Here we analyzed the expression of CMTM6 and PD-L1 in 185 GC tissues by immunohistochemistry and examined associations with clinicopathological characteristics and survival. We found that expression of CMTM6 or PD-L1 alone was not an independent prognostic factor in patients with GC after excluding other confounding factors. Co-expression of CMTM6 and PD-L1 was an independent prognostic factor in patients.
In our study, 15.1% patients had peritoneal metastasis. The reason for surgical treatment is that the imaging and physical signs of the patients are not manifested, and micrometastasis was found during the operation. N stage and M stage correlated with OS, and T stage was not an independent prognostic factor; this may be because of the high percentage of lymph node metastasis regardless of T stage. Borrmann type IV GC shows a specific biological behavior with a high degree of malignancy and accounts for 10–20% of all GC; the 5-year survival rate of this cancer type is only 0–17%. In our study, Borrmann type IV accounted for 22.7% of cases, closing to the highest proportion. Bowman type IV GC indicates lymph node metastasis, more common peritoneal metastasis, and late staging during surgery . Borrmann type IV was also an independent prognostic factor in our study.
CMTM6 plays different roles in different cancers. Guan et al. found that CMTM6 was highly expressed in glioblastoma multiforme and mesenchymal subtypes by analyzing CGGA, TCGA and other databases, and high expression of CMTM6 was related to poor prognosis . Cox model analysis showed that CMTM6 was an independent prognostic factor of glioma, which indicated that CMTM6 played an important role in tumor invasion and progression. Zhu et al. found that the expression of CMTM6 in hepatocellular carcinoma was significantly lower than that in adjacent non-tumor tissues through immunohistochemical detection, and the prognosis of cases with low CMTM6 expression was better . One possible mechanism is that CMTM6 binds with PD-L1 protein, decreases its ubiquitination and increases the half-life of PD-L1 protein, resulting in enhanced ability of tumor cells to inhibit T cells; the elimination of CMTM6 would reduce PD-L1 and improves OS. The conclusion of this study was different from that of Zhu et al. Our survival analysis showed that the OS of patients with high expression of CMTM6 was poor, which may be due to the difference of CMTM6 expression in different tissues, which determines the biological characteristics of different tissue tumors. The specific reasons need to be further explored. However, our results suggest that CMTM6 may be a new immune checkpoint molecule.
As an immunosuppressive molecule, PD-L1 can inhibit the activity of T cells through a variety of complex signaling pathways, thus promoting tumor progression [17–19]. The relationship between the expression of PD-L1 and the prognosis of patients has been controversial. A meta-analysis study involving 7308 patients found that high expression of PD-L1 was associated with poor prognosis (HR = 1.44, 95% CI: 1.18–1.76, P < 0.001), especially in GC (HR = 1.43, 95% CI: 1.05–1.94, P = 0.021) . Böger et al. suggested that high expression of PD-L1 was associated with good prognosis (HR = 0.753, 95% CI: 0.584–0.971, P = 0.029) . Our study also found that PD-L1 overexpression was not associated with poor prognosis, and this may be related to the difference in sample size and sample selection. Despite the rapid development of immune checkpoint blockade, a large proportion of patients still fail to benefit from anti- PD-L1 immunotherapy. Therefore, Das et al.  proposed a strategy for finding new immune checkpoints and adopting a combination of multiple immune checkpoint blockers.
CMTM6 has become another important immune checkpoint by regulating the anti-tumor immune effect mediated by T lymphocytes. However, whether CMTM6 can regulate PD-L1 and what role it plays in GC has not been studied. Our immunohistochemical results show that the expression of CMTM6 is positively correlated with the expression of PD-L1, and the expression level of CMTM6 and PD-L1 increases with the increase of malignant degree of GC. This indicates that the regulation of CMTM6 and PD-L1 signaling pathway in tumor microenvironment has a synergistic effect. Based on previous studies and our results, we speculate that CMTM6 may activate the transmission of related signals in the PD-L1 pathway or enhance the secretion of some cytokines in the tumor immune response, thus promoting the progression of GC. Mezzadra et al.  found that CMTM6 can enhance the ability of tumor cells expressing PD-L1 to inhibit T cells. The elimination of CMTM6 can decrease the expression of PD-L1 and then significantly reduce the inhibition of tumor-specific T cell activity, but the specific regulatory mechanism needs to be further studied. Another important finding is that the prognosis of patients with high expression of CMTM6 was poor, and if PD-L1 is also highly expressed in patients with high expression of CMTM6, these patients show a worse prognosis. Our findings suggest that PD-L1 depends on CMTM6 to perform its inhibitory function, and that the combination of high expression of CMTM6 and PD-L1 may be more suitable as a marker of GC than the individual markers. Whether CMTM6 can be combined with PD-L1 monoclonal antibody inhibitors as a new target for immunotherapy of GC will become a research focus in the future [24–25].
There are still some limitations in our study. Our study is a retrospective study, which may have certain selection bias. Our report also lacks in vitro and in vivo experiments and the underlying mechanism is still unknown. We plan to conduct this research in future studies. Our study is the first to study the clinicopathological correlation between CMTM6 and PD-L1 in GC; these findings may provide the experimental basis for the formation of dual-targeting drugs.