Purpose: Immunomodulatory drugs differ in mechanism-of-action from directly cytotoxic cancer therapies. Identifying factors predicting clinical response could guide patient selection and therapeutic optimization. Experimental Design: Patients (N = 41) with melanoma, non–small cell lung carcinoma (NSCLC), renal cell carcinoma (RCC), colorectal carcinoma, or castration-resistant prostate cancer were treated on an early-phase trial of anti–PD-1 (nivolumab) at one institution and had evaluable pretreatment tumor specimens. Immunoarchitectural features, including PD-1, PD-L1, and PD-L2 expression, patterns of immune cell infiltration, and lymphocyte subpopulations, were assessed for interrelationships and potential correlations with clinical outcomes. Results: Membranous (cell surface) PD-L1 expression by tumor cells and immune infiltrates varied significantly by tumor type and was most abundant in melanoma, NSCLC, and RCC. In the overall cohort, PD-L1 expression was geographically associated with infiltrating immune cells (P < 0.001), although lymphocyte-rich regions were not always associated with PD-L1 expression. Expression of PD-L1 by tumor cells and immune infiltrates was significantly associated with expression of PD-1 on lymphocytes. PD-L2, the second ligand for PD-1, was associated with PD-L1 expression. Tumor cell PD-L1 expression correlated with objective response to anti–PD-1 therapy, when analyzing either the specimen obtained closest to therapy or the highest scoring sample among multiple biopsies from individual patients. These correlations were stronger than borderline associations of PD-1 expression or the presence of intratumoral immune cell infiltrates with response. Conclusions: Tumor PD-L1 expression reflects an immune-active microenvironment and, while associated other immunosuppressive molecules, including PD-1 and PD-L2, is the single factor most closely correlated with response to anti–PD-1 blockade. Clin Cancer Res; 20(19); 5064–74. ©2014 AACR.

Although many human cancers such as melanoma express tumor antigens recognized by T cells, host immune responses often fail to control tumor growth for as yet unexplained reasons. Here, we found a strong association between melanocyte expression of B7-H1 (PD-L1), an immune-inhibitory molecule, and the presence of tumor-infiltrating lymphocytes (TILs) in human melanocytic lesions: 98% of B7-H1⁺ tumors were associated with TILs compared with only 28% of B7-H1⁻ tumors. Indeed, B7-H1⁺ melanocytes were almost always localized immediately adjacent to TILs. B7-H1/TIL colocalization was identified not only in melanomas but also in inflamed benign nevi, indicating that B7-H1 expression may represent a host response to tissue inflammation. Interferon-γ, a primary inducer of B7-H1 expression, was detected at the interface of B7-H1⁺ tumors and TILs, whereas none was found in B7-H1⁻ tumors. Therefore, TILs may actually trigger their own inhibition by secreting cytokines that drive tumor B7-H1 expression. Consistent with this hypothesis, overall survival of patients with B7-H1⁺ metastatic melanoma was significantly prolonged compared with that of patients with B7-H1⁻ metastatic melanoma. Therefore, induction of the B7-H1/PD-1 pathway may represent an adaptive immune resistance mechanism exerted by tumor cells in response to endogenous antitumor activity and may explain how melanomas escape immune destruction despite endogenous antitumor immune responses. These observations suggest that therapies that block this pathway may benefit patients with B7-H1⁺ tumors.

