Type I interferons (IFNs) are cytokines exhibiting antiviral and antitumor effects, including multiple activities on immune cells. However, the importance of these cytokines in the early events leading to the generation of an immune response is still unclear. Here, we have investigated the effects of type I IFNs on freshly isolated granulocyte/macrophage colony-stimulating factor (GM-CSF)–treated human monocytes in terms of dendritic cell (DC) differentiation and activity in vitro and in severe combined immunodeficiency mice reconstituted with human peripheral blood leukocytes (hu-PBL-SCID) mice. Type I IFNs induced a surprisingly rapid maturation of monocytes into short-lived tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)–expressing DCs endowed with potent functional activities, superior with respect to the interleukin (IL)-4/GM-CSF treatment, as shown by FACS® analyses, mixed leukocyte reaction assays with allogeneic PBLs, and lymphocyte proliferation responses to HIV-1–pulsed autologous DCs. Type I IFN induced IL-15 production and strongly promoted a T helper cell type 1 response. Notably, injection of IFN-treated HIV-1–pulsed DCs in SCID mice reconstituted with autologous PBLs resulted in the generation of a potent primary immune response, as evaluated by the detection of human antibodies to various HIV-1 antigens. These results provide a rationale for using type I IFNs as vaccine adjuvants and support the concept that a natural alliance between these cytokines and monocytes/DCs represents an important early mechanism for connecting innate and adaptive immunity.

IntroductionDendritic cells (DCs) are the most potent antigen-presenting cells playing a pivotal role in the induction of the immune response. [1][2][3] DCs are located in peripheral tissues, in sites where they can optimally survey for incoming pathogens. The interaction of DCs with pathogens leads to migration to secondary lymphoid organs where they initiate a specific immune response. Notably, the migration capability of DCs is strictly regulated by their response to soluble factors, namely chemokines 4,5 that characterize maturation stage and shape functional activities of DCs.Chemokines represent a family of 8-to 10-kDa secreted proteins capable of regulating migration and activation not only of leukocytes, including DCs, but also of stromal cells. 6,7 It is well documented that migration of DCs is tightly regulated as a function of maturation. [8][9][10][11][12] In particular, immature DCs respond to many CC and CXC chemokines, such as MIP-1␣, MIP-1␤, RANTES, and MIP-3␣, whereas mature DCs have lost their responsiveness to most of these chemokines, as a result of down-regulation of receptor expression or activity. However, mature DCs have been reported to respond to MIP-3␤/ELC and 6Ckine/SLC as a consequence of an up-regulation of their receptor (CCR7). Of interest, studies in knock-out mice for CCR7 have shown the crucial importance of the CCR7/MIP-3␤ interaction for the generation of a primary immune response. 13 All this emphasizes the essential role of certain chemokines/chemokine receptors and DC migration properties in the generation of the immune response.DCs are derived from hematopoietic progenitor cells, 2,3 and distinct subtypes of human circulating DCs have been detected in the blood. 14,15 However, the mechanisms regulating generation, functions, and survival of blood-circulating DCs in response to infections are largely unknown. The rapid generation of active DCs endowed with potent migratory capabilities would be advantageous for a prompt immune response to incoming pathogens.Blood monocytes are highly versatile cells playing crucial roles in the maintenance of immune homeostasis. These cells circulate in the bloodstream, transmigrate through vascular endothelium, and localize in peripheral and mucosal tissues, where they differentiate into different cell types. 16,17 Monocyte-derived mature DCs are currently generated in vitro by 2 sequential treatments, 18 leading first to the so-called "immature DCs," after exposure for several days to both granulocytemacrophage colony-stimulating factor (GM-CSF) and interleukin 4 (IL-4), and then to mature DCs, after a subsequent addition of stimuli such as lipopolysaccharide (LPS), CD40L, or virus infection. However, the in vivo relevance of monocyte differentiation into DCs remains unclear, especially because exposure of monocytes to IL-4 can hardly mimic the cytokine milieu likely to be present under in vivo conditions at the infection site.Although results indicate that DC maturation can occur directly from monocytes during transendothelial migration...

