Joanna Groom

Disrupted cardiac development but normal hematopoiesis in mice deficient in the second CXCL12/SDF-1 receptor, CXCR7

Chemotactic cytokines (chemokines) attract immune cells, although their original evolutionary role may relate more closely with embryonic development. We noted differential expression of the chemokine receptor CXCR7 (RDC-1) on marginal zone B cells, a cell type associated with autoimmune diseases. We generated Cxcr7−/− mice but found that CXCR7 deficiency had little effect on B cell composition. However, most Cxcr7−/− mice died at birth with ventricular septal defects and semilunar heart valve malformation. Conditional deletion of Cxcr7 in endothelium, using Tie2-Cre transgenic mice, recapitulated this phenotype. Gene profiling of Cxcr7−/− heart valve leaflets revealed a defect in the expression of factors essential for valve formation, vessel protection, or endothelial cell growth and survival. We confirmed that the principal chemokine ligand for CXCR7 was CXCL12/SDF-1, which also binds CXCR4. CXCL12 did not induce signaling through CXCR7; however, CXCR7 formed functional heterodimers with CXCR4 and enhanced CXCL12-induced signaling. Our results reveal a specialized role for CXCR7 in endothelial biology and valve development and highlight the distinct developmental role of evolutionary conserved chemokine receptors such as CXCR7 and CXCR4.

B Cell-Activating Factor Belonging to the TNF Family (BAFF)-R Is the Principal BAFF Receptor Facilitating BAFF Costimulation of Circulating T and B Cells

BAFF (B cell-activating factor belonging to the TNF family) is a cell survival and maturation factor for B cells, and overproduction of BAFF is associated with systemic autoimmune disease. BAFF binds to three receptors, BAFF-R, transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), and B cell maturation Ag (BCMA). Using specific mAbs, BAFF-R was found to be the predominant BAFF receptor expressed on peripheral B cells, in both humans and mice, and antagonist mAbs to BAFF-R blocked BAFF-mediated costimulation of anti-μ responses. The other BAFF receptors showed a much more restricted expression pattern, suggestive of specialized roles. BCMA was expressed by germinal center B cells, while TACI was expressed predominantly by splenic transitional type 2 and marginal zone B cells, as well as activated B cells, but was notably absent from germinal center B cells. BAFF was also an effective costimulator for T cells, and this costimulation occurs entirely through BAFF-R. BAFF-R, but not TACI or BCMA, was expressed on activated/memory subsets of T cells, and T cells from BAFF-R mutant A/WySnJ mice failed to respond to BAFF costimulation. Thus, BAFF-R is important not only for splenic B cell maturation, but is the major mediator of BAFF-dependent costimulatory responses in peripheral B and T cells.

BAFF and MyD88 signals promote a lupuslike disease independent of T cells

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease characterized by the production of autoantibodies. However, the underlying cause of disease appears to relate to defects in T cell tolerance or T cell help to B cells. Transgenic (Tg) mice overexpressing the cytokine B cell–activating factor of the tumor necrosis factor family (BAFF) develop an autoimmune disorder similar to SLE and show impaired B cell tolerance and altered T cell differentiation. We generated BAFF Tg mice that were completely deficient in T cells, and, surprisingly, these mice developed an SLE-like disease indistinguishable from that of BAFF Tg mice. Autoimmunity in BAFF Tg mice did, however, require B cell–intrinsic signals through the Toll-like receptor (TLR)–associated signaling adaptor MyD88, which controlled the production of proinflammatory autoantibody isotypes. TLR7/9 activation strongly up-regulated expression of transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), which is a receptor for BAFF involved in B cell responses to T cell–independent antigens. Moreover, BAFF enhanced TLR7/9 expression on B cells and TLR-mediated production of autoantibodies. Therefore, autoimmunity in BAFF Tg mice results from altered B cell tolerance, but requires TLR signaling and is independent of T cell help. It is possible that SLE patients with elevated levels of BAFF show a similar basis for disease.

