Melanoma is an immune-modulated malignancy and immune checkpoint modulators which inhibit PD-1 function (pembrolizumab, nivolumab) have demonstrated clinical efficacy as treatment for patients with stage IV melanoma. Pembrolizumab across a range of doses in phase I investigation has demonstrated clinical efficacy with RR approximately 27%. By better understanding how NK cell function and exhaustion interplays with PD1 function and activity, potentially more efficacious combination therapies can be developed. The pharmacodynamic studies to be performed as part of this trial will provide such information.