Differential expression of TIM-3 between primary and metastatic sites in renal cell carcinoma

Of all patients with kidney cancer, 20–30% of them were diagnosed as metastatic renal cell carcinoma (RCC) at initial diagnosis, and about 20–40% of localized renal cell carcinoma would develop distant metastasis in spite of having been treated with radical nephrectomy [1]. All along kidney cancer was recognized as immunogenic tumors. Thus in the 1990s, non-specific immunotherapy with cytokines was seen as the standard treatment of mRCC. However, due to the low efficacy of cytokine drugs, the objective response rate was only 5–27%, and the median progression-free survival (PFS) was only 3–5 months, accompanied by evident side effects. Compared with cytokines, targeted agents showed significant tumor response and beneficial survival outcomes. Nevertheless, the median survival time was still only 8–30 months (according to different prognostic risk group) [2‐4]. Therefore, development of more effective drugs to improve patients’ survival outcomes is needed.

Recent studies demonstrated that immune checkpoints have played key roles in the mechanism of immune escape [5‐8]. In the tumor immunologic microenvironment, the most critical part of antitumor activity is the T cell activation. Activated T cells can effectively recognize and kill tumor cells. With the breakthroughs in molecular immunology, researchers have found the presence of a variety of co-stimulatory and co-inhibitory signal receptors—the two common regulation of T cell activation—on T cell surface [9]. The co-stimulatory signal receptor is like an “accelerator” to promote T cell activation, thereby facilitating the immune cells killing tumor cell. Co-inhibitory signal receptor is like a “brake” trigger leading to T cell inactivation by tumor cells using its immunological checkpoints to combine with co-inhibitory signal receptor. Ultimately, tumor cells survive, or rather the occurrence of immune escape [10].

Through analyzing the expression of immune checkpoints in tumor and the effect of checkpoints inhibitors, researchers found a positive correlation between immune checkpoints (such as PD-L1) over-expression and the treatment response in lung cancer and melanoma [11, 12]. However, there was no such correlation observed in RCC [13]. Due to the significant heterogeneity of RCC, immune checkpoints may express differently between primary and metastatic tumor. This differential expression suggest that the evaluation of expressions of immune checkpoints in metastasis might offer more accurate prediction of treatment response of immunotherapy [14, 15]. Several studies have shown that PD-L1 differentially expressed between the primary and metastasis in myeloma and RCC [14, 15]. However, it is unclear that whether TIM-3 (T cell immunoglobulin and mucin-domain containing-3), another immune checkpoint, is in the same situation with that of PD-L1 in RCC.

TIM-3 is an immune checkpoint (co-inhibitory signal receptor) located in T-cell [16]. Despite the functional role of Tim-3 was first described to negatively regulate the Th1 response, it seems to play a more complicated role in regulating anti-tumor response [16]. In fact, stimulation of Tim-3 is considered to act as both inhibitory and activating signal, which has been demonstrated in infectious diseases, autoimmune disorders and cancer immunity [17‐20].

Therefore, in the present study, we aimed to compare the differential expression of TIM-3 in the primary and metastatic sites of RCC, and additionally, to find the role of TIM-3 in predicting patient’s prognosis.

This was the first study that aimed to analyze the differential expressions of TIM-3 between the primary and metastatic tumors among mRCC patients. Meanwhile, based on the analyses of correlation and survival, we determined the relationship between the expression of TIM-3 and clinicopathological parameters, and its value in predicting patient’s prognosis.

Researchers have found a positive correlation between immune checkpoints (such as PD-L1) expression and treatment effect in lung cancer and melanoma [11, 12], but there was no such correlation in RCC [13]. Furthermore, published evidences have demonstrated significant differential expressions of immune checkpoints between the primary and metastatic tumors [14, 21]. TIM-3 is one of these immune checkpoints and is located in T-cell [16]. Due to the significant heterogeneity of RCC, immunological factors like TIM-3 might express differently between primary and metastatic tumors.

The expressions of TIM-3 either in the primary or metastasis suggested a better prognosis for mRCC patients in our study, especially, its expression in metastases could more accurately predict the prognosis of patients (with higher PA values both in PFS and OS models). Interestingly, the results of TIM-3 expression in predicting better prognosis of mRCC were contradictory to other tumors [22‐26]. The exact reason of this phenomenon observed in mRCC remains unclear. According to our analysis about current reported studies, the possible reasons might include: 1) Most studies reported the prognostic role of TIM-3 in localized RCC, and TIM-3 may play different roles in different tumor stages; 2) Current evidences on the relationship between TIM-3 expression and prognosis are controversial among different tumor types. A study reported that TIM-3 was a protective factor in acute myeloid leukemia [20]. The same results were seen in pancreatic cancer, prostate cancer, usual vulvar intraepithelial neoplasia and colorectal cancer [27‐30]. Additionally, in the ESMO 2017, Torras et al. reported that TIM-3 was differentially expressed between sunitinib refractory and sensitive groups, and was associated with benefit to sunitinib treatment in mRCC patients [31]. 3) The functional outcomes were different depending on the expressions of immune check points in tumor or tumor infiltrating immune cells. High expression of CTLA-4 on CD8 + T cells was associated with shorter OS, but TIM-3 was positively correlated with prolonged OS when expressed on CD4 + T cells [27]. 4) Different biological functions of TIM-3 might be related to different expression localization. Similar to PD-1, TIM-3 was mainly expressed in cellular membranes, but it was also expressed in the nucleus in prostate cancer and colorectal cancer [28, 30]. In bladder cancer and esophageal cancer, TIM-3 was both expressed in cellular membranes and cytoplasm [24, 32]. 5) TIM-3 also appears as a co-stimulatory signal in other non-tumor disease states [18‐20].

In fact, the mechanism of TIM-3 in anti-tumor immune responses is complex. Some studies showed that TIM-3 could act as a co-stimulatory signal receptor and enhance the killing effect of cytotoxic T cells and other immune cells [17‐19]. Other studies also showed that TIM-3 might act as a “rheostat,” thus orderly fine-tuning cellular responses [16]. It is unclear under what circumstances TIM-3 could appear to be co-suppressed or co-stimulatory signals, and whether co-suppression signals depend on the co-expression of other molecules still needs further validation, such as Ceacam-1 or crosstalk between different ligands of TIM-3 [16].

Multivariate analysis of the present study suggested that the expression of TIM-3 in metastases was significantly higher than that in primary tumors in predicting the prognosis of patients. It was further clarified that the expression of TIM-3 in metastatic tumors might be more indicative to patient’s prognosis. Therefore, more attention should be paid to the assessment of metastatic sites in clinical practice and research, to improve the efficacy of immunotherapy for mRCC and finally realize individualized treatment strategy.

This study was the first to evaluate the differential expression of TIM-3 in primary and metastatic RCC tumors, and is the largest study to date. However, this study still has the following limitations: 1) retrospective study design, which may induce potential selection bias; 2) limited number of paired patients; 3) different types of samples were included (frozen, resection and aspiration). Therefore, subsequent studies should focus on the observation of differences between expression and prognosis on different types of tumor.

The expression differences of TIM-3 were significant between the primary and metastatic tumors. The assessment of immunological checkpoint-related protein in primary tumor might not be able to provide adequate information for clinicians to evaluate or predict the patient’s treatment-related efficacy and prognosis. The expressions of immune check points in metastatic lesions of mRCC should be given more attention, and their accurate diagnosis might be one of the effective ways to realize individual treatment. What’s more, the role of TIM-3 plays in mRCC remains to be verified.