Aim
Gastric cancer (GC) remains the third leading cause of cancer‑related death worldwide. Unfortunately, only a subset of GC patients, characterised by tumours with high microsatellite instability (MSI), responds to immune checkpoint (ICI) therapy. In the context of GC, the mechanisms of how MSI improves ICI therapy responses remain poorly understood. 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.
Methods
To study the functional and mechanistic effects of loss of MLH1 protein, we established MLH1-deficient (Kras-, Pi3kca-, Tp53-mutant) murine tumour organoids via CRISPR/Cas9 technology. These organoids were subcutaneously allografted into immunocompetent C57BL/6 mice to study tumour progression, immune surveillance, and responses to immunotherapy in vivo.
Results
MSI testing confirmed that MLH1-proficient parental organoids are MSI low, whereas MLH1-deficient organoids are MSI high. MLH1-deficient tumours were considerably smaller compared to MLH1-proficient tumours. MLH1‑deficient tumours showed a significantly higher mutational burden, predicted neoantigen load, and number of infiltrating CD8+ T cells. Additionally, MLH1-deficient tumours were similar in size to MLH1-proficient tumours when allografted into Rag1-/- mice. 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.
Conclusion
Taken together, we provide evidence that loss of MLH1 leads to high MSI in gastric tumours, reduces tumour growth and increases response to ICI 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.