This is a phase 1b study of pembrolizumab combined with pralatrexate (Arm A), pralatrexate + decitabine (Arm B), or decitabine (Arm C) in patients with R/R PTCL and CTCL…Patients who responded to treatment demonstrated a more consistent decline in levels of TNFα, MIP-3α, and IL-10 during cycle 1...These clinical data suggest that the integration of pembrolizumab on an epigenetic backbone is safe and demonstrates encouraging responses in heavily treated patients with PTCL and CTCL. These correlative data suggest that specific cytokines (TNFα, MIP-3α, and IL-10) and changes in circulating T-cell subpopulations may have predictive value as biomarkers of disease response.