Chronic variable stress activates hematopoietic stem cells

Abstract

Exposure to psychosocial stress is a risk factor for many diseases, including atherosclerosis. Although incompletely understood, interaction between the psyche and the immune system provides one potential mechanism linking stress and disease inception and progression. Known cross-talk between the brain and immune system includes the hypothalamic-pituitary-adrenal axis, which centrally drives glucocorticoid production in the adrenal cortex, and the sympathetic-adrenal-medullary axis, which controls stress-induced catecholamine release in support of the fight-or-flight reflex. It remains unknown, however, whether chronic stress changes hematopoietic stem cell activity. Here we show that stress increases proliferation of these most primitive hematopoietic progenitors, giving rise to higher levels of disease-promoting inflammatory leukocytes. We found that chronic stress induced monocytosis and neutrophilia in humans. While investigating the source of leukocytosis in mice, we discovered that stress activates upstream hematopoietic stem cells. Under conditions of chronic variable stress in mice, sympathetic nerve fibers released surplus noradrenaline, which signaled bone marrow niche cells to decrease CXCL12 levels through the β3-adrenergic receptor. Consequently, hematopoietic stem cell proliferation was elevated, leading to an increased output of neutrophils and inflammatory monocytes. When atherosclerosis-prone Apoe(-/-) mice were subjected to chronic stress, accelerated hematopoiesis promoted plaque features associated with vulnerable lesions that cause myocardial infarction and stroke in humans.

Figure 1. Chronic stress increases proliferation of hematopoietic stem and progenitor cells in the bone marrow

a, Cohen’s perceived stress scale score in medical ICU residents (n = 10–15, Student’s t–test). b, Blood leukocytes in residents (n = 29, Wilcoxon test). c, Leukocytes in mouse blood and d, bone marrow after 3 weeks of stress (n = 9 per group, Student’s t–test). e, Gating for LSK and HSC. Proliferation was assessed after 3 weeks of stress (n = 5 per group, Mann–Whitney test). f, Bone marrow colony forming unit (CFU) assay after one week of stress (n = 5 per group, Mann–Whitney test). g, Bone marrow HSC and LSK after 3 weeks of stress (n = 10 per group, Student’s t–test). h, Long–term competitive repopulation assay using limiting dilutions of whole bone marrow from stressed or non–stressed mice (Poisson statistics for LT–HSC frequencies, P = 0.2 two–tailed t–test). i, Competitive reconstitution with 2×10⁶ bone marrow cells from stressed or non–stressed controls co–injected with equal numbers of naive competitor cells, followed by assessment of blood chimerism (n = 10 mice per group, one–way ANOVA). Mean ± s.e.m., * P < 0.05.

Figure 2. Hematopoietic progenitors in the bone marrow of stressed mice dilute DiD membrane dye faster

a, Intravital microscopy of the mouse calvarium after adoptive transfer of DiD–labelled LSK (white arrows) before and seven days after stress (n = 5 mice per group, Mann–Whitney test). Dotted lines outline bone. Scale bar indicates 50 μm. Single dots in graphs represent DiD⁺ cells’ target–to–background ratio before (upper panel) and after stress (lower panel). b, DiD fluorescence on day 0 and 7 days after adoptive transfer of DiD⁺ GFP⁺ LSK in non–stressed control (CT) or stressed mice (n = 5 per group). The bar graph illustrates the DiD fluorescence in GFP⁺ LSK (Mann–Whitney test). Mean ± s.e.m., * P < 0.05.

Figure 3. Stress–induced sympathetic nervous signaling regulates proliferation of bone marrow HSC via CXCL12

a, Noradrenaline ELISA after 3 weeks of stress (n = 8 per group, Student’s t–test). b, Immunoreactive staining for tyrosine hydroxylase (TH) in bone marrow. Scale bar indicates 10 μm. Bar graph shows TH–positive area (n = 5 mice per group, Mann–Whitney test). c, CXCL12 mRNA in bone marrow (n = 10 per group, one–way ANOVA). d, CXCL12 protein in bone marrow (n = 7 per group, one–way ANOVA). e, Dot plots and quantification of LSK and HSC (n = 5 per group, Mann–Whitney test). f, Effects of β₃ adrenoreceptor blocker on blood leukocytes (n = 5 per group, Mann–Whitney test). Mean ± s.e.m., * P < 0.05.

Figure 4. Chronic stress increases inflammation in mouse atherosclerotic plaques

a, Protease activity in aortic roots of ApoE^−/− mice measured by FMT–CT after 6 weeks of stress. Circles indicate aortic root (n = 5 per group, Mann–Whitney test). b, Immunoreactive staining of aortic roots for CD11b and Ly6G. Bar graphs show percentage of positive area per root (n = 9–10 per group, Student’s t–test). Scale bar indicates 200 μm. c, Gating and quantification of aortic myeloid cells (n = 10 per group, Student’s t–test). d, qPCR for inflammatory genes in aorta (n = 9–10 per group, Student’s t–test). e, Masson trichrome staining (n = 9 per group, Student’s t–test). Scale bar indicates 50 μm, arrows point at fibrous cap covering necrotic core. Bar graphs show fibrous cap thickness and necrotic core area. Mean ± s.e.m., *P < 0.05.