Hematopoiesis sculpted by pathogens: Toll-like receptors and inflammatory mediators directly activate stem cells

Hematopoietic stem cells (HSCs) repopulate the immune system during normal replenishment as well as under the burden of pathogen stress, but the respective outcomes of differentiation are not the same. Under homeostatic conditions such as those which accompany turnover of immune cell subsets, HSCs appear to co-equally prime genes associated with the major downstream lineages: lymphoid, myeloid, and megakaryocyte/erythroid. Recent studies reveal, however, that during pathogen exposure, hematopoiesis may yield progeny in proportions different than those produced under homeostasis. At least some of these effects may be due to pathogen engagement of Toll-like receptors (TLRs) expressed on HSCs. HSCs are also responsive to inflammatory cytokines that are produced in response to pathogen burden and are present in the bone marrow microenvironment. Thus, hematopoiesis is not a formulaic process that produces the same, predictable outcome regardless of the specific environmental context. Rather, hematopoiesis represents a dynamic biological system that can be appreciably responsive to environmental factors, an influence that extends to the level of the HSC itself. Knowledge of functional consequences of TLR ligation on HSCs may be therapeutically exploited and applied to treatment of hematopoietic insufficiency in the setting of infection and disease.