# Serum immunoreactivity to neurofilament-medium shows high sensitivity and specificity in patients with Behçet disease

## TL;DR

This study shows that a specific immune response to a brain protein is highly common and unique to Behçet disease patients, potentially offering a new diagnostic tool.

## Contribution

The study confirms and validates the high specificity and sensitivity of NF-M immunoreactivity in Behçet disease sera across an independent cohort.

## Key findings

- 97% of Behçet disease sera showed distinct NF-M immunoreactivity in axons.
- Sera from other diseases and healthy controls lacked this specific staining pattern.
- NF-M immunoreactivity in BD may cross-react with bacterial heat shock protein 65.

## Abstract

Behçet disease (BD) is a complex vasculitis with both autoimmune and autoinflammatory features. Despite specific clinical features, no laboratory tests are available for the diagnosis of BD. We recently found that BD sera exhibited immunoreactivity against neurofilament medium protein (NF-M). This study aimed to replicate this finding in an independent cohort and to assess the specificity and sensitivity of NF-M immunoreactivity in serum samples obtained from BD, systemic lupus erythematosus (SLE), multiple sclerosis (MS), psoriatic arthritis (PsA) and non-Behçet uveitis (NBU) patients as well as healthy donors.

Serum samples from 76 patients (33 BD, 16 MS, 15 SLE, 9 PsA and 3 NBU) and 22 healthy donors (totalling 98 sera) were analysed. Mouse brain tissue sections were immunolabelled with the sera and examined using confocal microscopy.

97% (32/33) of BD patient sera exhibited a distinct fine filamentous staining pattern consistent with NF-M protein immunolabelling in axons, while sera from healthy controls and patients with SLE, MS, PsA and NBU showed no similar staining. Conversely, MS patient sera displayed a thick filamentous staining pattern attributed to oligodendrocytes and their myelin-forming processes. SLE patient sera intensely labelled all cell nuclei, conforming to immunoreactivity against nuclear antigens.

These findings reveal the ubiquitous presence of NF-M immunoreactivity, reportedly cross-reacting with bacterial heat shock protein 65, in BD sera. This common and specific immunoreactivity may serve as a valuable tool for diagnosing BD. Additionally, the data confirm the unique potential of connective tissue-poor brain sections for identifying sero-immunoreactivity.