Persistence of Disease-Associated Anti-Citrullinated Protein Antibody-Expressing Memory B Cells in Rheumatoid Arthritis in Clinical Remission

OBJECTIVE

In rheumatoid arthritis (RA), autoreactive B cells are pathogenic drivers and sources of anti-citrullinated protein antibodies (ACPAs) that are a diagnostic biomarker and predictor of worse long-term prognosis. Yet, the immunobiologic significance of persistent ACPA production at the cellular level is poorly understood. This study was undertaken to investigate the representation of ACPA-expressing switched-memory B cells in RA.

METHODS

In a cross-sectional study of RA patients, we investigated the presence of continued defects in immune homeostasis as a function of disease activity. Using an enzyme-linked immunosorbent assay (ELISA) and a sensitive multiplex bead-based immunoassay, we characterized fine binding antibody specificities in sera, synovial fluid (SF), and B cell culture supernatants. In this manner, we determined the frequency and epitope reactivity patterns of ACPAs produced by SF B cells and switched-memory blood B cells and compared the latter to serum ACPA levels and disease activity scores.

RESULTS

Cultured B cells from SF were shown to spontaneously secrete ACPAs, while constitutive IgG autoantibody production by peripheral blood mononuclear cells (PBMCs) was substantially less frequent. After in vitro stimulation, PBMCs secreted IgG ACPA that was overwhelmingly from switched-memory B cells, across all patient groups treated with methotrexate and/or a tumor necrosis factor inhibitor. Intriguingly, the frequencies of ACPA-expressing switched-memory B cells significantly correlated with serum IgG anti-cyclic citrullinated peptide 3 (r = 0.57, P = 0.003). Moreover, treatment-induced clinical remission had little or no effect on the circulating burden of switched-memory ACPA-expressing B cells, in part explaining the continued dysregulation of humoral immunity.

CONCLUSION

Our findings rationalize why therapeutic cessation most often results in disease reactivation and clinical flare. Hence, a clinical disease activity score is not a reliable indicator of the resolution of pathologic recirculating B cell autoimmunity.

MCPIP1/Regnase is a negative feedback inhibitor of IL-17-mediated signaling and inflammation (CCR3P.200)

IL-17 came to prominence with the discovery of Th17 cells. IL-17 induces inflammatory pathology in autoimmunity and certain infections, and thus constraint of this pathway is an essential part of its regulation. However, to date our understanding of IL-17 signaling checkpoints remains quite limited. Here, we identify MCPIP1 (Regnase-1, Zc3h12a) as a novel negative feedback inhibitor of IL-17 signaling. IL-17 treatment of fibroblasts stimulated increased MCPIP1 expression, whereas MCPIP1 silencing enhanced IL-17-mediated signal transduction. Conversely, reconstitution of MCPIP1-/- cells restricted IL-17 signaling, which was dependent on the MCPIP1 endonuclease domain, not its deubiquitinase domain. In vivo, MCPIP1 deficiency enhanced IL-17-dependent autoimmunity and host resistance to infection. Namely, susceptibility to candidiasis was reduced in MCPIP1 haploinsufficient mice, whereas IL-17-dependent pathology in EAE and pulmonary inflammation was enhanced. In addition to its well-documented capacity to degrade cytokine mRNA transcripts such as Il6, we found that MCPIP1 regulates some, though not all, IL-17-dependent promoters. In addition, we identified a novel function for MCPIP1 in mediating decay of mRNA transcripts encoding inflammatory receptors, including IL-17R subunits, in a manner independent of their 3’ UTR elements. This is the first demonstration that MCPIP1 restricts IL-17R signal transduction, and that this enzyme regulates inflammatory receptor expression.

Role of Integrins in mediating Th17 Pathogenesis (CAM3P.203)

IL-23 plays a key role in autoimmune pathology, as evidenced by the resistance of IL-23 deficient animals to many models of autoimmune disease; susceptibility SNPs in the IL-23 receptor have also been identified in human disease including inflammatory bowel disease and psoriasis. We have previously demonstrated that IL-23 drives effector differentiation and proliferation of primary and memory Th17 cells in vivo. We have now identified a novel integrin that is expressed by Th17 cells in an IL-23-dependent manner. Inflammatory Th17 cells (generated in vitro in the presence of IL-23, IL-6 and IL-1) express high levels of this integrin, while ‘regulatory’ Th17 cells do not, as TGFβ suppresses expression in a dose-dependent manner. Integrin blockade rendered mice resistant to experimental autoimmune encephalomyelitis (EAE), and knockout mice were also resistant to EAE. T cells specifically require integrin expression for EAE induction. While effector functions such as IL-17 are not impaired, integrin knockout cells have reduced migration out of LN and into CNS. Together these data highlight a novel Th17-specific integrin that is regulated by IL-23, and may promote effector Th17 functions and/or proliferation.

