Flow cytometric analysis is superior to the Ham test and permits concomitant diagnosis of PNH in about 20% of patients with myelodysplasia (a rate similar to that seen in patients with aplastic anemia). The presence of GPI-anchored protein-deficient cells in myelodysplasia predicts responsiveness to immunosuppressive therapy. Early emergence of GPI-anchored protein-deficient hematopoiesis in a patient with marrow failure may point to an underlying immune pathogenesis.
The mechanism by which paroxysmal nocturnal hemoglobinuria (PNH) clones expand is unknown. PNH clones harbor PIGA mutations and do not synthesize glycosylphosphatidylinositol (GPI), resulting in deficiency of GPI-linked membrane proteins. GPI-deficient blood cells often expand in patients with aplastic anemia who sustain immune-mediated marrow injury putatively induced by cytotoxic cells, hence suggesting that the injury allows PNH clones to expand selectively. We previously reported that leukemic K562 cells preferentially survived natural killer (NK) cell-mediated cytotoxicity in vitro when they acquired PIGA mutations. We herein show that the survival is ascribable to the deficiency of stress-inducible GPI-linked membrane proteins ULBP1 and ULBP2, which activate NK and T cells. The ULBPs were detected on GPI-expressing but not on GPI-deficient K562 cells. IntroductionParoxysmal nocturnal hemoglobinuria (PNH) is an acquired stem cell disorder of clonal nature characterized by intravascular hemolysis, venous thrombosis, bone marrow (BM) dysfunction, and infrequent development of leukemia. [1][2][3][4] In the last 2 decades, virtually the entire mechanism leading to PNH hemolysis has been elucidated. [5][6][7][8] PNH stem cells acquire PIGA mutations and do not generate glycosylphosphatidylinositol (GPI), resulting in a deficiency of a series of GPI-linked membrane proteins, including complement regulatory molecules such as decay-accelerating factor (DAF) and CD59. PNH cells are then vulnerable to autologous complement. For example, PNH erythrocytes undergo complement-mediated hemolysis. Soon after, the molecular events leading to clinically serious hemolytic precipitation induced by infection were clarified. 9 On the other hand, more than half of patients with PNH show various cytopenias and decreased CD34 ϩ hematopoietic cells. [1][2][3][4][10][11][12] PNH closely associates with aplastic anemia and myelodysplastic syndromes (MDSs) that share BM failure and development of leukemia. 1,12,13 It has been indicated that immune-mediated BM injury underlies at least a part of the diseases. 1,12 Indeed, immunosuppressive therapy ameliorates their marrow failure. 1,12,14,15 Of interest, combination therapy with antithymocyte globulin and cyclosporine A, these immunosuppressants having individual target spectra, 16,17 gives better results than therapy with each 1 of the 2 immunosuppressants. 12,18 In general, both cytotoxic T lymphocytes (CTLs) of adaptive immunity and natural killer (NK) cells of innate immunity operate in combination rather than independently. [19][20][21][22] It is then conceivable that both cytotoxic cells exert critical roles in BM injury, 12,[23][24][25][26] although real features of the autoimmunity have not been delineated.The marked progress in understanding of PNH pathophysiology evokes the next concern about the etiology of PNH, and the mechanisms of both selective expansion of PNH clones 27 and development of leukemia. 28 Above all, we have focused on the expansion of affected cells to mani...
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