Tissue uptake of circulating thrombopoietin is increased in immune-mediated compared with irradiated thrombocytopenic mice

We have previously demonstrated a significant inverse correlation between c irculating thrombopoietin (TPO) levels and peripheral platelet (PLT) counts in patients with thrombocytopenia secondary to megakaryocytic hypoplasia b ut not in patients with immune thrombocytopenic purpura (ITP; Chang et al, Blood 88:3354, 1996). To test the hypothesis that the differences in the ci rculating TPO levels in these two types of thrombocytopenia are caused by d ifferences in the total capacity of Mpl receptor-mediated TPO clearance, th rombocytopenia was induced in female CD-1 mice either by sublethal irradiat ion (irradiated) or rabbit antimouse PLT serum (RAMPS) for 1 day (1 d RAMPS ) and 5 days (5 d RAMPS). A well-characterized murine model of autoimmune t hrombocytopenic purpura, male (NZW x BXSB) F-1 mice (W/B F-1), was also inc luded in this study. All thrombocytopenic mice and their controls received trace amounts of I-125-recombinant murine TPO (I-125-rmTPO) intravenously a nd were killed 3 hours postinjection. Blood cell-associated radioactivity w as significantly decreased in all 4 groups of thrombocytopenic mice. Signif icantly increased plasma and decreased whole spleen-associated radioactivit y was observed in the irradiated group compared with controls (P < .05). Wh ile a lesser but still significant increase in plasma and decrease in whole spleen-associated radioactivity was observed in the 1 d RAMPS mice (P < .0 5), there were no significant differences between the 5 d RAMPS nor the W/B F-1 male mice compared with controls, although whole spleen-associated rad ioactivity was higher in the W/B F-1 male. A significant inverse correlatio n of plasma and whole spleen-associated radioactivity was demonstrated in W /B F-1 male mice (r = -.91, n = 6, P < .05). There was also a decrease in b one (femur)/blood-associated radioactivity in the irradiated group compared with controls (P < .05), but a significant increase in 1 d and 5 d RAMPS m ice (P < .01). Furthermore, the I-125-rmTPO uptake capacity within the sple en and marrow of immune thrombocytopenic mice appeared to be associated wit h a higher megakaryocytic mass when tissue samples were examined by light m icroscopy. Internalization of I-125-rmTPO by megakaryocytes and PLTs in the spleens and marrows of ITP mice was also demonstrated directly using elect ron microscopic autoradiography. Labeled PLTs were also found within spleni c macrophages. Additionally, the mean PLT volumes of RAMPS mice were signif icantly higher than those of the control and irradiated mice (P < .05), as was the bound I-125-rmTPO (cpm) per million PLT (P < .05). Finally, signifi cantly decreased I-125-rmTPO degradation products were only found in the pl asma of the irradiated mice compared with control animals (P < .05). These data suggest that the lack of Mpl(+) cells in the mice with thrombocytopeni a secondary to megakaryocytic hypoplasia (irradiated) results in decreased uptake and degradation of TPO and higher circulating TPO levels. Furthermor e, these data also suggest that, after a brief TPO surge in response to imm une thrombocytopenia (1 d RAMPS), the lack of an inverse correlation of cir culating TPO with PLT counts during steady-state immune thrombocytopenic mi ce (5 d RAMPS + W/B F-1 male) is due, at least in part, to its uptake and d egradation by the high PLT turnover and increased mass of megakaryocytes. ( C) 1999 by The American Society of Hematology.