Substantial effort is ongoing to identify predictors of response and resistance to anti–PD-1/PD-L1 (anti–programmed cell death 1/programmed cell death ligand 1) immunotherapy. Expression of PD-L1 protein was the first candidate biomarker associated with response to anti–PD-1 therapy.^1,2 Multiple PD-L1 immunohistochemistry (IHC) assays have been approved by the US Food and Drug Administration (FDA) as a companion or complementary diagnostic for patients with non–small cell lung cancer (NSCLC), melanoma, bladder cancer, gastric cancer, and cervical carcinoma to help preselect patients for anti–PD-1/PD-L1 therapy.^3,4 These IHC assays have been used to stratify patients in clinical trials. Positive PD-L1 IHC has also recently been shown to enrich for response to combination therapy with anti–PD-1/cytotoxic T lymphocyte antigen-4 in patients with NSCLC.⁵ Quiz Ref ID However, although PD-L1 expression has been shown to correlate with response to therapy in certain tumor types, the association is not absolute. As a diagnostic assay, PD-L1 IHC has several limitations: multiple different assays are available, the predictive significance of tumor cell vs immune cell expression varies by tumor type,⁶ the scoring of immune cell PD-L1 expression by pathologists has poor interobserver reproducibility,⁷ and the PD-L1 expression is commonly reduced to a digital readout (+ vs −) without assessing its expression in the greater context of the tumor microenvironment (TME) (eg, association with immune cells suggests an adaptive pattern of expression).^8,9

Neoadjuvant anti-PD-1 may improve outcomes for patients with resectable NSCLC and provides a critical window for examining pathologic features associated with response. Resections showing major pathologic response to neoadjuvant therapy, defined as ≤10% residual viable tumor (RVT), may predict improved long-term patient outcome. However, %RVT calculations were developed in the context of chemotherapy (%cRVT). An immune-related %RVT (%irRVT) has yet to be developed. The first trial of neoadjuvant anti-PD-1 (nivolumab, NCT02259621) was just reported. We analyzed hematoxylin and eosin-stained slides from the post-treatment resection specimens of the 20 patients with non-small-cell lung carcinoma who underwent definitive surgery. Pretreatment tumor biopsies and preresection radiographic ‘tumor’ measurements were also assessed. We found that the regression bed (the area of immune-mediated tumor clearance) accounts for the previously noted discrepancy between CT imaging and pathologic assessment of residual tumor. The regression bed is characterized by (i) immune activation—dense tumor infiltrating lymphocytes with macrophages and tertiary lymphoid structures; (ii) massive tumor cell death—cholesterol clefts; and (iii) tissue repair—neovascularization and proliferative fibrosis (each feature enriched in major pathologic responders versus nonresponders, P < 0.05). This distinct constellation of histologic findings was not identified in any pretreatment specimens. Histopathologic features of the regression bed were used to develop ‘Immune-Related Pathologic Response Criteria’ (irPRC), and these criteria were shown to be reproducible amongst pathologists. Specifically, %irRVT had improved interobserver consistency compared with %cRVT [median per-case %RVT variability 5% (0%–29%) versus 10% (0%–58%), P = 0.007] and a twofold decrease in median standard deviation across pathologists within a sample (4.6 versus 2.2, P = 0.002). irPRC may be used to standardize pathologic assessment of immunotherapeutic efficacy. Long-term follow-up is needed to determine irPRC reliability as a surrogate for recurrence-free and overall survival.

Programmed death–ligand 1 (PD-L1) expression on tumor cells is essential for T cell impairment, and PD-L1 blockade therapy has shown unprecedented durable responses in several clinical studies. Although higher expression of PD-L1 on tumor cells is associated with a better immune response after Ab blockade, some PD-L1–negative patients also respond to this therapy. In the current study, we explored whether PD-L1 on tumor or host cells was essential for anti–PD-L1–mediated therapy in 2 different murine tumor models. Using real-time imaging in whole tumor tissues, we found that anti–PD-L1 Ab accumulates in tumor tissues, regardless of the status of PD-L1 expression on tumor cells. We further observed that, while PD-L1 on tumor cells was largely dispensable for the response to checkpoint blockade, PD-L1 in host myeloid cells was essential for this response. Additionally, PD-L1 signaling in defined antigen-presenting cells (APCs) negatively regulated and inhibited T cell activation. PD-L1 blockade inside tumors was not sufficient to mediate regression, as limiting T cell trafficking reduced the efficacy of the blockade. Together, these findings demonstrate that PD-L1 expressed in APCs, rather than on tumor cells, plays an essential role in checkpoint blockade therapy, providing an insight into the mechanisms of this therapy.