Aging of Bone Marrow Microenvironment Promotes Myeloid Bias of Hematopoietic Progenitors and Is a Target in Age-Related Myeloproliferative Neoplasms

Background: BM HSC accumulate during aging but are functionally impaired; however, it remains debated whether this aging results from HSC-intrinsic and/or -extrinsic mechanisms. Myeloid malignancies are more frequent in the elderly, but whether the aged microenvironment predisposes to these malignancies is unclear. Myeloid-biased HSC and especially platelet-primed HSC expand in aged mice. However, both platelet-primed and unprimed old HSCs exhibit myeloid bias, possibly suggesting a microenvironmental participation. Moreover, some hallmarks of murine hematopoietic aging (like increased platelets) are present in premature aging (Hutchinson-Gilford progeria syndrome, HGPS) and are exacerbated in age-related myeloproliferative neoplasms (MPN), where we found that niche heterogeneity can influence disease progression (ASH abstract ID 113495). Given that aging increases myelopoiesis in physiology, HGPS and MPN (despite their heterogenous genetic background), we hypothesized that shared microenvironmental alterations influence HSC lineage bias during aging

Results: HSCs, perisinusoidal megakaryocytes (Mk) and myelopoietic cytokines increase in the BM of HGPS mice, resembling normally aged mice. However, WT recipients of HGPS BM cells do not reproduce myeloid bias, indicating that premature aging affects hematopoiesis in HGPS in a non-HSC-autonomous manner. We have investigated niche contributions to lineage bias during physiological aging because HGPSmice do not tolerate myeloablation. Consistently with recent findings (Nat Med 2018;24:782-91) HSC niches decrease near bone (endosteal BM) and expand further from bone in aged mice, but we observed different vascular changes mainly consisting of halved endosteal transition zone vessels with associated Nes-GFP+ cells and 4-fold-increased non-endosteal capillaries with Nes-GFP+ cells. However, contrasting the afore mentioned study but in agreement with other reports, we found aging-related increase of BM noradrenergic innervation, which promotes megakaryopoiesis in WT mice, but not in mice lacking β₂ and β₃ adrenergic receptors (ARs). Yet, these receptors exhibit opposite effects on microenvironmental regulation of myelopoiesis. β₂-AR-agonist promotes Mk differentiation from human CD34⁺ HSCs co-cultured with MS-5 cells or in WT (but not IL-6 KO) primary murine BM culture. Consequently, adult β₂-AR KO mice exhibit decreased Mk-lineage cells, which correlated with reduced BM IL-6. In contrast, adult β₃-AR KO mice exhibit increased mature Mk near sinusoids, which correlated with tripled Cxcl12 mRNA expression in non-endosteal BM. Mechanistically β₃-AR regulates Cxcl12-dependent HSPC and megakaryocyte localization, nitric oxide (NO) production and expression of myelopoietic cytokines. In vitro, β₃-AR-agonist-treated stromal cells decrease human and murine HSPC differentiation into Mk. This effect requires Nos1-dependent NO production, since Nos1^-/- mice have high circulating platelets and β₃-AR agonist specifically increases lymphoid-biased HSCs in primary BM cultures in a Nos1-dependent manner. Finally, Mk increased in β₃-AR KO mice transplanted with MPN BM cells, whereas Mk decreased in MPN mice treated with β₃-AR agonist. To test the clinical relevance of our findings, we measured plasma nitrates and myelopoietic cytokines in MPN patients (median age=62) chronically treated only with β₃-AR agonist, who show improved myelofibrosis correlated by rescued nestin+ niches (Blood 2016;128:3108), which is consistent with previous results in mice (Nature 2014;512:78-81). We found inverse correlations between the concentration of nitrates and myelopoietic cytokines

Conclusions: microenvironmental aging promotes myeloid bias through similar cytokines (e.g. IL1β, IL6) during premature or physiological aging. In the latter, HSC supporting niches decrease near bone but expand further from bone. Increased BM sympathetic noradrenergic innervation promotes β₂-AR-IL-6-dependent myeloid bias. Reduction of endosteal niches decreases β₃-AR-NO-dependent inhibition of myelopoietic cytokines. However, chronic treatment with β₃-AR-agonist does not rejuvenate overall hematopoiesis but decreases exacerbated megakaryopoiesis in mice and humans with MPNs. Therefore, niche aging promotes myeloid bias and might represent a therapeutic target in age-related myeloproliferative disorders