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Platelet dysregulation is drastically increased with advanced age and contributes to making cardiovascular disorders the leading cause of death of elderly humans. Here, we reveal a direct differentiation pathway from hematopoietic stem cells into platelets that is progressively propagated upon aging. Remarkably, the aging-enriched platelet path is decoupled from all other hematopoietic lineages, including erythropoiesis, and operates as an additional layer in parallel with canonical platelet production. This results in two molecularly and functionally distinct populations of megakaryocyte progenitors. The age-induced megakaryocyte progenitors have a profoundly enhanced capacity to engraft, expand, restore, and reconstitute platelets in situ and upon transplantation and produce an additional platelet population in old mice. The two pools of co-existing platelets cause age-related thrombocytosis and dramatically increased thrombosis in vivo. Strikingly, aging-enriched platelets are functionally hyper-reactive compared with the canonical platelet populations. These findings reveal stem cell-based aging as a mechanism for platelet dysregulation and age-induced thrombosis. [Display omitted] • Aging leads to two parallel platelet specification paths from HSCs • The shortcut platelet pathway is perpetuated by highly expansive MkPs • Canonical MkPs are resilient to age-induced changes • The aging-induced shortcut path causes thrombocytosis and platelet hyper-reactivity With advanced age, platelets generated by a differentiation pathway that shortcuts the canonical progenitor cascade to directly make megakaryotic precursor cells from hematopoietic stem cells cause thrombocytosis and are more prone to thrombosis compared with canonically derived platelets. [ABSTRACT FROM AUTHOR]

Transcription factor (TF) expression levels drive developmental programs, including cell fate and function, and their measurement by flow cytometry allows for robust downstream analysis. However, significant batch-to-batch variability between replicative experiments precludes direct comparison of absolute values across experimental conditions. Here, we present a flow cytometry protocol to measure the relative abundance of multiple TFs simultaneously in single cells, allowing for direct comparison across experimental conditions/time points. This protocol uses bone marrow cells but can be adapted for other cell types. For complete details on the use and execution of this protocol, please refer to Manso et al. (2021) and Manso et al. (2019)., Competing Interests: B.A.M. is currently affiliated with the University of California Santa Cruz, Santa Cruz, CA, USA. This protocol is submitted in support of work done while affiliated with the Mayo Clinic, Rochester, MN, USA., (© 2021 The Author(s).)

TNFα is implicated in chronic lymphocytic leukemia (CLL) immunosuppression and disease progression. TNFα is constitutively produced by CLL B cells and is a negative regulator of bone marrow (BM) myelopoiesis. Here, we show that co-culture of CLL B cells with purified normal human hematopoietic stem and progenitor cells (HSPCs) directly altered protein levels of the myeloid and erythroid cell fate determinants PU.1 and GATA-2 at the single-cell level within transitional HSPC subsets, mimicking ex vivo expression patterns. Physical separation of CLL cells from control HSPCs or neutralizing TNFα abrogated upregulation of PU.1, yet restoration of GATA-2 required TNFα neutralization, suggesting both cell contact and soluble-factor-mediated regulation. We further show that CLL patient BM myeloid progenitors are diminished in frequency and function, an effect recapitulated by chronic exposure of control HSPCs to low-dose TNFα. These findings implicate CLL B-cell-derived TNFα in impaired BM myelopoiesis., Competing Interests: Research funding to S.A.P. has been provided from 10.13039/100014491Pharmacyclics, 10.13039/501100013650MorphoSys, Janssen, 10.13039/100004325AstraZeneca, 10.13039/100013252TG Therapeutics, 10.13039/100006436Celgene, 10.13039/100006483AbbVie, and Ascentage Pharma for clinical studies in which S.A.P. is a principal investigator. S.A.P. also participated in Advisory Board meetings of Pharmacyclics, AstraZeneca, Genentech, Gilead, GlaxoSmithKline, Verastem Oncology, and AbbVie (he was not personally compensated for his participation). Research funding to N.E.K. has been provided from Acerta Pharma BV, 10.13039/100006436Celgene, 10.13039/100004328Genentech, 10.13039/100014491Pharmacyclics, and Tolero Pharmaceutical. N.E.K. also participates in the data safety monitoring committees of Agios Pharm, Astra Zeneca, Celgene, Cytomx Therapeutics, Genentech, Infinity Pharm, Morpho-Sys, and Pharmacyclics (he was not personally compensated for his participation). All other authors declare no competing interests., (© 2020 The Author(s).)