Main

Immunotherapy has begun to revolutionise cancer treatment, by introducing therapies such as checkpoint inhibitors that target the host immune system instead of the tumour (Chen and Mellman, 2013). For an anticancer immune response, the programmed cell death protein-1 (PD-1) pathway is considered an important inhibitory mechanism regulating T-cell exhaustion (** and Yoon, 2016). PD-1, which belongs to the CD28 family of proteins, is a receptor expressed on a number of immune cells, including T cells, B cells, monocytes, NK cells, and dendritic cells (Park et al, 2016). PD-1 has two ligands, the programmed cell death protein ligand-1 (PD-L1) and programmed cell death protein-ligand-2 (PD-L2). Programmed cell death protein ligand-1 is expressed on T cells, B cells, dendritic cells, macrophages, mesenchymal stem cells, bone marrow-derived mast cells, and some non-haematopoietic cells, whereas PD-L2 is mainly expressed on antigen-presenting cells, including macrophages, dendritic cells, and non-haematopoietic tissues (** and Yoon, 2016). Several studies have already demonstrated that PD-L1 or PD-1 is highly expressed on tumour cells in gastric cancers (GCs) (Wu et al, 2006; Takaya et al, 2015; Zhang et al, 2015; Böger et al, 2016; Derks et al, 2016; Dong et al, 2016a; Eto et al, 2016; Kim et al, 2016; Takano et al, 2016; Cho et al, 2017). Interestingly, Wang et al (2016) suggested that GC patients with positive PD-L1 expression had a significantly shorter survival than PD-L1-negative patients, although the prognostic impact remained inconsistent.

The Cancer Genome Atlas data revealed that Epstein–Barr virus (EBV)-associated GC (EBVaGC) closely associated with a PIK3CA mutation, PD-L1/2 overexpression, EBV-CIMP, CDKN2A silencing, and immune cell signalling activation among GC subtypes. As previous studies have suggested that high PD-L1 expression is related to a good responsiveness to anti-PD-1/PD-L1 therapy in human cancers, the EBVaGC subtype, in particular, may be a potential candidate for anti-PD-1/PD-L1 therapy among GC subtypes (Wang et al, 2003; ** and Yoon, 2016). In the present study, str-PD-L2 expression was positively correlated with dense iTu- and/or str-TILs, yet not iTu-PD-L2 expression. Ohigashi et al. suggested that PD-L2 expression inversely correlated with tumour-infiltrating CD8+ T cells in 41 patients with oesophageal cancer. Thus, whether PD-L1 and PD-L2 are influenced by specific markers of TILs, such as CD4, CD8, and FOXP3, and interferon-γ would be an interesting focus for further studies of the tumour immunity of EBVaGC. Meanwhile, for the correlation between PD-L1 and PD-L2 expression, PD-L1 was found to be expressed independently of PD-L2 expression. This finding supports previous report, where iTu-PD-L2 expression was induced in the absence of PD-L1 co-expression tumours, in which case PD-L2 expression was less likely to possess PD-L1+ immune cells (Derks et al, 2015). These observations can also help to understand the exact role of PD-L2 on the natural outcomes of EBVaGC.

Tumour-infiltrating lymphocytes could be used to identify a subgroup of patients with excellent outcomes (Chang et al, 2014) and our previous study also showed an independent association between a high density of TILs and a favourable recurrence-free survival or DFS in 120 patients with EBVaGC (Kang et al, 2016). This finding supports that TILs exhibit a host cellular immune response against tumours, indicating that immunotherapy may have a potential role in patients with EBVaGC (Kang et al, 2015). Several studies have already demonstrated a correlation between TILs and PD-L1 expression in EBVaGC (Kawazoe et al, 2016; Ma et al, 2016; Saito et al, 2016). Interestingly, when combining the iTu-PD-L1 expression and TIL score, the patient group with iTu-PD-L1 negativity and str-TIL positivity was associated with a better DFS than the other EBVaGC patients. This result creates a very meaningful subgroup that can provide more information for tailored therapy for each group of patients. In particular, this finding can provide a novel strategy for subgroups with decreased immunoediting via the PD-1 pathway and a strengthened immune response by TILs, and vice versa, considering that the state of the immune equilibrium is a critical factor for therapeutic success (Chen and Mellman, 2013; Palucka and Coussens, 2016). In fact, alterations of TILs have already been implicated in immune homeostasis, as well as immune response in the PD-1 pathway (Menon et al, 2016).

Although the present data identified a significant prognostic role of PD-L1 expression in operable EBVaGC, these results should be cautiously interpreted due to potential limitations. The current study is a retrospective evaluation, and the evaluation and interpretation of PD-L1 and PD-L2 within tumours and/or a tumour microenvironment have not yet been standardised in GCs (Manson et al, 2016). Moreover, a standardised cutoff value for PD-L1- and PD-L2 positivity has not yet been clearly established (Festino et al, 2016), plus PD-L1 and PD-L2 expression cannot be used to predict treatment response to anti-PD-1/PD-L1 therapy in patients with EBVaGC. Nevertheless, the present study has several important differences with previous studies as follows: (1) a relatively large-scale cohort of EBVaGC patients with a Korean homogeneous ethnic identity (** and Yoon, 2016, 2) equivalent treatment application, (3) minimal loss to follow-up, (4) assessment of TILs, and (5) comprehensive interpretation of expression in cancer cells and immune cells as separate microenvironments of cancer cells.

Conclusion

In conclusion, str-PD-L1- and str-PD-L2 positivity was shown to be associated with dense iTu- and str-TILs in patients with EBVaGC. The current study also revealed significant prognostic impact of iTu-PD-L1 positivity by demonstrating an independent association with a poor DFS. Therefore, these findings support the concept that PD-L1 can be a prognostic indicator for predicting patient outcomes and a rationale for therapeutic targeting in EBVaGC.