Intrathecal BCR transcriptome in multiple sclerosis versus other neuroinflammation: Equally diverse and compartmentalized, but more mutated, biased and overlapping with the proteome

- A few clones dominate the CSF IGHV transcriptome in MS and other neuroinflammation.
- These dominant clones are responsible for intrathecal IgG synthesis in MS.
- In both groups, B cells may mature on both sides of the blood-CSF barrier.
- The CSF IGHV transcriptome in MS had VH4 bias and more mutations than controls.

The mechanisms driving the intrathecal synthesis of IgG in multiple sclerosis (MS) are unknown. We combined high-throughput sequencing of transcribed immunoglobulin heavy-chain variable (IGHV) genes and mass spectrometry to chart the diversity and compartmentalization of IgG-producing B cells in the cerebrospinal fluid (CSF) of MS patients and controls with other neuroinflammatory diseases. In both groups, a few clones dominated the intrathecal IGHV transcriptome. In most MS patients and some controls, dominant transcripts matched the CSF IgG. The IGHV transcripts in CSF of MS patients frequently carried IGHV4 genes and had more replacement mutations compared to controls. In both groups, dominant IGHV transcripts were identified within clusters of clonally related B cells that had identical or related IGHV transcripts in the blood. These findings suggest more pronounced affinity maturation, but an equal degree of diversity and compartmentalization of the intrathecal B-cell response in MS compared to other neuroinflammatory diseases.