We investigated the mechanisms through which infection regulates the formation of bone marrow-derived dendritic cells (BMDCs) in vitro. We mimicked infection by stimulating developing cells with molecules associated with bacteria and viruses and with inactivated influenza viruses. We showed that toll-like receptor (TLR) ligands act as modulators of haematopoiesis, and that signalling through different TLRs results in differing

effects on the production of BMDCs. We demonstrated that ligands for TLR3 and influenza viruses reduce the production of BMDCs, resulting in increased neutrophil numbers, and that ligands for TLR4 and TLR9 drive the production of plasmacytoid dendritic cells. Furthermore, there are distinct signalling mechanisms involved in these Proteases inhibitor effects. Signalling pathways triggered by EPZ-6438 purchase TLR4 and TLR9 involve MyD88 and are partially mediated by the cytokine tumour necrosis factor-α (TNF-α). Mechanisms activated by TLR3 were Tir-domain-containing adaptor-inducing interferon dependent. Haematopoietic modulation induced by inactivated influenza viruses was associated with the activation of an antiviral pathway mediated by type-1 interferons. Toll-like receptors (TLRs) are a family of pattern

recognition receptors (PRRs) which are involved in the recognition of pathogen-related molecular patterns (PAMPs) associated with bacteria, viruses and fungi. Although the importance of TLRs for innate and adaptive immunity has been well documented, recent studies have suggested that they may also have a role in tissue homeostasis. Rakoff-Nahoum et al.1 demonstrated

that signalling through TLR4 plays a role in the maintenance of epithelial homeostasis in the gut. They found that commensal bacteria are recognized by TLRs under normal steady-state conditions and that this interaction plays a role in maintaining gut epithelial cells and protecting the epithelium from injury. Inflammation has been shown to alter leucocyte production by reducing lymphopoiesis and promoting granulopoiesis in vivo; this bias towards granulopoiesis is generated by inflammation-induced tumour necrosis factor (TNF)-α initiating a reduction in the level of chemokines such as CXCL12.2,3 Borrow et al.4 demonstrated that influenza virus infection leads to a depletion of early B-lineage cells PD184352 (CI-1040) in the bone marrow. This depletion was mediated by a TNF receptor (TNFR)-dependent mechanism and involved the cytokines TNF-α and lymphotoxin (LT)-α. Taken together, these data show that infection and inflammation can influence the production of haematopoietic cells in vivo. On ligand binding, TLRs initiate signalling cascades that result ultimately in the production of cytokines and chemokines. These signalling cascades are mediated by the adaptor molecules MyD88 (all TLRs excluding TLR3)5 and Tir-domain-containing adaptor-inducing interferon (TRIF) (TLR3 and TLR4).