Molecular pathogenesis of bone marrow failure as a major cause of death in paroxysmal nocturnal hemoglobinuria

日本再生障碍性贫血和阵发性夜间血红蛋白尿（PNH）的患病率相对较高，表现为致命的骨髓衰竭。骨髓衰竭对免疫抑制治疗反应良好，提示骨髓衰竭是免疫介导的。然而，包括淋巴细胞识别的造血细胞上的分子靶点在内的发病机制尚不清楚。在本研究中，我们发现应激诱导膜蛋白（或NKG2D配体）如ULBP和MICA/B在PNH和再生障碍性贫血患者的粒细胞和骨髓CD34+细胞上都有病理表达，并且粒细胞在体外依赖于配体的表达而受到自体淋巴细胞的损伤（Hanaoka, et. al., Blood 2006）。然后，我们提出这些配体是淋巴细胞在骨髓衰竭综合征（如PNH和再生障碍性贫血）中免疫介导的造血功能障碍中的分子靶点。为了支持这一假设，我们前瞻性地分析了3例再生障碍性贫血- pnh综合征患者和2例再生障碍性贫血患者1 - 5年的临床病程。然后我们发现一种或多种胁迫诱导的NKG2D配体在它们的粒细胞上表达。该表达不仅与反映骨髓衰竭的全血细胞减少症密切相关，而且与骨髓衰竭对免疫抑制治疗的良好反应密切相关。因此，我们得出结论，至少部分特发性骨髓衰竭综合征（包括PNH和再生障碍性贫血）患者暴露于一定的应激条件下，诱导其血细胞上的NKG2D配体，该配体的表达触发或促进了NKG2D介导的自体淋巴细胞损伤，导致骨髓衰竭（Kawaguchi & Nakakuma, Int J Hematol 2007）。目前，我们正在用NKG2D及其配体抗体进行体外集落形成抑制实验，证实NKG2D介导的骨髓细胞损伤。

The prevalence of aplastic anemia and paroxysmal nocturnal hemoglobinuria (PNH) that manifest fatal marrow failure are relatively high in Japan. The marrow failure shows good response to immunosuppressive therapy, indicating that marrow failure is immune-mediated. However, the pathogenesis including molecular targets on hematopoietic cells recognized by lymphocytes is unknown yet.In the present study, we show both that stress-inducible membrane proteins (or NKG2D ligands) such as ULBP and MICA/B were pathologically expressed on granulocytes and bone marrow CD34+ cells of patients with PNH and aplastic anemia, and that the granulocytes were injured by autologous lymphocytes dependently on the ligand expression in vitro (Hanaoka, et. al., Blood 2006). We then propose that the ligands are the molecular targets for lymphocytes in immune-mediated impairment of hemotopoiesis in the marrow failure syndromes such as PNH and aplastic anemia. lb support the hypothesis, we prospectively analyzed the clinical courses for one up to five years of 3 patients with aplastic anemia-PNH syndromes and 2 patients with aplastic anemia. We then found that one or more of the sress-inducible NKG2D ligands were expressed on their granulocytes. The expression closely associated not only with pancytbpenia reflecting marrow failure, but also with a favorable response of marrow failure to immunosuppressive therapy.We thus conclude that at least a part of patients with idiopathic bone marrow failure syndromes including PNH and aplastic anemia are exposed to a certain stress to induce the NKG2D ligands on their blood cells and that the ligand expression triggers off or promotes the NKG2D-mediated blood cell injury by autologous lymphocytes, leading to marrow failure (Kawaguchi & Nakakuma, Int J Hematol 2007). Currently, we are confirming the NKG2D-mediated marrow cells injury by in vitro colony formation inhibition assays with antibodies to NKG2D and its ligands.