Immunotherapy is a new and emerging method for treating malignant tumors in recent years. It mainly aims to control and eliminate tumor cells by activating the body's anti-tumor immune response, providing broad prospects for the treatment of malignant tumors. However, only a small subset of patients can benefit from this novel therapy[6–10]. The binding of PD-1 and PD-L1 can generate inhibitory signals, and some signal pathways can induce tumor immune tolerance by inhibiting the activation of T cells[12]. Therefore, blocking the signaling pathways can induce T cell activation and kill tumor cells. Studies[5, 13, 14] have shown that the expression of PD-1 and PD-L1 is significantly elevated in various malignant tumor tissues, resulting in decreased activity of the body's immune cells. In the exploration of cervical cancer[15–17], the expression levels of PD-1 and PD-L1 are closely related to pathological characteristics such as lesion size, FIGO stage, tumor differentiation degree, and lymph node metastasis. In esophageal cancer[16, 18], among patients in stages I-II without lymph node metastasis, those with positive PD-L1 expression have a better prognosis, while in patients in stages III-IV with lymph node metastasis, those with positive PD-L1 expression have a worse prognosis. In this study, we also found a close relationship between the expression levels of PD-1 and PD-L1 and tumor infiltration depth and lymph node metastasis, which are related to patient survival rates. This suggests that PD-1 and PD-L1 may be involved in the progression of esophageal cancer and can serve as important markers for patient prognosis. Additionally, neurological invasion, vascular invasion, and tumor infiltration depth have been confirmed by many researchers as independent risk factors for the prognosis of esophageal cancer patients[19–20], a conclusion that is also supported by our study.
Tumor lymphocyte infiltration is considered a manifestation of the body's anti-tumor immune response, and tumor-infiltrating cells play important roles in the occurrence and development of tumors, and can predict the prognosis of a variety of tumors. Tregs are a subset of T cells expressing Foxp3, CD25, and CD4 as cell phenotypic characteristics, belonging to a part of CD4 + cells, and play an important role in forming moderate immune responses and maintaining normal physiological functions in the body. Tregs accumulate in the peripheral circulation and tumor sites of patients, helping tumors evade the host immune system. This type of T cell shows a significant increase in the ratio in the peripheral blood of patients with various malignant tumors including lung cancer[21] and breast cancer[22]. Studies[23] have suggested that compared to Tregs in peripheral blood, tumor-infiltrating Tregs exhibit more proliferative and immunosuppressive phenotypes, with increased expression of CTLA-4, CD25, GITR, 4-1BB, OX40, ICOS, LAG-3, TIM3, TIGIT, and PD-1. The presence of tumor-infiltrating regulatory T cells is associated with poor outcomes in solid cancers. FOXP3 is an important transcription factor in the development of Tregs. Research[24] has found a significant association between FOXP3 and various autoimmune diseases, and more and more scholars believe that there is a clear correlation between the occurrence and development of tumors and the host's immune status. As a marker for T cells, FOXP3 is rarely expressed in healthy cell tissues, but is abnormally expressed in various malignant tumors such as lung cancer[25], pancreatic cancer[26], thyroid cancer[27], and T-cell lymphoma[28], serving as a biological marker for malignant prognosis. Research[29]has confirmed that FOXP3 plays an important role in the production, activation, and functional regulation of Tregs, while Tregs act as the executors of immune function, inhibiting the proliferation of CD8 + infiltrating T cells, disrupting the balance between T lymphocyte subgroups, suppressing immune function, and promoting immune escape. Conversely, some views suggest that high expression of the FOXP3 protein is a protective factor for breast cancer[30] and hepatocellular carcinoma[31]. Our research results indicate that in esophageal cancer, an elevated level of FOXP3 expression predicts a relatively poorer prognosis for patients, consistent with the conclusions of WU studies etc[32], suggesting that blocking FOXP3 expression in cancer cells may provide a new treatment strategy for esophageal cancer.
As is well known, the role of CD4 + T cells in generating potent CD8 + effector cells has long been established[33]. In some preclinical [34]and clinical studies[35], the anti-tumor function of CD4 + T cells in the absence of CD8 + T cells has been reported, indicating a more direct role of CD4 + T cells in anti-tumor immunity. During the effector phase of anti-tumor immunity, CD4 + lymphocytes can directly kill tumor cells, while releasing cytokines to activate tumor-killing immune cells indirectly eliminating tumor cells. Therefore, CD4 + T lymphocytes demonstrate a positive anti-tumor effect in esophageal cancer, and elevated levels of CD4 + expression may be a good prognostic marker. CD8 + T cells are the terminal effector cells in cancer immunity that, through a series of fine regulations, cell proliferation, and differentiation, ultimately form cytotoxic T lymphocytes (CTLs). These CTLs can express perforins and granzymes, penetrate, and destroy virus-infected or tumor cell membranes. CD8 + T cells proliferate significantly in the tumor microenvironment, directly acting on tumor cells, inducing tumor cell apoptosis, and killing antigen-bearing target cells[36]. Numerous studies [37–38]have found that patients with lower CD4 + CD8 + cell ratios in esophageal cancer tissues have lower overall survival compared to those with higher CD4 + CD8 + cell ratios. Our research results demonstrate that deceased patients have lower levels of CD4 + and CD8 + expression compared to surviving patients. Patients with high expression of CD4 + and CD8 + have better survival benefits, consistent with the above research findings.
CD25 is highly expressed in hematologic malignancies, activated circulating immune cells, and Tregs, while it is low in most solid tumors. However, Tregs typically infiltrate into solid tumors. Immunological studies[39–40] have shown that CD4 + CD25 + Tregs can suppress the immune response of T cells to foreign and self-antigens, thereby maintaining immune tolerance and inhibiting the immune response to tumor cells, ultimately leading to immune escape of tumor cells. Clinical studies have also confirmed that CD4 + CD25 + Tregs are recruited to tumor sites by recombinant human C-C motif chemokine 22 protein (CCL22) secreted by ovarian cancer cells, resulting in the accumulation of CD4 + CD25 + Tregs in cancer tissues. An excessive amount of CD4 + CD25 + Tregs can lead to immune escape and promote tumor development[41]. Our research found that high expression of CD25 is one of the adverse prognostic factors for esophageal cancer patients. After adjusting for confounding factors, high expression of CD25 is positively correlated with the risk of patient death. This conclusion is consistent with the above-mentioned animal experiments and clinical research results. This suggests that for cancer treatment, targeted anti-tumor drugs based on CD25 are expected to bring survival benefits to more esophageal cancer patients.
This study has certain limitations. Firstly, this study is a retrospective analysis and based on single-center data, which may introduce some bias and errors. In addition, the experiment used immunohistochemistry to detect paraffin sections without genetic testing. In future studies, we will pay attention to and improve these shortcomings to provide more convincing data for clinical research.
In conclusion, there is a significant correlation between PD-1 pathway related immune cells and clinical pathological features of esophageal cancer; tumor size, poor tumor differentiation, deep tumor infiltration, lymph node metastasis, high expression of PD-1, PD-L1, FOXP3+, and CD25 + are independent risk factors affecting patient prognosis; high expression of CD4 + and CD8 + are protective factors. A predictive model based on the expression levels of PD-1, PD-L1, FOXP3 proteins, and Tregs infiltration has a high predictive effectiveness for the prognosis of esophageal cancer patients, which may provide clues for exploring new treatment modalities for esophageal cancer in the future.