Circulating endogenous thrombopoietin, interleukin-3, interleukin-6 and interleukin-11 levels in patients undergoing allogeneic bone marrow transplantation

Analysis of the kinetics of TPO uptake during platelet transfusion

BACKGROUND

It has been shown in several studies that platelets play a role in the removal of TPO from the circulation. For instance, in vitro studies have shown that platelets can bind and internalize TPO, and transfusion studies have shown that the concentration of circulating TPO decreased after platelet transfusion. In the current study, the in vivo kinetics of plasma TPO levels and TPO uptake by transfused platelets is analyzed in more detail.

STUDY DESIGN AND METHODS

Serial blood samples from patients who received a platelet transfusion were analyzed with respect to platelet count, plasma TPO concentration, and TPO content per platelet. In addition, the capacity of transfused platelets to bind TPO in vitro was assessed.

RESULTS

Platelet counts increased immediately after transfusion, but subsequently started to decrease. Conversely, TPO levels decreased significantly but then returned to baseline level by 44 hours after transfusion. Platelet count and plasma TPO concentration were inversely correlated (r(p) = -0.9; p<0.05). The decrease in TPO concentration upon transfusion was accompanied by a significant increase in the platelet-associated TPO concentration. After transfusion, platelets isolated from the patient still displayed functional TPO receptors, as indicated by their intact capacity to bind TPO in vitro.

CONCLUSION

The decrease in plasma TPO followed by the increase in platelet TPO provides evidence that platelets are responsible for the clearance of TPO in circulation. In vivo, platelets can bind and may degrade TPO upon platelet transfusion.

Measurement of thrombopoietic levels: clinical and biological relationships

Platelet production is primarily regulated by the thrombopoietic cytokine thrombopoietin (TPO). In most cases thrombopoietin serum levels are determined by the rate of c-mpl receptor-mediated degradation after TPO uptake into platelets and megakaryocytes. The contribution of increased TPO protein synthesis by a translational mechanism was recently appreciated as the cause for hereditary thrombocythemia and will have to be elucidated in other conditions of thrombocytosis in association with increased TPO levels.

Thrombopoietic cytokines in relation to platelet recovery after bone marrow transplantation

In order to evaluate the importance of different thrombopoietic stimulatory cytokines in accelerating platelet recovery after bone marrow transplantation (BMT), we assayed serial plasma concentrations of three cytokines, thrombopoietin (TPO), interleukin-6 (IL-6), and IL-11 through the course of platelet nadir and recovery after BMT. Both mean TPO and IL-6 levels showed a marked rise and later fall preceding or coincident with the platelet nadir and recovery, suggesting their potential role as circulating regulators or stimulators of thrombopoiesis. In contrast, IL-11 levels remained remarkably constant through the whole course suggesting that this cytokine, though capable of stimulating thrombopoiesis, does not serve as a circulating regulator of platelet production. Additionally, we assayed the levels of these three cytokines following initial platelet transfusion to assess the capacity of transfused platelets to adsorb these thrombopoietic cytokines from the plasma and reduce their circulating levels, thus potentially modifying their availability for stimulating megakaryocyte proliferation. No consistent falls in TPO, IL-6 or IL-11 levels were observed following the initial two platelet transfusions. These data support the importance of circulating TPO and IL-6 as hormones capable of stimulating platelet production. Their physiologic relevance as in vivo regulators of thrombopoiesis and clinical utility for therapy of thrombocytopenia need further investigation.

