This study provides that PD-1 plays an important role in occurrence, development and recurrence of peritoneal adenocarcinoma. Peritoneal adenocarcinoma is the most common pathologic type of peritoneal neoplasms. In recent years, the clinical therapy for peritoneal adenocarcinoma is becoming multi-modal, including cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC), almost there are still some patients suffering curative resection and receiving chemotherapy without clinical benefit. Immune checkpoint blockade with anti-PD-1 and Tim-3 antibodies have become a hot topic for the treatment of human malignancies18–19. As its name implies, PD-1 is encoded by PDCD1 gene, and is initially identified to induce programmed cell death20. Tim-3, another T cell inhibitory receptor co-expressed with PD-1 in exhausted T cells21.The application of immunosuppressive therapy in peritoneal neoplasms is rarely reported, and it remains unclear how PD-1 and Tim-3 impact on the prognosis of peritoneal neoplasms.
As we all known, antibodies that blocking the PD-1 pathway have demonstrated antitumor activity in cancer patients, and have gained approval treatment of several different cancers by FDA22–23. Kamphorst et al investigate in peripheral blood of lung cancer patients, 70% of patients with disease progression had a delayed PD-1 + CD8 + T cell response, whereas 80% of patients with clinical benefit expressed PD-1 + CD8 + T cell 24, that suggest peripheral blood may provide valuable insights to the therapy of lung cancer patients who responses to PD-1 targeted therapies. Furthermore, it has been reported by Bekos et al that in ovarian tumor tissues, the percentages of CD8, PD-1 and PD-L1 expressing subpopulations of tumor infiltrating leucocytes, differing in primary tumor tissues and metastatic intraperitoneal metastases 25. Moreover, in peripheral blood of patients with acute-on-chronic liver failure, PD-1 induced T lymphocyte dysfunction, that dysfunction might involve glycolysis inhibition. Recently, the mechanism of Tim-3 in the development of human cancer has been reported more and more. Benling Xu et al reported the upregulation of the inhibitory receptor Tim-3 may restrict T cell responses in colorectal cancer patients 26. Riyao Yang et al uncovered PD-1 and Tim-3 were co-expressed and their crosstalk regulated T cell exhaustion and suggested galectin-9 as a promising target for immunotherapy27.
Our data strengthened the assumption that within peritoneal adenocarcinoma patients, PD-1 + lymphocytes and Tim-3 + lymphocytes participated in the cancer development. It was also found the peritoneal adenocarcinoma patients presented with higher levels of circulating CD4 + T cells, CD8 + T cells, Treg cells, IL-6, IL-10, PD-1 + T lymphocytes, PD-1 + CD4 + T lymphocytes and PD-1 + CD8 + T lymphocytes, compared with health controls. However, compared with PD-1, no significant differences were discovered of Tim-3 in lymphocytes subsets. That suggested PD-1 was more correlated with the occurrence and development of peritoneal adenocarcinoma. Compared with poor grade, the patients with well/moderate grade adenocarcinoma contained increased percentage of PD-1 + T lymphocytes. Arial Y et al reported the increased percentage of PD-1 on circulating CD4 + and CD8 + T cells was associated with dysfunction of cell immunity after colorectal cancer operation 28. Benling Xu et al found that Tim-3 + PD-1 + CD8 + T cell increased in circulation was correlated with clinical cancer stage but not histologic grade 26. In the epithelial ovarian cancer patients, between primary ovarian tumor tissue and metastatic intraperitoneal implants, the infiltration rates of PD-1 and PD-L1(programmed cell death 1 ligand 1) had differences25.Our result suggested, both in the PB and tissue level, the percentages of circulating PD-1 + T lymphocytes and PD-1 + CD4 + T lymphocytes have closely linked with the histologic grade of peritoneal adenocarcinoma.
To further characterized PD-1 expression in the tissues of peritoneal adenocarcinoma, we analyzed PD-1 by immunohistochemistry. Consistent with the results of PD-1 differential levels in the peripheral blood, the expression of PD-1 presented the higher level in the well/moderate grade adenocarcinoma group compared with poor grade. Furthermore, compared with peripheral blood in the same patient, PD-1 expression in tissues was lower. The positive percentage of PD-1 + correlated with lymph node metastasis in peritoneal adenocarcinoma tissues. Higher level of PD-1 + positive percentage was discovered in lymph node metastasis group compared with no lymphatic metastasis group. The data suggest a mechanism of PD-1 involving in the immune tolerance in the peritoneal adenocarcinoma tissue level.
One novel finding in this study was that the percentages of CD4 + T lymphocytes and PD-1 + T lymphocytes in circulation were correlated with recurrence interval (less than 6 months, greater than two years). CD4 + T and PD-1 + T lymphocytes represented the higher percentages in less than 6 months groups compared with greater than two years groups. In clinical tumor treatment, the formation of drug resistance was the major obstacle to the successful chemotherapy, meanwhile, the recurrent tumor always accompanied with drug tolerance. There were many researches about monitoring and avoidance of drug resistance in human cancer. However, drug resistance seemed to be a multigene phenomenon. Increased drug efflux, DNA repair, and altered apoptotic pathway were responsible for the formation of acquired drug resistance 29. Furthermore, ATP-binding cassette (ABC) membrane transporters participated in multidrug resistance in many human cancer cell lines such as ABCB1 (MDR1) and ABCC1 (MRP1)30. What’s the exact mechanism had been unclear. That indicated PD-1 involved in a network of drug resistance related signaling pathways and played an indispensable role in drug resistance formation. These results consistent with the correlation between PD-1 and PFS.
These findings suggest that peripheral blood PD-1 + T cells hold promise as a predictive biomarker for peritoneal adenocarcinoma malignancy and recurrence. Nonetheless, the present study has several limitations. Firstly, additional research using a larger sample size is necessary to validate these results. Secondly, the exact mechanism of PD-1 in the occurrence, development and recurrence of peritoneal adenocarcinoma will further be studied. That may be required the further studies on the molecular mechanism of how PD-1-mediated immunosuppression, which may provide valuable guidance to the novel treatment for the clinical therapy of peritoneal adenocarcinoma. Thirdly, more methods such as fluorescent quantitative PCR were needed, to investigate the prognostic role of PD-1 in peritoneal adenocarcinoma.