High expression of ICRs in the tumor microenvironment suppresses the immune response against the tumor and causes the tumor cells to evade the immune system. The number of studies on ICRs has been increasing in recent years, and it has been shown that their blockages increase the effectiveness of cancer treatment (16). Determining the associations of clinical, pathological, and demographic characterizations with ICRs may contribute to the optimization of ICR-targeted treatments.
The success of immunotherapy, which includes targeted monoclonal antibody treatments, is directly related to the high expression of ICRs in cancerous tissue. Sasidharan et al. showed that genes expressing PD-1, CTLA-4, TIM-3, and LAG-3 in breast cancer patients had higher levels in breast cancer tissue than in normal breast tissue (17). It has also been examined the levels and prognostic values of 50 ICR genes in molecular subgroups of breast cancer, including PD-1, CTLA-4, TIGIT, LAG-3, and TIM-3 (18). The results demonstrated that the levels were higher in breast cancer patients than healthy subjects.
There have been only a few reports on ICR expression of PBLs in breast cancer patients. Elashi et al. found that transcriptomic expression of PD-1, CTLA-4, TIM-3, TIGIT, and PDL-1 were upregulated in the peripheral blood of primary breast cancer patients, but LAG-3 expression was downregulated (19).
In our study, no significant association was found between PD-1, CTLA-4, TIM-3, LAG-3, and TIGIT expression on PBLs and clinicopathological data. However, a negative correlation was observed between PD1+CD8+, TIGIT+CD16+ and CTLA-4+CD56+ cells in PBLs and their counterparts in TILs. This negative correlation may indicate a decrease in tumor-specific ICR expressing PBLs through migration of these lymphocytes from the blood to the tumor area, which may occur through specific chemokine receptor and chemokine interactions between immune cells and the tumor in tumor-associated stroma (20, 21). However, TIM-3 expression of CD8+ and CD16+CD56dim cells in PBLs and TILs has shown positive correlations, but the results did not reach statistical significance. This may have been due to the limited number of patients with PBL samples in our study. Therefore, our results suggest that ICR expression except for TIM3 on PBLs cannot be reliably used as an indicator of the immune response against tumors (22, 23).
In the era of the immune checkpoint blockade, PD1+CD8+ cells in PBL are also being investigated as a promising biomarker to monitor the effectiveness of anti-PD-1 and PDL-1 treatments (24). Kamphorst et al. demonstrated an increase of PD1+CD8+ cells in PBLs in patients diagnosed with advanced lung cancer and showed clinical benefit following PD-1-targeted therapy (25). Based on these contrary findings, more studies are needed to examine the role of ICR-expressing PBLs and their interactions with ICR-expressing TILs in cancer treatment.
Lei Tu et al. showed that high expression of PD-1, CTLA-4, LAG-3, and TIM-3 is a good prognostic marker in patients with breast cancer who received chemotherapy, and higher ICRs expressions were detected on TILs of these patients (26). The presence of high TIL levels was found as a marker of the immune response of the organism to tumor cells, which seems to be associated with increased survival and good prognosis. In a meta-analysis, Mao et al. found an increased percentage of TILs to be a good prognostic factor for triple-negative tumors (1). Furthermore, a high level of tumor-infiltrating CD8+ cytotoxic T lymphocytes was associated with prolonged survival and disease-free survival for patients with triple-negative and hormone receptor-positive molecular subtypes. In another meta-analysis, it has been shown that high TIL levels were associated with improved survival in patients with triple-negative breast cancer (2).
The prognostic significance of PD-L1 and high TIL expression has been investigated in breast cancer in many studies (27–30). In most of these reports, it was concluded that higher expression of PD-L1 was associated with poor prognosis in patients with aggressive tumor characteristics. Okabe et al. investigated the expression of PD-1/PD-L1 and the density of CD8+ and CD3+ lymphocytes in tumor tissue by immunohistochemical analysis (27). The study included 97 patients with early-stage breast cancer and it was revealed that those with high CD8+/PD-L1+ TIL levels had lower survival rates. In a meta-analysis by Zhang et al., high PD-L1 level was similarly found to be associated with decreased overall survival, lymph node positivity, high HG, hormone receptor negativity, and the triple-negative molecular subtype of breast cancer (28). PD-L1 positivity was found to be associated with a poor outcome in a meta-analysis by Wang et al., which included 8583 patients, and one by Kim et al., which included 7877 patients (29, 30).
Our findings also showed increased LAG-3 expression on CD16+CD56dim NK cell subset and TIGIT expression of CD8+ T cells on TILs in patients over 50 years of age, which is in concordant with some studies (31). Similarly, higher TIM-3 expressions on TILs in patients over 45 years of age were submitted (31). However, there are also many studies reporting higher ICRs expression on TILs in younger patients (32–34). Furthermore, in our cohort, patients with larger tumor size were more likely to have LAG-3 expression on CD8+ T cells, but the results did not reach statistical significance (p = 0.09). Burugu et al. also showed that LAG-3 expression is associated with large tumor size (35).
