Abstract 1424: Predicting response to immune checkpoint therapy using an mutation burden threshold

Treatment with antibodies to PD-1 or CTLA-4 leads to prolonged response in some cancer patients. Patients whose tumors have a high mutational burden have a better response to anti-CTLA-4 treatment with ipilimumab in melanoma5, and to anti-PD-1 treatment with pembrolizumab in non-small cell lung6 and colorectal cancer7. These results suggest that a sufficiently high non-synonymous somatic mutation burden in the tumor may lead to protein alterations that serve as neo-antigens to stimulate CD8 T-cell immune response. This immune response may be blocked by the tumor by engaging the immune checkpoint pathway, making such tumors especially vulnerable to immune checkpoint disruption. However, the specific criteria to identify patients likely to respond to immune checkpoint therapy have remained elusive so far. Using somatic mutation and RNA-Seq data from TCGA, we propose that there exists a mutational threshold, which we call the “Activated Immune Mutational” (AIM) Threshold, which can identify patients likely to respond to immune checkpoint therapy. Compared to AIM- patients (potential non-responders), AIM+ patients (potential responders) have tumors that meet four criteria: (1) a high non-synonymous mutation burden; (2) a high level of immune infiltration in the tumor; (3) a high CD8 T cell fraction in the leukocyte component of the immune infiltrate plus high expression of the T cell marker CD8A; and (4) a high expression of immune checkpoint genes PD-1, PD-L1, PD-L2, CTLA-4. We find that in melanoma, endometrial, colon, cervical and breast cancer, a clear AIM threshold satisfying all four criteria exists. The distribution of mutations in AIM- versus AIM+ tumors for these cancers is very different. In the former, there are high frequency somatic mutations in only a few genes, while in the latter, high frequency somatic mutations are found in many genes distributed over the whole genome. Pathological analysis of high-resolution images from 150 TCGA tumors (30 from each of the five tumor types) validated the presence of significantly higher lymphocytic infiltration in the AIM+ tumors compared to AIM- tumors. We show that AIM+ tumors can be identified using sequencing assays which are currently in clinical use. Finally, survival analysis in melanoma patients from TCGA treated with immunotherapy will be presented. Citation Format: Anshuman Panda, Anil Betigeri, Kalyanasundaram Subramanian, Kim Hirshfield, Lorna Rodriguez, Shridar Ganesan, Gyan Bhanot. Predicting response to immune checkpoint therapy using an mutation burden threshold. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1424.