Autoantibodies neutralizing thrombopoietin in a patient with amegakaryocytic thrombocytopenic purpura.

Amegakaryocytic thrombocytopenic purpura (AMTP), first reported by Korn,1 is a rare disease characterized by severe thrombocytopenia associated with a total absence or a selective decrease in bone marrow megakaryocytes. Previous studies suggest a variety of pathogenetic mechanisms for AMTP, such as the intrinsic defect at the megakaryocyte progenitor cell level and humoral and cellular suppression of megakaryocytic differentiation.2 Thrombopoietin (TPO), recently isolated, is the hematopoietic factor that potently stimulates megakaryocytopoiesis and platelet production.3 Since TPO is the principal regulator of platelet production, a decreased availability or function of TPO might lead to a form of AMTP. There was a recent report of a patient with pure red-cell aplasia who had a circulating autoantibody against erythropoietin, the primary regulator of red blood cell production.4 These findings suggest that a similar mechanism may be responsible for some cases of AMTP, that is, an autoantibody that blocks the action of endogenous TPO on megakaryocytopoiesis. To explore this possibility, we checked for the presence of antibodies against TPO in a patient (a 70-year-old woman) with AMTP who achieved remission following treatment with cyclosporin A (cyA) with some modifications, according to Hill’s report.5 As shown in Figure 1, the patient’s serum before therapy had the highest level of anti-TPO IgG antibodies during the scope of the study. With cyA therapy, the antibody levels declined and did not return to the pretreatment value until the end of the study. Endogenous TPO levels appear to be regulated by c-Mpl, the receptor for TPO, expressed on platelets and megakaryocytes, a mechanism of end-cell regulation. Previous studies showed that serum TPO levels were markedly elevated in patients with AMTP before therapy.6 In contrast to these findings, Figure 1 shows that in spite of severe thrombocytopenia, the pretreatment level of TPO (0.21 fmol/mL) was lower than baseline values of normal subjects (0.33 to 1.72 fmol/mL) as previously reported.7 Throughout the study period, circulating platelet counts appeared to increase in response to the elevation in endogenous TPO levels (Figure 1). In an effort to characterize the patient’s anti-TPO antibody, we examined the effect of the IgG fractions from the patient’s serum on the growth of megakaryocyte colonies from adherent cell-depleted normal human bone marrow mononuclear cells (Figure 2). The IgG fraction before cyA therapy or during therapy (October 14, 1997) completely or partially reduced the number of megakaryocyte colonies stimulated with 0.2 ng/mL of glycosylated recombinant human TPO, respectively. This inhibition of megakaryocyte colony formation was able to be overcome by increasing the concentration of human TPO in the cultures to 10 ng/mL. The IgG fractions,