“Natural” Th17 cells control Candida albicans infections in the oral mucosa (MUC7P.749)

Oropharyngeal candidiasis (OPC) is an opportunistic mucosal infection caused by the commensal fungus Candida albicans. To date, there is a paucity of research on oral mucosal immunity in general and to fungi in particular. Humans with rare mutations have revealed a vital protective role of IL-17 signaling to OPC. It has long been assumed that conventional “iTh17” cells produce IL-17. However, many innate populations also produce IL-17, but their role in oral immunity is unknown. Since rodents are naïve to C. albicans, we evaluated OPC in a murine model. T cell receptor rearrangement is required for immunity, since Rag1-/-, SCID and IL-7Rα-/- mice are susceptible to OPC. However, iTh17, NK and NKT cells were dispensable. We identified a CD3+CD4+CD44hiTCRβ+ tissue-resident population in tongue that produces IL-17 rapidly upon Candida exposure. These were phenotypically identical to ‘natural Th17’ cells, a recently described, thymically derived innate lymphocyte population whose function is highly enigmatic. Development of nTh17 and iTh17 cells involves differential AKT/mTOR pathways, with nTh17 cells requiring mTORC2, and iTH17 cells requiring mTORC1. Consistently, mice deficient in the mTORC2 subunit Rictor were highly susceptible to OPC, whereas mice treated with rapamycin, which selectively targets mTORC1, were similar to WT mice. Therefore, innate immunity to Candida requires nTh17 cells, whose presence in the oral mucosa ideally positions them to control oral pathogens.

IL-23 promotes autoimmune Th17 memory formation and function (59.6)

IL-23 is essential for the differentiation of pathogenic effector Th17 cells, but little is known about its requirement in memory Th17 responses. To investigate autoimmune Th17 memory, we developed a variation of the experimental autoimmune encephalomyelitis model, in which CD4+ T cells from MOG(35-55)-immunized donors were transferred into naïve recipients and rested before re-challenge. Following secondary recall, memory Th17 cells rapidly expanded in lymph nodes before trafficking to the central nervous system in high numbers, resulting in earlier onset and increased severity of clinical signs. Th17 memory cells were generated from IL-17+ and RORγt+ precursors, and stability of the Th17 phenotype was dependent on time. IL-23 was required during the primary response for generation of Th17 memory cells During the recall response, IL-17 production was not directly impacted by neutralizing IL-23R at secondary challenge, but the generation of effectors that co- expressed T-bet was strongly impaired. In addition, IL-23R blockade resulted in decreased proliferation and reduced total numbers of Th17 memory cells in the secondary response.

Th17 memory cells aren't fooled by sugar-coating

Klebsiella bacteria evade humoral immunity by altering their polysaccharide capsular antigens. In this issue of Immunity, Chen et al. (2011) propose a new vaccination strategy, demonstrating that Th17 memory cells can provide cross-protection against multiple serotypes of Klebsiella pneumoniae.

Foxp3+ regulatory T cells promote Th17 development in vivo through regulation of IL-2 (168.16)

Th17 development is driven by multiple cytokines, including TGF-β. Regulatory T cells (Tregs) promote Th17 development through production of TGF-β in vitro. However their role in vivo remains unclear, particularly since Tregs inhibit inflammation in many models of autoimmunity where Th17 cells are active. Using mice expressing Diphtheria toxin receptor under control of the Foxp3 promoter, Foxp3+ Tregs were depleted in adult mice during in vivo Th17 priming. Treg depletion resulted in a reduced frequency of antigen-specific IL-17 producers in draining lymph nodes and blood, correlating with reduced inflammatory skin responses. In contrast, Tregs did not promote IL-17 after initial activation stages. Treg production of TGF-β was not required for Th17 promotion, neither was suppression of Th1-associated cytokines. Rather, regulation of IL-2 availability and resultant signaling through CD25 and pSTAT5 by Tregs was found to play an important role.