Yanxia Cao

Antigen-Specific B Cells Are Required as APCs and Autoantibody-Producing Cells for Induction of Severe Autoimmune Arthritis

B cells play an important role in rheumatoid arthritis, but whether they are required as autoantibody-producing cells as well as APCs has not been determined. We assessed B cell autoantibody and APC functions in a murine model of autoimmune arthritis, proteoglycan (PG)-induced arthritis, using both B cell-deficient mice and Ig-deficient mice (mIgM) mice that express an H chain transgene encoding for membrane-bound, but not secreted, IgM. The IgH transgene, when paired with endogenous λ L chain, recognizes the hapten 4-hydroxy-3-nitro-phenyl acetyl and is expressed on 1–4% of B cells. B cell-deficient and mIgM mice do not develop arthritis after immunization with PG. In adoptive transfer of PG-induced arthritis into SCID mice, T cells from mIgM mice immunized with PG were unable to transfer disease even when B cells from PG-immunized wild-type mice were provided, suggesting that the T cells were not adequately primed and that Ag-specific B cells may be required. In fact, when PG was directly targeted to the B cell Ig receptor through a conjugate of 4-hydroxy-3-nitrophenyl acetyl-PG, T cells in mIgM mice were activated and competent to transfer arthritis. Such T cells caused mild arthritis in the absence of autoantibody, demonstrating a direct pathogenic role for T cells activated by Ag-specific B cells. Transfer of arthritic serum alone induced only mild and transient arthritis. However, both autoreactive T cells and autoantibody are required to cause severe arthritis, indicating that both B cell-mediated effector pathways contribute synergistically to autoimmune disease.

IL-27 Induces a Th1 Immune Response and Susceptibility to Experimental Arthritis

IL-27 is the newest member of the cytokine family comprised of IL-12 and IL-23. IL-27 was originally described as a cytokine that along with IL-12 induces the differentiation of naive precursor T cells into Th1 effector cells. This activity has been called into question based on evidence in infectious disease and autoimmune models in which IL-27 is not absolutely required for the generation of IFN-γ, and IL-27 plays a regulatory role in controlling inflammation. We have previously reported in proteoglycan-induced arthritis (PGIA), a model of rheumatoid arthritis, that severe arthritis is dependent on the production of IFN-γ. In this study, we report that IL-27 was expressed in spleen and joint tissues of arthritic mice. We determined the involvement of IL-27 in PGIA by assessing the progression of arthritis in IL-27R−/− mice. Development of arthritis in IL-27R−/− mice was delayed and severity reduced in comparison with IL-27R+/+ littermate controls. Histology confirmed a reduction in joint cellularity, cartilage destruction, and bone erosion. Diminished arthritis was associated with fewer T cells producing IFN-γ and decreased IFN-γ secretion overtime. Moreover, the frequency of IL-4- and IL-17-expressing T cells and the production of IL-4 and IL-17 were similar in IL-27R−/− mice and controls. Our results indicate that IL-27 is critically involved in the induction of inflammation in PGIA. IL-27 functions by inducing the differentiation of IFN-γ-producing T cells in vivo that are essential for the development of arthritis.

