Anti‐PD‐1 synergizes with cyclophosphamide to induce potent anti‐tumor vaccine effects through novel mechanisms

Abstract

Programmed death‐1 receptor (PD‐1) is expressed on T cells following TCR activation. Binding of this receptor to its cognate ligands, programmed death ligand (PDL)‐1 and PDL‐2, down‐regulates signals by the TCR, promoting T‐cell anergy and apoptosis, thus leading to immune suppression. Here, we find that using an anti‐PD‐1 antibody (CT‐011) with Treg‐cell depletion by low‐dose cyclophosphamide (CPM), combined with a tumor vaccine, induces synergistic antigen‐specific immune responses and reveals novel activities of each agent in this combination. This strategy led to complete regression of established tumors in a significant percentage of treated animals, with survival prolongation. We show for the first time that combining CT‐011 and CPM significantly increases the number of vaccine‐induced tumor‐infiltrating CD8⁺ T cells, with simultaneous decrease in infiltrating Treg cells. Interestingly, we find that CT‐011 prolongs Treg‐cell inhibition induced by CPM, leading to a sustainable significant synergistic decrease of splenic and tumor‐infiltrated Treg cells. Surprisingly, we find that the anti‐tumor effect elicited by the combination of CT‐011 and CPM is dependent on both CD8⁺ and CD4⁺ T‐cell responses, although the antigen we used is a class I MHC‐restricted peptide. Thus, we describe a novel and effective therapeutic approach by combining multiple strategies to target several tumor‐mediated immune inhibitory mechanisms.