In rheumatoid arthritis, naïve B cells prevail in the peripheral blood, whereas memory B cells accumulate in the joint compartment, express ZAP-70 and characterize the aggregate pattern

Objectives: To analyse B cell subsets in the peripheral blood of RA, non-RA patients and Healthy Controls (HC) and in the synovial compartment in order to understand whether there is a compartmentalization of some subsets. To examine the phenotypic characteristics of the B cells in the synovial fluid and tissue and their possible relationship with disease activity and with synovial tissue cells and their pattern of distribution. Methods: Twelve patients with RA, 15 non-RA arthritides (4 UA, 3 MCA-microcrystal arthritis, 1 SpA, 1 SLE, 4 MOA-monoarthritis, 2 PsA) were examined for B cell subsets in the peripheral blood (PB) and in the synovial fluid (SF) and underwent synovial biopsy. Ten HC matched for sex and age, were enrolled in the study as control group. Results: The percentage of B cells was similar in RA and in non-RA patients, both in peripheral blood and in the synovial compartment.Considering the peripheral blood compartment, the percentage of CD19+ B cells (8.0±4.8%), as well as the eBm5 (12.4±5.6%) were lower in RA than in HC (CD19+%: 10.8±3.6%, p=0.003; eBm5%: 16.0±7.1%; p=0.06). On the other hand, the percentage of CD19+Zap-70+ B cells were higher in RA than in HC (6.1±5.5%>median 5.0 vs 3.9±3.1%> median 3.1% in HC; p=0.05), whereas no differences were seen between RA and non-RA patients (4.1±0.3%, p =0.22).In the SF, the percentage of Bm2 +Bm2' were significantly lower than in the PB (3.2±3.2% vs 41.3±20.5% in peripheral blood, p=0.001); on the contrary Bm5 were significantly higher in the SF than in the PB (67.9±11.4% vs 21.3±21.1%, p=0.001). These data have been confirmed by a significant increase of CD27+ B cells in the SF vs the PB (58.8±18.1% Vs 25.1±14.9%, p=0.001). More particularly, the percentage of CD27+IgD- was significantly higher in SF (54.7±16.7%) compared to PB (11.1±7.9% in PB, p=0.001).Dividing RA patients with high disease activity (DAS44 > 3.7) from those with a DAS< 3.7, the analysis revealed that Bm1 in the SF were higher in active RA (18.9±8.4% Vs 10.7±5.3%, p=0.04), whereas eBm5+Bm5 B cells were lower in active RA (75.4±9.4% Vs 85.9±6.4%, p=0.03). Zap-70+ B cells correlated with the total number of leukocytes (r=0.32, p=0.04), inversely with Bm2+Bm2'%(r=-0.56, p=0.05), and with the Bm2+Bm2'/eBm5+Bm5 ratio (r=-0.54, p=0.05). Moreover, the IHC analysis of CD68, CD3, CD20, CD27-CD20, CD38-CD20 and CD138 positive cells in the synovial tissue showed an increased trend in RA vs non-RA synovial tissues (p=ns). Zap-70+ B cells correlated significantly with CD38+ B cells (r=-0.79, p=0.04). Considering the RA synovial biopsies with the aggregate and diffuse patterns, CD27+ B cells characterized the pseudo-follicular/aggregate pattern (128.6±89.3/hpf vs 40.7±31.9/hpf; p=0.03). Conclusion: There is a characteristic accumulation of memory B cells in the joint compartment, with CD27 characterizing the aggregate pattern. In addition, data suggested that these cells are Zap-70+, which was previously shown to confer to B cells a longer life span. The synovial fluid of patients with active disease contained more "naïve" Bm1 and Bm2-Bm2' cells than patients with less active disease.