To explore the effects of sintilimab combined with chemotherapy on postoperative peripheral blood immune cell counts, inflammatory markers, and prognosis in patients with stage Ⅱ~Ⅲ colorectal cancer.

A retrospective analysis was conducted on 35 patients with stage Ⅱ~Ⅲ colorectal cancer admitted to the Department of Gastrointestinal Surgery, Beijing Shijitan Hospital, Capital Medical University, between January 2021 and December 2023. According to postoperative adjuvant treatment regimens, patients were divided into the observation group (n=17, sintilimab combined with XELOX/FOLFOX regimen) and the control group (n=18, XELOX/FOLFOX chemotherapy alone). Changes in neutrophil-to-lymphocyte ratio (NLR), monocyte-to-lymphocyte ratio (MLR), lymphocyte-to-monocyte ratio (LMR), and leukocyte subset counts were compared between the two groups before and after treatment. Kaplan-Meier method and Cox proportional hazards regression model were used to analyze progression-free survival (PFS) and prognostic factors in both groups.

In the observation group, lymphocyte count (1.40±0.63 vs 1.38±0.52, P=0.007) and eosinophil count (0.34±0.40 vs 0.16±0.18, P=0.004) were significantly increased after treatment compared with baseline. In the control group, monocyte count increased (0.36±0.13 vs 0.35±0.09, P=0.011), while eosinophil count decreased (0.12±0.07 vs 0.13±0.11, P=0.031) after treatment. Intergroup comparison showed that post-treatment monocyte count (0.49±0.22 vs 0.35±0.12, P=0.039) and eosinophil count (0.33±0.39 vs 0.12±0.06, P=0.030) in the observation group were significantly higher than those in the control group. Survival analysis showed that the 3-year PFS rate was significantly higher in the observation group than in the control group (88.2% vs 44.4%, P<0.05). Multivariate Cox proportional hazards regression analysis indicated that treatment regimen was an independent protective factor for prognosis (HR=0.038, 95%CI: 0.003~0.444, P=0.009), and sintilimab combined with chemotherapy significantly reduced the risk of disease progression.

Sintilimab combined with chemotherapy significantly improved postoperative peripheral blood lymphocyte and eosinophil levels, modulated tumor microenvironment immune status, and markedly increased 3-year progression-free survival. Peripheral blood immune cell counts represent potential biomarkers for monitoring immunotherapy response, though their long-term clinical implications and optimal patient selection warrant further investigation in prospective, large-scale studies.