Plasmacytoid dendritic cells efficiently cross-prime naive T cells in vivo after TLR activation.

Cross-presentation is a crucial mechanism in tumoral and microbial immunity because it allows internalized cell associated or exogenous antigens (Ags) to be delivered into the major histocompatibility complex I pathway. This pathway is important for the development of CD8(+) T-cell responses and for the induction of tolerance. In mice, cross-presentation is considered to be a unique property of CD8alpha+ conventional dendritic cells (DCs). Here we show that splenic plasmacytoid DCs (pDCs) efficiently capture exogenous Ags in vivo but are not able to cross-present these Ags at steady state. However, in vitro and in vivo stimulation by Toll-like receptor-7, or -9 or viruses licenses pDCs to cross-present soluble or particulate Ags by a transporter associated with antigen processing-dependent mechanism. Induction of cross-presentation confers to pDCs the ability to generate efficient effector CD8+ T-cell responses against exogenous Ags in vivo, showing that pDCs may play a crucial role in induction of adaptive immune responses against pathogens that do not infect tissues of hemopoietic origin. This study provides the first evidence for an in vivo role of splenic pDCs in Ag cross-presentation and T-cell cross-priming and suggests that pDCs may constitute an attractive target to boost the efficacy of vaccines based on cytotoxic T lymphocyte induction.

Purification of splenic dendritic cells induces maturation and capacity to stimulate Th1 response in vivo.

Dendritic cell (DC) maturation state is a key parameter for the issue of DC-T cell cognate interaction, which determines the outcome of T cell activation. Indeed, immature DCs induce tolerance while fully mature DCs generate immunity. Here we show that, in the absence of any deliberate activation signal, DCs freshly isolated from mouse spleen spontaneously produce IL-12 and tumor necrosis factor-alpha and up-regulate co-stimulation molecules, even when directly re-injected into their natural environment. Furthermore, after their isolation, these cells acquire the capacity to induce specific T(h)1 responses in vivo. These results demonstrate that the sole isolation of spleen DCs leads to the full maturation of these cells, which therefore cannot be considered as immature DCs. Moreover, we also show that the kinetics of DC activation do not influence the polarization of T(h) response in vivo challenging the idea that exhausted DCs induce preferentially T(h)2 response. Altogether, these observations should be taken into account in all experiments based on the transfer of ex vivo purified DCs.

Antigen targeting to CD11b allows efficient presentation of CD4+ and CD8+ T cell epitopes and in vivo Th1-polarized T cell priming.

Bordetella pertussis adenylate cyclase (CyaA) is an invasive bacterial toxin that delivers its N-terminal catalytic domain into the cytosol of eukaryotic cells bearing the alpha(M)beta(2) integrin (CD11b/CD18), such as myeloid dendritic cells. This allows use of engineered CyaA for targeted delivery of CD8(+) T cell epitopes into the MHC class I pathway of APC and induction of robust and protective cytotoxic responses. In this study, we demonstrate that CyaA can efficiently codeliver both a CD8(+) T cell epitope (OVA(257-264)) and a CD4(+) T cell epitope (MalE(100-114)) into, respectively, the conventional cytosolic or endocytic routes of processing of murine bone marrow-derived dendritic cells. Upon CyaA delivery, a strong potentiation of the MalE(100-114) CD4(+) T cell epitope presentation is observed as compared with the MalE protein, which depends on CyaA interaction with its CD11b receptor and its subsequent clathrin-mediated endocytosis. In vivo, CyaA induces strong and specific Th1 CD4(+) and CD8(+) T cell responses against, respectively, the MalE(100-114) and OVA(257-264) epitopes. These results underscore the potency of CyaA for design of new vaccines.

Murine plasmacytoid dendritic cells induce effector/memory CD8+ T-cell responses in vivo after viral stimulation.

Like their human counterparts, mouse plasmacytoid dendritic cells (pDCs) play a central role in innate immunity against viral infections, but their capacity to prime T cells in vivo remains unknown. We show here that virus-activated pDCs differentiate into antigen-presenting cells able to induce effector/memory CD8(+) T-cell responses in vivo against both epitopic peptides and endogenous antigen, whereas pDCs activated by synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG) acquire only the ability to recall antigen-experienced T-cell responses. We also show that immature pDCs are unable to induce effector or regulatory CD8(+) T-cell responses. Thus, murine pDCs take part in both innate and adaptive immune responses by directly priming naive CD8(+) T cells during viral infection.

Delivery of a MalE CD4(+)-T-cell epitope into the major histocompatibility complex class II antigen presentation pathway by Bordetella pertussis adenylate cyclase.

Recombinant adenylate cyclase toxoids are shown to deliver inserted foreign CD4(+)-T-cell epitopes into the major histocompatibility complex class II presentation pathway, inducing a specific CD4(+)-T-cell response in vivo and yielding in vitro stimulation of specific CD4(+) T cells at a 100-times-higher molar efficiency than the free peptide containing the epitope.