Abstract 1076: Neoantigens retention by patient derived xenograft model mediate autologous T cells activation in ovarian cancer

Ovarian cancer (OC) is the fifth leading cause of cancer death in the US, presenting a low mutational burden and a diverse degree of infiltrating T cells. Neoantigens derived from somatic mutations represent an attractive immunotherapeutic target, however, mouse models for the development of personalized immunotherapies are still poor and do not fully recapitulate the individualized nature of OC in patients. To address this hurdle, our study established a patient-derived xenograft (PDX) of an OC patient as segue to studying neoantigen-driven autologous T cell response. We first evaluated tumor mutations in the primary tumor (Pr) and two successive passages (P0, P1) via WES. As tumors from P0 and P1 have a higher mutational count than Pr, we hypothesize that this difference is caused by the loss of immune-surveillance in immunocompromised mice, which allows for the outgrowth of previously repressed clones. By using the variant allele frequency (VAF) to cluster 123 shared mutations, we identified three clusters with unique VAF dynamics (Fig 1A-D). Tumors from Pr, P0 and P1 had a conserved functional with a strong conservation of antigen presenting pathways (Fig 2A-E). In PDX P0 we identified 184 non-synonymous mutations, leading to 30 potential neoantigens with high affinity for the patient’s HLAs (Fig 3A-B). Neoantigens were ranked based on differential predicted HLA affinity of the WT versus mutated peptide (Fig 3C). T cell activation by neoantigens was tested in vitro via ELISA and flow cytometry. Interferon-γ production and upregulation of CD137 identified a core set of six neoantigens selectively recognized by patient’s autologous CD8+ T cells (Fig 4A-B). Of those, 3/6 neoantigens were common between PDX and primary tumor, corroborating the role of the patient’s own immune system in repressing the expansion of selected tumor clones (Fig 5). In vivo ACT studies showed that mice injected with neoantigen-stimulated autologous PBMCs (ACT_MUT) have reduced tumor growth when compared to mice injected with unstimulated cells (ACT_NP) (Fig 6A). ACT_MUT mice have higher levels of circulating T cells 15 days post-ACT and higher intratumoral T cells at end point than ACT_NP (Fig 6 B-C). We then sought to identify the TCR moieties that determine T cell response. Single cell TCRSeq analyses on the two strongest neoantigens identified multiple TCR activated by a single cancer neoantigen (Fig 7A-B), suggesting oligoclonal T cell activation. We tested this hypothesis by generating a motif with the most expanded clones and comparing it with the motif from clones that did not expend. Results indicate a significant difference in the frequency of amino acid in multiple CDR3 locations, suggesting the presence of an oligoclonal response to neoantigenic T cell stimulation. In conclusion, we have successfully established PDX models of OC that can be used to study and predict autologous T cell response to neoantigens. Citation Format: Muzamil Y. Want, Anna Konstorum, Ruea-Yea Huang, Richard Koya, Sebastiano Battaglia. Neoantigens retention by patient derived xenograft model mediate autologous T cells activation in ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1076.