Deletion of a rheumatoid arthritis risk enhancer in mouse disrupts Bim expression and immune homeostasis

Objectives This study aimed to characterise the in vivo function of a murine regulatory region syntenic to the human rheumatoid arthritis (RA)-associated enhancer at the ACOXL locus and its role in immunity. Methods Using CRISPR-Cas9, we generated the Bim-eΔ mouse model by excising a 15 kb genomic region syntenic to the human RA-associated locus tagged by rs13401811 from C57BL/6J zygotes. Bim expression was assessed by quantitative polymerase chain reaction. Thymocytes and splenocytes were profiled by flow cytometry, and whole-spleen RNA-seq was used to define transcriptional changes. Immune responses were tested using a Trichuris muris infection model and an antigen-induced arthritis (AIA) protocol. Results Bim-eΔ mice show reduced Bim expression vs wild-type counterparts. This indicates that Bim is a transcriptional target of this region. Enhancer loss was correlated with differences in T cell proportions across age- and sex-stratified cohorts consistent with altered thymic selection. Splenic T and B cells were altered, and the transcriptome skewed toward inflammation. Bim-eΔ and wild-type mice mounted comparable responses to T. muris , whereas Bim-eΔ mice developed a severe adverse response during the sensitisation phase of the AIA protocol. Conclusions A murine noncoding regulatory region modulates Bim expression and immunity in vivo . Enhancer deletion altered the splenic transcriptional profile and lymphocyte composition while revealing a context-dependent phenotype upon inflammatory in vivo challenge. These findings support a role for RA-associated regulatory architecture in shaping immunity and point to BIM regulation as a potential therapeutic lever for restoring immune balance.