Prognostic role of PD-L1 for HCC patients after potentially curative resection: a meta-analysis

Hepatocellular carcinoma (HCC) remains one of the most common cancers worldwide. It is ranked as the fifth leading cancer and is the second leading cause of cancer-related mortality [1]. Despite great advances in the past decade, the long-term survival of HCC is low, considering that surgical resection of HCC is only applicable to a small proportion of patients and that the rate of tumour recurrence after surgery of curative intent is high.

The predominant risk factor for HCC in epidemic regions is hepatic inflammatory response following chronic hepatitis B virus (HBV) or hepatitis C virus (HCV) infection [2]. Evidence has suggested that chronic inflammation in the liver creates an immunosuppressive microenvironment that permits HCC tumourigenesis and progression, providing a rationale for targeting the dysregulated tumour microenvironment to improve HCC treatment [3]. Currently, most studies on the mechanisms of HCC immune evasion have focused on the programmed death receptor 1 (PD-1)/programmed death ligand 1 (PD-L1) pathway. PD-1 is an immunoinhibitory receptor expressed in activated T and B cells and natural killer T cells. PD-L1, the major ligand of PD-1, is expressed in various immune cells such as antigen-presenting cells, as well as in endothelial cells. PD-1/PD-L1 ligation suppresses the activation of immune cells as well as the production of certain cytokines such as IFN-γ, thereby inducing immune suppression and peripheral tolerance [4]. Cancer cells hijack this pathway by over-expressing PD-L1 via various mechanisms to protect themselves from the host immune response. Immunotherapies targeting PD-1/PD-L1 immune checkpoints have shown promising efficacy in different malignancies, including HCC [5]. However, the response rate of the PD-1 inhibitor in advanced HCC is far from satisfying [6]. An urgent need exists in the field to further understand the mechanisms of aberrant PD-1/PD-L1 signalling by which HCC forms an immune-evasive tumour microenvironment and for leveraging anti-PD1 immune checkpoint blockade therapies against HCC.

Data on the prognostic role of PD-L1 expression in HCC remain inconsistent. Some studies [7‐11] and a meta-analysis [12] have shown that the expression of PD-L1 is correlated with a poor prognosis after hepatectomy, whereas other studies have reported non-homogeneous results [13‐15]. To further clarify the prognostic role of PD-L1, the relationship of PD-L1 expression with several important clinicopathological features and tumour-infiltrating T lymphocytes, we conducted this meta-analysis.

While numerous studies have established that aberrant expression of PD-L1 in cancer promoted cancer immune escape and blockade of the PD-L1-enhanced anti-tumour response [4, 5], the prognostic role of PD-L1 in HCC has remained inconsistent. Several previous studies [8, 11, 21] and meta-analyses [29‐31] have shown that high PD-L1 indicates a poorer OS and RFS, which could be explained by innate immune resistance. The innate immune resistance implies the constitutive expression of PD-L1, which is driven by intrinsic cellular oncological signalling in tumour cells and can lead to tumour-infiltrating immune cell inhibition [32, 33]. In this sense, PD-L1 expression reflects a lack of anti-tumour immunity and is reasonable to correlate with a significantly poorer prognosis [7‐11, 21]. Meanwhile, a few recent studies failed to confirm the negative prognostic effect of PD-L1 and even showed the opposite [13‐15, 23, 24]. For instance, Sideras et al. [15] reported that high PD-L1 in tumour cells indicated a significant better survival in HCC. The adaptive immune resistance was used to explain the inconsistency. The adaptive immune resistance implied a specific state of immune privilege of tumour cells acquired by the induced expression of PD-L1 to inhibit the pre-existing anti-tumour T cell response. The state can be reversed easily by blocking the PD1/PD-L1 interaction [34, 35]. From this point of view, the upregulation of PD-L1 represents the existence of immune-surveillance and could be correlated with a better prognosis [13‐15]. Both mechanisms may co-exist, and the predominant mechanism may shift from one to the other at different times depending on the cancer immunogenicity [36]. The prognostic role of PD-L1 tends to be the combined effect of these two mechanisms. In the present meta-analysis based on 17 studies with 2979 patients, we found no significant prognostic role of PD-L1 in HCC after potential curative hepatectomy. Additionally, the expression of PD-L1 was correlated with AFP, hepatitis history, and CD8+ TILs but not with other clinicopathological features. The robustness and reliability of these results was further validated by sensitivity analysis.

