A case report of clonal EBV-like memory CD4 + T cell activation in fatal checkpoint inhibitor-induced encephalitis

Checkpoint inhibitors produce durable responses in numerous metastatic cancers, but immune-related adverse events (irAEs) complicate and limit their benefit. IrAEs can affect organ systems idiosyncratically; presentations range from mild and self-limited to fulminantand fatal. The molecular mechanisms underlying irAEs are poorly understood. Here, we report a fatal case of encephalitisarising during anti-programmed cell death receptor 1 therapy in a patient with metastatic melanoma. Histologic analyses revealed robust T cell infiltration and prominent programmed death ligand 1 expression. We identified 209 reported cases in global pharmacovigilancedatabases (across multiple cancer types) of encephalitisassociated with checkpoint inhibitor regimens, with a 19% fatality rate. We performed further analyses from the index caseand two additional cases to shed light on this recurrent and fulminantirAE. Spatial and multi- omicanalyses pinpointed activated memory CD4+ T cells as highly enriched in the inflamed, affected region. We identified a highly oligoclonal T cell receptorrepertoire, which we localized to activated memory cytotoxic (CD45RO+ GZMB+Ki67+) CD4 cells. We also identified Epstein–Barr virus-specific T cell receptorsand EBV+ lymphocytes in the affected region, which we speculate contributed to neural inflammation in the index case. Collectively, the three cases studied here identify CD4+ and CD8+ T cells as culpritsof checkpoint inhibitor-associated immune encephalitis.