Notes From the 6th Global Arthritis Research Network (GARN) Meeting

GARN Meeting: Session 2

Friday, May 11, 2007 (pm)
B-cell biology in health and disease
Chairs: Saleh Ibrahim, Rostock, Germany and Wlodzimierz Maslinski, Warsaw, Poland

Edward A. Clark, MD, Seattle, WA, USA
Signaling pathways regulating B-cell survival and death

Edward A. Clark, MD, Seattle, WA, USA, introduced the audience to the topic of B-cell receptor (BCR) dependent molecular control of B-cell proliferation, survival and apoptosis. Antigen binding to BCR results in the transition of B-cells from G0 to G1 phase of cell cycle. The progression of B-cells from G1 phase of cell cycle into G2/S phase or cellular death by apoptosis is regulated by different molecules. Dr. Clark addressed the critical role of Bcl-2 interacting mediator (BIM), a protein belonging to the BH3-Bcl2 family, in the control of apoptosis of immature B-cells. B-cells derived from BIM knockout mice process through the G1 phase check point and proliferate. Furthermore, the data provide evidence on the role of B-cell adapted molecule of 32 kDa (Bam32) kinase in B-cell proliferation. The B-cells continue to G2/S phase, when the Bam32 kinase interacts with the phosphatidylinositol 3-, 4-, 5-bisphosphate [PI(3,4,5)P2] in the cellular membrane. Then, Bam32 activates Rac that in turn controls actin reorganization, BCR internalization, antigen presentation and membrane ruffling. In support of the above findings, B-cells derived from Bam32 knockout mice show defects in proliferation and progress through G2/S phase of cell cycle. A down regulation of B-cell proliferation can occur when the cytoplasmic proteins p47phox or gp91phox are expressed and produce reactive oxygen radicals. Dr. Clark finished his lecture by addressing the role of caspases, in particular caspase 6, in B-cell proliferation. Caspase 6 enhances B-cell entrance to the G1 phase and Notch1 is its candidate substrate.

Michael P. Cancro, MD, Philadelphia PA, USA
The Blyss/Baff family of ligands and receptors-molecular determinants of B-cell homeostasis

Michael P. Cancro, MD, Philadelphia, PA, USA, discussed in his talk the role of Blys in B-cell development and homeostasis. The BlyS/BAFF family consists of two ligands: a proliferation inducing ligand (APRIL) and BlyS. BlyS is able to bind three receptors: transmembrane activator and cyclophilin ligand interactor (TACI), B-cell maturation antigen (BCMA), and BAFF receptor 3 (BR3). Competition for BlyS controls selection during the transitional cell stage of B-cell development. Immature B-cells in the bone marrow show minimal BlyS binding activities. When these B-cells exit the bone marrow and pass through the transitional steps in the spleen, they display high binding activity for BlyS and express increased levels of BR3 and TACI receptors. Elimination of BlyS with gene deletion causes depletion of mature B-cell number. After a T-cell dependent response, distinct pools of B-cells are formed: short-lived primary antibody forming cells (AFCs), germinal centers (GC) B-cells, and long-lived memory B-cells. BR3 is expressed abundantly in GC B-cell pools, TACI in the AFC, and BCMA in memory B-cells. Since TACI is the only receptor expressed in the AFC, the APRIL-TACI interaction is important for the formation of antibodies. APRIL knockout mice have reduced IgA class switching. The memory B-cells likely use APRIL as a ligand for binding to its BCMA receptor. In conclusion, novel therapeutics might be developed for specific pools of B-cells by targeting the BlyS family of ligands and receptors.

