Background and Aims Thrombotic Thrombocytopenic Purpura (TTP) is due to deficiency or dysfunction of the metalloproteinase ADAMTS13, which cleaves von Willebrand factor. This results in increased platelet adhesion and thrombus formation in small blood vessels. Most cases are associated with autoantibodies (IgM/IgG/IgA) to ADAMTS13, which are somatically mutated (sub-classes being predominantly IgG4 subclass, followed by IgG1). Rituximab (RTX) is an effective treatment for patients with TTP and also for patients with Rheumatoid Arthritis (RA). In both conditions, remission can last for months or even years after RTX but in RA, relapse closer to B-cell return is more common. Factors controlling B-cell maturation, differentiation and autoantibody production probably impact remission. Survival and maturation of B-cells largely depends on interactions with the B-cell cytokine B-cell activating factor (BAFF). In order to explore the possible role of the BAFF/BAFF-receptor (BAFF-R or BR3) system in maintaining long remissions following RTX in TTP patients, we investigated B-cell phenotypes, serum BAFF levels and BAFF-R expression. The relationship between time after treatment, B-cell phenotype and BAFF-R expression with laboratory parameters in TTP patients was also explored.
Materials and Methods We compared B-cell phenotypes using FACS analysis (based on IgD/CD27 on CD19+ B-cells) between both diseases at B-cell return following RTX, RA (n = 5), TTP (n = 6) and in the TTP cohort of patients, also in patients in long-term remission (n = 13;10–68 months post-RTX). In this cross-sectional study, we also investigated the temporal relationship between serum BAFF and expression of BAFF-R on B-cell sub-populations. Comparisons between B-cell sub-populations, BAFF-R expression on B-cell sub-populations and between diseases were with non-parametric statistics and relationships with time and serum BAFF levels using Spearman’s rank correlation coefficients.
Results B-cell return after RTX followed similar patterns in TTP and RA, with naïve B-cells predominant and BAFF-R expression low in all B-cell subpopulations in both diseases. In TTP patients in long-term remission, naive B-cells remained high (p < 0.01) and post-switch memory B-cells (IgD-CD27+) low (p < 0.05) and BAFF-R expression reduced (p < 0.001 for all B-cell sub-populations compared with healthy controls. BAFF levels were most strongly inversely correlated with BAFF-R expression (MFI) on naïve (IgD+) B-cells (r2 > 0.45; p = 0.01).
Conclusions As extended periods of remission after RTX were associated with persistence of naïve B-cell phenotype in TTP and this may reflect the inability of potentially autoreactive B-cells to be selected into the memory compartment or into autoantibody-secreting cells. Reduced BAFF-R expression after RTX may decrease pro-survival B-cell signalling, perhaps reducing opportunities for ADAMTS13 specific B-cells to proliferate and/or undergo class-switch recombination.