B cell-intrinsic IRF8 transcriptionally reprograms antigen presentation to sustain CD8⁺ T cell antitumor immunity

Interferon regulatory factor 8 (IRF8) is a master transcription factor of myeloid differentiation, but whether IRF8 intrinsically controls B cell function in tumors remains unknown. Using paired single-cell transcriptomic and chromatin accessibility profiling of tumors from wild-type and Irf8-deficient mice, we identify a B cell-intrinsic IRF8 axis regulating antigen presentation and sustaining anti-tumor CD8 T cell immunity. IRF8 establishes conserved chromatin accessibility programs across myeloid cells and plasmablasts centered on antigen processing and MHC class I presentation, but engages distinct motifs by lineage: myeloid cells preferentially utilize ISRE and ETS-composite elements, whereas plasmablasts are selectively enriched for EICE elements, reflecting B lineage-specific IRF8-IRF4 cooperation. Loss of IRF8 disrupts these programs, skews B cells toward plasmablast differentiation and reduces antigen presentation machinery. B cell depletion accelerated tumor growth, while CD40 agonism activated B cells, expanded T cells, and enhanced anti-tumor immunity. B cell-specific IRF8 deletion alone accelerated tumor growth, establishing a cell-intrinsic requirement independent of myeloid IRF8 function. The IRF8-regulated B cell signature was enriched in PD-1 blockade cancer patient responders, and plasmablast abundance correlated with response in pembrolizumab-treated cancer patients. These findings establish IRF8 as a lineage-adapted regulator of antigen presentation and define the IRF8-B cell axis as a determinant of anti-tumor immunity.