TNF Deficiency Fails to Protect BAFF Transgenic Mice against Autoimmunity and Reveals a Predisposition to B Cell Lymphoma

TNF is well characterized as a mediator of inflammatory responses. TNF also facilitates organization of secondary lymphoid organs, particularly B cell follicles and germinal centers, a hallmark of T-dependent Ab responses. TNF also mediates defense against tumors. We examined the role of TNF in the development of inflammatory autoimmune disorders resembling systemic lupus erythematosus and Sjögren’s syndrome induced by excess B cell-activating factor belonging to the TNF family (BAFF), by generating BAFF-transgenic (Tg) mice lacking TNF. TNF−/− BAFF-Tg mice resembled TNF−/− mice, in that they lacked B cell follicles, follicular dendritic cells, and germinal centers, and have impaired responses to T-dependent Ags. Nevertheless, TNF−/− BAFF-Tg mice developed autoimmune disorders similar to that of BAFF-Tg mice. Disease in TNF−/− BAFF-Tg mice correlates with the expansion of transitional type 2 and marginal zone B cell populations and enhanced T-independent immune responses. TNF deficiency in BAFF-Tg mice also led to a surprisingly high incidence of B cell lymphomas (>35%), which most likely resulted from the combined effects of BAFF promotion of neoplastic B cell survival, coupled with lack of protective antitumor defense by TNF. Thus, TNF appears to be dispensable for BAFF-mediated autoimmune disorders and may, in fact, counter any proneoplastic effects of high levels of BAFF in diseases such as Sjögren’s syndrome, systemic lupus erythematosus, and rheumatoid arthritis.

Marginal-Zone B-Cells of Nonobese Diabetic Mice Expand With Diabetes Onset, Invade the Pancreatic Lymph Nodes, and Present Autoantigen to Diabetogenic T-Cells

OBJECTIVE—B-cells are important for disease pathogenesis in the nonobese diabetic (NOD) mouse model of type 1 diabetes. Recent studies demonstrate that marginal-zone B-cells (MZBs), which connect innate with adaptive immune responses, are increased in NOD mice. However, beyond this, the contribution of different B-cell subsets to diabetes pathogenesis is poorly understood. RESEARCH DESIGN AND METHODS—To better understand the role of different B-cell subsets in the etiology of type 1 diabetes, we have examined the MZB compartment in NOD mice, with respect to their number, distribution, and function. RESULTS—We demonstrate that splenic MZB numbers in female NOD mice undergo a marked, approximately threefold expansion between ∼12 and 16 weeks of age, coincident with the onset of frank diabetes. Functionally, NOD MZBs are hyperresponsive to toll-like receptor 9 ligation and CD40 ligation, as well as sphingosine-1-phosphate–dependent chemotactic cues, suggesting an increased sensitivity to selective innate- and activation-induced stimuli. Intriguingly, at 16 weeks of age, ∼80% of female NOD mice present with MZB-like cells in the pancreatic lymph node (PLN). These MZB-like cells express major histocompatibility complex class II and high levels of CD80 and CD86, and their presence in the PLN is associated with an increased frequency of activated Vβ4+ CD4+ T-cells. Significantly, we demonstrate that purified MZBs are able to present the autoantigen insulin to diabetogenic T-cells. CONCLUSIONS—These data are consistent with MZBs contributing to the pathogenesis of type 1 diabetes as antigen-presenting cells. By integrating innate-derived inflammatory signals with the activation of autoreactive T-cells, MZBs may help to direct T-cell responses against β-cell self-constituents.

Increased CD4+Foxp3+ T Cells in BAFF-Transgenic Mice Suppress T Cell Effector Responses

The cytokine B cell activation factor of the TNF family (BAFF) is considered to perform a proinflammatory function. This paradigm is particularly true for B cell-dependent immune responses; however the exact role for BAFF in regulating T cell immunity is ill-defined. To directly assess the effect of BAFF upon T cells, we analyzed T cell-dependent immune responses in BAFF-transgenic (Tg) mice. We found that T cell responses in BAFF-Tg mice are profoundly compromised, as indicated by their acceptance of islet allografts and delayed skin graft rejection. However, purified BAFF-Tg effector T cells could reject islet allografts with a normal kinetic, suggesting that the altered response did not relate to a defect in T cell function per se. Rather, we found that BAFF-Tg mice harbored an increased number of peripheral CD4+Foxp3+ T cells. A large proportion of the BAFF-expanded CD4+CD25+Foxp3+ regulatory T cells (Tregs) were CD62LlowCD103high and ICAM-1high, a phenotype consistent with an ability to home to inflammatory sites and prevent T cell effector responses. Indeed, depletion of the endogenous BAFF-Tg Tregs allowed allograft rejection to proceed, demonstrating that the increased Tregs were responsible for preventing alloimmunity. The ability of BAFF to promote Treg expansion was not T cell intrinsic, as Tregs did not express high levels of BAFF receptor 3, nor did excessive BAFF trigger NF-κB2 processing in Tregs. In contrast, we found that BAFF engendered Treg expansion through an indirect, B cell-dependent mechanism. Thus, under certain conditions, BAFF can play a surprising anti-inflammatory role in T cell biology by promoting the expansion of Treg cells.

