The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor initially identified as the receptor for dioxin. Almost half a century after its discovery, AHR is now recognized as a receptor for multiple physiological ligands, with important roles in health and disease. In this review, we discuss the role of AHR in the gut–brain axis and its potential value as a therapeutic target for immune-mediated diseases.

Neuromyelitis optica spectrum disorder (NMOSD) is characterized by inflammatory relapses that result in severe disability, including blindness and paralysis. Relapse prevention with safe and effective treatments is key to reducing long-term disability. We aim to compare the efficacy and safety of rituximab-the most commonly used treatment-with recently approved NMOSD-specific treatments-eculizumab, inebilizumab, satralizumab-and other common off-label NMOSD treatments-mycophenolate mofetil (MMF) and azathioprine. The primary outcomes are relapse-free survival and annualized relapse rate. Secondary outcomes are serious infectious adverse event (SIAE) and treatment-limiting adverse event (TLAE)-free survival. A retrospective cohort study of NMOSD was conducted on patients at the Mass General Brigham hospital network. Patients meeting 2015 NMOSD diagnostic criteria, who were seen between 2000 and 2024 were included. Poisson regression, frequentist negative binomial analysis with inverse probability of treatment weighting, and Cox proportional hazard models were used to assess relapse rates, relapse-free survival, and SIAEs. A total of 176 patients with NMOSD were followed for a median of 9 years (interquartile range: 5-14), contributing 691 relapse assessments. The median age at first attack was 42 years, and 83% were female. Compared with rituximab, relapse risk was significantly lower with C5 inhibitors (HR 0.12, 95% CI 0.07-0.24), inebilizumab (HR 0.22, 95% CI 0.12-0.65), and satralizumab (HR 0.19, 95% CI 0.11-0.42). Annualized relapse rates were the lowest for C5 inhibitors (0, 95% CI 0-0.063) and the highest for azathioprine (0.34, 95% CI 0.18-0.56). A composite outcome of relapse, SIAE, and TLAE favored C5 inhibitors (HR 0.22, 95% CI 0.05-0.67), while azathioprine (HR 2.33, 95% CI 1.08-4.86) and MMF (HR 1.75, 95% CI 1.02-2.95) showed increased risk compared with rituximab. C5 inhibitors had the lowest incidence of serious infections (incidence rate ratio 0.16, 95% CI 0.05-0.42 vs rituximab). Clinicians should consider using NMOSD-approved treatments given their favorable efficacy and safety profiles in the real-world setting. MMF and azathioprine should be avoided. We caution against rituximab as a default first-line given the cumulative risk of relapse, SIAEs, and TLAEs over time. This study provides Class III evidence that, in patients with NMOSD, FDA-approved disease-modifying therapies are associated with lower relapse rates and fewer serious adverse events compared with rituximab.

Supratentorial Lymphocytic Inflammation with Parenchymal Perivascular Enhancement Responsive to Steroids (SLIPPERS) is a rare variant of the CLIPPERS spectrum with less than ten reports published so far. There is ongoing discussion regarding whether SLIPPERS is a disease entity on its own or just an acronym encompassing many underlying diagnoses, such as sarcoidosis, vasculitis and anti-glial fibrillary acidic protein (GFAP)-associated disease. A 40-year-old woman presented with episodes of language and attention impairment. Magnetic resonance imaging (MRI) revealed T2/FLAIR hyperintense lesions in the subcortical white matter associated with a micronodular, curvilinear perivascular contrast-enhancement. Alternative diagnoses were excluded. There was a remarkable response to steroids. A relapse occurred after six years, and the biopsy showed perivascular T-cell lymphocytic infiltrate, without granulomas, vasculitis, or neoplasia. There was complete resolution of the relapse after steroids. This case represents the longest reported follow-up of a patient diagnosed with SLIPPERS, and brain biopsy after 6 years did not suggest alternative diagnoses. This report contributes to the discussion regarding the possibility that exclusive supratentorial CLIPPERS-like pathology might be an isolated disease entity, but more biopsy-proven cases with a longer follow-up are needed to support this hypothesis. Recently, GFAP astrocytopathy has been characterized and might correspond to a significant number of cases previously diagnosed as CLIPPERS or SLIPPERS.