The immunopathogenesis of seropositive rheumatoid arthritis: from triggering to targeting

Journal name:
Nature Reviews Immunology
Volume:
17,
Pages:
60–75
Year published:
DOI:
doi:10.1038/nri.2016.124
Published online

Abstract

Patients with rheumatoid arthritis can be divided into two major subsets characterized by the presence versus absence of antibodies to citrullinated protein antigens (ACPAs) and of rheumatoid factor (RF). The antibody-positive subset of disease, also known as seropositive rheumatoid arthritis, constitutes approximately two-thirds of all cases of rheumatoid arthritis and generally has a more severe disease course. ACPAs and RF are often present in the blood long before any signs of joint inflammation, which suggests that the triggering of autoimmunity may occur at sites other than the joints (for example, in the lung). This Review summarizes recent progress in our understanding of this gradual disease development in seropositive patients. We also emphasize the implications of this new understanding for the development of preventive and therapeutic strategies. Similar temporal and spatial separation of immune triggering and clinical manifestations, with novel opportunities for early intervention, may also occur in other immune-mediated diseases.

At a glance

Figures

  1. Genetic epidemiology of RA.
    Figure 1: Genetic epidemiology of RA.

    a | Genome-wide association studies have demonstrated major differences between the two subsets of rheumatoid arthritis (RA) as defined by the presence versus absence of antibodies to citrullinated protein antigens (ACPAs). Genetic association of RA with the HLA region (in particular, certain HLA-DR alleles; also known as the shared epitope (HLA-DR-SE) alleles) and with PTPN22 is mainly confined to the ACPA-positive subset of patients5, 120, 121. In the ACPA-negative subset, the overall heritability of disease is much lower122, and the influence of HLA genes is limited and different from ACPA-positive disease113. The P value for genome-wide significance is indicated by the horizontal blue lines. b | Pronounced gene–gene and gene–environment interactions are present in ACPA-positive RA but absent in ACPA-negative disease. The figure presents the combined effects of two genetic factors (HLA-DR-SE and PTPN22) and two environmental factors (smoking and alcohol consumption) on the lifetime risk of RA in the two disease subsets, and illustrates the increasingly complex pattern of determinants of disease. The data represent a combination of data on the lifetime risk of RA123 and on gene–gene and gene–environment interactions72, 124. c | The information regarding RA-associated genes, environmental factors and their interactions can be combined with information on active molecular pathways in different cell types and in different organs as studied, for example, by expression quantitative trait loci analyses. The graph illustrates such cell type-specific expression of genes that have been associated with disease through an analysis of single nucleotide polymorphisms (SNPs) in different cell types125. The graph shows a preferential expression of genes related to T cells in the context of SNPs related to seropositive RA. Hereby, CD4+ T cells, particularly effector T cells, have been implicated in the pathogenesis of ACPA-positive RA125, 126, 127. This combined view lends support to further studies of T cell specificity and effector function in patients with seropositive RA, including (but not necessarily limited to) T cells of T helper 1, T follicular helper and regulatory T cell phenotypes. Chr, chromosome; NK, natural killer. Part a is reproduced from Ref. 120. Part c is reproduced from Ref. 125.

  2. From triggering to targeting: a longitudinal perspective on the development of seropositive RA.
    Figure 2: From triggering to targeting: a longitudinal perspective on the development of seropositive RA.

    The figure illustrates a model for the longitudinal disease course of rheumatoid arthritis (RA). Interestingly, the pathology associated with anti-citrulline immunity often does not begin with arthritis; instead, the pathology can begin with bone loss and arthralgia before the onset of synovitis128, 129. The figure illustrates four schematic phases of disease development. Phase 1 (triggering): autoimmunity is triggered as a result of environmental stimuli acting together with the genotype at mucosal surfaces such as the lungs or possibly the gums or gut. Phase 2 (maturation): the autoimmune response gradually matures and involves epitope spreading and increasing titres of autoantibodies (antibodies to citrullinated protein antigens (ACPAs) and rheumatoid factor (RF)). This autoimmune response is restricted to sites outside the joints, possibly in the regional lymph nodes and other peripheral lymphoid organs. This phase is contingent on MHC class II-dependent T cell activation (as evidenced from genetic linkage studies) and the resulting interaction between T cells and B cells. Phase 3 (targeting): symptoms such as bone loss and joint pain (arthralgia) develop owing to certain ACPAs acting in synergy with RF. The binding of ACPAs to osteoclasts induces the production of the chemokine CXCL8, which functions as an autocrine growth factor for osteoclasts. CXCL8 also has the capacity to bind to its receptors CXCR1 and CXCR2 on nociceptive nerves and thereby cause pain. Phase 4 (fulminant disease): synovitis and the classical symptomatology of RA as defined by ACR/EULAR criteria develop. ACPAs may also promote the formation of neutrophil extracellular traps (NETosis) after binding to citrullinated histones in NETs. Moreover, citrullinated fibrinogen fragments, and potentially other citrullinated proteins, can bind to Toll-like receptors (TLRs) on synovial cells, leading to the production of interleukin-6 (IL-6), tumour necrosis factor (TNF) and matrix metalloproteinases (MMPs). The four phases depicted in the figure may occur sequentially; importantly, they may also co-occur or revert. The major message is that disease-inducing immunity may be triggered at one site by one mechanism, and thereafter target another site or organ much later, potentially using a different mechanism. We also postulate that an 'additional hit' may contribute to the targeting of the joint (see Fig. 5 for details). BCR, B cell receptor; TCR, T cell receptor.