Considering the significant between-study heterogeneity, we conducted further subgroup analysis and meta-regression analysis. High PD-L1 predicted a poorer RFS in Asian studies, while high PD-L1 predicted a better RFS in non-Asian patients; the difference between the subgroups was significant, and meta-regression analysis further confirmed the results. Together, these results indicated some racial differences in the prognostic role of PD-L1. Theoretically, several factors might also help explain the great heterogeneity. First, the positive rate of PD-L1 varied greatly across the included studies due to the different tested tissue samples, IHC assay, detection antibody, staining pattern, analytic method, and cut-off value. For instance, PD-L1 can be expressed in both tumour cells and immune cells in the intra-tumoural area of HCC tissues, with different biological and prognostic roles. As Liu et al. [27] reported, PD-L1 expressed on tumour cells was negatively correlated with the prognosis, while PD-L1 expressed on macrophages was positively correlated with the prognosis in HCC. Some included studies investigated the overall expression in the intra-tumoural area [10] or simply classified PD-L1 expression on tumour cells or immune cells by IHC [19]. Additionally, by combining the data published before 2015 or data including PD-L1 staining both in the cell membrane and/or cytoplasm, we found high PD-L1 significantly predicted a poorer survival. However, earlier studies might have used an invalid PD-L1 antibody [8, 11], while more recent studies utilized an antibody that distinguished between cytoplasmic and membranous patterns of PD-L1 staining [9, 15, 20, 22]. As a type I transmembrane molecule [37], membranous PD-L1 was proposed to be the most functionally relevant, and cytoplasmic staining represents intracellular stores of PD-L1 ready to be transported to the cell membrane with appropriate stimulation [38]. Failing to distinguish the expression pattern of PD-L1 in HCC can potentially confound the positive rate of PD-L1 and prognostic role of HCC. Moreover, the analytic method varied extensively. Some studies have utilized imaging software [7, 8, 10, 11, 13, 14, 19, 20, 22, 24], while others were based on intensity and distribution [9, 15, 21, 23]. Finally, the cut-off value used to evaluate high and low PD-L1 expression varied from 1 to 75%. Second, the different baseline characteristics of included patients and different follow-up periods might be another contributing factor. For example, the study of Calderaro et al. [19] only evaluated the prognostic role of PD-L1 in early recurrence within 2 years, while some other studies performed a significant longer follow up (both longer than 80 months) [21, 22]. However, our meta-regression analysis did not find a factor other than the region of research that significantly contributed to the level of heterogeneity, and the roles of the abovementioned factors warrant further investigation. Further studies should make great efforts to establish standardization regarding the methodology of PD-L1 assessment, such as the method to distinguish between tumour cells and immune cells, to distinguish between the expression patterns of PD-L1, to accurately calculate PD-L1 staining cells, and to set optimal cut-off values. Additionally, critical baseline patient characteristics such as AFP must be considered when estimating the prognostic role of PD-L1.

There have been several meta-analyses to date [12, 29, 31, 39‐43]. Most recently, Li et al. [43] finished a similar meta-analysis on the value of PD-L1 in HCC patients. The major difference between our study might be the included criteria. Li et al. included all studies that examined PD-L1 expression levels in clinical HCC tissues. They included the study from Zeng et al. [44] that evaluated most of the PD-L1 expression using flow cytometry, but not IHC, and the study from Dai et al. [45] that assessed the PD-L1 expression in peritumoural tissues. Additionally, they included the study from Finkelmeier et al. [46] that investigated soluble PD-L1 in serum. As pointed out in a previous study, using data from different assays or different tissue types would inevitably raise the heterogeneity more or less [12]. Therefore, our meta-analysis focused on only the immunohistochemical (IHC) assay of PD-L1 expression in tumour tissues (or tumour cells but not immune cells in the intra-tumoural area) and found some inconsistent results with previous meta-analyses. Although our results should be explained with great caution, our study possessed some strengths. To our knowledge, the present study is the largest meta-analysis to date with subgroup and meta-regression analyses providing extended evidence regarding the inter-study heterogeneity issues.

Several limitations existed in our study. First, most of the included studies were retrospective and of questionable quality, and data from the prospective high-quality study were lacking. Second, significant between-study heterogeneity was found. Nevertheless, we conducted these meta-analyses using random-effect models, and sensitivity analysis confirmed the reliability of our results. Third, we could not evaluate the prognostic role of PD-L1 based on critical prognostic factors such as TNM stage or HCC causes because most studies did not report sufficient data. For example, as Huang et al. [20] reported, high PD-L1 expression was correlated with a high HBV viral load. How the causes of HCC, such as hepatitis C virus or alcohol, influence the expression and prognostic role of PD-L1 remain unclear. Finally, our analysis indicated some racial differences exist in the prognostic role of PD-L1. However, data from non-Asian patients were limited, and the prognostic role of PD-L1 in these patients warrants further investigation.

Despite the limitations listed above, our meta-analyses found no significant prognostic role of PD-L1 in HCC patients after potential curative hepatectomy based on the included studies. The expression of PD-L1 was significantly correlated with AFP, hepatitis history, and TIL. The prognostic role of PD-L1 in HCC warrants further investigation.