MICA: Interleukin-21 in rheumatoid arthritis: exploring its therapeutic potential for the development of a novel targeted biologic therapy.

类风湿关节炎(RA)是一种慢性自身免疫性疾病，影响约1%的人口，是导致残疾和过早死亡的重要原因。这种疾病是由关节滑膜组织的慢性炎症引起的，其中聚集了激活的免疫细胞(巨噬细胞、树突状细胞、T细胞、B细胞和浆细胞)。抗CD20单抗去除B细胞对RA的治疗益处表明，自身反应性B细胞在RA的发病机制中起重要作用。同样，阻断炎性细胞因子(TNFa、IL-6)可导致显著的临床反应。然而，约30%-40%的患者对目前的治疗方法仍然无效。因此，在这一未得到满足的需求领域开发更好的治疗方法仍然至关重要，特别是在“耐药”患者中，识别额外的/替代的炎症途径可能导致新的治疗方法。IL-21是一种强大的多效性细胞因子，参与多种免疫细胞类型的激活/分化，包括B细胞和T细胞，也包括基质细胞，诱导金属蛋白酶和炎性细胞因子包括TNFa。IL-21是常见的细胞因子受体γ链结合家族成员，主要由活化的T细胞产生，包括Th17细胞和Tfh。TFH在次级淋巴器官的B细胞激活和抗体产生中起重要作用，并控制体液免疫反应。基于遗传学研究和RA患者表达增加，IL-21与RA有关，但其在RA关节中的致病作用及其作为治疗靶点的潜力尚未完全阐明。阻断IL-21受体(IL-21R)可显著改善RA动物模型的疾病进展，提示调节IL-21/IL-21R通路是一种很有前途的治疗方法。为了支持这一应用，我们对一大批RA患者进行了研究，结果表明：(1)IL-21/IL-21R途径在表现为异位淋巴结构(ELs)的局灶性病变的RA患者中显著增加，但在弥漫性滑膜炎患者中不表达；ii)滑膜IL-21蛋白与TFH样细胞亚群密切相关；iii)RA滑膜成纤维细胞，而不是RA真皮成纤维细胞，高水平表达IL-21R，提示IL-21不仅作用于免疫细胞，还作用于常驻滑膜细胞；4)当RA滑膜组织移植到免疫缺陷(SCID)小鼠体内并支持产生人类自身抗体时，IL-21和IL-21R的表达可维持数周。因此，我们最终的目标是通过以下4个步骤在RA中开发有效的IL-21靶向策略：步骤1)确定RA患者关节内IL-21的细胞来源和靶点。步骤2)在RA/SCID模型中，分析IL-21对RA关节免疫细胞和基质细胞的作用。步骤3)在人类RA/SCID模型中测试人类使用的IL21/IL21R阻断试剂(与工业合作伙伴在临床前开发中)。步骤4)评估IL-21在早期关节炎患者和使用抗TNFa或抗IL-6R生物制剂治疗的患者中对疾病转归和对现有传统和生物疗法的反应的贡献。通过解决RA中IL-21生物学的关键方面，这一建议将促进我们对IL-21在慢性炎症、B细胞自身免疫和ELS功能中的作用的了解，对其他自身免疫/慢性炎症性疾病具有重要的意义。人IL-21阻断试剂对病变组织滑膜炎症和体液自身免疫的有效性的概念性研究将推动IL-21治疗RA的新疗法的发展。最后，以患者为基础的研究可能会更好地识别IL-21介导的关节炎的关键反应者，并在早期临床试验中对受试者进行分层。

Rheumatoid arthritis (RA) is a chronic, autoimmune disease that affects ~1% of the population and an important cause of disability and premature mortality. The disease is driven by chronic inflammation of the joint synovial tissue in which there is accumulation of activated immune cells (macrophages, dendritic cells, T-cells, B-cells and plasma-cells). The therapeutic benefit of B-cell depletion with anti-CD20 mAb in RA demonstrated that autoreactive B-cells play a crucial pathogenic role. Likewise, blockade of inflammatory cytokines (TNFa, IL-6) results in significant clinical response. However, ~30-40% of patients remain refractory to current therapies. Thus, the development of better therapies in this area of unmet need remains of critical importance particularly in "resistant" patients where the identification of additional/alternative inflammatory pathways may lead to novel therapeutics. IL-21 is a potent pleiotropic cytokine involved in the activation/differentiation of many immune cell types including B and T cells but also stromal cells with the induction of metalloproteinases and inflammatory cytokines including TNFa. IL-21 is a member of the common cytokine receptor gamma chain-binding family and is mainly produced by activated T-cells including Th17 cells and TFh. TFh play a fundamental role in B cell activation and antibody production in secondary lymphoid organs and control humoral immune responses.IL-21 has been associated with RA based on genetic studies and increased expression in RA patients; however its pathogenic function in RA joints and its potential as a therapeutic target have not been fully addressed. Blockade of IL-21 receptor (IL-21R) considerably ameliorates disease progression in animal models of RA indicating that modulation of the IL-21/IL-21R pathway is a promising therapeutic approach. In support of this application we studied a large cohort of RA patients and demonstrated that i) the IL-21/IL-21R pathway is highly increased in a subset of RA patients presenting focal lesions called ectopic lymphoid structures (ELS) but not in patients with diffuse synovitis; ii) synovial IL-21 protein is expressed in strict association with a subset of TFh-like cells; iii) RA synovial fibroblasts, but not RA dermal fibroblasts, express high levels of IL-21R suggesting that IL-21 acts not only on infiltrating immune cells but also on resident synovial cells; iv) IL-21 and IL-21R expression is maintained for several weeks when RA synovial tissue is engrafted into immunodeficient (SCID) mice and support the production of human autoantibodies. Thus, we ultimately aim to develop effective IL-21 targeting strategies in RA through the following 4 steps: Step 1) Identification of the cellular sources and targets of IL-21 within the joints of RA patients. Step 2) Analysis of IL-21 function on immune and stromal cells isolated from the RA joints in vitro and in vivo in the RA/SCID model. Step 3) Testing IL21/IL21R blocking reagents for human use (in preclinical development with the industrial partner) in the human RA/SCID model. Step 4) Evaluating IL-21 contribution to disease outcome and response to existing traditional and biological therapies in patients with early arthritis and in those treated with anti-TNFa or anti-IL-6R Biologics.By addressing key aspects of IL-21 biology in RA this proposal will advance our knowledge of the role of IL-21 in chronic inflammation, B cell autoimmunity and ELS function with important implications for other autoimmune/chronic inflammatory diseases. Proof of concept studies of the effectiveness of human IL-21 blocking reagents on synovial inflammation and humoral autoimmunity in diseased tissue will advance the development of novel IL-21 therapeutics for the treatment of RA. Finally, the patient-based studies may enable better identification of key responders of IL-21 mediated arthritis and have a means of stratifying subjects in early clinical trials.