Activation of Dormant Hematopoietic Stem Cells In Vivo by the Endotoxin LPS.

Abstract 1613

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 deeply quiescent during homeostasis 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 and cytokines, 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 an 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 antiviral host defense. We now questioned whether endogenous IFNa is also produced in response to other types 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 the vicinity of HSCs and progenitors. In addition, LPS treatment in vivo induced a strong increase in HSC cycling. Surprisingly, since mice lacking the IFNa receptor (Ifnar^-/-) still respond to LPS this effect is independent of IFNAR signaling. LPS binds and signals via the TLR4-CD14 receptor complex and HSCs of mice lacking TLR4 (TLR4-/-) are no longer activated by LPS. Strikingly, LPS induced HSC activation correlated with increased expression of Sca-1, similar to what occurs upon IFNa treatment. Moreover, as for IFNa, the upregulation of Sca-1 is required for LPS induced proliferation, since Sca-1^-/- mice fail to respond to LPS stimulation.

In summary, these data suggest that not only virus inducible IFNa, but also infections by gram negative bacteria produced LPS induces cycling of progenitors and otherwise dormant HSCs. Finally, 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 dormant stem cells in response to injury signals.