Clare A. Notley

The importance of natural IgM: scavenger, protector and regulator.

The existence of IgM has been known for more than a century, but its importance in immunity and autoimmunity continues to emerge. Studies of mice deficient in secreted IgM have provided unexpected insights into its role in several diverse processes, from B cell survival to atherosclerosis, as well as in autoimmunity and protection against infection. Among the various distinct properties that underlie the functions of IgM, two stand out: its polyreactivity and its ability to facilitate the removal of apoptotic cells. In addition, new B cell-targeted therapies for the treatment of autoimmunity have been shown to cause a reduction in serum IgM, potentially disrupting the functions of this immunoregulatory molecule and increasing susceptibility to infection.

Germinal center B cells govern their own fate via antibody feedback

High-affinity antibodies reenter germinal centers (GCs) and limit antigen access, thus causing sustained directional evolution in GCs toward higher-affinity antibody production.

Adoptive therapy with redirected primary regulatory T cells results in antigen-specific suppression of arthritis.

Regulatory T cells (Tregs) can suppress a wide range of immune cells, making them an ideal candidate for the treatment of autoimmunity. The potential clinical translation of targeted therapy with antigen-specific Tregs is hampered by the difficulties of isolating rare specificities from the natural polyclonal T cell repertoire. Moreover, the initiating antigen is often unknown in autoimmune disease. Here we tested the ability of antigen-specific Tregs generated by retroviral gene transfer to ameliorate arthritis through linked suppression and therefore without cognate recognition of the disease-initiating antigen. We explored two distinct strategies: T cell receptor (TCR) gene transfer into purified CD4+CD25+ T cells was used to redirect the specificity of naturally occurring Tregs; and co-transfer of FoxP3 and TCR genes served to convert conventional CD4+ T cells into antigen-specific regulators. Following adoptive transfer into recipient mice, the gene-modified T cells engrafted efficiently and retained TCR and FoxP3 expression. Using an established arthritis model, we demonstrate antigen-driven accumulation of the gene modified T cells at the site of joint inflammation, which resulted in a local reduction in the number of inflammatory Th17 cells and a significant decrease in arthritic bone destruction. Together, we describe a robust strategy to rapidly generate antigen-specific regulatory T cells capable of highly targeted inhibition of tissue damage in the absence of systemic immune suppression. This opens the possibility to target Tregs to tissue-specific antigens for the treatment of autoimmune tissue damage without the knowledge of the disease-causing autoantigens recognized by pathogenic T cells.

Th17 cells are restrained by Treg cells via the inhibition of interleukin-6 in patients with rheumatoid arthritis responding to anti–tumor necrosis factor antibody therapy

OBJECTIVE The importance of interleukin-17 (IL-17) is underscored both by its resistance to control by Treg cells and the propensity of Treg cells to produce this highly inflammatory cytokine. This study sought to address whether Th17 cells are inhibited by Treg cells in rheumatoid arthritis (RA) patients responding to anti-tumor necrosis factor (anti-TNF) therapy, and if so defining the underlying mechanisms of suppression. METHODS Inhibition of Th17 cell responses was determined by Treg cell suppression assays. The Treg cell phenotype was analyzed using flow cytometry and enzyme-linked immunosorbent assay. Mechanisms of suppression were tested by cytokine addition or antibody blockade. RESULTS Th17 responses were inhibited by Treg cells from RA patients responding to the anti-TNF antibody adalimumab (Treg(ada) ), but not by Treg cells from healthy individuals or patients with active RA. Furthermore, Treg(ada) cells secreted less IL-17, even when exposed to proinflammatory monocytes from patients with active RA. Treg(ada) cells suppressed Th17 cells through the inhibition of monocyte-derived IL-6, but this effect was independent of IL-10 and transforming growth factor β, which mediated the suppression of Th1 responses. Adalimumab therapy led to a reduction in retinoic acid receptor-related orphan nuclear receptor C-positive Th17 cells and an increase in FoxP3+ Treg cells lacking expression of the transcription factor Helios. However, this acquisition of IL-17-suppressor function was not observed in RA patients responding to treatment with etanercept, a modified TNF receptor-Fc fusion protein. Indeed, there was no alteration in Treg cell number, function, or phenotype in etanercept-treated patients, and Th17 responses remained unchecked. CONCLUSION Th1 and Th17 responses are controlled through distinct mechanisms by Treg cells from patients responding to anti-TNF antibody therapy. Adalimumab therapy, but not etanercept therapy, induces a potent and stable Treg cell population with the potential to restrain the progression of IL-17-associated inflammation in RA via regulation of monocyte-derived IL-6.

Natural IgM is required for suppression of inflammatory arthritis by apoptotic cells.

