Simultaneous measurements of megakaryocyte-associated IgG (MAIgG) and platelet-associated IgG (PAIgG) in chronic idiopathic thrombocytopenic purpura

Immune pathophysiology of autoimmune thrombocytopenic purpura

Chronic autoimmune thrombocytopenic purpura (AITP) is an immune-mediated, bleeding disorder in which platelets are opsonized by autoantibodies and prematurely destroyed by phagocytic cells in the reticuloendothelial system. It is classed as an organ-specific autoimmune disease primarily mediated by immunoglobulin G (IgG) autoantibodies and its etiology appears to be similar to that observed for other organ-specific autoimmune diseases. Th1 cells are important in the process, and the costimulation of Th1 cells and B cells takes place in a cytokine milieu that is reminiscent of a proinflammatory process. Chronic AITP has classically been treated with nonspecific, immunosuppressive regimens (e.g., steroids). One of the most significant developments in the treatment of AITP in the last 20 years has been the use of intravenous immunoglobulin (IVIg) and anti-D preparations. These treatments confer benefit to patients with AITP by significantly raising platelet counts. Despite this, their exact mechanisms of action remain elusive. This review focuses on cell-mediated and cytokine abnormalities within AITP, and presents data related to the mechanism of action of anti-D.

Megakaryocytes and platelets in immune thrombocytopenic purpura

In idiopathic thrombocytopenic purpura, 90% of the patients have an elevated platelet-associated immunoglobulin. The most important related antigens are glycoprotein IIb-IIIa (GPIIb-IIIa) (16.7-83.3%), GPIb-IX (13.3-83%), GPIb (3.3-47.1%) and GPIIIa (21.6-33.3%), and less commonly GPIa-IIa, GPIV and GPV. Other related antigens can be platelet granule membrane protein, phospholipid, intraplatelet and cytoplasmic antigens, and rarely human platelet antigen (HPA) 1a and HLA-DR antigens. The marrow megakaryocytes are usually normal or increased in number with maturation impairment. There are discrepancies regarding megakaryocytopoiesis in vitro. A low dose of heparin could elevate the platelet number in certain cases. The expression of c-sis was reported to be inhibited in ITP, when the plasma beta-thromboglobin-platelet factor 4 level was elevated. In secondary immune thrombocytopenia, platelet antibodies can be (1) alloantibodies against the (HPA) system and (2) autoantibodies most commonly against platelet GPIIb-IIIa or GPIb-IX. Other antigens can be 30-52kDa proteins of the platelet membrane. Platelet survival is usually shortened, and marrow megakaryocytes are normal in number. Megakaryocyte colony-forming units could be reduced.