Gastric cancer (GC) remains the third leading cause of cancer‑related death worldwide. Unfortunately, only a subset of GC patients, characterized by tumours with high microsatellite instability (MSI), responds to immune checkpoint (ICI) therapy. Here we are undertaking a proof of principle study to demonstrate in novel GC mouse models that loss of mismatch repair protein MLH1 confers MSI phenotype and impairs tumour growth via altered anti-tumour immune responses. To study the functional and mechanistic effects of MLH1 protein loss, we established MLH1-deficient murine tumour organoids via CRISPR/Cas9 technology and subcutaneously allografted these organoids into immunocompetent mice to study tumour progression, immune surveillance, and responses to immunotherapy. Low passage MLH1-deficient organoid tumours grew similarly to MLH1-proficient tumours. However, after culturing organoids in vitro for a prolonged time prior to injection, subcutaneous allograft MLH1-deficient tumours were considerably smaller compared to MLH1-proficient tumours. MLH1‑deficient tumours showed a significantly higher number of CD8+ T cells. Interestingly, CD8- and CD4-depleting experiments showed an involvement of CD4+ T cells in the impairment of MLH1-deficient tumour growth. Treatment with anti-PD-1 reduced tumour mass further. We are currently investigating the underlying mechanism for the impaired growth of MLH1‑deficient organoid allograft tumours via tumour mutational burden analysis and neoantigen testing. Taken together, we provide evidence that loss of MLH1 reduces tumour growth and increases response to anti-PD-1 therapy. Our findings encourage further studies to investigate the mechanisms of impaired tumour growth after MLH1 loss in GC and may provide insights leading to improve ICI therapy responses for GC patients.