Effects Of Thrombopoietin Receptor Agonists On APRIL Plasma Levels In Patients With Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is an example of an autoimmune disease in which B-lymphocytes produce autoantibodies against platelets. Antibody-mediated platelet destruction and suboptimal platelet production leads to a decrease in platelet count. ITP patients with thrombocytopaenia have increased plasma levels of a proliferation-inducing ligand (APRIL), a factor that can promote B-cell maturation and survival.

Two new compounds that bind to the thrombopoietin receptor (TPO-R) and activate the megakaryopoiesis have been recently approved for the treatment of chronic ITP as second-line treatment.

It has been recently reported an improved regulatory T-cell activity in patients with chronic ITP treated with TPO-R agonists (TPO-RA) (Bao et al, 2010). So we aimed to evaluate the effect of TPO-RA treatment on APRIL plasma levels in ITP patients before (ITP-1) and after responding (ITP-2) to the treatment.

This was an observational and prospective study. Thirteen patients with chronic ITP in whom treatment with a TPO-RA was indicated, and thirty-three healthy controls were included. ITP patients were studied at two times: at inclusion (ITP-1), when platelet count was less than 30x10⁹/L for patients without concomitant medication or less than 65x10⁹/L for patients receiving corticosteroids or intravenous immunoglobulin; and after a response to TPO-RA therapy was elicited (ITP-2). The response to TPO-RA was defined as a platelet count >30x10⁹/L in patients without additional treatment or >65x10⁹/L for those with concomitant treatments.

EDTA-anticoagulated whole blood was centrifuged at 1,500 g for 15 min at 23°C to obtain platelet poor plasma which was then centrifuged at 10,000 g for 15 min at room temperature. Supernatant plasma was stored at –70°C until analysis. Plasma TPO and APRIL concentrations were determined using a commercially available enzyme-linked immunosorbent assay (ELISA, Duoset-R&D, Minneapolis, Mn, USA). Platelet counts were determined with a Coulter Ac. T Diff cell counter (Beckman Coulter, Madrid, Spain).

Comparisons of quantitative variables were made with ANOVA and Dunn test. Results were expressed as mean±SD. Correlations were calculated with Spearman test. Values of p≤0.05 were considered statistically significant.

Platelet count in the ITP-1 group ((23±17)x10⁹/L) increased after responding to TPO-RA to values similar to controls (controls: (233±77)x10⁹/L and ITP-2: (140±36)x10⁹/L).

TPO plasma level was higher in ITP-1 patients (30.05±26.81 pg/ml, p<0.005) than in healthy controls (7.36±11.74 pg/ml) but not significantly different when compared with the values of the ITP-2 group (26.81±17.62 pg/ml).

ITP-1 patients showed significantly higher APRIL plasma levels (37.60+28.73 ng/ml, p<0.0001) than controls (2.20±3.11 ng/ml) and ITP-2 patients (4.92+4.68 ng/ml), indicating that TPO-RA treatment caused a diminution in APRIL plasma levels.

When looking into the relationship between APRIL plasma levels and platelet count, a significant correlation was only found in the ITP-1 group (r=-0.5919, p<0.05). This supports the potential role of APRIL in the reduction of platelet counts in ITP patients.

ITP patients with thrombocytopaenia that responded to TPO-RA treatment increased their platelet count reducing plasma APRIL levels and without changing the moderately high levels of plasma TPO. Reductions in APRIL levels caused by TPO-RA treatment could be an additional mechanism that contributes to an increased platelet count in ITP patients treated with these agents.

Bao W, Bussel JB, Heck S, et al. Improved regulatory T-cell activity in patients with chronic immune thrombocytopenia treated with thrombopoietic agents. Blood. 2010 Nov 25;116(22):4639-4645

No relevant conflicts of interest to declare.