Endogenous thrombopoietin serum levels during multicycle chemotherapy

Little is known about the behaviour of endogenous thrombopoietin (TPO) serum levels during rapid sequences of dose-intensified chemotherapy. To characterize the relationship between TPO levels and platelet counts in this setting we serially measured both parameters over the entire treatment period of patients receiving multicycle polychemotherapy. We found TPO and platelet responses to be generally antagonistic through all cycles. However, a cross-correlation analysis indicated that TPO responses preceded platelet responses by approximately one day in all patients. The cumulative severity of thrombocytopenia observed over successive cycles was accompanied by an increasing TPO response which tended to grow overproportionally in relation to the degree of peripheral thrombocytopenia. These findings are consistent with a model suggesting that both platelet and megakaryocyte mass contribute to a receptor-dependent consumption process regulating the endogenous TPO level. In order to develop optimal schedules for exogenous TPO administration it might be important to consider endogenous TPO response characteristics.

Plasma thrombopoietin (TPO) levels and expression of TPO receptor on platelets in patients with myelodysplastic syndromes

Data on endogenous thrombopoietin (TPO) levels and their regulation in myelodysplastic syndromes (MDS) are sparse. We examined the plasma TPO level of 85 MDS patients by a sensitive enzyme immunoassay and the platelet expression of TPO receptor (TPO-R) protein, which metabolizes endogenous TPO, in 19 MDS patients with an equilibrium binding assay using 125I-TPO. The MDS patients had higher plasma TPO levels (7.0 +/- 9.3 fmol/ml) than 52 normal subjects (P < 0.0001). Refractory anaemia (RA) patients (n = 39) had higher plasma TPO levels than patients (n = 28) with RA with excess blasts (RAEB) or RAEB in transformation (RAEB-t) (P = 0.0002), irrespective of similar platelet counts in these groups. The plasma TPO level correlated inversely with the platelet count in RA patients (P = 0.0027) but not in RAEB and RAEB-t patients (P = 0.7865). These data suggest that the physiological pathway for TPO production and metabolism is conserved, at least partially, in RA, but deranged in RAEB/RAEB-t. The number of TPO-R per platelet was significantly smaller in 19 MDS patients (17.5 +/- 13.3) than in normals (P = 0.0014), but similar between RA patients and patients with RAEB and RAEB-t. Further, the bone marrow megakaryocyte count, determined in 31 MDS patients, was quite similar between RA patients and patients with RAEB or RAEB-t. Thus, in addition to thrombocytopenia, a reduced platelet TPO-R number may contribute to elevated plasma TPO levels in MDS, and a regulatory pathway for circulating TPO other than platelet TPO-R and marrow megakaryocytes, such as blasts expressing TPO-R, may operate in RAEB/RAEB-t.

Neutralization of biological activity and inhibition of receptor binding by antibodies against human thrombopoietin

Thrombopoietin (TPO) is a recently isolated cytokine that primarily regulates megakaryocytopoiesis and thrombopoiesis. We recently reported the development of a variety of antibodies (Abs) to synthetic peptides of human (h)TPO and to recombinant human TPO (rhTPO). In this study, we characterized the Abs and mapped immunologically distinct areas of the molecule. Among the five different antipeptide polyclonal Abs, only one, raised against synthetic peptide D8 to Q28, neutralized the TPO-dependent growth of FDCP-2 cells expressing human Mpl (FDCP-hMpl5 cells). One out of seven anti-rhTPO monoclonal Abs, designated as TN1, also showed neutralizing activity. TN1 was found to be specifically reactive with two proteolytic fragments, residues S1 to R117 and A60 to K122 of hTPO, indicating that the epitope(s) of TN1 is localized in residues A60 to R117 of the molecule. These two neutralizing Abs inhibited the binding of biotinylated rhTPO to FDCP-hMpl5 cells. On the other hand, the other Abs, which reacted with five polypeptides of S47 to D62, L108 to A126, N172 to A190, S262 to T284, and P306 to G332 of hTPO, did not show either the neutralizing activity or the ability to inhibit the binding of biotinylated rhTPO to the cell surface hMpl. These findings indicate that two regions, residues D8 to Q28 and A60 to R117 of hTPO, may contain the domains associated with its receptor, C-Mpl. These Abs characterized here are valuable for studying the structural analysis and the biological function of hTPO mediated by its receptor.