Development of Proteoglycan-Induced Arthritis Is Independent of IL-17

IL-17 is the hallmark cytokine for the newly identified subset of Th cells, Th17. Th17 cells are important instigators of inflammation in several models of autoimmune disease; in particular, collagen induced arthritis (CIA) and experimental autoimmune encephalomyelitis (EAE), which were previously characterized as Th1-mediated diseases. Although high levels of IFN-γ are secreted in CIA and EAE, disease is exacerbated in IFN-γ- or IFN-γ receptor-deficient mice due to the ability of IFN-γ to suppress IL-17 secretion. However, in proteoglycan-induced arthritis (PGIA), severe arthritis is dependent on the production of IFN-γ. We were therefore interested in determining the role of IL-17 in PGIA. We assessed the progression of arthritis in IL-17-deficient (IL-17−/−) mice and found the onset and severity of arthritis were equivalent in wild-type (WT) and IL-17−/− mice. Despite evidence that IL-17 is involved in neutrophil recruitment, synovial fluid from arthritic joints showed a comparable proportion of Gr1+ neutrophils in WT and IL-17−/− mice. IL-17 is also implicated in bone destruction in autoimmune arthritis, however, histological analysis of the arthritic joints from WT and IL-17−/− mice revealed a similar extent of joint cellularity, cartilage destruction, and bone erosion despite significantly reduced RANKL (receptor activator of NK-κB ligand) expression. There were only subtle differences between WT and IL-17−/− mice in proinflammatory cytokine expression, T cell proliferation, and autoantibody production. These data demonstrate that IL-17 is not absolutely required for autoimmune arthritis and that the production of other proinflammatory mediators is sufficient to compensate for the loss of IL-17 in PGIA.

Expression of CD80/86 on B Cells Is Essential for Autoreactive T Cell Activation and the Development of Arthritis

Depletion of B cells in rheumatoid arthritis is therapeutically efficacious. Yet, the mechanism by which B cells participate in the inflammatory process is unclear. We previously demonstrated that Ag-specific B cells have two important functions in the development of arthritis in a murine model of rheumatoid arthritis, proteoglycan (PG)-induced arthritis (PGIA). PG-specific B cells function as autoantibody-producing cells and as APCs that activate PG-specific T cells. Moreover, the costimulatory molecule CD86 is up-regulated on PG-specific B cells in response to stimulation with PG. To address the requirement for CD80/CD86 expression on B cells in the development of PGIA, we generated mixed bone marrow chimeras in which CD80/CD86 is specifically deleted on B cells and not on other APC populations. Chimeras with a specific deficiency in CD80/CD86 expression on B cells are resistant to the induction of PGIA. The concentration of PG-specific autoantibody is similar in mice sufficient or deficient for CD80/86-expressing B cells, which indicates that resistance to PGIA is not due to the suppression of PG-specific autoantibody production. CD80/86-deficient B cells failed to effectively activate PG-specific autoreactive T cells as indicated by the failure of T cells from PG-immunized CD80/86-deficient B cell chimeras to transfer arthritis into SCID mice. In vitro secondary recall responses to PG are also dependent on CD80/86-expressing B cells. These results demonstrate that a CD80/86:CD28 costimulatory interaction between B cells and T cells is required for autoreactive T cell activation and the induction of arthritis but not for B cell autoantibody production.

Suppression of Proteoglycan-Induced Arthritis by Anti-CD20 B Cell Depletion Therapy Is Mediated by Reduction in Autoantibodies and CD4+ T Cell Reactivity

B cells have been implicated in the pathogenesis of rheumatoid arthritis (RA) since the discovery of RA as an autoimmune disease. There is renewed interest in B cells in RA based on the clinical efficacy of B cell depletion therapy in RA patients. Although, reduced titers of rheumatoid factor and anti-cyclic citrullinated peptide Abs are recorded, the mechanisms that convey clinical improvement are incompletely understood. In the proteoglycan-induced arthritis (PGIA) mouse model of RA, we reported that Ag-specific B cells have two important functions in the development of arthritis. PG-specific B cells are required as autoantibody-producing cells as well as Ag-specific APCs. Herein we report on the effects of anti-CD20 mAb B cell depletion therapy in PGIA. Mice were sensitized to PG and treated with anti-CD20 Ab at a time when PG-specific autoantibodies and T cell activation were evident but before acute arthritis. In mice treated with anti-CD20 mAb, development of arthritis was significantly reduced in comparison to control mAb-treated mice. B cell depletion reduced the PG-specific autoantibody response. Furthermore, there was a significant reduction in the PG-specific CD4+ T cell recall response as well as significantly fewer PG-specific CD4+ T cells producing IFN-γ and IL-17, but not IL-4. The reduction in PG-specific T cells was confirmed by the inability of CD4+ T cells from B cell-depleted mice to adoptively transfer disease into SCID mice. Overall, B cell depletion during PGIA significantly reduced disease and inhibited both autoreactive B cell and T cell function.

