Vaccination with irradiated B16 melanoma cells expressing either GM-CSF (Gvax) or Flt3-ligand (Fvax) combined with antibody blockade of the negative T-cell costimulatory receptor cytotoxic T-lymphocyte antigen-4 (CTLA-4) promotes rejection of preimplanted tumors. Despite CTLA-4 blockade, T-cell proliferation and cytokine production can be inhibited by the interaction of programmed death-1 (PD-1) with its ligands PD-L1 and PD-L2 or by the interaction of PD-L1 with B7-1. Here, we show that the combination of CTLA-4 and PD-1 blockade is more than twice as effective as either alone in promoting the rejection of B16 melanomas in conjunction with Fvax. Adding αPD-L1 to this regimen results in rejection of 65% of preimplanted tumors vs. 10% with CTLA-4 blockade alone. Combination PD-1 and CTLA-4 blockade increases effector T-cell (Teff) infiltration, resulting in highly advantageous Teff-to-regulatory T-cell ratios with the tumor. The fraction of tumor-infiltrating Teffs expressing CTLA-4 and PD-1 increases, reflecting the proliferation and accumulation of cells that would otherwise be anergized. Combination blockade also synergistically increases Teff-to-myeloid-derived suppressor cell ratios within B16 melanomas. IFN-γ production increases in both the tumor and vaccine draining lymph nodes, as does the frequency of IFN-γ/TNF-α double-producing CD8 + T cells within the tumor. These results suggest that combination blockade of the PD-1/ PD-L1-and CTLA-4-negative costimulatory pathways allows tumorspecific T cells that would otherwise be inactivated to continue to expand and carry out effector functions, thereby shifting the tumor microenvironment from suppressive to inflammatory.he interaction between the T-cell receptor complex (TCR) and antigenic peptides bound in surface MHC molecules provides the specificity that defines adaptive T-cell immunity. In addition to TCR activation, costimulation via ligation of the coreceptor CD28 on T cells by B7 molecules on antigenpresenting cells (APCs) is required for optimal T-cell activation (1). Once mobilized, however, T cells begin to express other members of the CD28/B7 receptor family that attenuate the immune response through inhibition of proliferation and cytokine production (2). Cytotoxic T-lymphocyte antigen-4 (CTLA-4) is rapidly up-regulated following T-cell activation and binds to B7 molecules with a higher affinity than does CD28. The receptor programmed death-1 (PD-1) is also expressed on T cells following activation, where, on binding to its ligands PD-L1 and PD-L2, it promotes T-cell anergy, apoptosis, and exhaustion. Recently, an additional coinhibitory ligand/receptor interaction has been described that involves binding of PD-L1 on T cells to B7-1 on APCs or vice versa (3). Although the existence of so many layers of T-cell inhibition may seem surprising, the severe and sometimes fatal autoimmunity that results when even one of these pathways is disrupted attests to their necessity.Malignant transformation was classically defined by the ability to avoid the norm...