DNA mismatch repair-deficient (MMR-d) cancers present an abundance of neoantigens that is thought to explain their exceptional responsiveness to immune checkpoint blockade (ICB)(1,2). Here, in contrast to other cancer types(3-5), we observed that 20 out of 21 (95%) MMR-d cancers with genomic inactivation of beta 2-microglobulin (encoded by B2M) retained responsiveness to ICB, suggesting the involvement of immune effector cells other than CD8(+) T cells in this context. We next identified a strong association between B2M inactivation and increased infiltration by gamma delta T cells in MMR-d cancers. These gamma delta T cells mainly comprised the V delta 1 and V delta 3 subsets, and expressed high levels of PD-1, other activation markers, including cytotoxic molecules, and a broad repertoire of killer-cell immunoglobulin-like receptors. In vitro, PD-1(+) gamma delta T cells that were isolated from MMR-d colon cancers exhibited enhanced reactivity to human leukocyte antigen (HLA)-class-I-negative MMR-d colon cancer cell lines and B2M-knockout patient-derived tumour organoids compared with antigen-presentation-proficient cells. By comparing paired tumour samples from patients with MMR-d colon cancer that were obtained before and after dual PD-1 and CTLA-4 blockade, we found that immune checkpoint blockade substantially increased the frequency of gamma delta T cells in B2M-deficient cancers. Taken together, these data indicate that gamma delta T cells contribute to the response to immune checkpoint blockade in patients with HLA-class-I-negative MMR-d colon cancers, and underline the potential of gamma delta T cells in cancer immunotherapy.