  3. Local early immune activation in the lungs.
    Figure 3: Local early immune activation in the lungs.

    Various noxious agents (such as smoke particles, silica particles, textile dust and microorganisms) can cause insults in the lungs and, in the context of the susceptibility genes, trigger rheumatoid arthritis (RA)-associated immune reactions. Events inside the lungs involve the activation of dendritic cells (DCs), macrophages and B cells by Toll-like receptor (TLR) stimuli from particles in cigarette smoke, silica dust or textile dust or from components of an aberrant microbial flora42, 43, 44. This process may lead to peptidyl-arginine deiminase (PAD) activation and local extracellular citrullination and thereby the appearance of RA-associated neo-antigens. Another consequence may be the differentiation of B cells, whereby several different stimuli and specific recognition events can cause initial somatic mutations of immunoglobulin genes, enabling B cells to bind and present post-translationally modified autoantigens to T cells. Germinal centre-like structures (induced bronchus-associated lymphoid tissue (iBALT)) are formed in the lungs early in the development of RA–associated immune responses and disease35, 36, and T cell-dependent B cell activation promotes the local production of antibodies to citrullinated protein antigens (ACPAs) in lungs34. APC, antigen-presenting cell; TCR, T cell receptor.

  4. Involvement of adaptive immunity.
    Figure 4: Involvement of adaptive immunity.

    The figure depicts the interaction between the peptide–MHC complex on an antigen-presenting cell (APC) and the T cell receptor (TCR) on a T cell. Citrullinated peptides (as indicated by red rings) have been shown to bind the P4 pocket of the peptide-binding groove of the HLA-DR molecules associated with rheumatoid arthritis (RA); these genetically defined HLA-DR alleles are known as the shared epitope (HLA-DR-SE) alleles (Fig. 1). Neutral citrulline can be accommodated in the positively charged P4 pocket of the HLA-DR variants that predispose for RA, whereas the positively charged amino acid arginine is unlikely to fit into this structure66. Specific binding to the RA-associated P4 pocket has been demonstrated for peptides from vimentin, fibrinogen, α-enolase, aggrecan and cartilage intermediate-layer protein66, 68, 69. In addition, arginine or citrulline can be found at other positions in the antigenic peptide, including those facing towards the TCR. Examples of citrulline being recognized directly by the TCR at positions −2, −1 and 2 come from peptides of α-enolase and type II collagen69, 130, 141. Together, these findings indicate the importance of specific genetic associations of HLA-DR-SE alleles with RA in presenting disease-associated T cell epitopes. However, it is becoming increasingly clear from studies of both B cell and T cell responses in RA that this disease is not the result of an aberrant immune response to a single autoantigen but to many autoantigens, of which we still do not understand the relative importance and/or hierarchy.

  5. Development of ACPA-mediated disease as a precursor to RA.
    Figure 5: Development of ACPA-mediated disease as a precursor to RA.

    Autoantibodies (and in particular antibodies to citrullinated protein antigens (ACPAs)) generated at extra-articular sites and present in the circulation can mediate bone destruction and pain before the occurrence of chronic joint inflammation. Osteoclasts develop from monocyte precursors in the presence of receptor activator of nuclear factor-κB ligand (RANKL; also known as TNFSF11) and macrophage colony-stimulating factor (M-CSF). The physiological maturation of these precursor cells depends on the enzymatic activity of peptidyl arginine deiminases (PADs), leading to an increased expression of citrullinated targets in mature osteoclasts. Some ACPAs can bind these targets and activate osteoclasts to secrete CXCL8, which in turn induces further osteoclast development and maturation82. Antibodies incorporated in immune complexes, especially those that are a-sialylated83, 131, can also bind to the Fc receptors expressed on the surface of maturing osteoclasts to increase osteoclast differentiation through the activation of immunoreceptor tyrosine-based activation motif (ITAM) signalling. The activation of osteoclasts through antibodies, either alone or incorporated in immune complexes, finally leads to bone erosion, cortical fenestration and loss of trabecular bone in the absence of any sign of inflammation. ACPAs bound to osteoclasts trigger CXCL8 release, which in turn sensitizes or activates sensory neurons by binding to CXCR1 and CXCR2 (receptors for CXCL8) expressed on nociceptors87, 132, 133, 134, 135. ACPAs, but not other antibodies, induce mechanical and thermal hypersensitivity as well as a decrease in spontaneous locomotor activity, and these changes can largely be reversed by CXCR1 and CXCR2 blockade84.