Peter Lipsky, MD, NIH, Bethesda, MD, USA
B-cells and autoimmune disease

Peter Lipsky, MD, NIH, Bethesda, MD, USA, gave an insightful overview of the role of B-cells in the pathogenesis of systemic lupus erythematosus (SLE) indicating existing therapeutic opportunities and challenges. He explained the origins of autoantibodies in SLE drawing attention particularly to the process of de novo generation of autoreactive B-cells from nonautoreactive precursors that occurs in the peripheral compartment and is driven by molecular mechanism of somatic hypermutation of the immunoglobulin genes. This in turn implicates that the function of autoantigens in SLE might be to select B-cells induced to be autoreactive by somatic hypermutation rather than to stimulate B-cell autoreactivity. Dr. Lipsky also discussed the role of plasma cells (PCs) in lupus pathogenesis demonstrating high numbers of PCs in the blood of patients with SLE and their high correlation with disease activity as well as autoantibodies levels. Furthermore the pathological significance of CD27 (+) memory B-cells for systemic autoimmunity has been addressed. These cells have been shown to correlate with disease activity and even more interestingly are resistant to conventional immunosuppressive therapy, suggesting that new therapies targeting specifically memory B-cells might provide new opportunities to induce remission in SLE. In the last part of the lecture the role of IL-21/IL-21R in costimulation of B-cells during T- and B-cell interaction has been highlighted. Dr. Lipsky's group has recently identified a novel population of human memory B-cells that specifically respond to IL-21 and B-cell-activating factor belonging to the TNF family (BAFF/BLyS), rapidly differentiating into PC and thus contributing in an antigen-independent manner to serologic memory.

Chairs: Ladislav Šenolt, Prague, Czech Republic and Tsutomu Takeuchi, Saitama, Japan

Ari Theofilopoulos, MD, La Jolla, CA, USA
The role of Toll receptors and interferons in systemic autoimmunity

Ari Theofilopoulos, MD, La Jolla, CA, USA, discussed the current understanding of the causative role of Toll-like receptors (TLRs) and interferons (IFNs) in the systemic autoimmunity. Having introduced the family of IFNs (type I:IFN-α/β, type II: IFN-γ and III: IFN-σ) he presented evidence derived from reverse and forward genetic approaches on the central role of IFN-α/β in the pathogenesis of SLE. The speaker also addressed three issues of natural mechanisms preventing recognition of endogenous nucleic acids and TLRs engagement: 1) cellular compartmentalization, 2) DNA modification (for example, DNA methylation), and 3) endonucleases. In an attempt to find the answer to what causes systemic autoimmunity, Dr. Theofilopoulos and colleagues formulated a two-phase paradigm of type I IFN induction that involves TLR-dependent and TLR-independent pathways. According to this model, the initial phase of the SLE associated autoimmunity does not require activation of TLRs and is mediated by lymphoid dendritic cells (DCs), which take up endogenous apoptotic material and produce IFN-α/β. Under the effect of IFN-?/? DCs maturate into self-antigen presenting cells leading to the activation of autoreactive T helper cells. As a result of T-cell dependent stimulation and the effects of BLyS, APRIL and IFN-α/β autoreactive B-cells proliferate and differentiate into PC. Subsequently, immune complexes containing nucleic acids are formed and TLRs are triggered, inducing the TLR-dependent phase. This phase serves as an amplification loop encompassing further IFN-α/β production, enhanced B-cell proliferation and autoantibodies production. Presented pathways emerge as central pathogenic events in the course of SLE-associated autoimmunity, suggesting that IFN-α/β blockade as well as inhibition of TLRs engagement might be promising targets to treat lupus and related diseases.

Moncef Zouali, MD, Paris, France
Tracking the fate of autoreactive B lymphocytes

Moncef Zouali, MD, Paris, France, explained mechanisms of B-cell tolerance. He discussed the role of receptor editing in autoimmunity. Bone marrow immature B-cells express a variety of BCRs and are highly self reactive. In healthy subjects these cells undergo extensive BCR editing and only the nonself-reacting cells are released to the periphery. The remaining self-reactive cells that do not successfully accomplish rearrangement of BCR become anergic or undergo apoptosis. In lupus, self-reactive B-cells persist because they fail to efficiently accomplish the BCR receptor editing. In particular, the rearrangement of Vk locus of immunoglobulin gene is impaired. Dr. Zouali addressed also the molecular mechanisms underlying the drug induced SLE. Bone marrow B-cells pretreated with hydralazine or with a MEK-inhibitor can induce autoimmunity in the murine model. He concluded that the Erk signaling pathway contributes to the pathogenesis of hydralazine-induced lupus.

Session 2 summarized by Joanna Stanczyk and Emmanuel Karouzakis

6th Global Arthritis Research Network Meeting

• Introduction
• Notes From Session 1
• Notes From Session 2
• Notes From Session 3
• Notes From Session 4