Background/Purpose: Circulating autoantibodies like ACPA frequently develop years before symptoms of RA, during which the individual is at-risk for disease. Several lines of evidence suggest that these autoantibodies may be driven by microbial-mucosal interactions. We hypothesized that discrete gut-derived bacteria drive autoantibody generation.

Methods: Dual IgA/IgG family plasmablasts (n=94) were isolated from 4 individuals at-risk for RA, defined as serum anti-CCP+ (75%) and/or RF+ (75%), and from 2 anti-CCP+ individuals with early RA. Fab domains from the plasmablast-derived mAbs (PB-mAbs), selected for binding of RA-relevant antigens on a protein microarray, were expressed in a mouse IgG2a scaffold. The PB-mAbs were used to identify targeted fecal bacteria, and Ruminococcaceae(Rumino) strains were isolated from the feces of a human at-risk for RA. 16S sequencing identified seven Rumino family strains of interest, and through whole genome sequencing we identified an as yet unnamed species within genus Subdoligranulum (Sbg). Human PBMC were isolated and exposed to individual strains of Sbg to gauge memory responses by assessing in vitro CD154+ upregulation on CD4+ T cells. Germ free DBA/1 mice were colonized with each Sbg strain, as well as Prevotella copri and PBS as controls, and were monitored for stable colonization, autoantibody development, and joint swelling. Serum collected from arthritogenic Sbg monocolonized mice was injected into naive germ-free DBA/1 mice, and mice were monitored for joint swelling.

Results: All PB-mAbs bound RA-relevant antigens and 58/94 (62%) targeted families Lachno(spiraceae)/Rumino from a pool of fecal bacteria (derived from 5 healthy controls, 8 at-risk individuals, and 5 RA cases) at a disproportionately elevated level (56.31 ± 12.85% of all bacteria bound) compared to other taxa, suggesting cross-reactivity between bacterial and host antigens. We verified PB-mAb binding of Lachno/Rumino using our generated Sbg strains. We then observed an MHC class II-dependent memory T cell response in PBMCs against a subset of the Sbg strains in individuals with RA that was nearly absent in controls. To determine if Sbg could induce autoantibodies in vivo, germ-free mice were stably colonized with the Sbg strains, P. copri, or sterile PBS; within 14 days, joint swelling was observed only in mice monocolonized with a subset of the Sbg strains. Joint swelling was associated with an expansion of antibodies to RA-relevant antigens. Transfer of serum from affected Sbg strain colonized mice, but not P. copri or PBS controls, to naïve mice resulted in joint swelling.

Conclusion: A subset of circulating dual IgA/IgG PB-mAbs isolated from individuals at-risk for RA target both RA-relevant antigens and bacteria within families Lachno/Rumino. A specific strain from the genus Sbg established from an at-risk individual were recognized by T cells from RA cases. When germ-free mice were monocolonized with the specific Sbg strain, they developed joint swelling and serum autoantibodies capable of transferring the joint swelling phenotype. Our data suggests one model in which a strain of bacteria is capable of stimulating the development of pathogenic autoantibodies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/plasmablast-derived-autoantibodies-from-individuals-at-risk-for-ra-that-target-ra-relevant-antigens-are-polyreactive-with-arthritogenic-bacteria/