In Vivo Treatment with Prostaglandin E₂ (PGE2) Selectively Expands Short-Term Hematopoietic Stem Cells.

Parathyroid Hormone (PTH) expands hematopoietic stem cells (HSC) through activated osteoblasts in the bone marrow (BM). Since PTH stimulates osteoblastic production of Prostaglandin E2 (PGE2), we hypothesized that PGE2 could also regulate HSC. In vivo PGE2 treatment demonstrated a time and dose dependent increase in BM lineage⁻ Sca-1⁺ c-kit⁺ (LSK) BM mononuclear cells (BMMC) from PGE2 vs. vehicle treated mice (0.11 vs. 0.04% BMMC, P=0.0061, n=8 mice per treatment group), an effect superior to PTH (350 vs. 100% increase in LSK). There were no significant PGE2 effects on CFU-Cs or peripheral Hct, Plts or WBC counts compared to vehicle. Therefore PGE2-dependent cell expansion was not global across differentiated subsets, but was restricted to primitive hematopoietic cells, similar to the effects of PTH treatment. Consistent with a PGE2-dependent HSC increase, cells from PGE2 vs vehicle-treated mice had superior lymphomyeloid reconstitution by competitive repopulation analysis. However, this increase was short-lived: specifically, PGE2-dependent myeloid (CD11b⁺) reconstitution was no longer superior at 6 weeks, while the PGE2-dependent increase in lymphoid (CD3e⁺ and B220⁺) reconstitution ceased by 16 weeks. This surprising result suggests that in vivo PGE2 treatment selectively expands short-term HSC (or ST-HSC), which have highly proliferative properties, but limited self-renewal. To further confirm this targeted PGE2 effect, LSK subset analysis based on Flt3 and Thy1.1 expression was performed. Consistent with the competitive repopulation data, PGE2 treatment significantly increased Flt3⁺Thy1.1^int LSK ST-HSC (0.0273 vs 0.0140% n=4 in each group, p=0.0307) as well as Flt3⁺Thy1.1⁻ LSK Multipotent Progenitors (0.0305 vs 0.0195% n=4 in each group, p=0.0070), while Flt3⁻Thy1.1^int LSK Long-Term HSC or LT-HSC (0.0126 vs 0.0078% n=4 in each group, p=0.1069) were unchanged compared to vehicle treatment. ST vs LT-HSC activity can also be quantified by the in vivo clonogenic Colony Forming Unit-Spleen (CFU-S) assay, where day 8 CFU-S represent ST-HSC, while day 10–12 CFU-S represent LT-HSC. Consistent with a PGE2-dependent specific ST-HSC increase, BMMC from PGE2 treated mice gave rise to a significantly higher number of CFU-S_d8 compared to cells from vehicle treated mice (10.5 vs 4.75 CFU-S per 60,000 BMMC, n=4 in each group, p=0.0053), while CFU-S_d10 were unchanged (12.5 vs 11.5 CFU-S per 60,000 BMMC, n=6, p=0.4950). Finally, since ST-HSC confer radioprotection, PGE2-dependent ST-HSC expansion would be expected to improve survival of lethally irradiated recipients receiving limiting numbers of BMMC from PGE2 vs vehicle-treated mice. As predicted, recipients of BMMC from PGE2 treated mice had increased survival 30 days after transplantation compared to animals receiving BMMC from vehicle treated donors (150,000 donor cells: 80% vs 0% survival, p=0.0018; 75,000 donor cells: 53% vs 0% survival, p=0.0173). Taken together, these data demonstrate specific PGE2-dependent regulation of ST-HSC, and provide a unique and novel model to define control of HSC subsets. This finding implicates for the first time specialized regulation of HSC subsets. Moreover, these data indicate that selective therapeutic manipulation of ST-HSC could be exploited in clinical situations requiring rapid bone marrow reconstitution, such as in recovery from iatrogenic or pathologic myeloablative injury.