Single cell ATAC sequencing of SIINFEKL-reactive immune cell from intracranial murine GL261-SIINFEKL tumors 20 days after inoculation. SIINFEKL-reactive T cells were sorted based on dextramer staining. We show that loss of major histocompatibility complex (MHC) class II (MHCII)-restricted antigen presentation on bbm drives dysfunctional intratumoral tumor-reactive CD8+ T cell states through increased chromatin accessibility and expression of Tox, a critical regulator of T cell exhaustion.