PD-L1 and Survival in Solid Tumors: A Meta-Analysis

1Co‑stimulatory and co‑inhibitory receptors play a pivotal role in T cell biology, as they determine the functional outcome of T cell receptor (TCR) signaling and immune surveillance. However, the co-inhibitory mechanisms which are termed check-points elicited from cancer immunoediting can also facilitate cancer cell to escape from immunosurveillance. Despite the complexity of cancer immunoediting, growing evidences suggest that the co-inhibitory receptors, such as cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and programmed death 1 (PD-1), play a crucial role in cancer immunoediting, especially in the equilibrium and escape stages.

2Human programmed death-ligand 1 (PD-L1 or B7-H1), as a dominant ligand, plays a central role in antigen-specific T cell response mediating PD-1-dependent immune suppression. The abnormal expression of these ligands has been linked with prognosis and treatment response of multiple malignancies. For instance, overexpression of PD-L1 has been observed in different solid tumors including melanoma, colorectal cancer, lung cancer, pancreatic carcinoma and hepatocellular carcinoma. Recently, clinic trials demonstrate that various cancer patients can get survival benefit from immune check-point targeted treatment.

3Despite the clinical development of anti-PD-L1 therapies, the prognostic value of PD-L1 overexpression across different solid tumors is still unclear. Recently, it is reported that melanoma patients with PD-L1-expressing cells at the invasive tumor margin and inside tumors are more sensitive to anti-PD-1 therapy. Another two studies showed that across multiple cancer types, responses were observed in patients with tumors expressing high levels of PD-L1, especially when PD-L1 expressed on tumor-infiltrating immune cells. The evidence suggests along with development of PD-L1/PD-1 targeted therapy, some biomarkers are needed for guiding individualized anti-PD-1 therapy option. It would be desirable to explore whether PD-L1 overexpression is associated with worse outcome. Moreover, PD-L1 overexpression may serve as a potential biomarker for prognostic prediction and PD-L1/PD-1 targeted treatment option in solid tumors.

4Here, we present a meta-analysis evaluating the prognostic value of PD-L1 overexpression in solid tumor. The purpose of this study was to estimate the correlation of PD-L1 overexpression with survival in solid tumors, thereby shed more light on the development of PD-L1/PD-1 immune check-point targeted therapy and prognostic prediction.

5This meta-analysis was carried out in accordance with preferred reporting items for systematic reviews and meta-analyses statement.

6Pubmed, Web of Science and EBSCO were searched for studies evaluating the expression of PD-L1 and survival in solid tumors from 2002 to November 2014. The search terms included “programmed death-ligand 1” or “PD-L1” or “B7-H1” or “CD274” and “neoplasms” and the results were limited to human studies of solid tumors. In addition we used the entry “programmed death-ligand 1” or “PD-L1” or “B7-H1” or “CD274” and the name of each specific solid tumor to recognize additional studies. We identified a total of 350, 346 and 249 entries, respectively. Eligibility criteria were the measurement of PD-L1 by immunohistochemistry (IHC), availability of survival data for at least 3 years, and publication in English. Studies evaluating gene expression of PD-L1 measured by polymerase chain reaction were excluded from the analyses. Citation lists of retrieved articles were manually screened to ensure sensitivity of the search strategy. Study selection was based on the association of PD-L1 and survival. Inter-reviewer agreement was assessed using Cohen’s kappa coefficient. Disagreement was resolved by consensus.

7The primary endpoints were overall survival (OS) at 3 and 5 years. Tumors were classified by PD-L1 expression status using cut-offs as defined by individual studies.

8Two authors (Pin Wu and Dang Wu) independently extracted information using predefined data abstraction forms. The following details were extracted by 2 reviewers (Pin Wu and Dang Wu): tumor type, number of patients, antibody used for the evaluation, technique used to quantify PD-L1, and cut-off to determine PD-L1 positivity. Survival data were extracted from tables or Kaplan—Meier curves for both PD-L1 negative or low (control group) and PD-L1 positive or high expression (experimental group).

9The relative frequency of survival at 3 and 5 years between the control and experimental groups was expressed as an odds ratio (OR) and its 95% confidence interval (CI). A number of sensitivity analyses were prespecified.

10Data were extracted from the primary publications and analysed using RevMan 5.3 analysis software (Cochrane Collaboration, Copenhagen, Denmark). Estimates of ORs were weighted and pooled using the Mantel—Haenszel random effect model. Statistical heterogeneity was assessed using the Cochran’s Q and I2 statistics. Differences between subgroups were assessed using methods as previous described by Deeks et al. Sensitivity analyses were carried out for different analytical methods and cut-offs for defining PD-L1 expression. All statistical tests were two-sided, and statistical significance was defined as P less than 0.05. No correction was made for multiple statistical testing.