HER-2 positive and triple-negative molecular subtypes of breast cancer are known to be more immunogenic tumor types and have higher TIL rates than luminal subtypes (13, 14). NK cells are important effector cells against tumors, and increasing their activity will strengthen the immune response. Unlike T cells, NK cells have MHC-independent cytotoxicity, but the number and activity of NK cells are generally suppressed by immunosuppressive factors in the tumor microenvironment (36). Immune checkpoint receptors are one of these factors. As a result, immunotherapy modalities targeting NK cells, like immune checkpoint blockade, have become a rising star in cancer treatment in recent years (20, 21).
In the present study, we have demonstrated significantly elevated tumor-infiltrating cytotoxic CD16+CD56dim NK cell rates in patients with a high TIL ratio and high Ki-67 proliferation index. Moreover, a trend of increased CD16+CD56dim NK and LAG-3-expressing CD16−CD56bright NK cell subsets in patients with HER2 positivity were observed. These findings suggest that NK cells may play an important role in the immune response against HER2-positive or highly proliferating tumors, which is concordant with previous reports (37, 38). Furthermore, these findings also indicate that patients with a high Ki67 index and HER2 positivity might be suitable candidates for NK targeted immune checkpoint blockade.
Kim et al. showed that better therapeutic effects of chemotherapy were significantly associated with HER-2 positivity and higher NK cells in patients with breast cancer. Study examined the expression of PD-1, PD-L1, PD-L2, CTLA-4, LAG-3, and TIM-3 in breast cancer patients by flow cytometry and immunohistochemical analysis, and showed that ICR expressions were higher in triple-negative and HER-2 positive patients, and expression levels differed significantly even in tumors in the same molecular subgroup (39).
In concordance with previously published reports, our results showed that PD-1 and TIM-3 expression on CD8+ T cells and CTLA-4 expression on CD16+CD56dim NK cells were higher in patients with non-luminal tumors (32, 34). Nevertheless, more studies are needed to evaluate the ICR expression on CD16+CD56dim NK cells and the effectiveness of anti-ICR treatment in patients with luminal-type tumors.
Furthermore, our results also revealed that CD16+CD56dim NK cells and LAG-3 expressing CD8+ T cells on TILs were significantly higher in patients with high TIL density. Similarly, other studies also demonstrated high expression of ICRs in patients with high TIL levels (26, 40). Baitsch et al. reported that in patients with malignant melanoma, tumor-infiltrating CD8+ T cells express many inhibitory molecules, such as BTLA, TIM-3, LAG-3, KRLG-1, 2B4, CD160, PD-1 and CTLA-4 (41). Cabioglu et al. analyzed the expressions of PDL-1, PD-1, TIM-3, LAG-3, and CTLA-4 immunohistochemically in surgical specimens from patients with locally advanced triple-negative breast cancer after neoadjuvant chemotherapy, and showed that PD-1, PDL-1, CTLA-4, TIM-3 and LAG-3 in tumors and TILs were positively correlated with each other (42).
We recently reported that the new generation ICRs TIM-3, LAG-3, and TIGIT are highly expressed in locally advanced breast cancer with poor prognostic factors following neoadjuvant chemotherapy (43). Byun et al. also found that high TIM-3 expression on TILs was associated with high PD-1 and PD-L1 expression (33). Furthermore, Burugu et al. showed that there is a high rate of PD-1 expression on LAG-3+ TILs (35). In another study, high TIM-3 levels on TIL were related to high PD-1 and high LAG-3 levels on TILs (34). We similarly found that PD-1, CTLA-4, TIM-3, LAG-3, and TIGIT expressions on CD8+ T and CD16−CD56bright NK cells on TILs had high correlation with each other. These findings may indicate that patients who would benefit more from various immunotherapies can be identified by evaluating one of the ICRs, such as PD-1 and PD-L1, which are the most frequently used predictive markers in the immunotherapy era in clinical practice.
In conclusion, various ICRs were found to be highly co-expressed with each other on TILs, including PD-1, CTLA-4, LAG-3, TIM-3, and TIGIT. These receptors may synergistically suppress the response to the tumor, which may trigger immune escape mechanisms in the early stage of carcinogenesis. The high co-expression of ICRs, which may have a similar and synergistic effect in the tumor microenvironment, suggests that combined immunotherapy options could improve the success of cancer treatment. Furthermore, NK cells may play an important role in the immune response against HER2-positive or highly proliferating tumors. This suggest that patients with a high Ki67 index and HER2 positivity may be more likely to benefit from NK-targeted immune checkpoint blockade strategies that should be investigated in future studies.