Host genetic background affects response to immune checkpoint inhibitors in a B16 melanoma model using Diversity Outbred mice

Abstract Historically metastatic melanoma has been associated with poor prognosis. The advent of immune checkpoint inhibitors targeting CTLA-4 and PD-1 have greatly improved patient survival in metastatic melanoma, but a significant proportion of patients fail to respond to therapy. To explore the effect of genetic background on immune checkpoint inhibitor efficacy, we have developed a tumor immunotherapy model using genetically heterogeneous Diversity Outbred (DO) mice and the C57BL/6 syngeneic B16F0 melanoma model. To implement genetic diversity, DO mice were crossed with C57BL/6 to generate (C57BL/6xDO)F1 mice. Untreated (C57BL/6xDO)F1 mice (n=17) reliably develop B16F0 tumors after subcutaneous inoculation, with some variation in tumor onset (8.2 +/− 4.1 days, CV 49.9%). In a pilot immunotherapy study, 20 (C57BL/6xDO)F1 mice were treated with anti-PD-1/anti-CTLA4 on days 12, 14, and 16, after tumors were palpable (mean onset 6.8 +/− 4.6 days, CV 68.2%). Tumor tissue was collected on day 19 and CD8 T cell infiltration was analyzed by IHC and qPCR. Mice with reduced tumor growth showed an increase presence of CD8+ cells and IFN-g expression. To further test the role of genetics in response, a second cohort of C57BL/6xDO)F1 mice (n=95) was treated with combined anti-PD1/anti-CTLA-4 on days 3, 6, and 10 after B16F0 inoculation. In this larger trial, mice receiving therapy show wide variation in tumor onset up to 61 days (mean 13.6 +/− 8.9, CV 65.2%), with 10 never developing tumor by day 88. This data further suggests that the genetic background significantly influences response to treatment and validates further testing of this model and subsequent Quantitative Trait Locus analysis. Supported by NIH R37 CA220482.