Expansion in the Absence of TLR Signaling Hematopoietic Stem and Progenitor Cell Cutting Edge: Bacterial Infection Induces

Bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) can be activated by type I IFNs, TLR ago-nists, viruses, and bacteria to increase hematopoiesis. In this study, we report that endotoxin treatment in vivo induces TLR4, MyD88, and Toll/IL-1 resistance domain-containing adaptor-inducing IFN-b (TRIF)-dependent expansion of BM HSPCs. Bacterial infection by Staphylococcus aureus or cecal ligation and puncture also induces HSPC expansion, but MyD88, TRIF, type I IFN, cytokine, PG, or oxidative stress pathways are not required for their expansion. S. aureus-induced HSPC expansion in MyD88 2/2 TRIF 2/2 mice is also normal, but is associated with BM remodeling as granulocyte stores are released peripherally. Importantly, reduction in BM cellularity alone can reproduce HSPC expansion. These data show in vivo HSPC responses to bacterial infection are complex and not absolutely dependent upon key inflammatory signaling pathways. L ong-term reconstituting (LTR) hematopoietic stem cells (LT-HSCs) are the source of all circulating blood cells and are defined by their capacity for self-renewal and multilineage differentiation. Mouse hematopoietic stem and progenitor cells (HSPCs), including LT-HSCs, can be identified within the lineage 2/low Sca-1 + c-kit + (LSK) subset of cells (1–3), and LT-HSCs can be differentiated from those with short-lived, rapidly dividing HSPCs short-term reconstituting-HSCs (ST-HSC) by the expression of additional cell surface markers (4). Although LT-HSCs maintain blood cell production during homeostasis, HSPCs expand in vivo following IFN-a treatment (4–6), bone marrow (BM) ablation (7), or bleeding (8), dramatically increasing the production of leukocytes. TLRs are central to innate immunity by recognizing conserved molecular patterns found on pathogens. All TLRs, except TLR3, signal via the adaptor protein, MyD88, leading to NF-kB activation. TLR4, the receptor for LPS, and TLR3 also signal through the adaptor protein, Toll/IL-1 resistance domain-containing adaptor-inducing IFN-b (TRIF)/Toll-IL1 receptor domain-containing adaptor molecule 1, which leads to IFN-I production. HSPCs express TLRs and are activated by TLR2 and TLR4 agonists in vitro through MyD88 sig-naling (9). HSPCs are also activated in vivo by the TLR3 agonist polyinosinic-polycytidylic acid through IFN-I signal-ing (5, 6). Similarly, vaccinia virus infection causes MyD88-dependent HSPC expansion (10). Although Escherichia coli infection induces LSK expansion (11), the mechanism(s) of bacterial-induced HSPC activation have not been fully elucidated. In this study, we used two infectious models, Staphylococcus aureus infection and polymicrobial sepsis induced by cecal ligation and puncture (CLP), to examine the regulation of in vivo HSPC expansion. double knockout (DKO) mice were performed at Osaka University …