (adoptive T cell transfer and dual blockade of CTLA-4 and PD-1) [6].