Squamous cell esophageal carcinoma (ESCC) is the most common histological type of esophageal cancer (EC) in China (1, 2). Most patients have advanced or metastatic disease when diagnosed, with a 5-year survival rate of less than 20%. Immune checkpoint inhibitors (ICIs), mainly targeting programmed cell death receptor-1 (PD-1) and its ligand (PD-L1), have greatly improved outcomes and have been added to the current standard of care, which includes platinum-based chemotherapy. However, the current study demonstrated that the survival benefit from ICIs alone is limited, as patients frequently develop immune resistance, regardless of whether the tumor is immunogenic or whether the microenvironment is immune-suppressive (3). Thus, combination strategies for advanced ESCC patients receiving immunotherapy are required to overcome immune resistance and achieve optimal therapeutic benefits.

Radiotherapy (RT), another pillar of advanced EC treatment, can activate the innate and adaptive immune responses by enhancing the presentation of tumor antigens and increasing T lymphocyte infiltration to potentiate the effects of immunotherapy, which involves a variety of inflammatory cells, cytokines, and chemokines in the tumor microenvironment (4–7). Zhang and colleagues found that in locally advanced ESCC, RT plus anti–PD-1 antibody as first-line therapy is safe and feasible (8). Studies have shown that systemic inflammation is a hallmark of the development and progression of malignant tumors, which usually occurs when the balance between the inflammatory cells (neutrophils and monocytes) and tumor-specific lymphocytes becomes disrupted (9, 10). RT activates pro-inflammatory factors including interferons and chemokines that attract activated T cells into tumors (11). When the anti-cancer therapy triggered by RT works, the body improves the immune status by increasing the lymphocyte count and decreasing monocytes, which leads to increased lymphocyte count and lymphocyte-to-monocyte ratio (LMR) (12). Moreover, a number of inflammatory biomarkers (IBs) and their derived ratios have been investigated as prognostic indicators in various cancers. The systemic immune inflammation index (SII), an integrated indicator based on peripheral lymphocyte, neutrophil, and platelet counts, is a strong prognostic indicator for patients with several tumor types (13). Recent studies have revealed that higher neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR) are associated with poorer outcomes in ESCC (14, 15). Lymphocytes are important in promoting antitumor immunity, and a higher lymphocyte-to-monocyte ratio (LMR) generally indicates better survival and response to immunotherapy (15–17). The mentioned IBs and their derived ratios change when patients undergo RT and immunotherapy, as a result of differences in radiosensitivity among different immune cell types (18).

However, local relapses often occur following RT, suggesting RT-induced responses are inadequate to maintain antitumor immunity (19). Many preclinical studies (6, 19) have validated that locoregional tumor control increases when radiotherapy is combined with checkpoint blockade immunotherapy. Clinical studies in colorectal cancer and non-small cell lung cancer (NSCLC) also showed that RT synergized with ICIs and improved the therapeutic effect (20, 21). The increased infiltration of CD8 effector cells and increased ratio of CD8 effector cells to regulatory T cells may explain this immune-based mechanism for combinatorial efficacy (22). In addition, previous studies have reported that, as a result of spatial intratumor heterogeneity and temporal heterogeneity in ESCC, the bulk tumor might include a diverse collection of cells harboring distinct molecular signatures and cancer-related signaling pathways with differential levels of sensitivity to treatment (23–26). Inherent differences exist in the immune microenvironment of different metastatic sites, from the relatively immune-privileged brain protected by the blood-brain barrier to the lung and liver, which are constantly exposed to antigens and have a relatively immunotolerant microenvironment. Studies have also shown that stereotactic ablative RT (SAR) induces systemic immunologic changes that are dependent on the irradiated site (27). Consequently, we speculated that the synergistic effect of RT and immunotherapy on different metastases could produce different immune system changes; however, few studies have explored how to optimize RT with immunotherapy for advanced ESCC with multiple metastases to achieve the optimal combined response.

In this study, we collected the mentioned IBs to examine the immune activation effect among different irradiated sites during radioimmunotherapy and explore the potential factors related to short-term efficacy in advanced ESCC patients who received RT as well as immunotherapy.

As hematological IBs can reflect changes in the immune system, we used delta-IBs, including delta-NLR, delta-LMR, delta-PLR, delta-SII, delta-AEC, delta-WBC, delta-ALC, delta-ANC, delta-AMC, and delta-APC, to examine the immune activation of RT at different sites and their predictive effect on short-term efficacy. We found that brain irradiation may stimulate stronger immune activation than other extracranial organs, and lower delta-SII and earlier lines of ICIs were found to be independently associated with better short-term efficacy in patients with advanced ESCC who received RT and immunotherapy. To the best of our knowledge, this retrospective study is the first to investigate the immune activation of different irradiated sites as well as the relationship between delta-IBs and short-term efficacy in patients with advanced ESCC receiving radioimmunotherapy, and it may provide useful instructions for implementing individualized treatment regimens.

