Combined PARP14 Inhibition and PD-1 Blockade Promotes Cytotoxic T Cell Quiescence and Modulates Macrophage Polarization in Relapsed Melanoma

Programmed Cell Death 1 (PD-1) signaling blockade effectively restores immune surveillance to treat melanoma. However, chronic interferon-gamma (IFNγ) -driven immune homeostatic mechanisms in melanoma cells contribute to immune evasion and acquired resistance. We previously demonstrated that poly ADP ribosyl polymerase 14 (PARP14), an IFNγ-responsive gene product, in part mediates IFNγ-driven resistance, as its inhibition prolonged PD-1 blockade responses in preclinical models. Nevertheless, PARP14 inhibition alone could not achieve full tumor clearance, indicating additional resistance mechanisms. Herein, we identify a robust PARP14 catalytic inhibitor (PARP14i) gene signature associated with improved patient survival. We elucidate immune and tumor cell adaptations to PARP14 inhibition combined with PD-1 blockade using preclinical models. This combination therapy suppressed tumor-associated macrophages while increasing pro-inflammatory memory macrophages. Notably, it preserved cytotoxic T cell function by inducing a quiescent state, mitigating terminal exhaustion. Despite enhancing immune responses, adaptive resistance mechanisms emerged in tumor cells, engaging alternative immune evasion pathways. These findings indicate that this combination therapy primes immune cells for further therapeutic intervention, providing a promising strategy to overcome resistance and optimize treatment outcomes. Summary While immune checkpoint inhibitors, such as α-PD-1 therapy, provide significant clinical benefits, many tumors develop resistance and relapse. Re-treatment with α-PD-1 often fails because T cells lose their function (a process called exhaustion), and immune-suppressing cells like tumor-associated macrophages (TAMs) increase in the tumor environment, enabling tumors to evade immune attack. Recent studies have shown that a small molecule inhibitor of PARP14 can restore sensitivity to α-PD-1 therapy by remodeling the tumor environment. This includes increasing immune cell infiltration and improving antigen presentation, which helps the immune system recognize tumor cells. Using single-cell RNA sequencing, we discovered that this therapy induces a new state of T cell dormancy, where cytotoxic T cells temporarily stop attacking but retain the potential to be reactivated by additional treatments. This finding opens new opportunities to sustain immune responses and improve outcomes for patients who relapse after immunotherapy. Graphical Summary