KIF17 governs a trafficking checkpoint for tumor-specific MHC-II presentation and breast cancer immunity

Abstract Background Tumor-specific major histocompatibility complex class II (tsMHC-II) expression is associated with favorable prognosis and improved responsiveness to immune checkpoint blockade. However, the intracellular mechanisms controlling tsMHC-II trafficking and surface presentation remain poorly understood. This study investigated the molecular machinery regulating tsMHC-II surface presentation in breast cancer. Methods The tsMHC-II interactome was mapped across breast cancer subtypes using immunoprecipitation-coupled quantitative proteomics. Bioinformatic analyses, subcellular fractionation, flow cytometry, co-immunoprecipitation, proximity ligation, recombinant protein pull-down and structural modeling were used to characterize KIF17-mediated trafficking. Orthotopic 4T1 models, multiplex immunofluorescence, RNA sequencing and spectral flow cytometry were used to assess tumor growth and tumor microenvironment (TME) remodeling. Splenic T-cell coculture assays with MHC-II blockade were performed to evaluate CD4 + and CD8 + T-cell activation and tumor-cell killing. Patient specimens and public immunotherapy datasets were analyzed to assess clinical associations. Statistical analyses included two-tailed Student’s t-tests, analysis of variance, log-rank tests, correlation analysis, receiver operating characteristic analysis and random-effects meta-analysis. Results KIF17 was identified as a tsMHC-II-interacting protein that was downregulated in breast tumors and associated with immune activation and favorable survival. KIF17 interacted directly with HLA-DQB2 and increased its tumor-cell surface localization without altering total MHC-II abundance. In orthotopic models, Kif17 overexpression remodeled the TME and increased activated CD4⁺ and CD8⁺ T-cell and antigen-presenting-cell populations. In coculture assays, Kif17-overexpressing tumor cells promoted CD4⁺ T-cell activation, CD4⁺-supported CD8⁺ T-cell activation and tumor-cell killing. These responses were attenuated by MHC-II blockade or omission of CD4⁺ T cells. Kif17-overexpressing tumors also displayed improved tumor control and prolonged survival following programmed cell death protein 1 (PD-1) blockade. Elevated KIF17 expression was associated with favorable treatment outcomes in patient specimens and independent immunotherapy datasets. Conclusions KIF17 regulates an intracellular trafficking checkpoint controlling tsMHC-II surface presentation. These findings support a model in which KIF17 promotes coordinated CD4⁺ and CD8⁺ T-cell responses and establishes an immune context associated with improved responsiveness to PD-1 blockade.