Fig. 1: The changes in the osteo-hematopoietic niche with aging and in CHIP.

A Various cell types and secreted niche factors directly or indirectly regulate HSC activity in the adult BM microenvironment. Periarteriolar niches localized near endosteal spaces constitute a more “dormant or quiescent” niche, specialized in promoting HSC quiescence and self-renewal. Quiescent HSCs found in these spaces associate with periarteriolar nestin^bright MSCs, which express the pericyte marker neural–glial antigen 2 (NG2) or the smooth muscle marker α-smooth muscle actin (α-SMA), and Schwann cells, connected to the sympathetic nervous system. Osteolineage cells, including periarteriolar LEPR⁺ osteolectin⁺ osteogenic progenitors, support the maintenance of more committed hematopoietic progenitors, in particular the lymphoid lineage. Perisinusoidal niches, comprising nestin^dim cells that overlap with LEPR⁺ MSCs, CAR cells, and megakaryocytes, constitute a more “proliferative” niche where HSCs proliferate and migrate. CXCL12 and SCF, two key factors involved in HSC maintenance, are widely expressed in the periarteriolar/endosteal and perisinusoidal niches. HSC-derived progeny, such as macrophages, neutrophils, Tregs, and megakaryocytes can provide feedback and contribute to HSC maintenance and mobilization. B Age-related alterations of the BM niche that affect HSCs include changes in the vasculature, MSCs, and osteolineage cells, with concomitant altered secretion of niche factors (e.g. reduced CXCL12, SCF, Jagged 1, and OPN), increased adipogenic and decreased osteogenic differentiation of MSCs, and increased proinflammatory cytokine expression (e.g. IL-1β, IL-6), promoting myeloid and megakaryocytic differentiation skewing. Arteries, arterioles and type H vessels, which support osteogenesis, decline with age. Concomitantly, the endosteal niche is compromised with a reduction in the number of osteoblasts and OPN. Sympathetic neuropathy (through disrupted β-adrenergic signaling) has been identified as an important determinant of niche remodeling in the aging BM. C In individuals with CHIP, the presence of mutant myeloid progeny (i.e. monocytes, macrophages, neutrophils) in the BM microenvironment contributes to proinflammatory cytokine expression, further increasing inflammaging in the BM and activating inflammatory transcriptional programs in aged endothelial and stromal niche cells. DNMT3A^mut macrophages may promote osteoclastogenesis by secreting proinflammatory cytokines, including IL-20, leading to accelerated bone loss and frailty. The resulting bone resorption bias sustains the inflammatory milieu and releases growth/niche factors that may support clonal growth and aggravate CH over time. The well-described upregulation of inflammatory mediators in TET2^mut monocytes/macrophages may contribute to further remodeling of vascular niches. TET2 mutations have also been shown to contribute to repressing NK cell function. Overall, CHIP-driven remodeling of supportive BM niches can facilitate immune evasion and activate survival pathways favoring malignant clonal expansion. DARC duffy antigen receptor for chemokines (also known as ACKR1), NES nestin, OLN osteolectin, OPN osteopontin, PCs plasma cells. This image was created with BioRender.com.