The programmed cell death 1 (PD-1)/PD-1 ligand 1 (PD-L1) pathway has emerged as a critical inhibitory pathway regulating T-cell response in urological malignancies. Nevertheless, the high degree of non-responders demonstrates that we are still far from completely understand the events underlying tumor immune resistance. Although the expression of PD-L1 in tumor tissue has been correlated with clinical response to anti PD-1 inhibitors, the ability of this marker to discriminate the subgroup of patients who derive benefit from immunotherapy is suboptimal. Circulating tumor cells (CTCs), as an accessible source of tumor for biologic characterization that can be serially obtained with minimally invasive procedure, hold significant promise to facilitate treatment-specific biomarkers discovery. It has been recently demonstrated that the persistence of PD-L1 positive CTCs is indicative for treatment resistance in metastatic chemo-refractory patients. Furthermore, it has been previously suggested that PD-L1 positive CTCs may represent a small population of partial epithelial mesenchymal transition (EMT)-transformed cancer cells. The bidirectional crosstalk between PD-L1 expression and EMT is well described, although the molecular determinants of this association remain incompletely understood. Although only limited data exist, preliminary research suggests that PD-L1 is expressed in CTCs with mesenchymal traits. Basing upon previous observation that the presence of PD-L1 on CTCs apparently predicts resistance to the anti PD-1 nivolumab in metastatic lung cancer patients, we aim to investigate the co-expression of PD-L1 and EMT markers in CTCs from bladder cancer patients, as a possible molecular background of immune escape.
Immune dysfunction associated with urothelial carcinomas of the urinary bladder has been documented by several evidences highlighting the hypothesis that immunotherapy can alter the process of carcinogenesis and influence the natural history of the disease [3].
Overall, 12% of bladder tumors cells are expressing PD-L1 as well as the 27% of tumor infiltrating immune cells, and up to 50% of malignant urothelial cells in carcinoma in situ [11]. The expression of PD-L1 on urothelial carcinomas demonstrated to be predictive of BCG failure in NMIBC and to be associated with higher mortality following cystectomy in patients with organ-limited disease and in those with advanced metastatic disease.
It has been postulated indeed that PD-L1 expression in bladder cancer cells might be a mechanism whereby tumor cells progressively invade the bladder wall disrupting host immunity that normally counteract tumor progression. Thus, it is conceivable that a marked PD-L1 expression may contribute to the decline in the effectiveness of the adjuvant BCG therapy for NMIBC over time and facilitate the progression of NMIBC to muscle invasive disease.
For these reasons the PD-1/PD-L1 pathway has been identified as an attractive therapeutic target for the treatment of bladder cancer similar to the findings in other epithelial tumors such as renal cell cancer, lung cancer, and melanoma [12].
Despite the fact that PD-1/PD-L1 blockade has activity across a number of cancers, in most studies, less than 50 % of patients respond to treatment, indicating a need for predictive biomarkers.
CTCs are regarded as surrogates for early metastatic spread of disease and significant interest has been focused on their role as prognostic in different cancer types, including bladder cancer. In addition, the molecular characterization of CTCs may represent a tool to investigate the basic mechanism of cancer biology since CTCs have the potential to give information regarding the intratumour heterogeneity and tumour evolution [13].
The demonstration that PD-L1 positive CTCs are EMT transformed cells might help to identify a population of patients who are not responder to immune checkpoint inhibitors. Besides, increasing evidence indicates that EMT-like processes can alter immune functions also through the induction of immunosuppression. EMT has been indeed found to be able to mediate immunosuppressive tumor microenvironment and a mechanistic link has been recently demonstrated between EMT and immune evasion through the elevation of multiple immune checkpoints [8-9]. This might open a new scenario in the identification of biomarkers to select patients with high risk NMIBC and those with advanced bladder cancer (MIBC, MBC) who could be candidate to immunotherapies as well as in the design of new combination strategies targeting EMT and PD-L1.
Although the complex mechanisms connecting EMT and the activation of an immune escape program in cancer remain incompletely understood, the EMT-related upregulation of PD-L1 expression may potentially be qualified as a mechanism of resistance to immune-checkpoint inhibitors and certainly deserves further investigation.
References
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