The clearance of dying cells is vital for re-establishing tolerance during inflammation and has potent immunoregulatory consequences. Because natural IgM plays a key role in the removal of apoptotic cells, we investigated whether the immune modulatory properties of apoptotic cells depended on its presence. Using an Ab-independent, Ag-induced model of inflammatory arthritis, we tested whether natural IgM is essential for the arthritis-suppressing properties of apoptotic cells. Whereas administration of apoptotic cells reduced joint inflammation and damage in normal mice accompanied by suppression of the Th17 response, no protection was afforded in secreted IgM-deficient (Sμ–) mice. The enhanced production of IL-10 by T cells from draining lymph nodes and splenic marginal zone B cells, driven by the infusion of apoptotic cells, was abrogated in the absence of natural IgM. Apoptotic cells were present shortly after administration in the splenic marginal zone, but their removal was substantially delayed in the absence of natural IgM. Incubation of apoptotic cells with natural IgM in vitro restored their arthritis-suppressing properties in Sμ– mice. Moreover, these IgM-coated apoptotic cells were cleared rapidly after injection from the spleens of Sμ– mice. Our results demonstrate that natural IgM is a critical factor in a chain of events triggered by the administration of apoptotic cells that promote IL-10–secreting B and T cells and restrain the development of inflammation.

The yin and yang of regulatory T cells and inflammation in RA

Rheumatoid arthritis (RA) is characterized by chronic inflammation leading to joint destruction. Regulatory T (TREG) cells are potent suppressors of autoimmunity, but are not capable of controlling every aspect of the inflammatory reaction. We have found that TREG-cell function is abnormal in patients with RA, and that a distinct population of TREG cells with potent suppressive properties is induced after therapy with inhibitors of tumor necrosis factor. In this Review, we discuss the mutual interactions between the opposing forces of TREG cells and inflammation in the context of RA. Therapeutic approaches that enhance TREG-cell function whilst controlling inflammation are likely to be the most effective strategies for restoring immune tolerance in patients with this disease.

Secreted IgM Enhances B Cell Receptor Signaling and Promotes Splenic but Impairs Peritoneal B Cell Survival

B cell survival has a central role in maintaining immune responses to foreign organisms while curbing autoimmunity. In this study, we show that mature B cell survival is impaired and B cell turnover is accelerated in the spleen of mice lacking secreted IgM. Although in vitro responses to BCR cross-linking were normal, there was a marked reduction in basal ERK and global tyrosine phosphorylation in splenic B cells from serum IgM-deficient mice, suggesting diminished interaction with cognate Ag in vivo. The provision of BAFF either in vitro or in vivo reversed the increase in B cell apoptosis, demonstrating that other survival signals can compensate for the loss of secreted IgM in the spleen. In striking contrast to the splenic compartment, peritoneal B cell survival was enhanced in secreted IgM-deficient mice, despite a similar reduction in basal BCR signaling compared with wild type mice. These results suggest that secreted IgM acts as an adjuvant, boosting BCR signals to maintain survival and maintenance of mature splenic B cells while increasing B cell apoptosis in the peritoneum. BAFF administration mitigated the consequences of secreted IgM deficiency on B cell survival in the spleen but not in the peritoneum. This work provides new insight into the regulation of B cell signaling and homeostasis in different peripheral compartments by secreted IgM.

Engulfment of Activated Apoptotic Cells Abolishes TGF-β–Mediated Immunoregulation via the Induction of IL-6

Phagocytosis of apoptotic cells (ACs) is usually a potent immunoregulatory signal but can also promote inflammation. In this article, we show that administration of apoptotic dendritic cells (DCs) inhibited inflammation in vivo through increasing production of TGF-β from intrinsic DCs and B cells. However, ACs derived from LPS-activated DCs failed to restrain inflammation because of a short-lived but marked IL-6 response, which abolished the increase in TGF-β. Inhibition of IL-6 restored the protective anti-inflammatory properties of aACs and the TGF-β response. DCs isolated from mice that had received resting but not activated ACs could transfer the suppression of inflammation to recipient mice. These transferred DCs stimulated B cell TGF-β production and relied on an intact B cell compartment to limit inflammation. These results highlight how the activation state of AC governs their ability to control inflammation through reciprocal regulation of IL-6 and TGF-β.

Induced CD8+FoxP3+ Treg Cells in Rheumatoid Arthritis Are Modulated by p38 Phosphorylation and Monocytes Expressing Membrane Tumor Necrosis Factor α and CD86

Limiting the severity of inflammation and promoting its eventual resolution are vital for protecting host tissues both in autoimmunity and chronic infection. The aim of this study was to determine the suitability of repurposing anti‐CD3 monoclonal antibody (mAb) therapy for rheumatoid arthritis (RA) by analyzing its ability to induce CD8+FoxP3+ Treg cells from peripheral blood mononuclear cells (PBMCs).

Production of IL-17: What's STAT got to do with it?

Th17 cells are important mediators of autoimmunity, yet the mechanisms by which they are controlled are not fully understood. Studies in mice, including a recent article in Nature Immunology by Yang et al., show that IL-2 is an important inhibitory factor for the differentiation of Th17 cells, inducing phosphorylation of STAT5, which outcompetes STAT3 binding at the IL-17 locus. In humans however, IL-2 appears to be crucial for Th17 differentiation, yet inhibits the expansion of antigen-specific Th17 clones, again via a STAT5 mechanism. Here we discuss how the article by Yang et al. offers a novel mechanism to explain how changes in the balance of different cytokines in the inflammatory environment may alter the stability or phenotype of regulatory T cells and T helper cell subsets.