POS0460 ASSOCIATION OF ANTI-CITRULLINATED PROTEIN ANTIBODIES OF IgA SUBCLASSES WITH SUSTAINED REMISSION AND FLARE IN RHEUMATOID ARTHRITIS

Background:Understanding key mechanisms of flare development and sustained remission is one of the acute goals in modern rheumatology. Anti-citrullinated protein antibodies (ACPA) are the most abundant and specific autoantibodies in rheumatoid arthritis (RA) patients. However, the impact of ACPA of IgA isotype is poorly defined. IgA ACPA were previously shown to have a higher percentage of IgA2 in comparison to total IgA; and a correlation between IgA2% ACPA with the DAS28 score was observed in a previous study [1]. Of note, IgA1 and IgA2 were shown to exhibit different effector functions, with IgA2 being pro-inflammatory, which might be the background for its role in RA [1].Objectives:We aimed to investigate, whether IgA ACPA could be used as a predictive factor for flare development in RA; and to look further into the changes in IgA ACPA levels in patients remaining in stable remission versus patients developing flare.Methods:We analysed serum of 111 patients from a multicentre randomized controlled trial ‘RETRO’. The study observational period was 12 months. Patients in the trial had to be in stable remission (DAS28-ESR<2.6) for a minimum of 6 months and were randomized into 3 different treatment arms: continuation of treatment, tapering by 50% or a gradual tapering until discontinuation [2]. IgA ACPA concentrations were measured with an enzyme-linked immunosorbent assay on CCP2-pre-coated plates.Results:60% of patients had IgG-ACPA. IgA ACPA levels were higher among the IgG-ACPA-positive patients (median 4.7 versus 2.24 µg/ml, p<0.0001). Baseline IgA1 and 2 ACPA levels were not different between patients who had a flare later on in the study period and those remaining in remission, showing no predictive value for flare development. However, the percentage of IgA2 in ACPA was correlating with the first registered DAS28 after flare (r=0.36, p=0.046). After the 12 months study period, IgA2 ACPA as well as IgA2% ACPA decreased significantly in patients who remained in stable remission by 17.5% (median, p<0.0001) and 13.6% (p=0.0006), respectively. By contrast, there was no significant change in IgA2 ACPA levels over time in patients who developed a flare. IgA1 ACPA levels remained stable over time. Disease management strategies did not seem to influence IgA ACPA levels in a specific way, as baseline levels were similar between patients on biological and conventional DMARDs and changes in levels after 12 months did not depend on the assignment to either of the study arms.Conclusion:Neither IgA1 nor IgA2 ACPA levels were predictive of flare development or associated with treatment strategies (though rituximab, JAK-inhibitors and abatacept were not amongst treatment options). However, in patients remaining in sustained remission after 1 year a decrease in IgA2 and IgA2% ACPA was observed and IgA2% ACPA was associated with DAS28 score registered after flare. This could be an indication towards ACPA of IgA2 isotype contributing to the severity of flare, alongside other factors, and its reduction being associated with a prolonged state of remission.References:[1]Steffen U, Koeleman CA, Sokolova MV, et al. IgA subclasses have different effector functions associated with distinct glycosylation profiles. Nat Commun 11, 120 (2020).[2]Haschka J, Englbrecht M, Hueber AJ, et al. Relapse rates in patients with rheumatoid arthritis in stable remission tapering or stopping antirheumatic therapy: interim results from the prospective randomised controlled RETRO study. Ann Rheum Dis. 75:45-51 (2016).Disclosure of Interests:None declared

Human B cell clonal expansion and convergent antibody responses to SARS-CoV-2

SUMMARYDuring virus infection B cells are critical for the production of antibodies and protective immunity. Here we show that the human B cell compartment in patients with diagnostically confirmed SARS-CoV-2 and clinical COVID-19 is rapidly altered with the early recruitment of B cells expressing a limited subset of IGHV genes, progressing to a highly polyclonal response of B cells with broader IGHV gene usage and extensive class switching to IgG and IgA subclasses with limited somatic hypermutation in the initial weeks of infection. We identify extensive convergence of antibody sequences across SARS-CoV-2 patients, highlighting stereotyped naïve responses to this virus. Notably, sequence-based detection in COVID-19 patients of convergent B cell clonotypes previously reported in SARS-CoV infection predicts the presence of SARS-CoV/SARS-CoV-2 cross-reactive antibody titers specific for the receptor-binding domain. These findings offer molecular insights into shared features of human B cell responses to SARS-CoV-2 and other zoonotic spillover coronaviruses.

B cell clonal expansion and convergent antibody responses to SARS-CoV-2

During virus infection B cells are critical for the production of antibodies and protective immunity. Establishment of a diverse antibody repertoire occurs by rearrangement of germline DNA at the immunoglobulin heavy and light chain loci to encode the membrane-bound form of antibodies, the B cell antigen receptor. Little is known about the B cells and antigen receptors stimulated by the novel human coronavirus SARS-CoV-2. Here we show that the human B cell compartment in patients with diagnostically confirmed SARS-CoV-2 and clinical COVID-19 is rapidly altered with the early recruitment of B cells expressing a limited subset of V genes, and extensive activation of IgG and IgA subclasses without significant somatic mutation. We detect expansion of B cell clones as well as convergent antibodies with highly similar sequences across SARS-CoV-2 patients, highlighting stereotyped naïve responses to this virus. A shared convergent B cell clonotype in SARS-CoV-2 infected patients was previously seen in patients with SARS. These findings offer molecular insights into shared features of human B cell responses to SARS-CoV-2 and other zoonotic spillover coronaviruses.

IgA subclasses have different effector functions associated with distinct glycosylation profiles

Monomeric serum immunoglobulin A (IgA) can contribute to the development of various autoimmune diseases, but the regulation of serum IgA effector functions is not well defined. Here, we show that the two IgA subclasses (IgA1 and IgA2) differ in their effect on immune cells due to distinct binding and signaling properties. Whereas IgA2 acts pro-inflammatory on neutrophils and macrophages, IgA1 does not have pronounced effects. Moreover, IgA1 and IgA2 have different glycosylation profiles, with IgA1 possessing more sialic acid than IgA2. Removal of sialic acid increases the pro-inflammatory capacity of IgA1, making it comparable to IgA2. Of note, disease-specific autoantibodies in patients with rheumatoid arthritis display a shift toward the pro-inflammatory IgA2 subclass, which is associated with higher disease activity. Taken together, these data demonstrate that IgA effector functions depend on subclass and glycosylation, and that disturbances in subclass balance are associated with autoimmune disease.

Human IgA binds a diverse array of commensal bacteria

In humans, several grams of IgA are secreted every day in the intestinal lumen. While only one IgA isotype exists in mice, humans secrete IgA1 and IgA2, whose respective relations with the microbiota remain elusive. We compared the binding patterns of both polyclonal IgA subclasses to commensals and glycan arrays and determined the reactivity profile of native human monoclonal IgA antibodies. While most commensals are dually targeted by IgA1 and IgA2 in the small intestine, IgA1+IgA2+ and IgA1−IgA2+ bacteria coexist in the colon lumen, where Bacteroidetes is preferentially targeted by IgA2. We also observed that galactose-α terminated glycans are almost exclusively recognized by IgA2. Although bearing signs of affinity maturation, gut-derived IgA monoclonal antibodies are cross-reactive in the sense that they bind to multiple bacterial targets. Private anticarbohydrate-binding patterns, observed at clonal level as well, could explain these apparently opposing features of IgA, being at the same time cross-reactive and selective in its interactions with the microbiota.