Background: Ankylosing spondylitis (AS) is a chronic immune mediated inflammatory rheumatic disease, in which primarily the sacroiliac joints and the spine are affected. Extra-skeletal manifestations (ESM) include uveitis, psoriasis, inflammatory bowel disease and peripheral arthritis. In studies into the pathogenesis of AS, B-cells have received little attention most likely due to the lack of auto-antibodies ¹ . A B-cell subset that has been particularly associated with autoreactivity is characterized by low expression of CD21. These CD21 ^-/low B-cells are increased in systemic autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus and primary Sjögren syndrome (pSS) ² . At least part of the CD21 ^-/low B-cells are considered to represent anergic, autoreactive B-cells, that fail to become activated through conventional B-cell receptor and CD40 signaling ² .

Objectives: To phenotypically study the peripheral B-cell compartment in in the blood of AS patients compared to pSS patients, a typical B-cell-associated autoimmune disease, and healthy controls (HC). Special emphasis was given to the CD21 ^-/low compartment.

Methods: The proportions and phenotype of peripheral B-cells were assessed in cryopreserved peripheral blood mononuclear cells of 45 AS patients (62% male, mean age 49.2±13.2 years, mean ASDAS 2.5±1.0), 20 age-matched patients with pSS (20% male, mean age 50.6±12.0, median (IQR) ESSDAI 3±6.25) and 30 age- and sex-matched HCs, using 15-color flow-cytometry analysis. Differences between groups were tested using the Independent Samples t-test or Mann-Whitney U test depending on the distribution of variables. Associations between CD21 ^-/low B-cells and clinical parameters were explored using the Pearson or Spearman correlation coefficient.

Results: The percentage of total B-cells in AS patients did not differ from pSS patients and HCs. In AS patients, percentages of CD27 ⁺ memory B-cells and CD27 ^- IgD ⁺ naïve B-cells were also similar to HCs, whereas CD27 ⁺ IgD ^- memory B-cells were significantly reduced in pSS patients, as expected. The proportions of CD27 ^- CD38 ^low CD21 ^-/low B-cells among total B-cells were significantly increased in both AS (median 6.4%, p<0.0001) and pSS patients (median 7.8%, p<0.0001) compared to HCs (median 4.9%). Interestingly, only in AS patients, expression of chemokine receptors CXCR3 and CXCR5 was significantly elevated on CD27 ^- CD38 ^low CD21 ^-/low B-cells compared to HCs (p<0.001 and p<0.01, respectively). In comparison to HCs the expression of the immune markers T-bet and CD11c by CD27 ^- CD38 ^low CD21 ^-/low B-cells was significantly lower in AS patients (p<0.01 and p<0.01, respectively). The distribution of IgM and IgD expression within the CD27 ^- CD38 ^low CD21 ^-/low B-cell population was similar between all three study groups. Regarding the association between CD27 ^- CD38 ^low CD21 ^-/low B-cells and clinical parameters in AS patients, we observed a positive correlation with age (r=0.347, p=0.02) and erythrocyte sedimentation rate (ρ=0.386, p=0.01). Furthermore, AS patients with ESM showed increased proportions of CD27 ^- CD38 ^low CD21 ^-/low B-cells compared to patients without ESM (p<0.05).

Conclusion: In this cross-sectional study, we observed an increased proportion of circulating CD27 ^- CD38 ^low CD21 ^-/low B-cells in AS patients, similar as in patients with pSS, a typical B-cell-mediated autoimmune disease. The elevated expression of CXCR3 on CD27 ^- CD38 ^low CD21 ^-/low B-cells in AS patients is suggestive for active involvement in the inflammatory response. These findings are indicative of B-cell involvement in the pathogenesis of AS, against current dogma.

REFERENCES:

[1]Ranganathan V et al. Nat Rev Rheumatol . 2017;13(6):359-367.

[2]Thorarinsdottir K et al. Scand J Immunol . 2015;82(3):254-261.

Disclosure of Interests: Rick Wilbrink: None declared, Anneke Spoorenberg: None declared, Suzanne Arends: None declared, Frans G.M. Kroese Speakers bureau: BMS, Roche, Janssen-Cilag, Consultant of: BMS, Grant/research support from: BMS, Gwenny M. Verstappen: None declared