Variation in Mesenchymal KITL/SCF and IGF1 Expression at Middle Age Underlies Steady-State Hematopoietic Stem Cell Aging

Intrinsic molecular programs and extrinsic factors including pro-inflammatory molecules are understood to regulate hematopoietic aging. This is based on foundational studies using genetic perturbation to evaluate causality. However, individual organisms exhibit natural variation in hematopoietic aging phenotypes and the molecular basis of this heterogeneity is poorly understood. Here, we generated individual single cell transcriptomic profiles of hematopoietic and non-hematopoietic cell types in five young adult and nine middle-aged C57BL/6J female mice, providing a web-accessible transcriptomic resource for the field. Among all assessed cell types, hematopoietic stem cells (HSCs) exhibited the greatest phenotypic variation in expansion among individual middle-aged mice. We computationally pooled samples to define modules representing the molecular signatures of middle-aged HSCs and interrogated which extrinsic regulatory cell types and factors would predict variance in these signatures between individual middle-aged mice. Decline in signaling mediated by ADIPOQ, KITL and IGF1 from mesenchymal stromal cells (MSCs) was predicted to have the greatest transcriptional impact on middle-aged HSCs, as opposed to signaling mediated by endothelial cells or mature hematopoietic cell types. In individual middle-aged mice, lower expression of Kitl and Igf1 in MSCs highly correlated with reduced lymphoid lineage commitment of HSCs and increased signatures of differentiation-inactive HSCs. These signatures were independent of expression of aging-associated pro-inflammatory cytokines including IL1β, IL6, TNFα and RANTES. In sum, we find that Kitl and Igf1 expression are co-regulated and variable between individual mice at middle age and expression of these factors is predictive of HSC activation and lymphoid commitment independently of inflammation.