Cerebrospinal Fluid Interleukin-6 in Immune Checkpoint Inhibitor-Induced Autoimmune Meningoencephalitis

Immune checkpoint inhibitors (ICIs) have proven clinical benefits in various advanced cancers. However, despite their significant therapeutic efficacy, ICIs induce immune-related adverse events. Among these events, autoimmune meningoencephalitis often has severe effects on patients' outcomes, but its specific clinical features are still unclear. Here, we report two cases of ICI-associated meningoencephalitis with elevated interleukin-6 (IL-6) levels in the cerebrospinal fluid (CSF). A 47-year-old woman (Case 1) with renal cell carcinoma developed severe headache after a seventh nivolumab administration. A neurological examination revealed jolt accentuation signs and hyperreflexia in all extremities. CSF analysis revealed a high IL-6 value (6,620 pg/mL) with marked pleocytosis. A 70-year-old woman (Case 2) who received an initial administration of nivolumab plus ipilimumab for renal cell carcinoma developed alterations of consciousness. She presented with impaired consciousness, neck stiffness, and hyperreflexia in all extremities. CSF analysis demonstrated a high IL-6 value (49.3 pg/mL) with mild pleocytosis. Both patients were treated with steroid pulse therapy (methylprednisolone 1,000 mg/day, 3 days), followed by the administration of oral predonisolone. The symptoms and laboratory findings improved in both cases. CSF IL-6 values were proportional to the severity of meningoencephalitis and other clinical parameters. These findings may help elucidate the mechanisms of central nervous system complications that are caused by ICIs.