Acute infection usually triggers the mobilization of myeloid cells, in particular neutrophils and monocytes, from the BM to infected tissues. This is accompanied by the proliferation

and differentiation LBH589 mouse of HSPCs in the BM to maintain the supply of myeloid cells. During most bacterial, viral, and fungal infections, myelopoiesis therefore becomes the predominant form of cellular production, with the development of other lineages (lymphoid and erythroid) inhibited. Myelopoiesis is also commonly accompanied by alterations in the cellular composition and/or functional characteristics of BM HSPCs [5, 6]. In fact, inflammatory cytokines secreted during infection-induced emergency myelopoiesis reduce the expression of growth and

retention factors for lymphopoiesis, and BM lymphocytes are therefore mobilized to secondary lymphoid organs [6]. Emergency myelopoiesis may consist of granulopoiesis (especially neutrophil production), monopoiesis (generation of monocytes and macrophages) or both, depending on the specific microbe as well as the route and severity RXDX-106 nmr of infection. Several cytokines and transcription factors have been implicated in emergency myelopoiesis, although the molecular mechanisms underlying its regulation have not been clearly defined yet. In many cases it is not even yet clear which cells are responsible for instructing the emergency response. Moreover, HSPCs appear to respond to both “pull” and “push” signals (reviewed in [7]).

“Pull” signals are exerted on HSPCs by the differentiation of more committed progenitors and the mobilization of differentiated cells from the BM to infected tissues, which induces HSPCs to replace those cells. Myelopoiesis can also be driven by “push” signals, such as myelopoietic factors produced by differentiated cells of hematopoietic (e.g. tissue macrophages) or nonhematopoietic (e.g. epithelial cells) origin, which sense the infection. For example, in mice chronically infected with Mycobacterium avium, increased HSC proliferation Dichloromethane dehalogenase has been shown to be part of the primary immune response, rather than a compensatory response to progenitor depletion as it occurs in the absence of peripheral cytopenia [7, 8]. Several cytokines have been shown to induce myeloid cell production by HSPCs, including type I and II IFNs, TNF-α and IL-6 [5, 7, 9, 10]. In this review we will focus on a new paradigm that has emerged over the past decade: the delivery of myelopoiesis-inducing “push” signals by microbial components directly sensed by HSPCs. Differentiated innate immune cells such as macrophages and neutrophils recognize characteristic molecular signatures of microbes using pattern recognition receptors (PRRs).