Background: The mechanism of response and resistance to immune checkpoint inhibition (ICI) remains to be defined in urothelial cancer. Methods: The abacus study explored neo-adjuvant atezolizumab prior to cystectomy in muscle-invasive bladder cancer (MIBC, NCT02662309). In this post hoc exploratory analysis, we utilized samples from the Abacus. Nanostring GeoMx Digital Spatial Profiler (DSP), a spatial transcriptomics platform enabling comprehensive characterization of the entire human transcriptome while preserving spatial information, was performed on Formalin Fixed Paraffin Embedded (FFPE) tissue from 12 paired biopsies (3 pCR, 3 MPR, and 6 relapse tumors). To guide region selection and tissue segmentation on the GeoMx DSP, we utilized SYTO 13, PanCK, CD45, and CD3 markers. Subsequently, we quantified the entire transcriptome in selected regions and conducted differential gene expression analysis using the DSP data analysis suite and BioTuring Lens platforms. Results: Tumor cells of the MPR samples showed upregulation of MHC class II genes (log2FC >1.52, q< 5.5E-04, p< 9.7E-07). pCR samples showed numerous tertiary lymphoid structures (TLS) in the remaining tumor bed. We then performed MHC class II (HLA-DR) immunohistochemistry staining to confirm this transcriptome-level finding on available FFPE samples from Abacus. The MPR response group (n=4) had significantly higher levels of MHC class II expression on tumor cells after the treatment than relapse (n=12) and stable disease (n=15) patients (p< 0.001). Conclusions: Upregulation of MHC class II on tumor cells in anti-PD-L1 responding patients is a novel finding. Together with the TLS presence, they offer insights into potential mechanisms underlying treatment response, thus aiding in identifying predictive biomarkers for ICI therapy in MIBC.