Impact of IgA and IgG ACPA on the immune system and disease onset

抗瓜氨酸化蛋白抗体（ACPA）对类风湿关节炎（RA）具有极强的特异性，是严重疾病的一个强有力的指标。虽然大多数ACPA是IgG同型，但在许多患者中，它们也以IgA的形式存在。人类拥有两个IgA亚类：IgA1和IgA2。在过去的资助期内，我们发现这两个亚类对骨髓细胞的作用不同，其中IgA2具有更强的诱导中性粒细胞和巨噬细胞形成中性粒细胞胞外陷阱（NET）和促炎细胞因子释放的能力（Steffen et al., Nature Communications 2020）。我们发现，与血清总IgA相比，ACPA显示出更高的IgA2百分比。ACPA中IgA2百分比与已有疾病的RA患者的疾病严重程度相关（Steffen et al., Nature Communications 2020），与缓解期RA患者的复发严重程度相关（Sokolova et al.，提交）。在亚极大胶原诱导关节炎（CIA）模型中，瓜氨酸化vimentin的联合免疫增加了发病率和疾病严重程度，提示ACPA直接导致关节炎的爆发。有趣的是，在两个独立队列中，ra前患者的血清ACPA、IgA和IgG同型水平在疾病发病前后均有所下降。这可能是ACPA从血清向受影响组织（如滑膜）转移增加的迹象。在第二个资助期，我们建议进一步研究IgA和IgG ACPA对RA临床发病的影响。我们的目的是研究瓜氨酸化蛋白（自身）免疫对适应性和先天免疫系统以及滑膜炎的影响，采用小鼠亚极大CIA模型。此外，我们将分析关节炎发病前后滑膜组织中ACPA抗原表达的时空变化。研究结果将适用于不同阶段的RA前期和已确诊RA患者。

Anti-citrullinated protein antibodies (ACPA) are extremely specific for rheumatoid arthritis (RA) and represent a strong indicator for severe disease. Although the majority of ACPA is of the IgG isotype, in many patients, they are present in the form of IgA as well. Humans possess two IgA subclasses: IgA1 and IgA2. In the past funding period, we discovered that these two subclasses act differently on myeloid cells with IgA2 having a stronger capacity to induce neutrophil extracellular trap (NET) formation and proinflammatory cytokine release by neutrophils and macrophages (Steffen et al., Nature Communications 2020). We found that ACPA display a higher IgA2 percentage compared to total serum IgA. The IgA2 percentage in ACPA correlated with disease severity of RA patients with established disease (Steffen et al., Nature Communications 2020) and with the severity of relapse in RA patients in remission (Sokolova et al., submitted). In a model of submaximal collagen induced arthritis (CIA), co-immunization against citrullinated vimentin increased the incidence and disease severity, suggesting that ACPA directly contribute to the outbreak of arthritis. Interestingly, serum ACPA levels of both, IgA and IgG isotypes declined in pre-RA patients of two independent cohorts around disease onset. This may be a sign of increased transmigration of ACPA from the serum to affected tissue, such as the synovium. For the second funding period, we propose to further investigate the impact of IgA and IgG ACPA on the clinical onset of RA. We aim to investigate the effects of (auto)immunity against citrullinated proteins on the adaptive and innate immune system as well as synovitis employing the murine model of submaximal CIA. In addition, we will analyze spatial and temporal changes in the expression of ACPA antigens in the synovial tissue around the onset of arthritis. The results will be translated to patients with different stages of pre-RA and established RA.