Thrombopoietin Administration Alleviates Hematopoietic Stem Cells Intrinsic Long Term Ionizing Radiation Damages. Identification of Preferential Cellular Expansion Sites

Hematopoietic stem cells (HSC) are indispensable for the integrity of complex and long-lived organisms since they can reconstitute the hematopoietic system for life and achieve long term repopulation of lethally irradiated mice. Exposure of an organism to ionizing radiation (IR) causes dose dependant bone marrow suppression and challenge the replenishment capacity of HSC. Yet, the precise damages that are generated remain largely unexplored. To better understand these effects, phenotypic and functional changes in the stem/progenitor compartments of sublethally irradiated mice were monitored over a ten week period after radiation exposure. We report that shortly after sublethal IR-exposure, HSC, defined by their repopulating ability, still segregate in the Hoechst dye excluding side population (SP); yet, their Sca-1 (S) and c-Kit (K) expression levels are increased and severely reduced, respectively, with a concurrent increase in the proportion of SP^SK cells positive for established indicators of HSC presence: CD150⁺/CD105⁺ and Tie2⁺. Virtually all HSCs quickly but transiently mobilize to replenish the bone marrow of myelo-ablated mice. Ten weeks after, whereas bone marrow cellularity has recovered and hematopoietic homeostasis is restored, major phenotypic modifications can be observed within the c-Kit⁺ Sca-1⁺ Lin^−/low (KSL) stem/progenitor compartment: CD150⁺/Flk2⁻ and Flk2⁺ KSL cell frequencies are increased and dramatically reduced, respectively. CD150⁺ KSL cells also show impaired reconstitution capacity, accrued γ-H2AX foci and increased tendency to apoptosis. This demonstrates that the KSL compartment is not properly restored 10 weeks after sublethal exposure, and that long-term IR-induced injury to the bone marrow proceeds, at least partially, through direct damage to the stem cell pool. Since thrombopoietin (TPO) has been shown to reduce haematopoietic injury when administered immediately after exposure to radiations, we asked whether TPO could restore the permanent IR-induced damage we observed in the HSC compartment. We first found in competitive transplant experiments that a single TPO administration rescued the impaired reconstitution capacity of HSC’s from animals exposed to sublethal IR. In addition, we observed that TPO injection right after irradiation considerably attenuates IR-induced long-term injury to the stem/progenitor compartment. Finally, the use of marrow cells from transgenic ubiquitous luciferase-expressing donors combined with bioluminescence imaging technology provided a valuable strategy that allowed visualizing HSC homing improvements of TPO-treated compared to untreated irradiated donors, and enabled the identification of a preferential cellular expansion sites which were inaccessible to investigation in most studies. Electronic microscopy analysis revealed that these sites show also differential activity of megakaryocytopoiesis with marked differences in the proplatelets reaching the vascular sinus. Altogether, our data provide novel insights in the cellular response of HSC to IR and the beneficial effects of TPO administration to these cells.