B cells are increasingly recognized as key drivers of local inflammation in rheumatoid arthritis (RA). This preprint characterizes B-cell differentiation and activation at the site of inflammation by comparing paired blood and synovial fluid samples from patients with ACPA-positive RA, seronegative RA and spondyloarthritis. The primary goal was to define the phenotypic states, differentiation stages and surface receptor organization of synovial B cells, with attention to disease- and serostatus-specific features.
The analysis used paired peripheral blood and synovial fluid specimens from three groups: ACPA-positive RA (N=12), seronegative RA (N=10) and spondyloarthritis (N=8). Phenotypic profiling included flow cytometric characterization and single-cell spatial proteomics to map surface receptor organization on defined B-cell subsets. Sample pairing allowed direct comparison between circulating and intra-articular B-cell states.
Across all three disease groups, the authors report a notable enrichment of CD11c+CD21- B cells within the synovial fluid compartment. These cells composed on average 67% of CD19+ cells in ACPA-positive RA, 49% in seronegative RA and 51% in spondyloarthritis. This consistent overrepresentation suggests that CD11c+CD21- B cells are a dominant B-cell population at inflamed joints in multiple inflammatory arthritides.
Within ACPA-positive RA samples the study identified increased proportions of CD11c+IgG+CD27low cells and multiple stages of pre-plasmablasts. These observations indicate enhanced extrafollicular plasmablast differentiation in ACPA+ disease compared with other groups. The phenotype of these cells supports an antigen-experienced, activated state that is biased toward antibody-secreting lineage commitment at the local tissue site.
Phenotypic markers on the CD11c+CD21- population pointed to active B-cell receptor (BCR) activation, increased proliferation and antigen-presentation potential. The combination of these features defines a functional program compatible with local antigen sensing, expansion and provision of help to T cells. The authors emphasize that although an activated, antigen-experienced synovial B-cell profile was observed across disease groups, important differences in differentiation trajectories were apparent by serostatus.
The B-cell signatures were paralleled by distinct T-cell programs in the synovial compartment. In ACPA-positive RA, a LAG3+CTLA4+TIGIT+ peripheral helper T-cell program was detected, consistent with a helper phenotype that can support B-cell differentiation. In contrast, seronegative RA showed a CCR6+ Th17-skewing T-cell profile. These divergent T-cell environments may contribute to the differential B-cell differentiation and activation observed between seropositive and seronegative disease.
Single-cell spatial proteomics revealed a distinctive surface architecture on CD11chighCD21low B cells. A notable feature was a BCR co-receptor–tetraspanin signature, including increased co-localization of CD72 and CD81, which was elevated in RA. Additional immune-modules identified in CD11c+ cells included CD44–CD52/CD59 and MHCII–CD40/CD84, modules that were absent in other B-cell subsets. These surface receptor organizations implicate enhanced BCR responsiveness, relative complement resistance and increased capacity for antigen presentation.
The findings support a model in which CD11c+CD21- B cells are a heterogeneous but dominant synovial B-cell population with functional specializations relevant to local inflammation in RA. While an activated antigen-experienced profile was shared across RA and spondyloarthritis, ACPA-positive RA displayed features of augmented extrafollicular plasmablast differentiation and a permissive peripheral helper T-cell milieu. The identified surface receptor signatures may underlie heightened BCR sensitivity, complement resistance and antigen presentation capacity, offering mechanistic insight into why these B cells persist and contribute to joint inflammation. As a preprint, these data have not been peer reviewed; the report focuses on phenotypic and proteomic characterization rather than clinical outcomes or therapeutic interventions.
The work lists funding from the Swedish Rheumatism Association, the Swedish Research Council and King Gustaf V’s 80-year Foundation. A competing interest statement notes that two authors are employed by Pixelgen Technologies AB. Additional methodological or dataset details beyond those summarized here were not reported in the abstract of the source.