Different classes of anti-modified protein antibodies are induced on exposure to antigens expressing only one type of modification.

OBJECTIVES: Autoantibodies against post-translationally modified proteins (anti-modified protein antibodies or AMPAs) are a hallmark of rheumatoid arthritis (RA). A variety of classes of AMPAs against different modifications on proteins, such as citrullination, carbamylation and acetylation, have now been described in RA. At present, there is no conceptual framework explaining the concurrent presence or mutual relationship of different AMPA responses in RA. Here, we aimed to gain understanding of the co-occurrence of AMPA by postulating that the AMPA response shares a common 'background' that can evolve into different classes of AMPAs. METHODS: Mice were immunised with modified antigens and analysed for AMPA responses. In addition, reactivity of AMPA purified from patients with RA towards differently modified antigens was determined. RESULTS: Immunisation with carbamylated proteins induced AMPAs recognising carbamylated proteins and also acetylated proteins. Similarly, acetylated proteins generated (autoreactive) AMPAs against other modifications as well. Analysis of anti-citrullinated protein antibodies from patients with RA revealed that these also display reactivity to acetylated and carbamylated antigens. Similarly, anti-carbamylated protein antibodies showed cross-reactivity against all three post-translational modifications. CONCLUSIONS: Different AMPA responses can emerge from exposure to only a single type of modified protein. These findings indicate that different AMPA responses can originate from a common B-cell response that diversifies into multiple distinct AMPA responses and explain the presence of multiple AMPAs in RA, one of the hallmarks of the disease.

Fc gamma receptor binding profile of anti-citrullinated protein antibodies in immune complexes suggests a role for FcγRI in the pathogenesis of synovial inflammation.

OBJECTIVES: Anti-citrullinated protein antibodies (ACPA) are highly specific for rheumatoid arthritis (RA). Here, we studied binding of ACPA-IgG immune complexes (IC) to individual Fc gamma receptors (FcγR) to identify potential effector mechanisms by which ACPA could contribute to RA pathogenesis. METHODS: ACPA-IgG1 and control IgG1(IgG1 depleted of ACPA-IgG1) were isolated from plasma and synovial fluid (SF) of RA patients by affinity chromatography using CCP2 peptides. Subsequently, IC were generated using fluorescently labelled F(ab')2 fragments against the F(ab')2 region of IgG, or by using citrullinated fibrinogen. IC were incubated with FcγR-transfected CHO cell lines or neutrophils from healthy donors. FcγR binding of IC was analysed by flow cytometry in the presence or absence of specific blocking antibodies. RESULTS: ACPA-IgG1 IC predominantly bound to FcγRI and FcγRIIIA on FcγR-transfected CHO cell lines, while much lower binding was observed to FcγRIIA and FcγRIIB. ACPA-IgG1 IC showed reduced binding to FcγRIIIA compared to control IgG1 IC, in line with enhanced ACPA-IgG1 Fc core-fucosylation. Neutrophils activated in vitro to induce de novo expression of FcγRI showed binding of ACPA-IgG IC, and blocking studies revealed that almost 30% of ACPA-IgG IC binding to activated neutrophils was mediated by FcγRI. CONCLUSIONS: Our studies show that ACPA-IgG1 IC bind predominately to activating FcγRI and FcγRIIIA, and highlight FcγRI expressed by activated neutrophils as relevant receptor for these IC. As neutrophils isolated from SF exhibit an activated state and express FcγRI in the synovial compartment, this IC-binding could contribute to driving disease pathogenesis in RA.

Variable domain glycosylation of ACPA-IgG: A missing link in the maturation of the ACPA response?

