The P47 GTPase Lrg-47 Links Host Defense and HSC Proliferation.

Hematopoietic stem cells (HSCs) are largely quiescent, self-renewing cells that give rise to all adult blood lineages. They have a remarkable capacity to respond to proliferative stimuli from a variety of insults, exiting their quiescent phase and undergoing a period of self-renewal and differentiation in order to restore hematopoietic homeostasis. Yet, while some molecular controls on the cycle of HSC activation and quiescence have been elucidated, little is known about the mechanisms linking this process to natural signals, such as the stress of an immune response to chronic infection. Previous work in our laboratory examined gene expression changes in a cycle of activation, expansion, and return to quiescence in HSCs stimulated by the chemotherapeutic agent 5-fluorouracil (5FU). One family of genes identified by this approach encodes the interferon-inducible p47 GTPases, which has been previously characterized solely in the context of the immune response to intracellular infections. Interestingly, mice deficient for one member of this family, Lrg-47, had been observed to develop a major hematopoietic defect in the face of infectious challenge. This combination of findings led us to test the novel hypothesis that Lrg-47 might regulate HSC function in the face of either chemical or pathogenic stress. We found that, while Lrg-47 −/ − mice have largely normal blood counts under homeostasis, these animals exhibited impaired hematopoietic recovery from stress in the form of sublethal irradiation or 5FU treatment. Furthermore, we show that HSCs deficient in Lrg-47 are profoundly compromised in functional repopulation assays, even when given extreme advantages over competitor marrow. We also observed, in BrdU labeling studies, that Lrg-47 −/ − HSCs display dysregulated HSC proliferation, with substantially increased turnover at baseline. We found that this inappropriate proliferation has a functional consequence, as Lrg-47 −/ − mice have a delayed expansion of the HSC compartment following 5FU treatment. Finally, in order to investigate the response of stem and progenitor populations to infectious stimuli, we challenged mice by infection with Mycobacterium avium. Phenotypic and functional assays demonstrated that, while wild-type progenitors expand dramatically in response to infection, Lrg-47 −/ − mice show a stark defect. Our findings thus point to a dual role for Lrg-47 - first as an inhibitor of HSC proliferation in the steady state, and second as a key regulator of HSCs in the response to injury. Overall, our results establish a link between the response to infection and HSC activation, and demonstrate a novel function for a member of the p47GTPase family. Additionally, as Lrg-47 has been chiefly known as an effector of IFNgamma signalling, our results suggest a possible mechanism for the inhibitory effects of this cytokine on hematopoiesis, with implications for human diseases with interferon-related pathogenesis. Most generally, our findings reveal an unexpected intersection point between the fields of stem cell biology and the immune response to pathogens, and suggest that defects in HSC function should be considered as possible determinants of host susceptibility to infection.