11The search results have been shown in Fig 1. The primary literature research retrieved 945 records. After screening the title of citations, 603 records were excluded because of the non-relevance with the theme and duplicated literatures. Next, 314 citations were excluded after screening abstracts of the records. Then we carefully read the full text of the left citations and at last 28 studies were included.

12We identified 28 studies using IHC techniques for the assessment of PD-L1 expression. Characteristics of included studies are shown in Table 1. Four studies evaluated urothelial cancer, two evaluated colorectal cancer, three evaluated esophageal cancer, three evaluated gastric cancer, three evaluated hepatocellular carcinoma, four evaluated lung cancer, three evaluated melanoma, two evaluated oropharyngeal squamous cell carcinoma, and one each evaluated cervical cancer, glioblastoma, malignant mesotheliomas, ovarian cancer and pancreatic cancer. A total of 3107 patients were included in these studies.

13A description of the antibodies used in the included studies is shown in Table 2. Various antibodies were used for the evaluation of PD-L1 expression. Seven studies used clone 5H1, five studies used clone MIH1, four studies used clone 2H11, two studies used clone 27A2 or ab82059, and one each used clone 236A/E7, NBP1-03220, ab58810 or primary antibody respectively. Four studies did not report the clone of PD-L1 antibody. The cut-off value for PD-L1 overexpression depended on the staining score and the method used. Among the group determined as PD-L1 overexpressed, the median overexpression of PD-L1 staining was 45.4%. Esophageal cancer, gastric cancer, and oropharyngeal squamous cell carcinoma had the highest expression of PD-L1, with more than 66.7% of tumors considered overexpression. Levels of PD-L1 overexpression in urothelial cancer, pancreatic carcinoma, colorectal cancer, melanoma, lung cancer, hepatocellular carcinoma, esophageal cancer, glioblastoma and cervical carcinoma ranged from 19% to 54.3%.

14A total of 25 studies reported data for OS at 3-years. Results showed that PD-L1 overexpression was associated with worse 3-year OS of solid tumors (OR = 2.43, 95% CI = 1.60 to 3.70, P < 0.0001) (Fig 2). There was significant heterogeneity among studies (Cochran’s Q P < 0.00001, I2 = 76%), so we conducted subgroup meta-analysis to explore whether the heterogeneity is due to different cancer types.

15Three studies provided 3-year OS for lung cancer, 2 studies for hepatocellular carcinoma, 2 studies for melanoma, 2 studies for oropharyngeal squamous cell carcinoma, 4 studies for urothelial cancer, 3 studies for esophageal cancer, and 3 studies for gastric cancer. In the stratified analysis by cancer types, PD-L1 overexpression was associated with worse 3-year OS of esophageal cancer (OR = 3.96, 95% CI = 2.10 to 7.46, P < 0.0001) (Fig 3A), gastric cancer (OR = 3.84, 95% CI = 1.88 to 7.85, P = 0.0002) (Fig 3B), hepatocellular carcinoma (OR = 2.28, 95% CI = 1.34 to 3.90, P = 0.002) (Fig 3C) and urothelial cancer (OR = 3.74, 95% CI = 1.14 to 12.32, P = 0.03) (Fig 3D). However, there was no association between PD-L1 overexpression and 3-year OS of lung cancer (OR = 1.57, 95% CI = 0.38 to 6.48, P = 0.54) (S1A Fig), melanoma (OR = 0.97, 95% CI = 0.09 to 10.14, P = 0.98) (S1B Fig) and oropharyngeal squamous cell carcinoma (OR = 1.32, 95% CI = 0.73 to 2.38, P = 0.36) (S1C Fig).

16A total of 25 studies reported data for OS at 5-years. Similar to the 3-year OS data, PD-L1 overexpression was significantly associated with worse 5-year OS of solid tumors (OR = 2.23, 95% CI = 1.40 to 3.55, P = 0.0008) (Fig 4). There was also high heterogeneity among studies (Cochran’s Q P < 0.00001, I2 = 79%), so we conducted subgroup meta-analysis according to different cancer types.

17Four studies provided 5-year OS for lung cancer, 3 studies for hepatocellular carcinoma, 2 studies for melanoma, 2 studies for oropharyngeal squamous cell carcinoma, 3 studies for urothelial cancer, and 3 studies for esophageal cancer. PD-L1 overexpression was associated with worse 5-year OS of esophageal cancer (OR = 4.95, 95% CI = 2.66 to 9.24, P < 0.00001) (Fig 5A), gastric cancer (OR = 3.47, 95% CI = 1.91 to 6.32, P < 0.0001) (Fig 5B) and colorectal cancer (OR = 2.02, 95% CI = 1.18 to 3.46, P = 0.01) (Fig 5C). However, there was no significant association between PD-L1 overexpression and the 5-year OS of urothelial cancer (OR = 2.61, 95% CI = 1.00 to 6.85, P = 0.05) (S2A Fig), hepatocellular carcinoma (OR = 17.10, 95% CI = 0.19 to 1505.43, P = 0.21) (S2B Fig), lung cancer (OR = 0.91, 95% CI = 0.32 to 2.63, P = 0.60) (S2C Fig), melanoma (OR = 1.12, 95% CI = 0.09 to 13.64, P = 0.93) (S2D Fig) and oropharyngeal squamous cell carcinoma (OR = 1.13, 95% CI = 0.58 to 2.20, P = 0.71) (S2E Fig).