Collagen-induced arthritis is exacerbated in IL-10-deficient mice

IL-10 is a potent immunoregulatory cytokine attenuating a wide range of immune effector and inflammatory responses. In the present study, we assess whether endogenous levels of IL-10 function to regulate the incidence and severity of collagen-induced arthritis. DBA/1 wildtype (WT), heterozygous (IL-10+/-) and homozygous (IL-10-/-) IL-10-deficient mice were immunized with type II collagen. Development of arthritis was monitored over time, and collagen-specific cytokine production and anticollagen antibodies were assessed. Arthritis developed progressively in mice immunized with collagen, and 100% of the WT, IL-10+/-, and IL-10-/- mice were arthritic at 35 days. However, the severity of arthritis in the IL-10-/- mice was significantly greater than that in WT or IL-1+/- animals. Disease severity was associated with reduced IFN-γ levels and a dramatic increase in CD11b-positive macrophages. Paradoxically, both the IgG1 and IgG2a anticollagen antibody responses were also significantly reduced. These data demonstrate that IL-10 is capable of controlling disease severity through a mechanism that involves IFN-γ. Since IL-10 levels are elevated in rheumatoid arthritis synovial fluid, these findings may have relevance to rheumatoid arthritis.

IFN-γ Regulates the Requirement for IL-17 in Proteoglycan-Induced Arthritis

The contribution of the proinflammatory cytokines IFN-γ and IL-17 to the pathogenesis of experimental arthritis is controversial. In proteoglycan (PG)-induced arthritis (PGIA), severe arthritis is dependent on the production of IFN-γ, whereas IL-17 is dispensable. In collagen-induced arthritis and Ag-induced arthritis, although high levels of IFN-γ are secreted, disease is exacerbated in IFN-γ or IFN-γ receptor-deficient mice due to the ability of IFN-γ to suppress IL-17 expression. In the current study, we investigated the effect of IFN-γ on the IL-17 response and its consequences in PGIA. In PG-immunized IFN-γ−/− mice, despite reduction in arthritis, the PG-specific CD4+ T cell IL-17 response was significantly increased. Elevated IL-17 contributed to development of arthritis, as disease in IFN-γ/IL-17−/− was significantly reduced in comparison with either IFN-γ−/− or IL-17−/− mice. A contribution of IFN-γ and IL-17 to the development of arthritis was also identified in T-bet−/− mice. PG-specific CD4+ T cells from T-bet−/− mice produced reduced IFN-γ and elevated concentrations of IL-17. Both IFN-γ and IL-17 contribute to arthritis, as T-bet−/− mice lacking IL-17 (T-bet/IL-17−/−) were resistant, whereas wild-type, T-bet−/−, and IL-17−/− mice were susceptible to PGIA. T cell proliferation and autoantibody production did not correlate with development of disease; however, expression of cytokines and chemokines in joint tissues demonstrate that IFN-γ and IL-17 cooperatively contribute to inflammation. These results demonstrate that both IFN-γ and IL-17 have the potential to induce PGIA, but it is the strength of the IFN-γ response that regulates the contribution of each of these Th effector cytokines to disease.

Endocytic sequestration of the B cell antigen receptor and toll-like receptor 9 in anergic cells

