Immune dysfunction contributes to comorbid depression in patients with multiple sclerosis.

BACKGROUND

Depression affects approximately 30% of people with multiple sclerosis (MS) and worsens outcomes, yet its immunopathogenesis remains unclear. We investigated whether immune dysfunction contributes to MS-associated depression by integrating genomic and single-cell data.

METHODS

We analyzed genome-wide association study (GWAS) summary statistics from the International Multiple Sclerosis Genetics Consortium (IMSGC, MS) and the Psychiatric Genomics Consortium (PGC, depression) using linkage disequilibrium score regression (LDSC), bidirectional two-sample Mendelian randomization (2SMR), and multi-trait analysis of GWAS (MTAG). In addition, we performed exploratory single-cell RNA sequencing (scRNA-seq) of peripheral blood mononuclear cells (PBMCs) from four MS patients (two with depression and two without) to characterize immune cell phenotypes and functions.

RESULTS

LDSC revealed a significant positive genetic correlation between MS and depression (rg = 0.108, P = 0.035), while 2SMR analysis did not support a causal relationship. MTAG identified 10 shared loci concentrated in the chromosome-6 MHC region and enriched for antigen processing/presentation pathways. scRNA-seq revealed expansion of GZMB+ effector CD8+ T cells in MS with depression and a dominant TCR clonotype (CASGVGGNTIYF) predicted to recognize the neuronal antigen 5-hydroxytryptamine receptor 1F (5-HT1F) receptor as well as Epstein-Barr nuclear antigens (EBNA3/4). ERGO-II simulations predicted strong binding (score > 0.80) between 5-HT1F peptides and HLA-B allotypes (B35, B27, B07, B53, B18). Functionally, 5-HT1F-specific CD8+ T cells displayed a terminally differentiated cytotoxic phenotype, consistent with chronic antigenic stimulation.

CONCLUSIONS

This study provides integrative evidence that immune dysfunction, particularly CD8+ T cell-mediated HLA-B restricted targeting of 5-HT1F-expressing neurons, may underlie depression in MS.