LAUSR

There is strong evidence that immunotherapy-mediated tumor rejection can be driven by the reinvigoration of tumor-specific CD8+ T cells recognizing neoantigens derived from tumor somatic mutations (citations). Thus, it is possible that the relative abundance and/or characteristics of these tumor-reactive, mutation-specific CD8+ T cells can be used as predictive biomarkers of response to immunotherapy. However, only a fraction of these potential neoantigens are usually immunogenic and in addition, these tumor-reactive, mutation-specific CD8+ cells are present only at low frequencies in blood, making it difficult to reliably identify these effector cells. Here, mass cytometry and highly-multiplexed combinatorial tetramer staining together with cellular barcoding was used to profile immune cells in longitudinally collected PBMCs from 14 non-small cell lung cancer (NSCLC) patients treated with anti-PD-L1 (atezolizumab) antibody to compare patients with objective response (n=8) and progressive disease (n=6). Although no significant phenotypic differences were detected in bulk CD8+ T cells, greater insight was gained from a parallel analysis using highly multiplexed peptide-MHC multimer staining to screen and profile antigen-specific T cells. A longitudinal analysis was performed using peripheral blood CD8+ T cells for 800 candidate tumor neoantigens and 73 known viral-derived control peptides across all patient samples. In addition to virus antigen-specific T cells, a total of 20 different neoantigen-specific T cell populations were detected and their high dimensional profiles were compared. We found that neoantigen-specific T cells were more frequently detected in responding patients and their phenotypes were almost entirely distinct from non-responding patients. Neoantigen-specific T cells from responding patients showed a differentiated effector phenotype with high expression of KLRG1, 2B4 (CD244) and CD57, similar to CD8+ T cells associated with CMV and some types of EBV infection. In contrast, more memory-like phenotypic profiles, with high CD27 and CD127 expression, were observed for neoantigen-specific CD8+ T cells from patients with progressive disease. In addition to the utility of this approach for the ex vivo identification, characterization, and longitudinal tracking of rare tumor-specific T cells, this study supports further research into assessing whether the presence of late-differentiated neoantigen-specific T cells could be used as a predictor of response to checkpoint blockade. Citation Format: Mahesh Yadav, Michael Fehlings, Suchit Jhunjhunwala, Bill O9Gorman, Priti Hegde, Leesun Kim, Alessandra Nardin, Susan Flynn, Hermi Sumatoh, Marcus Ballinger, David Shames, Boon Heng Lee, Evan Newell. Late-differentiated effector neoantigen-specific CD8+ T cells are enriched in non-small cell lung cancer patients responding to atezolizumab treatment [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 4055.