In autoimmune prone murine strains, sequential engagement of the B cell antigen receptor (BCR) on the cell surface and toll-like receptors (TLRs) in late endosomes is necessary and sufficient for secretion of autoantibodies. However, ubiquitous nucleoprotein self-antigens fail to elicit productive TLR activation, and break self-tolerance in anergic DNA-reactive B cells. The mechanisms limiting TLR activation in these cells are largely unknown. Here, we demonstrate that in anergic 3H9/Vκ8 and Ars/A1 B cells the normal endocytic transit of both the ligated BCR and TLR9 into late endosomes is abrogated. The BCR and TLR9 arrest together just outside late endosomes, indicating that they enter this compartment along a single, regulated endocytic route. Access to late endosomes could be restored by reversing anergy through several methods, including conferring genetic susceptibility to autoimmunity, complementing proximal BCR signaling or by preventing BCR binding to self-antigen. Downstream of the BCR, JNK, which is activated in naive but not anergic B cells, regulated entry into late endosomes. Restoration of BCR and TLR9 endocytic trafficking rescued TLR9 activation by BCR-captured ligands. These results indicate that B cell anergy is reinforced by the exclusion of both TLRs and their BCR captured ligands from subcellular environments necessary for TLR activation.

B7-H1 expression on non-B and non-T cells promotes distinct effects on T- and B-cell responses in autoimmune arthritis

The immune system has developed several regulatory mechanisms to maintain homeostasis of adaptive immune responses. T‐cell programmed death (PD)‐1 recognition of B7‐H1 (PD‐L1) expressed on APC and non‐lymphoid tissue regulates T‐cell activation. We show that B7‐H1−/− mice exhibit exacerbated proteoglycan (PG)‐induced arthritis and increased Th‐1 CD4+ T‐cell responses. Unexpectedly, the PG‐specific antibody response in B7‐H1−/− mice was diminished. A reduction in the number of peanut agglutinin+ GC coincided with a decrease in CD19+ GL‐7+ CD95+ GC B cells that was a result of increased caspase‐induced apoptosis. The percent of CD38+ CD138+ emerging plasma cells was decreased. B7‐H1−/− mice exhibited an increased frequency of CD4+ PD‐1hi CXCR5hi ICOShi CD62Llo T follicular helper cells that displayed a hyperactive phenotype with increased expression of mRNA transcripts for Bcl6, IL‐21, and the apoptosis‐inducer molecule FasL. In cell transfer of B7‐H1−/− cells into SCID mice, non‐B and non‐T cells were sufficient to normalize the antibody response, T‐cell hyperactivity, and the development of PG‐induced arthritis. These findings indicate that B7‐H1 on non‐B and non‐T cells signals through PD‐1 on T effector cells to prevent excessive activation and reduce autoimmune arthritis. Furthermore, these findings demonstrate a novel role for B7‐H1 expression in promoting B‐cell survival by regulating the activation of T follicular helper cell.

B cell depletion enhances T regulatory cell activity essential in the suppression of arthritis.

The efficacy of B cell-depletion therapy in rheumatoid arthritis has driven interest in understanding the mechanism. Because the decrease in autoantibodies in rheumatoid arthritis does not necessarily correlate with clinical outcome, other mechanisms may be operative. We previously reported that in proteoglycan-induced arthritis (PGIA), B cell-depletion inhibits autoreactive T cell responses. Recent studies in B cell-depletion therapy also indicate a role for B cells in suppressing regulatory mechanisms. In this study, we demonstrate that B cells inhibited both the expansion and function of T regulatory (Treg) cells in PGIA. Using an anti-CD20 mAb, we depleted B cells from mice with PGIA and assessed the Treg cell population. Compared to control Ab-treated mice, Treg cell percentages were elevated in B cell-depleted mice, with a higher proportion of CD4+ T cells expressing Foxp3 and CD25. On a per-cell basis, CD4+CD25+ cells from B cell-depleted mice expressed increased amounts of Foxp3 and were significantly more suppressive than those from control Ab-treated mice. The depletion of Treg cells with an anti-CD25 mAb concurrent with B cell-depletion therapy restored the severity of PGIA to levels equal to untreated mice. Although titers of autoantibodies did not recover to untreated levels, CD4+ T cell recall responses to the immunizing Ag returned as measured by T cell proliferation and cytokine production. Thus, B cells have the capacity to regulate inflammatory responses by enhancing effector T cells along with suppressing Treg cells.