The current normative treatment for advanced ESCC is a combination of PD-1 agents and platinum-based chemotherapy agents. RT has historically been used to relieve the symptoms of relapse and metastases. Evidence suggests that radiation may eliminate tumors by activating local and/or systemic immune responses, particularly when combined with immunostimulatory agents such as ICIs. Many preclinical studies have shown that radiation may modulate the tumor microenvironment by enhancing the release of neoantigens, upregulating the expression of MHC molecules in cancer cells, increasing effector T-cell infiltration, and activating the cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) innate immune response (28). Consistent with our study, Wu et al. showed that brain irradiation induced the best immune activation effect when compared to other organs in advanced NSCLC (12). For strong immune activation of the brain in advanced ESCC, we speculated that irradiation of the brain destroyed the blood-brain barrier (BBB); thus, ICIs were able penetrate the brain and exert their pharmacodynamic effect (29). Therefore, patients with advanced ESCC and brain metastasis, even without symptoms, should receive RT as early as possible to activate the immune system. A clinical trial with a larger sample size is warranted.

Elevated SII denotes increased neutrophil and platelet counts and/or lymphocytopenia. Neutrophils have been shown to accelerate tumorigenesis by releasing genotoxic DNA substances, stimulating tumor cell proliferation by secreting PGE2, activating tumor angiogenesis by releasing Bv8 and matrix metalloproteinase 9 (MMP9), and promoting tumor cell migration, invasion, and extracellular matrix degradation (30). Tumor-activated platelets contribute greatly to tumor progression, metastasis, and immunosuppression via C-type lectin-like immune receptor 2 (CLEC- 2) (31). Contrary to neutrophils and platelets, T-lymphocytes have been shown to inhibit tumor proliferation and metastasis, induce cytotoxic cell death, and foster antitumor immune responses (32). In recent years, a growing number of studies have demonstrated the predictive value of SII in patients with ESCC undergoing surgery and neoadjuvant therapy (33–36). However, limited studies are available that use delta-SII to predict the efficacy for any tumor. In locally advanced non-squamous NSCLC, Biswas et al. observed that SII is an informative mid-treatment marker of overall survival and progression-free survival (37). Wang et al. found that the pre-/post-RT SII ratio and mid-RT SII ratio were potentially effective markers for predicting ESCC prognosis (33). Similarly, in this study, an elevated delta-SII, referring to the changes in SII before and during RT, indicated worse short-term efficacy in patients with advanced ESCC who received radioimmunotherapy. The AUC of the delta-SII was maximal in these four indicators, suggesting delta-SII has the best predictive value for short-term efficacy in this population.

Of the enrolled 121 patients, six received SBRT with a DCR of 83.8%, including four SD and one PR patients; five of them received HFRT with a DCR of 80.0%, including three SD and one PR patients, while the DCR in patients who received CFRT was 74.5%. Better efficacy was observed in patients who underwent SBRT and HFRT. Limited by the small sample size, more convincing research with a larger population is necessary to clarify the currently unclear mechanism. Regrettably, we did not find any significant association between short-term efficacy and different irradiation sites in this study. A possible explanation might be that metastases of EC mainly involve lymph nodes, while bone, liver, and brain metastases are relatively few.

Our study had several limitations. First, because of its retrospective nature, it was inevitably affected by loss of data, clinical bias of treatment choice, and unavailability of fresh serum specimens, which leads to a lack of molecular research. Moreover, the follow-up of our study was not long enough, as this study mainly explored the relationships between characteristics and short-term efficacy, not survival. Third, chemotherapy regimens were not recorded in this study, and different systemic regimens among the irradiation groups may have affected therapeutic efficacy. Therefore, a prospective study with a large sample size, more detailed RT parameters, such as dose, fraction, and sequencing of radiation combined with immunotherapy, and more molecular-biological index is needed to investigate the mechanism and optimal combination therapy strategy in advanced ESCC.

In conclusion, for patients with advanced ESCC receiving radioimmunotherapy, brain irradiation may trigger stronger immune activation than that of extracranial organs. Delta-SII and line of ICIs have predictive value for short-term efficacy, which may provide guidance for individualized treatment regimens.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Institutional review board of Shandong Cancer Hospital and Institute. Written informed consent for participation was not required for this study in accordance with the national legislation and the institutional requirements.

XM and XH designed the study and provided funding support. ML, GC and ZG collected and analyzed the data of the clinical trial. ML drafted the manuscript. XM and XH reviewed and edited the manuscript. All authors contributed to the article and approved the submitted version.