Stress-Induced Activation of Dormant Hematopoietic Stem Cells In Vivo,

Abstract

3390 Maintenance of the blood system is dependent on dormant haematopoietic stem cells (HSCs), which are characterized by pluripotency and life long self-renewal capacity. In order to both maintain a supply of mature blood cells and not exhaust HSCs throughout the lifespan of the organism, most adult HSCs remain quiescent and only a limited number are cycling at any given time. The balance between self-renewal and differentiation of HSCs is controlled by external factors such as chemokines, as well as interactions of HSCs with its niche environment. We have recently shown that the cytokine IFNa very efficiently activates dormant HSCs in vivo. Within hours after treatment of mice with IFNa HSCs exit G0 and enter the active cell cycle. In general, IFNa is produced in response to viral infections by cells of the immune system, and plays an important role in the host defense against the viral infection. We now questioned whether endogenous IFNa is also produced in response to other forms of bone marrow stress and whether this affects the proliferation rate of HSCs. To monitor IFNa production in the bone marrow in vivo, we have generated MxCre; ROSA-R26-EYFP mice and found that treatment with both the chemotherapeutic agent 5-FU as well as the endotoxin LPS leads to the production of IFNa in HSCs and progenitors. In addition, LPS treatment in vivo induced a strong increase in proliferation of HSCs. In contrast to the direct effect of IFNa on HSCs, in vivo and in vitro experiments have shown that the LPS induced activation of HSCs is triggered via an indirect effect of LPS on CD11b+ cells in the bone marrow. Activation of these cells via TLR4 signaling then results in increased proliferation of the HSCs, a mechanism we are currently investigating in more detail. Interestingly, LPS induced activation correlated with increased expression of Sca-1 on HSCs, similar to the increased Sca-1 expression upon IFNa treatment. As for IFNa, the upregulation of Sca-1 is required for LPS induced proliferation, since Sca-1−/− mice do not respond to LPS stimulation. Furthermore, cDNA array comparisons between HSCs treated with IFNa or LPS suggest a more common mechanism of activation, independent of the source leading to the activation. In summary, these data suggest that in addition to viral infection also other forms of bone marrow stress, like LPS, result in activation of quiescent HSCs in the bone marrow, probably via similar mechanisms. Furthermore, both IFNa and LPS induced activation of HSCs are dependent on the up-regulation of Sca-1, suggesting a more general role for Sca-1 in the activation of stem cells. Disclosures: No relevant conflicts of interest to declare.

Hypersensitivity response has negligible impact on Hematopoietic Stem Cells

“…Previous studies using other stimuli reported significant changes in HSCs' surface markers, most notably SCA1 and CD150 (Baldridge et al, 2010;Essers et al, 2009Essers et al, , 2011Flach et al, 2014;Haas et al, 2015;Walter et al, 2015). A functional role for SCA1 was reported (Bradfute et al, 2005), while CD150 may not have an essential function in HSCs (Kim et al, 2006) but changes dramatically following stimulation.…”

“…While HSCs are deeply quiescent at naive state ( Orkin and Zon, 2008 ; Pietras, 2017 ), they can rapidly activate following immune stimulation ( Bujanover et al., 2018 ; de Bruin et al., 2014 ; Essers et al., 2009 , 2011 ; Haas et al., 2015 ). Therefore, we performed cell-cycle analysis of BM cells from control phosphate-buffered saline (PBS)-treated, Ova-hypersensitized, and pIpC-treated mice.…”

Rapid Activation of Hematopoietic Stem Cells

“…The activation is largely IFNα (Essers et al, 2011;Essers et al, 2009). Interestingly, pIpC was reported to not only activate, but to also skew differentiation into megakaryocytes (Haas et al, 2015), presumably to enhance coagulation.…”

“…Immune stimulation was further reported to induce trained immunity in HSCs including metabolic and epigenetic myeloid skew (de Laval et al, 2020; Mitroulis et al, 2018). Either viral- or bacterial stimuli induce the quiescent HSCs to proliferate – and give rise to more effector cells (Essers et al, 2011; Pietras et al, 2014; Takizawa et al, 2017; Takizawa et al, 2011; Walter et al, 2015; Yanez et al, 2009). Surprisingly, while previous studies extensively studied the days and months following immune stimulation, the acute early stages of HSC’s activation are relatively less studied.…”

“…pIpC stimulation activates type-I interferon response, leading to the activation of HSCs in the BM (Essers et al, 2009; Sato et al, 2009). Such HSC activation depends largely on IFNα (Essers et al, 2011; Essers et al, 2009). Interestingly, pIpC was also reported to skew HSC differentiation into megakaryocytes (Haas et al, 2015), presumably to enhance coagulation.…”