# Relationship between HLA-DRB1 shared epitope alleles and peripheral blood monocyte counts in Japanese patients with rheumatoid arthritis

## TL;DR

This study found that certain HLA-DRB1 alleles are linked to higher monocyte counts and anti-CCP antibody levels in Japanese rheumatoid arthritis patients.

## Contribution

First study to show a significant association between HLA-DRB1 shared epitope alleles and monocyte counts in RA.

## Key findings

- SE-positive patients, especially those with S3P alleles, had significantly higher monocyte counts.
- S3P alleles, particularly HLA-DRB1 *04:05, were independently associated with higher monocyte counts and anti-CCP Ab titers.
- HLA-DRB1 *01:01 affected monocyte counts but not anti-CCP Ab levels.

## Abstract

To investigate the relationship between Human Leukocyte Antigen-DR beta 1 (HLA-DRB1) shared epitope (SE) alleles and peripheral blood monocyte counts in disease-modifying antirheumatic drug-naïve patients with rheumatoid arthritis (RA), and also the relationship between specific SE alleles and anti-cyclic citrullinated peptide antibody (anti-CCP Ab) titers.

This retrospective single-center study included 86 Japanese patients with RA. HLA-DRB1 genotyping was performed, and SE alleles associated with a high risk of developing RA were classified into the S2 (*04:01) and S3P (*01:01, *01:02, *04:04, *04:05, *04:08, and *10:01) categories. Patients were stratified based on monocyte count tertiles. The relationships between monocyte counts at diagnosis and clinical, serological, and genetic factors were analyzed. Logistic regression was used to identify independent factors associated with high monocyte counts.

SE-positive patients, particularly those with S3P alleles, had significantly higher monocyte counts than SE-negative patients. A multivariate analysis revealed that male sex and S3P positivity, particularly HLA-DRB1 *01:01 or *04:05, were independently associated with higher monocyte counts. Patients carrying at least one S3P allele had significantly higher anti-CCP Ab titers, with patients homozygous for HLA-DRB1 *04:05 having the highest levels. A similar relationship was not found with HLA-DRB1 *01:01 despite its strong effect on monocyte counts.

This is the first study to demonstrate a significant association between SE alleles and peripheral blood monocyte counts in RA. The results obtained suggest that specific SE alleles, particularly S3P alleles, contribute to the early pathogenesis of RA by enhancing monocyte-driven immune activation and anti-CCP Ab production.