Interrogation Of Neoantigen-Specific Cd8 T Cells In Peripheral Blood Following Pd-L1 Blockade In Patients With Metastatic Urothelial Carcinoma (Muc).

3075 Background: Proliferation of CD8 T cells can be detected in the blood of cancer patients (pts) following a single dose of immune checkpoint blockade (ICB) and tends to be more robust in responding pts. Furthermore, tumor mutational burden (TMB) is seen to predict outcome to ICB across cancers. Mutation-derived neoepitopes presented on the tumor cell surface is believed to be recognized by T cells and are thus critical for tumor clearance. However, the capacity to mount a neoantigen T cell response and the kinetics in relation to ICB remain poorly understood. Methods: 24 pts with mUC were treated with atezolizumab (anti-PD-L1) 1200mg q3w on IMVigor 210 at MSKCC and included in here. Pt-specific neoepitopes were predicted based on whole-exome and RNA sequencing of pre-treatment archival tumors using the MuPeXI platform. Using DNA-barcode labelled pMHC multimers, we investigated CD8 T cell recognition of mutation-derived neoepitopes by screening pt PBMC samples pre- and post-treatment with atezolizumab (n = 85 PBMC samples). The kinetics of neoepitope-specific CD8 T cells were assessed for association with durable clinical benefit (DCB; defined as progression free survival > 6 mo). Results: Neoepitope peptide libraries of between 200-587 peptides were generated per pt (mean = 260 peptides per pt). 31 out of a combined 56 possible pt HLA types across the cohort were utilized for T cell analyses (mean four HLAs per pt). MHC multimer-based screening of pt PBMCs revealed detection of neoepitope-specific CD8 T cells in 22 of 24 pts pre-treatment (range one to 14 neoepitope responses) and 21 of 22 pts post-treatment (up to 273 weeks after trial start; one to 19 neoepitope responses). There were large inter- and intra-patient variations of neoepitope-specific CD8 T cell responses during treatment with the largest increases occurring at the 3-wk, post-treatment initiation timepoint. We observed that pts with DCB tend to raise a broader neoantigen T cell response than patients without DCB. 38% of pts without DCB and 67% of pts with DCB exhibited an increase in neoepitope-specific CD8 T cell responses within 3 wks of treatment initiation. Conclusions: Using high-throughput screening, pt-specific neoepitope reactive CD8 T cells could be detected pre- and post-treatment in pts with mUC treated with atezolizumab. Phenotypic characterization of neoepitope reactive CD8 T cells is ongoing. These data may help elucidate the dynamics and characteristics of the T cells of highest relevance to the ICB-induced, anti-tumor immune response.