Transitional cells represent a crucial step in the differentiation and selection of the mature B cell compartment. Human transitional B cells have previously been variably identified based on the high level of expression of CD10, CD24, and CD38 relative to mature B cell populations and are expanded in the peripheral blood following rituximab-induced B cell-depletion at reconstitution. In this study, we take advantage of the gradual acquisition of the ABCB1 transporter during B cell maturation to delineate refined subsets of transitional B cells, including a late transitional B cell subset with a phenotype intermediate between T2 and mature naive. This late transitional subset appears temporally following the T1 and T2 populations in the peripheral compartment after rituximab-induced B cell reconstitution (and is thus termed T3) and is more abundant in normal peripheral blood than T1 and T2 cells. The identity of this subset as a developmental intermediate between early transitional and mature naive B cells was further supported by its ability to differentiate to naive during in vitro culture. Later transitional B cells, including T2 and T3, are found at comparatively increased frequencies in cord blood and spleen but were relatively rare in bone marrow. Additional studies demonstrate that transitional B cells mature across a developmental continuum with gradual up-regulation of mature markers, concomitant loss of immature markers, and increased responsiveness to BCR cross-linking in terms of proliferation, calcium flux, and survival. The characterization of multiple transitional B cell subpopulations provides important insights into human B cell development.

Objective. Recent data suggest that the reconstituting peripheral B cell compartment after B cell depletion therapy may be functionally immature, with a preponderance of transitional B cells and a paucity of memory B cells. This study was undertaken to determine the magnitude, duration, and cause of these defects in rituximab-treated systemic lupus erythematosus (SLE) patients.Methods. Fifteen patients with SLE previously treated with rituximab as part of a phase I/II doseescalation study were evaluated during a long-term followup (mean followup period 41 months). B cells from peripheral blood and tonsils were assessed using multicolor flow cytometry, and their developmental pathway was classified based on the expression of defined surface markers. Rituximab is a chimeric mouse/human monoclonal antibody directed against the B cell-specific antigen CD20, which depletes B lymphocytes in vivo from the pre-B cell stage in bone marrow, when CD20 is first expressed, to the mature B cell stage. Due to its efficacy in the depletion of both normal and malignant B cells, rituximab represents an effective treatment for B cell lymphomas and has emerged as a promising treatment for multiple autoimmune diseases, including systemic lupus erythematosus (SLE), as we and other investigators have previously described (1-4).However, the long-term immunologic effects of rituximab, the mechanism(s) whereby B cell depletion Dr.

Objective We postulated that proteasome inhibition (PI) may be useful in the treatment of SLE by targeting plasmacytoid dendritic cells (pDCs) and plasma cells (PCs), both critical to disease pathogenesis. Methods Lupus prone mice were treated with the non-selective PIs carfilzomib and bortezomib, the LMP7-selective immunoproteasome inhibitor ONX 0914, or vehicle control. Tissues were harvested and analyzed by flow cytometry using standard markers. Nephritis was monitored by proteinuria and kidney harvest. Serum anti-dsDNA levels were measured by ELISA and total IgG and dsDNA antibody secreting cells (ASC) by ELIspot. Human PBMCs or mouse bone marrow cells were incubated with TLR agonists and PIs and interferon α measured by ELISA and flow cytometry. Results Early treatment of lupus prone mice with the dual targeting PIs carfilzomib or bortezomib or the immunoproteasome specific inhibitor ONX 0914 prevented disease progression, and treatment of mice with established disease dramatically abrogated nephritis. Treatment had profound effects on plasma cells with greater reductions in autoreactive than total IgG ASCs, an effect that became more pronounced with prolonged treatment, and was reflected in decreasing serum autoantibodies. Remarkably, proteasome inhibition efficiently suppressed production of interferon α by toll-like receptor activated pDCs in vitro and in vivo, an effect mediated by both an inhibition of pDC survival and function. Conclusions Inhibition of the immunoproteasome is equally efficacious to dual targeting agents in preventing lupus disease progression by targeting two critical pathways in disease pathogenesis, type I interferon activation and autoantibody production by plasma cells.

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Rheumatoid arthritis (RA) is mediated by a proinflammatory cytokine network with TNF at its apex. Accordingly, drugs that block TNF have demonstrated significant efficacy in the treatment of RA. A great deal of experimental evidence also strongly implicates B cells in the pathogenesis of RA. Yet, it remains unclear whether these two important players and the therapies that target them are mechanistically linked. In this study we demonstrate that RA patients on anti-TNF (etanercept) display a paucity of follicular dendritic cell networks and germinal center (GC) structures accompanied by a reduction in CD38+ GC B cells and peripheral blood memory B cell lymphopenia compared with healthy controls and RA patients on methotrexate. This study provides initial evidence in humans to support the notion that anti-TNF treatment disrupts GC reactions at least in part via effects on follicular dendritic cells.