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The evidence for T-cell–mediated regression of human cancers such as non–small-cell lung carcinoma, renal cell carcinoma, and—in particular—melanoma after immunotherapy is strong. Anti-CTLA4 (ipilimumab) treatment has been approved for treatment of meta-static melanoma,1 and antibody-mediated blockade of PD-1, a second inhibitory receptor on T cells, has shown highly encouraging results in early clinical trials.2,3 Although the clinical activity of these treatments is apparent, it is still unknown which T-cell reactivities are involved in immunotherapy-induced cancer regression.4 T-cell reactivity against nonmutated tumor-associated self-antigens has been analyzed in patients treated with ipilimumab or with autologous tumor-infiltrating T cells, but the magnitude of the T-cell responses observed has been relatively modest.5,6 In part on the basis of such data, recognition of patient-specific mutant epitopes (hereafter referred to as neoantigens) has been suggested to be a potentially important component.7 A potential involvement of mutated epitopes in T-cell control would also fit well with the observation that the mutation load in sun-exposed melanomas is particularly high.8-10 Intriguingly, on the basis of animal model data, it has recently been suggested that (therapy-induced) analysis of T-cell reactivity against patient-specific neoantigens may be feasible through exploitation of cancer genome data.11,12 However, human data have thus far been lacking. Here we report a case of a patient with stage IV melanoma who exhibited a clinical response to ipilimumab treatment. Cancer exome–guided analysis of T-cell reactivity in this patient revealed reactivity against two neoantigens, including a dominant T-cell response against a mutant epitope of the ATR (ataxia telangiectasia and Rad3 related) gene product that increased strongly after ipilimumab treatment. These data provide the first demonstration (to our knowledge) of cancer exome–guided analysis to dissect the effects of melanoma immunotherapy.

9085 Background: Evidence for T cell mediated regression of human cancer in particular melanoma following immunotherapy is strong. Anti-CTLA4 treatment has been approved for treatment of metastatic melanoma and blockade of PD-1 has shown encouraging results. However, it is unknown which T cell reactivities are involved in cancer regression. Reactivity against non-mutated tumor self-antigens has been analyzed in patients treated with Ipilimumab or with autologous TILs, but the size of these responses are modest. Therefore, T cell recognition of patient-specific mutant epitopes may be a potentially important component. Animal model data recently suggested that analysis of T cell reactivity against patient-specific neo-antigens may be feasible through exploitation of cancer genome data. However, human data have thus far been lacking. Methods: To address this we have used MHC class I peptide exchange technology allowing production of very large collections of pMHC complexes, together with a pMHC "combinatorial coding" strategy for parallel detection of dozens of different T cell populations within a single sample. Results: From a melanoma patient responding to ipilimumab treatment, we identified tumor specific mutations via exome sequencing of tumor material. The exome contained 1,075 non-synonymous mutations. Possible MHC epitopes covering these mutations were predicted based on; 1) predicted to bind the patient’s MHC; 2) predicted to be cleaved by the proteasome; 3) genes of which the mutated peptides arose had evidence of RNA expression. The analysis yielded 1,952 epitopes restricted to the HLA-A and HLA-B. To screen for T cell reactivity against these epitopes we used the pMHC combinatorial coding approach. We found T cell reactivity against 2 neo-antigens, including a dominant T cell response against a mutant epitope of the ATR gene product. Analysis of PBMC samples collected before and during Ipilimumab therapy showed that this particular response increased strongly after treatment from 0.06% to 0.28% of CD8 T cells after being stable in magnitude for 10 months. Conclusions: These data provide the first demonstration of cancer exome-guided analysis to dissect the effects of melanoma immunotherapy.