The crosstalk between extrinsic niche-derived and intrinsic hematopoietic stem cell (HSC) factors controlling HSC maintenance remain elusive. Here we demonstrate that amphiregulin (AREG) from bone marrow (BM) leptin receptor (LepR+) niche cells is an important factor that mediates the crosstalk between BM niche and HSCs in stem cell maintenance. Mice deficient for the DNA repair gene Brca2 specifically in LepR+ cells (LepR-Cre;Brca2fl/fl) exhibit increased frequencies of total and myeloid-biased HSCs. Furthermore, HSCs from LepR-Cre;Brca2fl/fl mice show compromised repopulation, increased expansion of donor-derived myeloid-biased HSCs and myeloid output. Brca2-deficient BM LepR+ cells exhibit persistent DNA damage-inducible overproduction of AREG. Ex vivo treatment of WT HSCs, or systemic treatment of C57BL/6 mice, with recombinant AREG impairs repopulation, leading to HSC exhaustion. Conversely, inhibition of AREG by anti-AREG neutralizing antibody or deletion of the Areg gene in LepR-Cre;Brca2fl/fl mice rescues the HSC defects caused by AREG. Mechanistically, AREG activates the PI3K/AKT/mTOR pathway, promotes HSC cycling and compromises HSC quiescence. Finally, we demonstrate that BM LepR+ niche cells from other DNA repair-deficient and aged mice also show persistent DNA damage-associated overexpression of AREG, which exerts similar negative effects on HSC maintenance. We have therefore identified an important factor regulating HSCs function under conditions of DNA repair deficiency and aging.

The crosstalk between the BM microenvironment (niche) and hematopoietic stem cells (HSCs) is critical for HSC regeneration. Here, we show that in mice, deletion of the Fanconi anemia (FA) genes Fanca and Fancc dampened HSC regeneration through direct effects on HSCs and indirect effects on BM niche cells. FA HSCs showed persistent upregulation of the Wnt target Prox1 in response to total body irradiation (TBI). Accordingly, lineage-specific deletion of Prox1 improved long-term repopulation of the irradiated FA HSCs. Forced expression of Prox1 in WT HSCs mimicked the defective repopulation phenotype of FA HSCs. WT mice but not FA mice showed significant induction by TBI of BM stromal Wnt5a protein. Mechanistically, FA proteins regulated stromal Wnt5a expression, possibly through modulating the Wnt5a transcription activator Pax2. Wnt5a treatment of irradiated FA mice enhanced HSC regeneration. Conversely, Wnt5a neutralization inhibited HSC regeneration after TBI. Wnt5a secreted by LepR+CXCL12+ BM stromal cells inhibited β-catenin accumulation, thereby repressing Prox1 transcription in irradiated HSCs. The detrimental effect of deregulated Wnt5a/Prox1 signaling on HSC regeneration was also observed in patients with FA and aged mice. Irradiation induced upregulation of Prox1 in the HSCs of aged mice, and deletion of Prox1 in aged HSCs improved HSC regeneration. Treatment of aged mice with Wnt5a enhanced hematopoietic repopulation. Collectively, these findings identified the paracrine Wnt5a/Prox1 signaling axis as a regulator of HSC regeneration under conditions of injury and aging.

The immune receptor TREM1 (Triggering receptor expressed on myeloid cells 1) is a master regulator of inflammatory response. Compelling evidence suggests important pathological roles for TREM1 in various types of solid tumors. However, the role of TREM1 in hematologic malignancies is not known. Our previous study demonstrates that TREM1 cooperates with diminished DNA damage response to induce expansion of pre-leukemic hematopoietic stem cells (HSCs) in mice deficient for the Fanconi anemia gene Fanca. Here we investigate TREM1 in leukemogenesis using mouse models of the DNA repair-deficient Fanca-/- and the oncogenic MLL-AF9 or KrasG12D. We found that Trem1 was highly expressed in pre-leukemic HSCs and leukemia stem cells (LSCs). By selective deletion of the Trem1 gene in the hematopoietic compartment, we showed that ablation of Trem1 reduced leukemogenic activity of the pre-leukemic HSCs and LSCs in mice. Trem1 was required for the proliferation of the pre-leukemic HSCs and LSCs. Further analysis revealed that Trem1 expression in pre-leukemic HSCs and LSCs was associated with persistent DNA damage, prolonged oncogenic stress, and a strong inflammatory signature. Targeting several top Trem1 inflammatory signatures inhibits the proliferation of pre-leukemic HSCs and LSCs. Collectively, our observations uncover previously unknown expression and function of TREM1 in malignant stem cells, and identify TREM1 as a driver of leukemogenesis.

Oral presentation at ASH meeting 2021 Session Name: 506. Hematopoiesis and Stem Cells: Microenvironment Session Date: Saturday, December 11, 2021 Session Time: 2:00 PM - 3:30 PM Presentation Time: 2:30 PM Room: Georgia World Congress Center, B216-B217 https://ash.confex.com/ash/2021/webprogram/Paper144762.html