GEOapplication/xmlftp://ftp.ncbi.nlm.nih.gov/geo/series/GSE326nnn/GSE326904/primaryOK200OtherMus musculusOtherhttps://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE326904GEOGSEfalseCardiac tertiary immune niches drive immune activation in immune checkpoint inhibitor myocarditis [CosMx]Using integrated spatial and single-cell analyses in a pharmacological murine model, we identified regional infiltration of inflammatory myeloid cells and PD-1⁺CD8⁺ T cells in the heart that organize into fibroblast-rich immune structures, which we term Tertiary T Cell Niches (TTCNs). TTCNs serve as hubs for T cell activation, sharing features of tertiary lymphoid structures. Complementary TCR analyses revealed clonal expansion of cardiac T cells following ICI treatment. Together, these findings suggest that cardiac tertiary immune structures play a central role in activating and recruiting immune cells in ICI myocarditis and highlight pathways that could mitigate ICI cardiotoxicity.2026/04/06GSE326904GSM9643340GSM964333936279326904Mus musculus