Abstract 5678: Developing a preclinical immunotherapy platform using syngeneic mouse models of human melanoma

Melanomais an aggressive and lethal disease with no efficacious therapies for a broad subset of late stage patients. Current immunotherapies, including immune checkpointblockade (ICB), may prolong survival in certain patients, but generate responses in less than 40% of the treated cohort. This demands a better understanding of molecular mechanisms underlying the lack of response and acquired resistance to ICB. However, functional studies are limited in patients, and current preclinical studies are handicapped by the absence of appropriate mouse models that recapitulate the pathological and immunological diversity of human melanomas. Here we develop four syngeneic melanomamouse models with human-relevant genetic modifications and carcinogenic agents, which we hypothesize will mirror the spectrum of responses to ICB and offer a platform for future mechanistic studies in melanoma. The models are: 1) neonatal ultraviolet radiation (UV)-induced melanomain a HGF-transgenic mouse, in which melanocyte localization at the epidermal-dermal junction mimics human distribution (HU); 2) 7,12-Dimethylbenz(a)anthracene ( DMBA)-induced melanomain a HGF-tg and Cdk4R24C mouse (HC4D); 3) UV-induced melanomain a BrafCA/+; HGF-tg; Cdkn2aflox/+; Tyr-CreERT2-t mouse (BHCU); and 4) UV-induced melanomain a BrafCA/+; Ptenflox/+; Cdkn2aflox/+; Tyr-CreERT2-tg mouse (BPCU). Exome sequencing of the four models reveals a high correlation with mutational subtypes previously described in human melanoma. BPCU and BHCU represent different Braf mutant patient populations and HU and HC4D represent triple wildtype melanoma(non-BRAF, -NRAS, -NF1). The four mouse models demonstrate distinct responses to ICB with anti- CTLA-4treatment. While HU and HC4D melanomasshow high or partial sensitivity to anti- CTLA-4, respectively, BPCU and BHCU do not respond to treatment. In vivo vaccination assays demonstrate that the anti- CTLA-4response in our models is linked to increased tumor immunogenicity. However, the number of non-synonymous mutations and antigen presentation functionality do not correlate with ICB efficacy. Tumor infiltration by T cells was assessed by CD3 and FoxP3 immunostaining and gene expression analysis. Although clear differential gene expression profiles are noted among the four models and in those tumors responding to the treatment, we unexpectedly found that “hot” melanomas(e.g., showing upregulation of inflammatory pathways and high T-cell infiltration) do not necessarily predict ICB efficacy. These results suggest that additional mechanisms could help determine the response or intrinsic resistance to anti- CTLA-4and open new avenues for future research and treatment. Overall, our study offers four genetically and phenotypically distinct mouse models representing diverse human melanomasubtypes as powerful tools for the mechanistic study of the response to immunotherapies in melanoma. Citation Format: Corinne Rauck, Eva Perez-Guijarro, Zoe W. Ohler, Rajaa El E. Meskini, Howard Yang, Suman Vodnala, Cari Graff-Cherry, Sung Chin, Anyen Fon, Helen Michael, Maxwell Lee, Terry Van Dyke, Shyam Sharan, Glenn Merlino, Chi-Ping Day. Developing a preclinical immunotherapy platform using syngeneic mouse models of human melanoma[abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5678.