Subgroup-specific predictive biomarkers in peripheral blood lymphocyte subsets for immune checkpoint inhibitor response in advanced esophageal squamous cell carcinoma: a prospective study

Background: Immune checkpoint inhibitors (ICIs) significantly improve the survival of patients with advanced esophageal squamous cell carcinoma (ESCC). However, reliable biomarkers for predicting treatment response and prognosis have not yet been identified. This study investigated the relationship between specific lymphocytes in peripheral blood prior to treatment and the efficacy of ICIs, as well as prognosis, in patients with advanced ESCC. Methods: Peripheral blood samples were prospectively collected from 97 patients with stage III–IVB ESCC before ICI treatment. Flow cytometry was used to detect and quantify peripheral blood lymphocyte subsets, including natural killer (NK) cells, B cells, T cells, CD4+ T cells, and CD8+ T cells, along with the CD4+/CD8+ T cells ratio. Treatment response was classified according to the Response Evaluation Criteria in Solid Tumors (RECIST, version 1.1). The patients with a complete response (CR) or partial response (PR) were allocated to the response group, while those with stable disease (SD) or progressive disease (PD) were allocated to the non-response group. The Chi-squared test and U test were used to assess the qualitative and quantitative differences between groups, and receiver operating characteristic (ROC) curves were used to evaluate the predictive efficacy of lymphocyte subpopulations for ICI response. Kaplan-Meier and log-rank tests were used to compare disease-free survival among the groups. Results: Of the 97 patients, 42.3% (41/97) responded to ICI treatment. The baseline characteristics, including age, gender, and ICI drugs, were balanced between the response and non-response groups. The patients in the response group exhibited significantly higher baseline CD4+ T-cell counts (42.60 vs. 34.45, P=0.001) and CD4+/CD8+ ratios (2.02 vs. 1.54, P=0.01). The area under the curve (AUC) for the CD4+ T-cell count and the CD4+/CD8+ ratio were 0.701 and 0.648, respectively. Stratified analyses revealed variations in predictive efficacy based on staging and ICI drugs. In the stage III patients, higher AUC values were observed (0.883 for the CD4+ T-cell count and 0.833 for the CD4+/CD8+ ratio). Conversely, in the stage IV patients, a correlation was only observed between low B-cell counts and an improved response (6.40 vs. 8.65, P=0.02), with an AUC of only 0.510. Higher CD4+ T-cell counts were associated with improved responses in the patients receiving anti-programmed death-ligand 1 (PD-L1) therapy (40.00 vs. 30.30, P=0.03), with an AUC of 0.806. The results of the patients who received anti-programmed cell death protein 1 (PD-1) therapy were consistent with those of the overall population, but the corresponding AUC values were lower (0.693 and 0.638, respectively). The prognostic analysis revealed that lower NK cell counts before treatment were correlated with longer disease-free survival. Conclusions: In patients with advanced ESCC, specific lymphocyte subsets in peripheral blood before treatment are closely associated with ICI efficacy and patient prognosis, and thus could potentially guide treatment decisions.