Background: Axial spondyloarthritis (axSpA) pathogenesis integrates autoinflammatory and autoimmune pathways, initiated by gut dysbiosis and intestinal barrier dysfunction. Innate immune cells (ILC3, γδ T cells, MAIT cells) drive early IL-23/IL-17 axis activation at entheses, amplified by adaptive Th17/Tc17 cells that perpetuate synovial inflammation. Emerging evidence suggests that B cells may also contribute to disease pathogenesis and altered numbers of circulating B cells have been observed in the peripheral blood of patients with axSpa. However, to our knowledge, circulating B-cell subpopulations have not been investigated as potential biomarkers of clinical outcomes in axSpA.

Objectives: To characterize circulating B-cell subpopulation frequencies in the peripheral blood of axSpA patients and their association with response to anti-TNF therapy.

Methods: This non-interventional study included 42 biologic- and targeted synthetic DMARD-naïve patients with axial spondyloarthritis (axSpA) requiring treatment escalation. Median (IQR) age was 44 years (36-56), 55% were female, 79% HLA B27+ and 52% had radiographic axSpA. Median (IQR) ASDAS-CRP was 3.29 (2.98-3.72). Of these, 31 patients initiated anti–TNF therapy according to routine clinical practice and were prospectively followed, while 11 patients who initiated anti-IL-17 or JAK inhibitor therapy were excluded from the prospective analysis because of the small sample size. Peripheral blood samples were collected prior to treatment escalation after written informed consent was obtained. For each patient, an age- and sex-matched healthy control (HC) was included. Peripheral blood mononuclear cells (PBMCs) were isolated by Ficoll-Hypaque density gradient centrifugation and stained with antibodies against CD19, CD20, CD27, CD24, IgD and CD38 for flow cytometric analysis.

Results: Compared with healthy controls (HC; n=42), patients with axial spondyloarthritis (axSpA; n=42) exhibited significantly increased numbers of circulating CD19 ⁺ B cells. In addition, axSpA B cells displayed an altered subpopulation distribution, characterized by increased proportions of CD19 ⁺ CD24 ^hi CD38 ^hi transitional B cells, CD19 ⁺ CD27 ⁻ IgD ⁺ naïve B cells, and CD19 ⁺ CD27 ⁻ IgD ⁻ double-negative B cells. In contrast, the proportion of CD19 ⁺ CD27 ⁺ IgD ⁺ non-switched memory B cells was reduced, while frequencies of CD19 ⁺ CD27 ⁺ IgD ⁻ switched memory B cells and CD19 ⁺ CD27 ⁺ CD20 ⁻ CD38 ^hi plasmablasts were comparable to those observed in healthy controls. These B cell population frequencies did not differ across axSpA subgroups defined by HLA-B27 status or radiographic classification. Among the 31 axSpA patients who initiated anti–TNF therapy, 15 achieved a good clinical response (responders), whereas 16 showed an incomplete response (non-responders). A good clinical response was defined as an ASDAS-CRP improvement ≥ 1.1, and achievement of ASDAS-CRP < 2.1 at 12 weeks of treatment. Notably, responders exhibited significantly higher baseline frequencies of circulating transitional B cells and lower baseline frequencies of circulating double-negative B cells compared with non-responders, while no differences were observed in other B cell subsets. Follow-up samples were available from 14 axSpA patients 6 months after anti-TNF initiation, at which time a significant reduction in total circulating B cell numbers and transitional B cell proportions was observed, whereas frequencies of the remaining B cell subpopulations remained unchanged.

Conclusions: In biologic and tsDMARD-naïve axSpA, higher circulating transitional and lower double negative B-cell frequencies associate with anti-TNF response, positioning these subpopulations as candidate biomarkers for precision treatment selection.

REFERENCES: NIL.

Acknowledgments: NIL.

Disclosure of Interests: Beatriz Nieto-Carvalhal: None declared, Mariela del Carmen Uyaguari Morocho: None declared, Diana Peiteado: None declared, Alejandro Villalba: None declared, Irene Monjo-Henry: None declared, Sara García-Carazo: None declared, Eugenio De Miguel: None declared, Maria-Eugenia Miranda-Carus BMS, Gebro Pharma.