Abstract 891 Cytokines in Disease and Therapy Poster Symposium, Monday, 5/3
Thrombopoietin (Tpo) is a potent stimulator of megakaryocytopoiesis in vitro. Its efficacy has also been demonstrated in vivo in adults with certain thrombocytopenias. It is unknown, however, whether thrombocytopenic neonates would respond to rTpo. To assess this we obtained bone marrow and peripheral blood megakaryocyte progenitors from 4 thrombocytopenic neonates and 4 healthy adults. We cultured these progenitors under identical conditions in a serum-free, collagen-based culture system, in the presence of increasing concentrations of rTpo (0, 0.1, 1, 10, 50, and 100 ng/mL). The marrow obtained from the neonates generated three times more megakaryocyte (MK) colonies than the marrow obtained from normal adults (mean ±SD, 129±39 vs. 45±28 colonies per 105 cells plated, at a rTpo concentration of 100 ng/mL). In neonates and in adults, a percent of maximal colony count vs. Tpo concentration curve showed a dose-dependent increase in the number of megakaryocyte colonies produced. However, the concentration at which a maximal number of colonies was counted (plateau) was 10 ng/mL in the neonates, compared with 50 ng/mL in the adults. This difference accounted for a larger area under the curve for the neonatal progenitors (p=0.004). Once the plateau of the neonatal curve was reached, further increases in the concentration of rTpo did not result in higher colony counts, but increased the percentage of large MK-colonies (>50 cells), from 17% at a rTpo concentration of 10 ng/mL, to 26% at 100 ng/mL. This was in contrast to the response of adult progenitors, which under the highest rTpo concentration generated only 2% large colonies. To determine the relationship between peripheral blood- and marrow-derived progenitors, we also cultured progenitors obtained from peripheral blood of each subject. We found a significant linear relationship between peripheral blood and marrow progenitors, both for neonates and adults (p=0.0001), indicating that the response of bone marrow cells to rTpo can be estimated from the circulating progenitors. However, for each subject, the number of progenitors in the blood was only a moderately accurate predictor of the number of progenitors in the bone marrow. We conclude that; 1) MK progenitors of thrombocytopenic neonates respond to rTpo at lower concentrations than do progenitors of healthy adults; 2) MK progenitors of thrombocytopenic neonates seem to have a higher proliferative potential than adult progenitors in response to rTpo alone in vitro ; 3) A linear relationship exists between the rTpo response of MK progenitors in the blood vs. marrow. Based on these findings, we speculate that thrombocytopenic neonates are likely to respond to somewhat lower doses of rTpo than those effective in adults.
