Modulation of irradiation-induced thrombocytopenia and alteration of megakaryocyte size and number with thrombopoietin from human embryonic kidney cells : a comparison with interleukin-6 and granulocyte macrophage-colony stimulating factor

Previous work showed that mice treated with platelet specific antiserum prior to whole-body irradiation did not suffer the degree or duration of thrombocytopenia as did irradiated control mice. Release and action of endogenous thrombopoietin in response to thrombocytopenia caused by antiserum injections was hypothesized to be responsible for alleviating radiation-induced thrombocytopenia. The following studies demonstrate that a partially purified thrombocytopoiesis-stimulating factor (TSF or thrombopoietin) mimics the effects of antiserum treatment in hematopoietically suppressed mice. Male C3H mice were exposed to 3.0 or 4.5 Gy of ¹³⁷Cs and injected with a total dose of 4 units (U) of TSF. Human serum albumin (HSA), rabbit anti-mouse platelet serum (RAMPS) and normal rabbit serum (NRS)-injected mice, along with unirradiated mice served as controls. Thrombocytopoietic recovery of mice was evaluated by determination of peripheral platelet counts and %³⁵S incorporation into platelets after treatment with TSF or HSA on days 7-14 and after RAMPS and NRS treatment on days 10 and 14. The results showed that TSF-treated and RAMPS-treated mice had higher platelet counts in comparison to HSA-treated and NRS-treated control mice after exposure to both 3.0 and 4.5 Gy of irradiation. Further, %³⁵S incorporation levels were significantly higher for TSF-treated mice compared to HSA-treated mice exposed to 4.5 Gy and for RAMPS-treated mice compared to NRS-treated mice after 3.0 Gy. Additional experiments using 227 U/mouse of granulocyte macrophage-colony stimulating factor (GM-CSF), 40,000 U/mouse of interleukin-6 (IL-6) or a combined dose of GM-CSF and IL-6 did not show elevated platelet counts or %³⁵S incorporation into platelets on days 10 and 14 compared to other mice treated with bovine serum albumin (BSA). Additionally, megakaryocyte sizes and numbers were evaluated in mice exposed to 3.0 Gy for TSF-treated and HSA-treated mice on days 7-14 and for RAMPS-treated and NRS-treated mice on days 10 and 14. The megakaryocyte compartment of irradiated mice responded similarly to RAMPS and TSF treatment. RAMPS-treated mice had significantly larger megakaryocytes on day 14 compared to values for NRS-treated mice, while TSF-treated mice had significantly larger megakaryocytes on all days assessed compared to HSA-treated control mice. On the other hand, megakaryocyte numbers were significantly depressed in RAMPS-treated mice on day 10 and for TSF-treated mice on days 7- 10 and 14 compared to their respective controls. Assessment of megakaryocyte sizes and numbers for mice treated with IL-6, GM-CSF or a combined dose of both cytokines revealed that GM-CSF significantly increased megakaryocyte numbers on days 10 and 14, but there was a decrease in megakaryocyte sizes compared to BSA-treated control mice on day 10. Treatment with both factors also significantly increased megakaryocyte numbers on day 14 and decreased sizes on day 14 compared to BSA-treated mice, while lL-6 failed to alter megakaryocyte numbers or sizes on either day. The results from the.se studies indicate another similarity in the biological actions of endogenous and exogenous thrombopoietin. Moreso, the ability of thrombopoietin to enhance platelet recovery in hematopoietically suppressed mice suggests a clinical potential for thrombopoietin in treating patients with bleeding diatheses or as a supportive therapy in treating patients undergoing chemotherapeutic treatment.