To determine the effect of excessive production of BAFF on the distribution and function of B cell subsets in patients with primary Sjögren's syndrome (SS).
The phenotype of B lymphocytes was analyzed by flow cytometry. Differences in the expression level of membrane IgD and CD38 were used to identify B lymphocyte subsets evolving from naive Bm1 through memory Bm5 cells. Based on our finding of a low expression of CD45RA, we sorted Bm2/Bm2' cells to determine the time course of translocation of the CD19 molecule and the B cell receptor into lipid rafts, by confocal microscopy. Serum levels of BAFF were measured by an enzyme-linked immunosorbent assay developed in-house.
"Circulating" Bm2/Bm2' cells were expanded in patients with primary SS compared with rheumatic disease controls and with normal controls. In addition, these B cell subsets were functionally abnormal. Prolonged residency of the B cell receptor in lipid rafts in these cells was associated with elevated CD19 expression in B cells, most notably, Bm2 and Bm2' cells, obtained from the patients with primary SS. BAFF levels were higher in the patients than in the normal controls and correlated with the percentage of Bm2/Bm2' cells and their expression of CD19 in primary SS patients. These correlations were confirmed by placing sorted Bm1 or Bm2 cells from normal controls in culture in the presence or absence of BAFF.
Bm2/Bm2' cells express more CD19 molecules in primary SS patients than in normal controls. BAFF might participate in this elevated expression of CD19. These patients might be suitable candidates for treatment with BAFF antagonists.