Transforming growth factor-beta 1 (TGF-β1) down-regulates IgA Fc-receptor (CD89) expression on human monocytes

IgA is the predominant immunoglobulin in human secretions and the second most important immunoglobulin in the circulation on a quantitative basis. The clearance of IgA is dependent on the function of at least three types of receptors. One of these receptors recognizes the Fc portion of the IgA molecule, FcαR, which has been cloned recently. FcαR, also designated CD89, is found on a number of cells, including human glomerular mesangial cells, and monocytes. In this study we analysed the effect of TGF-β1, a cytokine with strong immunosuppressive function, on the expression of CD89 on freshly isolated monocytes. We found that TGF-β1 down-regulates CD89 expression on human peripheral blood monocytes in a dose-dependent fashion. Optimal down-regulation occurred at a concentration of 5 ng/ml. The down-regulation of CD89 by TGF-β1 is linear in time, with a mean down-regulation of 34 ± 13% after 24 h. Also at the mRNA level, CD89 expression was down-regulated by TGF-β1, suggesting regulation of CD89 at the transcriptional level. Monocytes pre-treated with TGF-β1 displayed a reduced response to IgA, as measured by IL-6 production by monocytes, in contrast to monocytes pre-treated with medium alone. These results suggest an important role for TGF-β1 in the regulation of CD89. This down-regulation may have direct consequences for the handling of IgA by human monocytes

Antibodies and B cells recognising citrullinated proteins display a broad cross-reactivity towards other post-translational modifications

Objective Autoantibodies against antigens carrying distinct post-translational modifications (PTMs), such as citrulline, homocitrulline or acetyllysine, are hallmarks of rheumatoid arthritis (RA). The relation between these anti-modified protein antibody (AMPA)-classes is poorly understood as is the ability of different PTM-antigens to activate B-cell receptors (BCRs) directed against citrullinated proteins (CP). Insights into the nature of PTMs able to activate such B cells are pivotal to understand the a € evolution' of the autoimmune response conceivable underlying the disease. Here, we investigated the cross-reactivity of monoclonal AMPA and the ability of different types of PTM-antigens to activate CP-reactive BCRs. Methods BCR sequences from B cells isolated using citrullinated or acetylated antigens were used to produce monoclonal antibodies (mAb) followed by a detailed analysis of their cross-reactivity towards PTM-antigens. Ramos B-cell transfectants expressing CP-reactive IgG BCRs were generated and their activation on stimulation with PTM-antigens investigated. Results Most mAbs were highly cross-reactive towards multiple PTMs, while no reactivity was observed to the unmodified controls. B cells carrying CP-reactive BCRs showed activation on stimulation with various types of PTM-antigens. Conclusions Our study illustrates that AMPA exhibit a high cross-reactivity towards at least two PTMs indicating that their recognition pattern is not confined to one type of modification. Furthermore, our data show that CP-reactive B cells are not only activated by citrullinated, but also by carbamylated and/or acetylated antigens. These data are vital for the understanding of the breach of B-cell tolerance against PTM-antigens and the possible contribution of these antigens to RA-pathogenesis

Autoantibodies recognizing carbamylated proteins are present in sera of patients with rheumatoid arthritis and predict joint damage

Autoimmune responses against posttranslationally modified antigens are a hallmark of several autoimmune diseases. For example, antibodies against citrullinated protein antigens (ACPA) have shown their relevance for the prognosis and diagnosis of rheumatoid arthritis (RA), and have been implicated in disease pathogenesis. It is conceivable that other autoantibody systems, recognizing other posttranslationally modified proteins, are also present in RA. Here, we describe the presence of an autoantibody system that discriminates between citrulline- and homocitrulline-containing antigens in the sera of RA-patients. IgG antibodies recognizing carbamylated (homocitrulline-containing) antigens were present in sera of over 45% of RA-patients. Likewise, anticarbamylated protein (anti-CarP) IgA antibodies were observed in 43% of RA-sera. ACPA and anti-CarP antibodies are distinct autoantibodies because, in selected double-positive patients, the anti-CarP antibody binding to carbamylated antigens could be inhibited by carbamylated antigens, but not by control or citrullinated antigens. Similarly, ACPA-binding to citrullinated antigens could only be inhibited by citrullinated antigens. In line with this observation, 16% of ACPA-negative RA-patients, as measured by a standard ACPA assay, harbored IgG anti-CarP antibodies, whereas 30% of these patients tested positive for IgA anti-CarP antibodies. The presence of anti-CarP antibodies was predictive for a more severe disease course in ACPA-negative patients as measured by radiological progression. Taken together, these data show the presence of a unique autoantibody system recognizing carbamylated, but not citrullinated, protein antigens. These antibodies are predictive for a more severe clinical course in ACPA-negative RA-patients, indicating that anti-CarP antibodies are a unique and relevant serological marker for ACPA-negative RA