Background/Purpose: We previously demonstrated that intestinal bacteria within the families Lachnospiraceae and Ruminococcaceae are preferentially targeted by ACPA-reactive plasmablast-derived monoclonal antibodies (PB-mAbs) from dual IgA/IgG isotype families derived from individuals at-risk for RA. Furthermore, intestinal colonization of mice with a PB-mAb reactive primary Ruminococcaceae genus Subdoligranulum strain, designated strain 7, from an at-risk individual drove Th17 cell responses as well as serum RA-related autoantibodies and inflammatory joint swelling in monocolonized mice in the absence of other arthritogenic factors. Development of arthritis required B and T cells in mice, and could be transferred with serum to naïve mice. Given these findings, we hypothesized that the presence of this arthritogenic Subdoligranulum strain 7 is specific to those at risk for and with RA and that a specific anti-strain 7 immune response is found in human RA.

Methods: Feces from individuals at-risk for (n=12, defined as being serum anti-CCP3 positive) and with early RA (n=12, < 1 year from diagnosis), as well as healthy controls (n=12), were screened for the presence of Subdoligranulum strain 7 by a strain-specific qPCR with a sensitivity of 0.0002% abundance in feces. Human PBMCs from individuals with early RA (n=11) and healthy controls (n=12) were exposed to strains 1 and 7 as well as influenza, and T cell activation based on CD69 and CD154 upregulation were evaluated by flow cytometry. CD45RA-/CXCR3+/CCR4-/CCR6- cell markers were used as to define Th1 cells while CD45RA-/CXCR3-/CCR4+/CCR6+ were used for Th17 cells.

Results: Subdoligranulum strain 7 was detectable in the feces at a mean abundance 1.637 x 10⁷± 3.059 x 10⁷ CFUs in ~16% of individuals at-risk for and with early RA, but not detected in healthy individuals (Table 1). ACPA-reactive PB-mAbs bound strains 1 and 7, and digestion of cell wall associated proteins with 100 µg/mL Proteinase K decreased binding by 48%. In addition, informative PB-mAbs could immunoprecipitate a molecule of 10 kD from a cell wall-enriched fraction in strain 7 but not strain 1. Finally, strain 7 but not strain 1 induced T cell activation in human PBMCs from individuals with RA, but not controls (Fig. 1, P< 0.01). This response was HLA class II dependent and skewed towards a Th17 phenotype, as compared to influenza that induced a Th1-like response (P=0.0078, Fig. 2).

Conclusion: The preferential presence of, and immune responses against, a primary human-derived strain of Subdoligranulum in individuals at-risk for or with early RA, but not in controls, supports the conclusion that this is an arthritogenic bacterium of relevance in a subset of RA risk and patient populations. The finding of PB-mAb-reactive antigen associated with the cell wall fraction of Subdoligranulum, suggests that a cross-reactive protein may be stimulating systemic adaptive immunity in RA through molecular mimicry. Taken alongside our murine findings, these data implicate Subdoligranulum strain 7 as a candidate mucosal driver of RA development in a subset of individuals.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/an-arthritogenic-strain-of-subdoligranulum-specifically-detectable-in-the-feces-of-individuals-at-risk-for-and-with-rheumatoid-arthritis-is-bound-by-acpa-and-stimulates-th17-cell-activation-in-those/