Anti-citrullinated Protein Antibodies (ACPA) are excellent markers for Rheumatoid arthritis (RA) and are postulated to have a pathogenic role in the disease process. A multistep model for the evolution of the ACPA response in RA was proposed in which an initial break of tolerance causes, as "first hit", "silent" production of ACPA without any clinical symptoms. The model further proposes that the ACPA immune response matures upon a certain (unknown) trigger, a "second hit", which leads to epitope spreading, an increase in ACPA titres and extended isotype usage before clinical RA manifestations. These occurrences are indicative of an expansion of the citrulline-specific B cell response, though ACPA remain of low avidity even in established disease. This persistence of low avidity is puzzling, as the typical signs of maturation of the immune response seem to be uncoupled from the classical process of affinity maturation. In fact, it suggests that B cells expressing ACPA could bypass selection mechanisms that otherwise control the expansion of auto-reactive B cells. In the established, chronic phase, we recently found that ACPA-IgG are extensively glycosylated in the variable (Fab) domain. More than 90% of ACPA-IgG molecules carry Fab glycans that are highly sialylated. This molecular feature is striking and may provide a missing link in our understanding of the maturation of the ACPA immune response. This review, therefore, describes the current knowledge about ACPA Fab glycosylation in the pathogenesis of RA.

Rheumatoid factors do not preferentially bind to ACPA-IgG or IgG with altered galactosylation.

Objectives: Recent reports describe interactions between the two most prominent RA-related autoantibodies, RFs and ACPAs. The main aim of the present study was to investigate whether RFs preferentially interact with ACPA-IgG over non-ACPA IgG. Additionally, interactions of RFs with IgG with altered galactose content in the Fc domain were examined, since ACPA-IgGs have been shown to have decreased Fc galactose content in RF+ patients. Methods: (Auto)antibody interactions were studied in a surface plasmon resonance imaging assay and with ELISA. Target antibodies were isolated from RA patient plasma (polyclonal ACPA- and non-ACPA-IgG) or recombinantly produced to obtain monoclonal IgG with well-defined Fc galactose content. Interacting autoantibodies were studied using autoantibody positive patient sera and two recombinantly produced IgM-RFs. Results: The sera from 41 RF+ RA patients showed similar RF binding to ACPA- and non-ACPA-IgG and no differences in binding to IgG with normal, high or low levels of Fc galactosylation. Two monoclonal IgM-RFs, one interacting with the CH2-CH3 interface and one binding close to the C-terminal end of the CH3 domain showed no influence of the Fc glycan on IgG binding by IgM-RF. Conclusion: Although interactions between RF and ACPA may play a role in inflammatory processes in RA, RFs do not preferentially interact with ACPA-IgG over non-ACPA-IgG nor with agalatosylated IgG over IgG with normal or high galactosylation.

Synovial fluid mononuclear cells provide an environment for long-term survival of antibody-secreting cells and promote the spontaneous production of anti-citrullinated protein antibodies.

OBJECTIVES: In rheumatoid arthritis (RA), observations point to a crucial role for (autoreactive) B cells in disease pathogenesis. Here, we studied whether cells from the synovial environment impact on the longevity of autoreactive B cell responses against citrullinated antigens. METHODS: Synovial fluid mononuclear cells and peripheral blood mononuclear cells (SFMC/PBMC) were obtained from patients with established RA and assessed for the presence of B cell subpopulations. Cells spontaneously secreting anti-citrullinated protein antibodies (ACPA-IgG) directly ex vivo were detected by antigen-specific Enzyme-Linked ImmunoSpot (ELISpot) assay. SFMC and PBMC were cultured to assess the degree of spontaneous ACPA-IgG secretion. Cells surviving for several weeks were characterised by carboxyfluorescein succinimidyl ester (CFSE) labelling and Ki-67 staining. RESULTS: Cells spontaneously secreting ACPA-IgG were readily detectable in peripheral blood and synovial fluid (SF) of patients with ACPA-positive RA. SFMC showed an up to 200-fold increase in ex vivo ACPA-IgG secretion compared with PBMC despite lower numbers of B cells in SFMC. ELISpot confirmed the presence of spontaneously ACPA-IgG-secreting cells, accounting for up to 50% (median 12%) of all IgG-secreting cells in SF. ACPA-IgG secretion was remarkably stable in SFMC cultures, maintained upon depletion of the CD20+ B cell compartment and detectable for several months. CFSE labelling and Ki-67 staining confirmed the long-term survival of non-dividing plasma cells (PCs). CONCLUSIONS: This study demonstrates a high frequency of differentiated, spontaneously ACPA-IgG-secreting cells in SF. These cells are supported by SFMC for prolonged survival and autoantibody secretion, demonstrating that the synovial compartment is equipped to function as inflammatory niche for PC survival.