Necroptosis in Esophageal Squamous Cell Carcinoma: An Independent Prognostic Factor and Its Correlation with Tumor-Infiltrating Lymphocytes

SIMPLE SUMMARY: Necroptosis is a regulated form of necrotic cell death that plays pivotal roles in cancer biology, including tumorigenesis, metastasis, and cancer immunity. However, the significance of necroptosis in esophageal squamous cell carcinoma has remained largely unknown. In addition, its correlation with the tissue microenvironment has not yet been explored. In this study, we first investigated the diagnostic and prognostic significance of mixed lineage kinase domain-like protein (MLKL) and phosphorylated MLKL (pMLKL), both of which are currently considered the most reliable markers for detecting necroptosis. We also investigated the correlations between the status of MLKL/pMLKL and tumor-infiltrating lymphocytes) in esophageal squamous cell carcinoma patients. ABSTRACT: Necroptosis is a pivotal process in cancer biology; however, the clinical significance of necroptosis in esophageal squamous cell carcinoma (ESCC) has remained unknown. Therefore, in this study, we aimed to verify the potential involvement of necroptosis in the clinical outcome, chemotherapeutic resistance, and tumor microenvironment of ESCC. Mixed lineage kinase domain-like protein (MLKL) and phosphorylated MLKL (pMLKL) were immunohistochemically examined in 88 surgically resected specimens following neoadjuvant chemotherapy (NAC) and 53 pre-therapeutic biopsy specimens, respectively. Tumor-infiltrating lymphocytes (TILs) were also evaluated by immunolocalizing CD3, CD8, and forkhead box protein 3 (FOXP3) in the residual tumors after NAC. High pMLKL status in the post-NAC resected specimens was significantly correlated with worse prognosis in ESCC patients. Multivariate analysis demonstrated that a high pMLKL status was an independent prognostic factor. In pre-NAC biopsy specimens, a high pMLKL status was significantly associated with a lower therapeutic efficacy. CD8+ TILs were significantly lower in the high-pMLKL group. FOXP3+ TILs were significantly higher in both high-MLKL and high-pMLKL groups. We first demonstrated pMLKL status as an independent prognostic factor in ESCC patients. Our study revealed the possible involvement of necroptosis in the immunosuppressive microenvironment, resulting in the attenuated therapeutic efficacy of NAC and eventual adverse clinical outcomes in ESCC.