Upon recognition of pathogen-associated molecular patterns (PAMPs), i.e. danger signals and

sensing of the inflammatory cytokine environment, DCs undergo rapid maturation. The extent of their activation depends on the initial triggering stimuli 5 that can directly impact the fate of CD8+ T cells differentiation 1. In mice infected by Listeria monocytogenes (Lm), inadequate cDC activation correlates with impaired development of protective CD8+ T-cell memory 6–8. Evidence accumulated over the past years suggested that CD8α+ cDCs PI3K inhibitor play a unique role in priming CD8+ T cells, in particular because of intrinsic features of their MHC class I processing machinery 9. CD8α+ cDCs have also been shown to be endowed with optimized functional characteristics to induce pathogen- and tumor-specific CD8+ T cells to differentiate into primary effector cells 10–13. However, whether these cells or even CD8α− cDCs, independently of their respective capacity to process MHC class I-associated antigens, are capable of integrating all pathogen-derived signals signaling pathway and conveying them to naïve CD8+ T cells to become long-lasting pathogen-specific

protective memory cells in vivo is not known. While both cytosolic and/or extracellular-derived signals likely contribute to such cDC licensing, the relative impact of these signals has not been extensively investigated. Lack of such knowledge is mostly due to technical limitations. In fact, adoptive transfer of DC subsets from immunized animals has been difficult to interpret since these cells contain virulent pathogens that can directly infect recipient hosts and activate long-term immunity. Selective in vivo depletion of APC subsets also suffered from the specificity of the depletion 4, 14. To circumvent these issues, we designed Oxymatrine an experimental system in which APC subsets could be purified from mice immunized with the intracellular bacterium Lm lacking the SecA2 auxiliary secretion system (secA2− or ΔSecA2 Lm) 15, 16 which induce protective immunity only upon infection with high numbers of bacteria (107). SecA2−Lm also exhibit impaired spreading from cell to cell and do not efficiently infect APCs from recipient mice. Thus, taking advantage

of this experimental set-up, we could ask whether a subset of cDC is indeed more efficient at inducing protective CD8+ T-cell memory in vivo. We previously demonstrated that mice immunized with low numbers (106) of secA2−Lm develop memory CD8+ T cells that do not protect against a secondary infection with wt bacteria 16, 17. Since SecA2 partially controls the secretion of a subset of bacterial proteins, we hypothesized that induction of protective memory CD8+ T cells may require the secretion of a sufficient amount of at least one SecA2 substrate protein inside the cytosol of infected host cells to generate the appropriate priming environment. Therefore, we reasoned that the cytosolic signaling defect should be restored by immunizing mice with an increased dose of secA2−Lm.