Association of BAFF/BLyS overexpression and altered B cell differentiation with Sjögren's syndrome.

BAFF (BLyS, TALL-1, THANK, zTNF4) is a member of the TNF superfamily that specifically regulates B lymphocyte proliferation and survival. Mice transgenic (Tg) for BAFF develop an autoimmune condition similar to systemic lupus erythematosus. We now demonstrate that BAFF Tg mice, as they age, develop a secondary pathology reminiscent of Sjögren's syndrome (SS), which is manifested by severe sialadenitis, decreased saliva production, and destruction of submaxillary glands. In humans, SS also correlates with elevated levels of circulating BAFF, as well as a dramatic upregulation of BAFF expression in inflamed salivary glands. A likely explanation for disease in BAFF Tg mice is excessive survival signals to autoreactive B cells, possibly as they pass through a critical tolerance checkpoint while maturing in the spleen. The marginal zone (MZ) B cell compartment, one of the enlarged B cell subsets in the spleen of BAFF Tg mice, is a potential reservoir of autoreactive B cells. Interestingly, B cells with an MZ-like phenotype infiltrate the salivary glands of BAFF Tg mice, suggesting that cells of this compartment potentially participate in tissue damage in SS and possibly other autoimmune diseases. We conclude that altered B cell differentiation and tolerance induced by excess BAFF may be central to SS pathogenesis.

Cutting edge: BAFF regulates CD21/35 and CD23 expression independent of its B cell survival function.

Herein we demonstrate that B cell-activating factor of the TNF family (BAFF), a B cell survival factor, also regulates CD21/35 and CD23 expression. BAFF blockade in wild-type mice down-modulates CD21/35 and CD23 on B cells while survival remains intact, and BAFF exposure causes elevated CD21/35 and CD23 expression. Similar down-modulation is observed in bcl-2-transgenic mice treated with a BAFF inhibitor. This is the first evidence that BAFF has a function independent of B cell survival. Reports using CD21/35 and CD23 expression to assess splenic B cell subsets in BAFF-null mice concluded a lack of B cells beyond the immature stage. Since CD21/35 and CD23 are inadequate for delineating B cell subpopulations in BAFF-null mice, we used expression of BAFF-R and several B cell markers to identify more mature splenic B cells in these mice. These data broaden our understanding of BAFF function and correct the view that BAFF-null mice lack mature B cells.

Cutting edge: germinal centers formed in the absence of B cell-activating factor belonging to the TNF family exhibit impaired maturation and function.

Germinal centers (GCs) form in B cell follicles and require specific signals for development and maintenance. B cell-activating factor belonging to the TNF family (BAFF) is a fundamental B cell survival factor and therefore may influence GC reactions and subsequent Ab responses. To test this possibility, the effect of BAFF neutralization in immunized mice was assessed. Using B cell maturation Ag-Fc, we demonstrate that BAFF blockade does not inhibit GC formation or somatic hypermutation. However, GCs in B cell maturation Ag-Fc-treated mice dissipated more rapidly than those of control mice and did not form a mature follicular dendritic cell reticulum. Examination of immunized BAFF-null mice validated the BAFF-independent nature of GC formation. Furthermore, Ab responses, including high-affinity responses, were attenuated. This is the first evidence that BAFF is required for maintenance, but not initiation, of the GC reaction, and it further hints that somatic hypermutation within the GC and selection of Ag-specific high-affinity Ab could be uncoupled.

The contribution of Fc effector mechanisms in the efficacy of anti-CD154 immunotherapy depends on the nature of the immune challenge.

Blockade of the CD154-CD40 co-stimulatory pathway with anti-CD154 mAbs has shown impressive efficacy in models of autoimmunity and allotransplantation. Clinical benefit was also demonstrated in systemic lupus erythematosus (SLE) and idiopathic thrombocytopenia patients with the humanized anti-CD154 mAb, 5C8 (hu5C8). However, thromboembolic complications that occurred during the course of the hu5C8 clinical trials have proven to be a major setback to the field and safe alternative therapeutics targeting the CD154-CD40 pathway are of great interest. Recently, effector mechanisms have been shown to play a part in anti-CD154 mAb-induced transplant acceptance in murine models, while this issue remains unresolved for humoral-mediated models. Herein, aglycosyl anti-CD154 mAbs with reduced binding to FcgammaR and complement were used as a novel means to test the role of effector mechanisms in non-human primate and murine models not amenable to gene knockout technology. While aglycosyl hu5C8 mAb was relatively ineffective in rhesus renal and islet allotransplantation, it inhibited primary and secondary humoral responses to a protein immunogen in cynomolgus monkeys. Moreover, an aglycosyl, chimeric MR1 mAb (muMR1) prolonged survival and inhibited pathogenic auto-antibody production in a murine model of SLE. Thus, the mechanisms required for efficacy of anti-CD154 mAbs depend on the nature of the immune challenge.