The data reported by Takemura and coworkers [3] suggest that B cells are absolutely required to support the activation and effector function of synovial CD4+ T cells, extending their functional contribution to the disease process far beyond autoantibody production [4]. The nature of this supportive interaction is as yet unclear, although two possibilities seem likely: B cells present antigens to autoreactive T cells, driving their activation; or the presence of B cells in synovial tissue generates survival signals for T cells.
In order to appreciate how B cells function in rheumatoid synovitis, it is important to understand their role in the histological architecture of synovial lesions. The joints of some RA patients exhibit what appear to be functional ectopic germinal centers (Fig. 1a), with T cells and B cells arranged around a network of dendritic cells. Lesions in other patients do not form follicular microstructures, but rather contain aggregates of T cells and B cells (Fig. 1b), whereas still other lesions have a diffuse arrangement of T cells and B cells (Fig. 1c). These patterns of synovial histology are stable over time and consistent within patients, with tissues from distinct joints exhibiting the same type of inflammatory lesion.
Interestingly, these patterns can be correlated with bio-markers for B cell activity. The relationship between B cells and the establishment of complex lymphoid architectures has been studied in a large cohort of synovial biopsies, all collected from patients with active rheumatoid synovitis (authors' unpublished data). As a first step, B cell activity in the tissues was determined by quantifying immunoglobulin transcription. Interestingly, rheumatoid synovium can display a wide range of IgG production, varying over several orders of magnitude. Tissues containing germinal centers tend to have the highest levels of IgG mRNA, tissues containing B cell aggregates have intermediate levels, and tissues with a diffuse arrangement of T cells and B cells tend to have the lowest levels.
Clearly, the functional significance of these patterns makes it critical to understand how lymphoid organogenesis occurs in synovial tissue. Organization of cells in lymphoid organs and in inflammatory lesions is orchestrated by chemokines – soluble messengers that stimulate chemotactic behavior. Two such molecules, namely CC chemokine ligand (CCL)2 and CCL18, were previously proposed to play a role in recruitment of lymphocytes and macrophages to rheumatoid lesions. Analysis of the levels of these molecules in synovial tissue, however, reveals no significant correlation between patterns of B cell organization and levels of CCL2 or CCL18 mRNA production, suggesting that other chemokines are more important in directing how B cells are recruited to the synovial membrane and determining how they arrive at a distinct site in the synovial microenvironment.
Indeed, analysis of other chemokines reveals an intriguing relationship between lesion architecture and chemokine levels. Takemura and coworkers [5] found that two chemokines, namely CXC chemokine ligand (CXCL)13 and CCL21, are major determinants of germinal center formation. CXCL13 has been implicated in the recruitment of B cells, whereas CCL21 has been shown to attract T cells. Again, both of these chemokines are found at high levels in synovial tissues that are capable of forming germinal centers and at low levels in other rheumatoid lesions (Fig. 2b, c). Thus, CXCL13 and CCL21 are not only involved in regulating the positioning of T cells and B cells in structured lymphoid organs; they also serve a critical role in supporting the infrastructure of the rheumatoid lesion. The end result is a process of ectopic lymphoid neogenesis, providing the disease lesion with robust structural support. It is important to recognize that this process is directly related to the functioning of B cells and relies on their contributions far beyond the release of autoantibodies.
Takemura and coworkers [5] provided a clue to the process by which B cells may regulate ectopic germinal center formation. They observed that two members of the TNF superfamily, lymphotoxin (LT)-α and LT-β, were differentially expressed in tissues with distinct B cell organization. These proteins form α1β2 heterotrimers and, along with CXLC13 and CCL21, have been implicated in lymph node formation in mouse knockout models. There is now evidence that in rheumatoid synovitis LT-β is supplied by B cells. Analysis of LT-β mRNA production in synovial tissue reveals a strong correlation with levels of IgG production, two markers closely related to synovial architecture (Fig. 2a). The relationship between LT-β production and IgG production was the first hint that B cells are the cellular source of the TNF-like ligand. Indeed, immunohistochemical analysis has identified expression of LT-β on a subset of B cells. Specifically, LT-β+ B lymphocytes are positioned in the follicular mantle zone, assigning a critical contribution of that structure to ectopic lymphoid organogenesis.
Braun and coworkers [6] demonstrated recently that LT-α1β2 has functions beyond regulating how B cells are embedded into a three-dimensional network. LT-α1β2 stimulation also causes profound changes in the function of synovial fibroblasts. In particular, LT-α1β2 stimulation led to the induction of a number of factors in fibroblast-like synoviocytes that contribute to inflammation and T cell recruitment. These include IL-1β; matrix metalloproteinases; the T cell attracting chemokines CCL2, CCL5, and CCL8; and cell adhesion molecules required for efficient attachment of lymphocytes. These data support a critical role for LT-β producing B cells in sustaining principal disease pathways in rheumatoid synovitis. Such disease pathways include the process of lymphoid organogenesis as well as sustaining the activation of tissue resident fibroblasts.