Immunotherapy for Colorectal Cancer with Mismatch Repair Deficiency: What MSI Testing Means for You

Research-informed explainer — last updated April 12, 2026

For the roughly 15% of colorectal cancer patients whose tumors have mismatch repair deficiency (dMMR) or high microsatellite instability (MSI-H), immunotherapy has transformed both early-stage and metastatic disease — including a landmark 2022 trial in which every single patient with locally advanced rectal cancer achieved complete clinical remission with PD-1 blockade alone, potentially avoiding surgery altogether. For the remaining 85% with proficient mismatch repair tumors, immunotherapy has largely failed — making MSI testing one of the most consequential tests a colorectal cancer patient can receive.

The foundational science and clinical evidence for this comes from researchers across leading institutions. The 2015 New England Journal of Medicine trial establishing that dMMR tumors respond to PD-1 blockade — the paper that earned the lead author a Nobel Prize nomination and resulted in the first FDA approval of a cancer drug based on a molecular biomarker rather than tumor type — was co-authored by investigators associated with Johns Hopkins. Atif Zaheer, Associate Professor of Radiology at Johns Hopkins, contributed to the imaging and biomarker research context for this work (9,310 citations for the PD-1 blockade in MMR-deficient tumors study, and 6,515 citations for the subsequent Science paper confirming MMR deficiency predicts immunotherapy response across 12 cancer types). Andrea Cercek, Section Head of Colorectal Cancer and Co-Director of the Center for Young Onset Colorectal and GI Cancer at Memorial Sloan Kettering, is the first author of the extraordinary dostarlimab rectal cancer trial (1,472 citations). Michael Overman, Professor of Gastrointestinal Medical Oncology at MD Anderson Cancer Center, published the CheckMate 142 trial establishing nivolumab as effective in MSI-H metastatic colorectal cancer (2,759 citations) and the nivolumab plus ipilimumab combination trial in this population (2,023 citations).

What mismatch repair deficiency means and why it matters

DNA mismatch repair (MMR) is a cellular quality-control system that fixes errors introduced during DNA replication. Tumors with deficient MMR — caused either by inherited Lynch syndrome mutations or by sporadic silencing of MMR genes — accumulate thousands of small insertions and deletions across the genome. These errors generate an enormous number of abnormal protein fragments (neoantigens) that the immune system can potentially recognize.

MSI testing measures the downstream consequence of dMMR: short repetitive DNA sequences (microsatellites) that become unstable when MMR fails. MSI-H (high microsatellite instability) and dMMR are essentially equivalent findings from different assay methods and are treated identically for treatment purposes.

The critical insight is that dMMR tumors are "immunogenic" — already partially recognized by the immune system — but suppressed by PD-1/PD-L1 checkpoint signaling. Blocking that checkpoint unleashes an immune response that had already begun.

How to get tested: MMR/MSI testing is typically performed by immunohistochemistry (IHC) for MMR proteins on the tumor biopsy or surgical specimen. Most major guidelines recommend universal MMR testing for all colorectal cancer patients. If your pathology report has not mentioned MSI or MMR status, ask your oncologist to verify whether the test was run.

The dostarlimab rectal cancer trial: a result that changed practice

In 2022, Andrea Cercek and colleagues at Memorial Sloan Kettering published a trial in which 12 consecutive patients with dMMR locally advanced rectal cancer received dostarlimab — a PD-1 inhibitor — for approximately 6 months before planned chemoradiotherapy and surgery. All 12 patients achieved a complete clinical response. None of the 12 underwent surgery or chemoradiotherapy. At median follow-up, none had experienced disease recurrence.

The result was unprecedented in oncology. The standard treatment for locally advanced rectal cancer involves combined chemoradiation, surgery (often requiring a permanent colostomy), and adjuvant chemotherapy — a treatment sequence associated with major bowel, urinary, and sexual function effects. The possibility of avoiding all of that with a few months of immunotherapy was immediately recognized as potentially practice-changing.

Important caveats: the trial enrolled only 12 patients at the time of first publication, all with dMMR tumors (15% of rectal cancers). Long-term recurrence data is still maturing. This approach is being studied in larger trials, and outside a clinical trial, patients should discuss whether surveillance without surgery is appropriate at a high-volume center with experience managing this approach.

Metastatic MSI-H colorectal cancer: nivolumab and the combination regimen

For patients with metastatic dMMR/MSI-H colorectal cancer, Michael Overman's CheckMate 142 trial showed that nivolumab produced an objective response rate of 31% and a disease control rate of 69% in heavily pretreated patients, with responses durable at 12 months in 81% of responders. The subsequent combination trial showed nivolumab plus ipilimumab produced an objective response rate of 55% and 12-month overall survival of 85% — making it the preferred first-line option for metastatic MSI-H disease based on updated guidelines.

Pembrolizumab (KEYNOTE-158) has also shown high response rates in MSI-H solid tumors and was the first drug approved by the FDA based on a tumor agnostic biomarker (MMR/MSI status) rather than site of origin — meaning the approval covers any MSI-H solid tumor, not just colorectal cancer.

Why MSI-negative colorectal cancer does not respond to immunotherapy

The majority of colorectal cancers — approximately 85% — are microsatellite stable (MSS). These tumors have functional mismatch repair, fewer neoantigens, and a "cold" tumor immune microenvironment with few infiltrating T cells. Multiple trials of PD-1 inhibitors in MSS colorectal cancer have failed to show meaningful response rates. Research into why KRAS mutations specifically promote immune evasion in MSS colorectal cancer (the KRAS-IRF2 axis, published by Overman and colleagues) is helping explain the biology — and may eventually point toward combination strategies to make cold tumors immunotherapy-responsive.

What happens if immunotherapy stops working in MSI-H disease?

A subset of MSI-H tumors eventually develop resistance to checkpoint blockade. Current active research areas include combining PD-1 inhibitors with anti-angiogenic agents, VEGF inhibitors, MEK inhibitors, and other immune-modulating strategies. If you experience progression on first-line PD-1 blockade, enrollment in a clinical trial should be a high priority.

Questions to ask your doctor

• Has my tumor been tested for MMR/MSI status? If so, what were the results?
• If my tumor is dMMR/MSI-H, is immunotherapy now appropriate as my first treatment, even before chemotherapy?
• For locally advanced rectal cancer, am I eligible for a trial using neoadjuvant immunotherapy to potentially avoid surgery?
• If my tumor is MSS, are there any clinical trials testing immunotherapy combinations that might still benefit me?
• If I have Lynch syndrome or a family history of colorectal cancer, should my family members be tested?

The bottom line

MSI/MMR testing is the single most important biomarker test for colorectal cancer patients — it determines whether immunotherapy is likely to be highly effective or largely futile. Patients with dMMR/MSI-H tumors now have access to first-line immunotherapy regimens that consistently outperform chemotherapy in metastatic disease, and to potentially surgery-sparing neoadjuvant approaches in rectal cancer. Every newly diagnosed colorectal cancer patient should confirm that this test has been performed and that they understand the result.