Identification of host-intrinsic resistance mechanisms to immune checkpoint inhibitors (ICI) in Diversity Outbred mice

The advent of immune checkpoint inhibitors (ICI) for cancer therapy has improved outcomes for a variety of malignancies, however many a significant proportion of patients fail to benefit. While tumor-intrinsic mechanisms are likely involved in therapy resistance, it is unclear to what extent genetic heterogeneity of the host plays in response. To investigate this in a pre-clinical setting, we utilize the Diversity Outbred (DO) and Collaborative Cross (CC) mouse models. DO mice are genetically distinct animals generated by cross-breeding 8 inbred founder strains, and CC mice are recombinant inbred mice generated from the same 8 founders as DO. We find a wide variation in response to anti-PD-1/anti-CTLA-4 ICI in B16F0-bearing (C57BL/6xDO)F1 mice (n=142) with ~30% showing partial response with delayed tumor onset and ~13% showing complete tumor elimination with protection against subsequent contralateral rechallenge. Genetic linkage analysis using the R/qtl2 package identifies genomic loci associated with therapeutic outcomes. Importantly, the MHC locus was not significantly associated with response. Of the strongly associated loci, the region within chromosome 13 was further validated in CC mice bearing the positive (NZO) versus negative (C57BL/6) driver genotypes at this locus. ICI response was detected in 2/3 (C57BL/6xCC)F1 predicted responder models, with no response in predicted non-responders. This locus solely contains the murine prolactin family. Prolactin is a known immunomodulating cytokine with association to various autoimmune disorders. To directly test whether prolactin influences ICI response rates, we implanted inbred C57BL/6 mice, which notoriously fail to respond to ICI alone against B16F0 tumors, with subcutaneous slow-release prolactin pellets to induce mild hyperprolactinemia. Combined ICI with prolactin shows an interaction effect against B16F0, with 5/8 mice exhibiting slowed tumor growth relative to controls. To test whether this host-derived phenomenon applies to additional tumor models, we tested ICI response to Lewis Lung Carcinoma (LLC) in a (C57BL/6xCC)F1 model where response to B16F0 was observed. Relative to inbred C57BL/6 mice, where no response to ICI is observed, this (C57BL/6xCC)F1 model shows a similar response to ICI against LLC as was detected for B16F0. Collectively, this study shows that host genetics play a role in ICI response and that prolactin may be a positive regulator of ICI efficacy.Citation Format: Justin Hackett, James Glassbrook, Maria Muniz, Heather M. Gibson. Identification of host-intrinsic resistance mechanisms to immune checkpoint inhibitors (ICI) in Diversity Outbred mice [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2021 Oct 5-6. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(1 Suppl):Abstract nr P060.