IgLON5 autoimmunity secondary to immune checkpoint inhibitor.

IgLON5 autoimmunity is characterized by a diverse range of clinical presentations, including neuropsychiatric symptoms, sleep disturbances, gait instability, and bulbar symptoms, that are usually insidiously progressive. While some individuals with specific HLA haplotypes may be more susceptible to developing anti-IgLON5 disease, this antibody is typically not associated with a paraneoplastic etiology nor known to be induced by immune checkpoint inhibitors (ICI). We present a clinical and serological workup of a patient who developed symptoms of IgLON5 autoimmunity following treatment with pembrolizumab. He was found to have IgLON5 antibodies present in both the serum and cerebrospinal fluid, but he also expressed high-risk HLA haplotypes. This case suggests that immune checkpoint inhibitors (ICI) may promote the development of IgLON5 autoimmunity, particularly in those with high-risk HLA haplotyes.