Microbial sensing by hematopoietic stem and progenitor cells: Vigilance against infections and immune education of myeloid cells.

Bone marrow hematopoietic stem and progenitor cells (HSPCs) express pattern recognition receptors such as Toll-like receptors (TLRs) to sense microbial products and activation of these innate immune receptors induces cytokine expression and redirects bone marrow hematopoiesis towards the increased production of myeloid cells. Secreted cytokines by HSPCs in response to TLR ligands can act in an autocrine or paracrine manner to regulate hematopoiesis. Moreover, tonic activation of HSPCs by microbiota-derived coumpouds might educate HSPCs to produce superior myeloid cells equipped with innate memory responses to combat pathogens. While hematopoietic stem cell activation through TLRs meets the increased demand for blood leukocytes to protect the host against infection, persistant exposure to inflammatory cytokines or microbial products might impair their function and even induce malignant transformation. This mini-review highlights the potential outcomes of HSPCs in response to TLR ligands.