Background: Anti-modified protein antibodies (AMPAs) targeting post-translational modifications (PTMs) are a hallmark of rheumatoid arthritis (RA), with anti-citrullinated protein antibodies (ACPAs) being the most prominent and disease-specific. In contrast, anti-acetylated protein antibodies (AAPAs) are also detectable at the IgM level in healthy individuals, raising questions about the earliest stages of AMPA development.

Objectives: To investigate how AMPA responses emerge in RA and to determine whether AAPA IgM represents a common starting point for PTM recognition and the subsequent development of broader AMPA responses.

Methods: To assess whether AAPA IgM precedes other AMPAs, we longitudinally measured AAPA and ACPA IgM in individuals before RA onset using in-house ELISAs. In parallel, single AAPA- and ACPA-expressing B cells were isolated from RA patients and analysed for B cell receptor (BCR) configuration to determine whether autoreactivity can arise directly from the germline repertoire.

Results: In asymptomatic, pre-RA individuals, AAPA IgM was more frequently detected than ACPA IgM (27.3% vs 11.7%). Towards disease onset, ACPA IgM positivity rose to 51.2%, while AAPA IgM positivity remained stable. Additionally, AAPA IgM presence was not required for the development of other AMPAs detected upon symptom onset. At the cellular level, four fully germline-configured BCRs were identified among AAPA- and ACPA-expressing B cells (1 AAPA, 3 ACPAs). These germline BCRs retained autoreactivity to their cognate PTM and in several cases, displayed inherent cross-reactivity to additional PTMs.

Conclusions: Although AAPA IgM emerges earlier than ACPA IgM, it does not appear to seed the broader AMPA response. The identification of germline-encoded AAPA- and ACPA-expressing B cells with intrinsic PTM cross-reactivity provides the first direct evidence that AMPA autoreactivity can exist in the naïve repertoire and may arise from distinct PTM-reactive B cells, rather than a common ancestor. These findings challenge the view that AMPA responses require extensive somatic hypermutation and instead suggest that multiple autoreactive germline B cell clones may independently initiate autoimmunity, shifting the focus of RA pathogenesis toward very early B-cell tolerance checkpoints. Targeting naïve B cell activation before clinical disease onset may therefore represent a promising strategy for preventive intervention.

REFERENCES: NIL.

Acknowledgments: NIL.

Disclosure of Interests: None declared.