A new experimental approach to address whether TLR agonists can stimulate HSPCs in vivo has been recently used. Purified Lin− or LKS+ cells from the BM of B6Ly5.1 mice (CD45.1+) were transplanted

into TLR2−/−, TLR4−/−, or MyD88−/− mice (CD45.2+), which were then injected with pure ligands for TLR2, TLR4, or TLR9 (Pam3CSK4, LPS, and CpG ODN, respectively). Recipient mouse cells Idasanutlin are not capable of recognizing or responding to the injected TLR ligands; therefore, any responses observed in the transplanted cells must be due to direct recognition of the agonists by TLRs expressed by the donor HSPCs. Transplanted HSPCs were detected in the BM and spleen of recipient mice and, in response to TLR ligand injection, these cells differentiated preferentially into macrophages, demonstrating unequivocally that HSPCs can respond directly to TLR agonists in vivo, and that the engagement of these receptors induces macrophage differentiation [21] (Fig. 2). A similar in vivo transplantation approach was used to Bortezomib molecular weight study the effect of C. albicans infection on HSPCs [20]. Transplanted Lin− cells were detected in the spleen and BM of recipient mice,

and they differentiated preferentially to macrophages in response to both live and inactivated yeast. Macrophage generation was dependent on TLR2, but independent of TLR4 (Fig. 2). These results indicate that TLR2-mediated recognition of C. albicans by HSPCs helps to replace and/or to increase cells that constitute the first line of defense against the fungus, and suggest that TLR2-mediated signaling leads to programming of early progenitors to rapidly replenish the innate immune system and generate the mature cells most urgently needed to deal with the pathogen. Direct microbial detection by HSPCs, of course, requires colocalization. HSPCs can be found

as resident or migratory populations in uninfected and infected tissues [45, 46], where microbes could induce them to differentiate by extramedullary hematopoiesis. PRKD3 HSPCs located in infected tissues are more likely to have an opportunity to directly detect microbial components than the majority of HSPCs, which reside in the BM. However, HSPCs in the heavily vascularized BM may also be exposed to circulating microbial components, or even to intact microbes following BM invasion during systemic infection. We have previously detected fungal cells in the BM of mice with invasive candidiasis, albeit at lower numbers than in peripheral tissues, but theoretically at sufficient levels to induce measurable activation of HSPCs [26, 42]. The concept of microbial components directly stimulating HSPCs to trigger the rapid generation of myeloid cells to boost the immune response against the infection is certainly attractive.