Immune microenvironment and treatment response of dMMR gastric cancer: Heterogeneity and implications for immunotherapy

Deficient mismatch repair (dMMR) in gastric cancer is associated with a high tumor mutation burden (TMB) and increased neoantigen load, which may enhance sensitivity to immunotherapy. Nevertheless, heterogeneous responses among patients with dMMR suggest complex tumor immune microenvironment (TIME) regulation. The present study aimed to integrate a case of dMMR gastric cancer with multi-cohort bioinformatic analyses to assess TIME determinants of immunotherapy outcomes, focusing on hematological immune cell subsets. Pre- and post-immunotherapy TIME changes were evaluated in a patient with dMMR gastric cancer using immunohistochemistry analysis [CD8⁺, CD4⁺, forkhead box P3 (FoxP3)⁺ regulatory T cells (Tregs), M1/M2 macrophages and programmed death-ligand 1 (PD-L1)]. Moreover, bioinformatics analyses assessed microsatellite instability (MSI) status, immune checkpoints [cytotoxic T-lymphocyte-associated protein 4 (CTLA4), CD274 and programmed cell death protein 1 (PDCD1)], TMB and TIME cell infiltration across The Cancer Genome Atlas (TCGA)-stomach adenocarcinoma (STAD; n=383), Kim (n=45), Peking University Cancer Hospital (n=31) and Esophagogastric Cancer-Memorial Sloan Kettering-2017 (EGC-MSK-2017; n=28) cohorts using CIBERSORT (LM22) and R-based statistical approaches. The case report demonstrated that, prior to treatment, the tumor was deemed 'immune cold', characterized by low levels of CD8⁺ T cells, M1 macrophages and Tregs. Post-treatment, total T cell infiltration increased, driven primarily by CD4⁺ T cells, while CD8⁺ T cell levels remained low. FoxP3⁺ T cell density (0.18 cells/mm²) indicated potential immune suppression; B cell infiltration (CD20⁺, 17.07 cells/mm²) did not markedly increase, with no tertiary lymphoid structures (TLS) detected. Moreover, bioinformatics cross-cohort analyses revealed favorable immunological features in microsatellite instability (MSI)-high (MSI-H) gastric cancer compared with in MSI-low (MSI-L)/microsatellite stability (MSS) gastric cancer. In TCGA-STAD dataset, MSI-H tumors had significantly higher TMB, neoantigen load, infiltration of memory-activated CD4⁺ T cells, activated natural killer (NK) cells, mast cells, M1 macrophages and immune checkpoint markers (CTLA4, CD274 and PDCD1; all P<0.05); CD8⁺ T cells differed significantly between MSI-H and MSI-L (P<0.05), but not between MSI-H and MSS cancer. MSI-H was associated with a higher TMB in the Kim and EGC-MSK-2017 cohorts, a higher immunotherapy response rate in the Kim cohort (P<0.05) and trends toward improved OS in TCGA-STAD [hazard ratio (HR), 0.68] and EGC-MSK-2017 (HR, 0.42) cohorts. All survival differences were non-significant (P>0.05). In conclusion, the results demonstrated that in dMMR/MSI-H tumors (which typically exhibit high TMB and neoantigen load), a TIME characterized by low CD8⁺ T cells, high Tregs, absent TLS and low PD-L1, could be indicative of immunotherapy resistance. Cross-cohort analysis further revealed that despite favorable immunological features (higher TMB, neoantigen load, memory-activated CD4⁺ T cells, activated NK cells, mast cells, M1 macrophages and immune checkpoint expression), MSI-H gastric cancers have heterogeneous immunotherapy outcomes. Collectively, dMMR/MSI-H status alone is insufficient for outcome prediction, highlighting the need for individualized TIME evaluation in patients with gastric cancer receiving immunotherapy.