  6. The inflammatory cascade in established RA.
    Figure 6: The inflammatory cascade in established RA.

    This figure illustrates how antibodies to citrullinated protein antigens (ACPAs) and rheumatoid factor (RF) may contribute to joint inflammation. a | Binding of ACPAs to osteoclasts can induce CXCL8 secretion, which promotes neutrophil chemoattraction. Neutrophils can also migrate to joints in response to non-specific inflammatory stimuli (caused by 'additional hits'; not shown). CXCL8 promotes neutrophils to release neutrophil extracellular traps (NETs), which can be further enhanced by ACPAs binding to citrullinated histones exposed in the NETs. b | Moreover, expression of citrullinated proteins in articular cartilage might allow ACPAs97, 136 to bind the cartilage surface and initiate local inflammation through immune complex-mediated mechanisms. c | ACPAs might also bind citrullinated proteins directly in the synovial membrane following minor insults (such as trauma or infection). d | Immune complexes between citrullinated proteins (such as fibrinogen) and ACPA-IgG can drive inflammation (including the production of IL-6 and tumour necrosis factor (TNF), which are a hallmark of the synovial inflammation in rheumatoid arthritis (RA)102) by engaging both Toll-like receptor 4 (TLR4) and Fc receptors91. Irrespective of the initiating event, changes associated with normally transient joint inflammation may be sufficient to allow ACPAs and RF to exert their pro-inflammatory effects selectively in the joints, where inflammation may promote the expression as well as citrullination of proteins. e | As a consequence, both ACPA- and RF-dependent events could contribute to the initiation and propagation of chronic synovial inflammation, and further synergize with T cell activation137 and enhance the local production of autoantibodies77, 138. APC, antigen-presenting cell; MMPs, matrix metalloproteinases; TCR, T cell receptor.

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Affiliations

  1. Rheumatology Unit, Department of Medicine at Solna, Karolinska University Hospital, Karolinska Institute, 171 76 Stockholm, Sweden.

    • Vivianne Malmström,
    • Anca I. Catrina &
    • Lars Klareskog

Competing interests statement

The authors have received financial support (to their host institution, the Karolinska Institute) from the following companies: Janssen, Pfizer, Bristol-Myers Squibb (BMS), AbbVie, UCB, Sobi and Roche. Support has also been given to the Rheumatology Unit of the Karolinska Institute within the projects BTCure and ULTRA-DD, which are supported by the Innovative Medicines Initiative of the European Union (EU) and by several companies (Janssen, UCB, GlaxoSmithKline, Pfizer and Thermo Fisher) according to EU rules. V.M., A.I.C. and L.K. have all given lectures supported by pharmacological companies (BMS, Pfizer, Janssen, Sobi and Novartis) within their duties of the Karolinska Institute (and with financial compensation to the Karolinska Institute). V.M., A.I.C. and L.K. have filed patent applications related to the therapeutic use of CXCL8 and peptidyl-arginine deiminase blockade. V.M. and L.K. have also filed patent applications related to tolerization with citrullinated peptides (supported by a European Research Council Proof of Concept (ERC PoC) grant). All patent applications are handled within a research and innovation foundation named Vectis.

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  • Vivianne Malmström

    Vivianne Malmström received her Ph.D. from Lund University in Sweden and her postdoctoral training from the University of Oxford, UK. She has worked on murine adaptive immunity in the context of arthritis (and colitis), and her research is currently focused on human T cell and B cell responses in the setting of inflammatory rheumatic diseases such as rheumatoid arthritis. Malmström was appointed a professor of rheumatological immunology, Karolinska Institute, Sweden, in 2014.

  • Anca I. Catrina

    Anca I. Catrina received her Ph.D. and her postdoctoral training from the Karolinska Institute in Sweden. She is currently serving as an adjunct professor in rheumatology and medical head of the early arthritis clinic at the Karolinska University Hospital and Institute. Her research focuses on the pathogenic mechanisms responsible for the early initiation of rheumatoid arthritis, as well as the clinical interventions that might prevent the development of rheumatoid arthritis. More recently, she and her group have investigated the role of antibodies to citrullinated protein antigens and osteoclast interaction in antibody-mediated bone loss and pain.

  • Lars Klareskog

    Lars Klareskog received both his Ph.D. and postdoctoral training in immunology from Uppsala University in Sweden. Following his training as a rheumatologist, he was appointed a professor of rheumatology at the Karolinska Institute in 1993. His research combines genetic epidemiology with immunology to elucidate how interactions between genes and environment trigger autoimmune reactions that may ultimately cause arthritis.

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