B-cell response and thrombotic complications in COVID-19

摘要 COVID-19的严重性/致死性与功能失调的炎症免疫反应和过度的 驱动止血和血栓形成的途径的接合，但这两种表现之间的联系不是 明白与症状轻微的患者相比，重症COVID-19患者的IgG反应性更强 对SARS-CoV-2病毒及其刺突蛋白受体结合域（RBD）的免疫应答，以及 CD 11 c + CD 21- B细胞和浆母细胞区室显著增加。血栓形成的发生率和 严重COVID-19中的炎症性疾病是前所未有的，表现为血浆炎症性增加， 标志物，如IL-6、肿瘤坏死因子α、C-反应蛋白和补体途径激活 等等，以及参与炎症和凝血反应的细胞成分的失调活化 包括血小板内皮细胞单核细胞和中性粒细胞血栓形成的表现范围从 动脉、静脉和组织微血栓形成至血栓栓塞，以静脉血栓为主 血栓栓塞在与肝素相关的灾难性血栓形成中观察到类似的表现- 血小板减少症和血栓形成（HIT）。患有灾难性HIT的患者会突然发作， 由于血栓形成前血小板，多发性动脉和静脉血栓，但有时没有肝素暴露， 激活识别与肝素多糖复合的血小板因子4（PF 4/H）的IgG。使用方法 在以前的HIT研究中，我们研究了严重COVID-19患者，并确定了PF 4/H- 与HIT患者中发现的致病性抗体非常相似的反应性促血栓形成IgG抗体 激活血小板的能力。令人惊讶的是，患者血浆中的PF 4/H抗体水平与患者的免疫功能相关。 对SARS-CoV-2刺突蛋白的受体结合域（RBD）具有特异性的抗体水平。我们克隆 能够激活血小板的RBD特异性抗体。与那些只承认RBD的人相比， 同时识别RBD和PF 4/H的B细胞中，CD 11 c+、CD 21-和CXCR 3+细胞显著增多，这标志着 滤泡外B细胞亚群在严重的COVID-19中强劲扩增。基于这些发现，我们假设 SARS-CoV-2感染驱动RBD特异性B细胞亚群通过滤泡外淋巴细胞应答， 途径，并产生血小板活化抗体，有助于血栓形成并发症，但不 严重COVID-19中的病毒中和。为了验证我们的假设，我们将1）研究血栓前 RBD特异性抗体在住院COVID-19患者血浆中的活性; 2）研究 在严重的COVID-19中产生RBD特异性血小板活化抗体的B细胞; 3）研究 控制RBD和PF 4/H交叉反应性B细胞亲和力成熟的发育途径。我们的建议 研究了COVID-19血栓性并发症中一种新的B细胞/血小板轴， 的发病率/死亡率在这种疾病的复杂发病机制，并提出新的治疗。

Abstract COVID-19 severity/lethality is associated with a dysfunctional inflammatory immune response and a hyper- engagement of pathways driving hemostasis and thrombosis, but the link between the two manifestations is not understood. Compared to patients with mild symptoms, severe COVID-19 patients have stronger IgG reactivity to SARS-CoV-2 virus and its spike protein receptor binding domain (RBD) and a robust B-cell response with a marked increase of the CD11c+CD21- B cells and plasmablast compartments. The incidence of thrombosis and inflammatory disease in severe COVID-19 is unprecedented, manifested by increased plasma inflammatory markers, such as IL-6, tumor necrosis factor alpha, C-reactive protein, and activation of compliment pathway etc., and dysregulated activation of cellular components participating in inflammatory and coagulation responses that include platelets, endothelial, monocytes, and neutrophils. The thrombotic manifestation ranges from arterial, venous, and tissue micro thrombosis to thromboembolism and is predominated by venous thromboembolism. Similar manifestations are observed in catastrophic thrombosis associated with heparin- induced thrombocytopenia and thrombosis (HIT). Patients with catastrophic HIT have the sudden onset of multiple arterial and venous thrombi with, but sometime without, heparin exposure, due to prothrombotic platelet- activating IgGs that recognize platelet factor 4 complexed with heparin polysaccharide (PF4/H). Using methods employed in previous studies of HIT, we studied patients with severe COVID-19 patients and identified PF4/H- reactive, pro-thrombotic IgG antibodies that closely resemble pathogenic antibodies found in patients with HIT in their ability to activate platelets. Surprisingly, levels of PF4/H antibodies in the patient plasma correlated with levels of antibodies specific for the receptor binding domain (RBD) of the SARS-CoV-2 spike protein. We cloned RBD-specific antibodies that are able to activate platelets. Compared to those that recognize RBD alone, significantly more B cells recognizing both RBD and PF4/H were CD11c+, CD21- and CXCR3+, which mark a subset of extrafollicular B cells robustly expanded in severe COVID-19. Based on these findings, we hypothesize that SARS-CoV-2 infection drives a subset of RBD-specific B cells to respond via an extrafollicular pathway and generate platelet-activating antibodies that contribute to thrombotic complications but not virus neutralization in severe COVID-19. To test our hypothesis, we will 1) investigate the prothrombotic activity of RBD-specific antibodies in the plasma of hospitalized COVID-19 patients; 2) investigate the expansion of B cells that make RBD-specific platelet-activating antibodies in severe COVID-19; 3) investigate the developmental pathway that governs affinity maturation of RBD and PF4/H cross-reactive B cells. Our proposal studies a novel B-cell/platelet axis in thrombotic complications in COVID-19 and should unravel a large portion of the complex pathogenesis of morbidity/mortality in this disease and suggest new treatment.