RA NS = 59 AS NS = 61 HC NS = 60 Pvalue * Pvalue # CD19+ B cells BAFF-R 14.4 ± 2 14.2 ± 1.4 12.7 ± 1 NS   TACI 4.1 ± 0.4 2.6 ± 0.1 2.6 ± 0.1 0.03 RA vs HC: NS; RA vs AS: 0.0075 BCMA 7.6 ± 1 3.4 ± 0.3 3.5 ± 0.3 < 0.0001 RA vs HC: < 0.0001 Naïve B cells BAFF-R 14.4 ± 1.9 14.5 ± 1.4 13 ± 1 NS   TACI 2.2 ± 0.3 2 ± 0.1 1.9 ± 0.1 NS   BCMA 3.7 ± 1 1.8 ± 0.1 1.9 ± 0.1 0.006 RA vs HC: 0.004; RA vs AS: 0.002 Transitional B cells BAFF-R 8.3 ± 0.9 10.7 ± 1.1 10.1 ± 0.8 NS   TACI 19.8 ± 4.5 8.5 ± 2.1 10.3 ± 3.1 0.06   BCMA 12.9 ± 4.7 5.7 ± 1.3 4.8 ± 1.2 NS   Memory B cells BAFF-R 14.5 ± 1.8 14.6 ± 1.5 13.2 ± 0.9 NS   TACI 2.6 ± 0.2 2.7 ± 0.2 2.7 ± 0.1 0.01 RA vs HC: 0.001 BCMA 3.3 ± 0.2 2.6 ± 0.1 2.8 ± 0.1 NS   Pre GC B cells BAFF-R 11.9 ± 2.4 12.3 ± 1.3 10.7 ± 0.9 NS   TACI 16.2 ± 5.1 5.5 ± 0.8 7.9 ± 1.2 NS   BCMA 5.9 ± 1.1 3.7 ± 0.3 4.3 ± 0.5 NS   Post GC B cells BAFF-R 15.8 ± 2.3 14.3 ± 1.5 13 ± 0.9 NS   TACI 2.5 ± 0.1 2.6 ± 1.2 2.7 ± 0.1 0.06   BCMA 3.1 ± 0.1 2.5 ± 0.1 2.5 ± 0.1 0.0006 RA vs HC: 0.001; RA vs AS: < 0.001 Plasmablasts BAFF-R 5.9 ± 1.7 4.4 ± 0.6 3.9 ± 0.4 NS   TACI 6 ± 1.3 4.1 ± 0.4 3.7 ± 0.3 NS   BCMA 4.4 ± 0.4 3.3 ± 0.2 3.8 ± 0.3 0.06   Results are expressed as the mean ± SEM of the RFI corresponding to the ratio between the mean fluorescence intensity (MFI) observed with the marker on the B cell subset and the MFI obtained with the isotype control. *Kruskal- Wallis test [level of significance was <0.05] #Post hoc Dunn test [level of significance was < 0.016]

Table 3: Expression of the receptors for BAFF and APRIL (BAFF-R, TACI and BCMA) on the different B cell subpopulations in patients with rheumatoid arthritis (RA), ankylosing spondylitis (AS) and healthy controls (HC).