Direct Toll-like receptor-mediated stimulation of hematopoietic stem and progenitor cells occurs in vivo and promotes differentiation toward macrophages.

As Toll-like receptors (TLRs) are expressed by hematopoietic stem and progenitor cells (HSPCs), they may play a role in hematopoiesis in response to pathogens during infection. We show here that TLR2, TLR4, and TLR9 agonists (tripalmitoyl-S-glyceryl-L-Cys-Ser-(Lys)4 [Pam3CSK4], lipopolysaccharide [LPS], and CpG oligodeoxynucleotide [ODN]) induce the in vitro differentiation of purified murine lineage negative cells (Lin(-) ) as well as HSPCs (identified as Lin(-) c-Kit(+) Sca-1(+) IL-7Rα(-) [LKS] cells) toward macrophages (Mph), through a myeloid differentiation factor 88 (MyD88)-dependent pathway. In order to investigate the possible direct interaction of soluble microorganism-associated molecular patterns and TLRs on HSPCs in vivo, we designed a new experimental approach: purified Lin(-) and LKS cells from bone marrow of B6Ly5.1 mice (CD45.1 alloantigen) were transplanted into TLR2(-/-) , TLR4(-/-) , or MyD88(-/-) mice (CD45.2 alloantigen), which were then injected with soluble TLR ligands (Pam3CSK4, LPS, or ODN, respectively). As recipient mouse cells do not recognize the TLR ligands injected, interference by soluble mediators secreted by recipient cells is negligible. Transplanted cells were detected in the spleen and bone marrow of recipient mice, and in response to soluble TLR ligands, cells differentiated preferentially to Mph. These results show, for the first time, that HSPCs may be directly stimulated by TLR agonists in vivo, and that the engagement of these receptors induces differentiation toward Mph. Therefore, HSPCs may sense pathogen or pathogen-derived products directly during infection, inducing a rapid generation of cells of the innate immune system.