Background: Cisplatin-based chemotherapy has been associated with functional cures (i.e., DDFTFS) in a small subset of patients with mUC. However, the mechanistic basis for this phenomenon has remained elusive. Immune checkpoint blockade may enhance the ability of cisplatin-based chemotherapy to cure mUC, particularly if the latter is achieved by promoting antitumor immunity. From a phase II trial evaluating a phased schedule of gemcitabine and cisplatin (GC) followed by GC plus Ipilimumab (Ipi) for mUC, 4/36 patients were treatment-free disease-free >7 years after initiation of treatment. We sought to identify the immunological mechanisms associated with functional cure of such patients. Methods: Whole exome sequencing was performed on pre-treatment archival tumor tissue. Neoantigen binding affinity was predicted using NetMHCcons. Neoantigen-specific T cell response was assessed via IFNγ ELISpot assays after co-culture of patient-derived peripheral blood CD4+ and CD8+ T cells with autologous neoantigen-pulsed antigen presenting B cells. Serum protein soluble analytes were measured using the Olink Target 96 Immuno-Oncology Panel at the pre-treatment, post-GC, and post-GC+Ipi timepoints. Results: CD4+ and CD8+ neoantigen-specific T cell reactivity was measured at serial timepoints in 2 patients achieving DDFTFS and 5 patients not achieving DDFTFS. Strikingly, peripheral blood CD4+ and CD8+ T cell reactivity to a predicted neoantigen emerged on cycle 6 day 1 (after 2 cycles of GC followed by 4 cycles of GC + Ipi) in 2/2 patients achieving DDFTFS (variant in TP53 or DHX9) but in 0/5 patients not achieving DDFTFS. Peripheral serum proteomic analysis at serial timepoints (n = 35 patients) revealed that pre-treatment, patients achieving DDFTFS demonstrated an immune activated phenotype with elevations in cytotoxic adaptive immunity markers and immune checkpoints (Table). After the addition of Ipi, patients achieving DDFTFS demonstrated decreased markers of tumor-promoting inflammation but increased markers of adaptive immunity, co-stimulation, and cytotoxicity. Conclusions: Anti-tumor immunity may underlie functional cures achieved in patients with mUC treated with cisplatin-based chemotherapy +/- immune checkpoint blockade. Probing the mechanistic basis for DDFTFS may facilitate the identification of pre- and on-treatment features, and therapeutic components and sequences, necessary to extend this ultimate therapeutic goal to a larger subset of patients.