18Removal of studies which did not report the PD-L1 antibody clone did not influence results for 3-year or 5-year OS (OR = 2.26, 95% CI = 1.43 to 3.57; OR = 2.14, 95% CI = 1.31 to 3.51, respectively). However, removal of the studies that did not report the cut-off value of PD-L1 overexpression increased the association between PD-L1 and worse 3-year and 5-year OS compared with PD-L1 low expression (OR = 2.95, 95% CI = 2.03 to 4.28; OR = 2.70, 95% CI = 1.67 to 4.37, respectively). Exclusion of these studies did not reduce heterogeneity for 5-year OS (Cochran’s Q P = 0.0002, I2 = 60%; Cochran’s Q P < 0.00001, I2 = 68%, respectively).

19PD-L1 overexpression has been observed in a substantial number of solid tumors. Moreover, numerous studies have demonstrated that PD-L1 plays a key role in cancer immune escape. A decade ago some studies reported that blockade of PD-L1 could improve antitumor immunity. Recently, several studies show that therapies targeting PD-L1/PD-1 display clinical responses in patients with multiple cancer types expressing high levels of PD-L1. These leading studies highlight that PD-L1 may serve as a biomarker for prognostic prediction and PD-L1/PD-1 targeted treatment option in solid tumors. In this study, we meta-analyzed the published data about the expression of PD-L1 in solid tumors and their association with survival for studies that evaluated PD-L1 by IHC.

20Results showed that overexpression of PD-L1 was associated with worse 3-year OS for all studies analyzed except for one study of melanoma and lung cancer, respectively. Studies reporting 5-year OS data demonstrated that overexpression of PD-L1 is associated with worse outcome except for one study of cervical carcinoma, melanoma, oropharyngeal squamous cell carcinoma and two study of lung cancer. Among the tumor types evaluated, esophageal cancer was the tumor type most linked with worse 3-year and 5-year outcome for patients with high levels of PD-L1. Overall, PD-L1 overexpression was reported to be associated with worse 5-year outcome of patients with digestive tract-derived tumors such as esophageal cancer, gastric cancer and colorectal cancer. In addition, one study on hepatocellular carcinoma showed that PD-L1 expression on macrophages in tumors was significantly higher than paired normal tissues and correlated with tumor stage. These results suggest that PD-L1 has an important role in the immune escape and progress of tumors especially digestive tract-derived.

21A recent study reported that epithelial-originated cancer patients with positive expression of PD-L1 on tumor tissues were associated with significantly poorer OS when compared to those with negative expression of PD-L1. Consistently, our study showed that overexpression of PD-L1 in the tumor tissues of most epithelial-originated cancer types was associated with worse 3-year and 5-year OS except for lung cancer, melanoma and oral squamous cell carcinoma. However, recent clinic trials showed that patients with melanoma and lung cancer could acquire clinical benefit from anti-PD-L1 treatment. In our meta-analysis, we noted that the results of studies using different clone of PD-L1 antibodies were conflicting in melanoma (S1A and S1B Fig) and lung cancer (S2C Fig). The difference between technologies used in studies may partly account for the contradictory results. Further studies are needed to confirm the impact of antibodies on the results of studies.

22Besides, the PD-1 expression state of tumor infiltrating lymphocytes is another key point of the PD-1/PD-L1-mediated tumor immune escape. A recent study suggests that pre-existing CD8+ T cells distinctly located at the invasive tumor margin are associated with expression of the PD-1/PD-L1 immune inhibitory axis and may predict response to therapy. Moreover, another study showed that across multiple cancer types, responses were observed in patients with tumors expressing high levels of PD-L1, especially when PD-L1 was expressed on tumor-infiltrating immune cells. The evidence implys that further study is needed to clarify the different prognostic and therapeutic prediction value of both PD-1 and PD-L1 expression on various cell types in solid tumor tissue.

23Our analyses show that overexpression of PD-L1 in solid tumor tissues, as measured by IHC, is associated with worse prognosis in different tumor types, which suggests that the development of strategies against the PD-L1/PD-1 axis would be a promising therapeutic approach for solid tumors. Moreover, further studies are required to investigate the potential role of PD-L1 expression in solid tumors for prognostic prediction as well as PD-L1 targeted treatment decision.