Lipopolysaccharide-induced activation of haematopoietic stem cells in vivo

Haematopoietic stem cells (HSCs) are the best-characterised adult stem cells and ensure life-long maintenance of the blood system. Under homeostatic conditions most of the HSCs are found in a quiescent state, but can be activated in response to stress (Wilson et al., 2008). Infection is a major source of natural stress to the haematopoietic system, leading to exhaustion of large amounts of immune cells during the defence against invading pathogens. Reestablishment of homeostasis following infection depends on the replacement of immune cells by haematopoietic progenitors. Interestingly, \nrecent studies have indicated that HSCs themselves can be activated in response to infection (Essers et al., 2009; Sato et al., 2009; Baldridge et al., 2010; Takizawa et al., 2011). However, whether HSCs are able to directly sense the infectious state remains unclear. In this work, we have investigated the in vivo short-term response of HSCs to lipopolysaccharide (LPS), a major cell wall component of Gram-negative bacteria. Low dose administration of LPS to mice induces a rapid TLR4 dependent proliferation of even the most dormant HSCs, without impairing their long-term repopulating potential. This \nactivation is accompanied by upregulation of stem cell antigen 1 (Sca-1) and dependent on the expression of Sca-1 on HSCs. Both in vitro and in vivo data demonstrate that activation of HSCs via LPS is not a direct effect of LPS on HSCs. Instead, LPS treatment stimulates myeloid cells in the BM, most probably inflammatory monocytes, to produce inflammatory cytokines, which in turn induce the activation of HSCs. Furthermore, our results suggest that the activation of HSCs is mediated by initial cell-cell-interaction of \nthese myeloid cells with HSCs. Using genetic mouse models we show that the activation of HSCs is regulated by combined IFNa, IFNg and TNFa signalling. Moreover, TNFa alone induces a Sca-1 dependent activation of HSCs in vivo, making these cells susceptible to chemotherapy. In addition to IFN and TNFa signalling also IL-1b signalling is necessary to mediate the LPS-induced activation of HSCs. Interestingly, IL-1b itself leads to increased \nproliferation of HSCs, most likely through direct activation of IL-1 signalling in these cells. Thus, our work provides novel cellular and genetic insights into the identity of LPS induced cytokine production, leading to HSC activation. Thus, providing a way to rapidly induce the production of mature immune cells upon infection, leading to a fast restoration of homeostasis upon bacterial infections. Furthermore, our work provides new insights into the role of the pro-inflammatory cytokines TNFa and IL-1b in the regulation of HSC function.