Affinity maturation in ectopic germinal centers

In silico subtraction approach reveals a close lineage relationship between follicular dendritic cells and BP3(hi) stromal cells isolated from SCID mice

Organization of the stromal compartments in secondary lymphoid tissue is a prerequisite for an efficient immune reaction. In particular, follicular dendritic cells (FDC) are pivotal for the activation and differentiation of B cells. To investigate the development of FDC, FDC together with tightly associated B cells (FDC networks) were micro-dissected from frozen tissue sections and follicular B cells were sorted by FACS. Using an in silico subtraction approach, gene expression of FDC was determined and compared with that of follicular stromal cells micro-dissected from the spleen of SCID mice. Nearly 90% of the FDC genes were expressed in follicular stromal cells of the SCID mouse, providing further evidence that FDC develop from the residual network of reticular cells. Thus, it suggests that rather minor modifications in the gene expression profile are sufficient for differentiation into mature FDC. The analysis of different immune-deficient mouse strains shows that a complex pattern of gene regulation controls the development of residual stromal cells into mature FDC. The in silico subtraction approach provides a molecular framework within which to determine the diverse roles of FDC in support of B cells and to investigate the differentiation of FDC from their mesenchymal precursor cells.

BLyS and APRIL in rheumatoid arthritis

The cytokines B lymphocyte stimulator (BLyS) and a proliferation-inducing ligand (APRIL) enhance autoimmune disease by sustaining B cell activation. In RA, B cells contribute to the formation of 3 functionally distinct types of lymphoid microarchitectures in the inflamed synovium: ectopic GCs; T cell-B cell aggregates lacking GC reactions; and unorganized, diffuse infiltrates. We examined 72 tissues representing the 3 types of synovitis for BLyS and APRIL production and for expression of APRIL/BLyS receptors. Biologic effects of BLyS and APRIL were explored by treating human synovium-SCID mouse chimeras with the APRIL and BLyS decoy receptor transmembrane activator and CAML interactor:Fc (TACI:Fc). GC+ synovitis had the highest levels of APRIL, produced exclusively by CD83+ DCs. BLyS was present in similar levels in all tissue types and derived exclusively from CD68+ macrophages. In GC+ synovitis, treatment with TACI:Fc resulted in GC destruction and marked inhibition of IFN-gamma and Ig transcription. In contrast, inhibition of APRIL and BLyS in aggregate and diffuse synovitis left Ig levels unaffected and enhanced IFN-gamma production. These differential immunomodulatory effects correlated with the presence of TACI+ T cells in aggregate and diffuse synovitis and their absence in GC+ synovitis. We propose that BLyS and APRIL regulate B cell as well as T cell function and have pro- and antiinflammatory activities in RA.

[Do B cells play an important role in the pathogenesis of rheumatoid arthritis?]

The role of B cells for the pathogenesis of rheumatoid arthritis (RA) has been debated for a long time. Here we show that chronic inflammation in the affected joints leads to the development of ectopic germinal centers. A micro-environment is established which supports B cell activation and differentiation. Plasma cells may develop which secrete autoantibodies of high affinity directly into the synovial tissue. Antigen/antibody complex formation, the activation of the complement cascade and the stimulation of macrophages may contribute to the destruction of joints. Furthermore, B cells are efficient antigen presenting cells. They seem to play a pivotal role in the activation of synovial T cells and the induction of cytokine secretion. The success of B cell depletion therapy by using the monoclonal antibody Rituximab further emphasized the importance of B cells in the pathogenesis of RA.

Antigen-specific memory B cell development

Helper T (Th) cell-regulated B cell immunity progresses in an ordered cascade of cellular development that culminates in the production of antigen-specific memory B cells. The recognition of peptide MHC class II complexes on activated antigen-presenting cells is critical for effective Th cell selection, clonal expansion, and effector Th cell function development (Phase I). Cognate effector Th cell-B cell interactions then promote the development of either short-lived plasma cells (PCs) or germinal centers (GCs) (Phase II). These GCs expand, diversify, and select high-affinity variants of antigen-specific B cells for entry into the long-lived memory B cell compartment (Phase III). Upon antigen rechallenge, memory B cells rapidly expand and differentiate into PCs under the cognate control of memory Th cells (Phase IV). We review the cellular and molecular regulators of this dynamic process with emphasis on the multiple memory B cell fates that develop in vivo.

Targeting plasma cells in autoimmune diseases

Antibodies specific for self-antigens mediate life-threatening pathology in several autoimmune diseases. Clearly the ability to target the plasma cells (PCs) producing the autoantibodies would be of great clinical benefit. Current immunosuppressive therapies are based on the premise that autoreactive PCs are short-lived and replenished from ongoing immune responses. However, recent results question this assumption and suggest that optimizing the treatment of severe autoimmune conditions will require a significant investment in elucidating the details of PC biology.

B cell abnormalities in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic, multisystem autoimmune disease characterized by the differentiation of short- and long-lived immunoglobulin secreting plasma cells that secrete pathogenic autoantibodies. Ectopic germinal centers and plasma cells secreting autoantibodies have been observed in lupus nephritis kidneys. Candidate genetic susceptibility loci for SLE include genes that affect differentiation and survival of plasma cells, such as those that influence activation, proliferation, cytokine and chemokine secretion/responsiveness, and apoptosis of the T and B cells that are involved in humoral immunity generated in germinal centers, as well as genes that are involved in presentation and clearance of apoptotic material and autoantigens by antigen presenting cells and other phagocytes. Emerging data have demonstrated that B lymphocytes are active participants in humoral immune responses that lead to T-dependent and T-independent differentiation of immunoglobulin-secreting plasma cells by homotypic CD154–CD40 interactions as well as continued stimulation by B cell activating factor through B cell maturation antigen, B cell activating factor receptor and transmembrane activater.

Clues to the etiology of autoimmune diseases through analysis of immunoglobulin genes

The role of autoantibodies in the etiology of autoimmune diseases remains unclear. However, an examination of the sequences of these autoantibodies can be informative. Antibody sequences that violate constraints normally imposed during ontogeny and during development point to a failure of regulation. The existence of clonally related sequences indicates that production of these antibodies may frequently be driven by self-antigen. A better understanding of the mechanisms that normally constrain the composition of the antibody repertoire and of the nature of the inciting and/or driving antigens may yield new insights into both the pathogenesis and potential treatment of these crippling diseases.