Development of nephritis but not sialadenitis in autoimmune-prone BAFF transgenic mice lacking marginal zone B cells

B cell‐activating factor belonging to the TNF family (BAFF) is a B cell survival factor required for B cell maturation. BAFF transgenic (Tg) mice develop autoimmune disorders characterized by autoantibody production, which leads to nephritis and salivary gland destruction (sialadenitis), features reminiscent of systemic lupus erythematosus and Sjögrens syndrome (SS), respectively. Disease in BAFF Tg mice correlates with the expansion of the marginal zone (MZ) B cell compartment and the abnormal presence of MZ‐like B cells in the blood, LN and inflamed salivary glands, suggesting a role for these cells in BAFF‐induced autoimmunity. Lymphotoxin‐β (LTβ)‐deficient mice show disrupted splenic architecture, lack MZ B cells and some peripheral LN, and are unable to mount T cell‐dependent immune responses. BAFF Tg mice lacking LTβ (LTβ▵‐BTg) retained these defects, yet still developed nephritis associated with the presence of B‐1 B cells in the kidneys. However, in contrast to old BAFF Tg mice, aging LTβ▵‐BTg mice no longer developed sialadenitis. Thus, autoimmune disorders in BAFF Tg mice are possibly events coordinated by MZ and B‐1 B cells at separate anatomical sites.

B cells flying solo.

Systemic autoimmunity such as systemic lupus erythematosus (SLE) is associated with the loss of B‐cell tolerance, B‐cell dysregulation and autoantibody production. While some autoantibodies may contribute to the pathology seen with SLE, numerous studies have shown that dysregulation of T‐cell function is another critical aspect driving disease. The positive results obtained in clinical trials using T‐cell‐ or B‐cell‐specific treatments have suggested that cooperation between T and B cells probably underlies disease progression in many patients. A similar cooperative mechanism seemed to explain SLE developing in mice overexpressing the B‐cell‐activating factor from the tumor necrosis factor family (BAFF). However, surprisingly, T‐cell‐deficient BAFF transgenic (Tg) mice develop SLE similar to T‐cell‐sufficient BAFF Tg mice, and the disease was linked to innate activation of B cells and production of proinflammatory autoantibody isotypes. In conclusion, dysregulated innate activation of B cells alone can drive disease independently of T cells, and as such this aspect represents a new pathogenic mechanism in autoimmunity.

Levels of BAFF in Serum in Primary Biliary Cirrhosis and Autoimmune Diabetes

Levels of the B-cell activating cytokine BAFF are increased in serum in various autoimmune disease, and particularly Sjögrens syndrome in which there is evident B-lymphocyte proliferation. Studies in two autoimmune disease in which B-cell proliferation is less evident, primary biliary cirrhosis (PBC), and adult-onset Type 1 diabetes, showed serum levels of BAFF to be mostly in the normal range. A single raised level among eight sera tested in one patient studied with autoimmune hepatitis (AH) coincided with a relapse of the disease. Increased levels of BAFF in human sera, indexing a potent antigenic drive on B-cell production and survival in some autoimmune diseases, may mark only particular stages in the evolution of such diseases.

Development of autoimmune nephritis in genetically asplenic and splenectomized BAFF transgenic mice

B cell activating factor belonging to the TNF family (BAFF or BLyS) is a critical B cell survival factor essential for B cell maturation. BAFF transgenic (Tg) mice develop autoimmunity resembling Systemic Lupus Erythematosus (SLE) in a T cell-independent but toll-like receptor (TLR) signalling-dependent manner, requiring TLR-induced innate B cell-derived pro-inflammatory autoantibody deposition in the kidneys. Importantly, neutralizing BAFF in the clinic shows efficacy in patients with SLE, confirming its critical role in the progression of this disease in both humans and mouse models. The specific B cell types that produce autoantibodies in BAFF Tg mice are TLR-activated innate marginal zone (MZ) B cells and B1 cells, but not follicular B cells. Interestingly, in BAFF Tg mice MZ-like B cells infiltrate salivary glands whereas B1 B cells infiltrate the kidneys. To ascertain the relevance of B1 and MZ-like B cells in the development of nephritis in BAFF Tg mice, we generated genetically asplenic as well as splenectomized BAFF Tg animals. BAFF Tg mice born without a spleen lack MZ B cells, have very reduced B1a B cell numbers but a normal B1b B cell compartment. Loss of these B cell subsets failed to protect BAFF Tg mice against nephritis indicating that B1b B cells are an important subset for the development of autoimmune nephritis in BAFF Tg mice. Thus the spleen is dispensable for the development of autoimmune nephritis in BAFF Tg mice and points toward a pathogenic role for innate B1 B cells. Identifying similar innate Bxa0cells in humans may offer the possibility of more targeted B cell therapies.