IntroductionOver the past years, it has become apparent that type-I interferons (IFNs) affect adaptive immunity through their effects on monocytes. In particular, IFN-␣ has been shown to act as a potent inducer of the rapid differentiation of human monocytes into highly activated and partially mature dendritic cells (DCs), known as IFN-DCs. 1 We demonstrated previously that human monocytes exposed to granulocyte macrophage colony-stimulating factor (GM-CSF) and IFN-␣ are rapidly induced to express a set of membrane molecules involved in antigen (Ag) presentation and T-cell costimulation, as well as to strongly promote T helper (Th)-1 response and CD8 ϩ T cell cross-priming. 2 Moreover, IFN-DCs were shown to cross-present very efficiently low amounts of nonstructural-3 protein (NS3) of hepatitis C virus (HCV) to a specific CD8 ϩ T cell clone, even in the absence of CD4 ϩ T-cell help. 2 The cross-presentation efficiency of DCs is not dictated solely by their Ag capture capability 3 ; it also is affected by critical factors such as (1) the route of Ag uptake, (2) acidification-sensitive Ag degradation in endosomal-lysosomal compartments, and (3) Ag entry into the major histocompatibility complex class-I (MHC-I) pathway. [4][5][6][7][8] At present, 4 nonmutually exclusive models have been proposed to explain cross-presentation. [8][9][10] In the canonical cytosolic pathway, endocytosed Ags are translocated into the cytosol, where they are degraded by the proteasome, and then the antigenic peptides are transported into the lumen of the endoplasmic reticulum (ER) by the transporters associated with Ag processing (TAPs), 9,10 or alternatively, reimported from the cytoplasm into the early endosomes and loaded onto endosomal 12 According to a less well-defined TAP-and proteasomeindependent endosomal pathway, Ags can be processed by endosomal proteases and loaded onto MHC-I molecules directly within early and late endosomes and lysosomes. [13][14][15] An additional model involves the delivery of components of the ER to endocytic organelles or the transport of incoming Ags to the ER. [16][17][18] Here, we have investigated the mechanisms underlying the superior efficiency of IFN-DCs in the cross-presentation of soluble proteins, by studying (1) the Ag uptake and trafficking to the class-I processing pathway, (2) the maturation kinetic of the organelles containing the internalized proteins, (3) the Ag stability within endosomes, and (4) the Ag processing and cross-presentation to specific CD8 ϩ T cells. The results reveal that IFN-DCs exhibit a delayed endosomal acidification associated with a prolonged Ag survival and retention in the early endosomal compartment, as well as with Ag trafficking to recycling pathways. In IFN-DCs, both early and recycling endosomal compartments serve as important stores of MHC-I molecules, allowing rapid presentation of exogenous Ags. These findings provide novel mechanistic insight into the cross-presentation efficiency of IFN-DCs and underscore the potential advantage of using these cellula...

Dendritic cells (DC) generated after a short-term exposure of monocytes to IFN-a and GM-CSF (IFN-DC) are highly effective in inducing cross-priming of CD8 + T cells against viral antigens. We have investigated the mechanisms responsible for the special attitude of these DC and compared their activity with that of reference DC. Antigen uptake and endosomal processing capabilities were similar for IFN-DC and IL-4-derived DC. Both DC types efficiently cross-presented soluble HCV NS3 protein to the specific CD8 + T cell clone, even though IFN-DC were superior in cross-presenting low amounts of viral antigens. Moreover, when DC were pulsed with inactivated HIV-1 and injected into hu-PBL-SCID mice, the generation of virus-specific CD8 + T cells was markedly higher in animals immunized with IFN-DC than in mice immunized with CD40L-matured IL-4-DC. Of interest, in experiments with purified CD8 + T cells, IFN-DC were superior with respect to CD40L-matured IL-4-DC in inducing in vitro cross-priming of HIV-specific CD8 + T cells. This property correlated with enhanced potential to express the specific subunits of the IL-23 and IL-27 cytokines. These results suggest that IFN-DC are directly licensed for an efficient CD8 + T cell priming by mechanisms likely involving enhanced antigen presentation and special attitude to produce IL-12 family cytokines.

A major challenge of AIDS research is the development of therapeutic vaccine strategies capable of inducing the humoral and cellular arms of the immune responses against HIV-1. In this work, we evaluated the capability of DCs pulsed with aldrithiol-2–inactivated HIV-1 in inducing a protective antiviral human immune response in SCID mice reconstituted with human PBL (hu-PBL-SCID mice). Immunization of hu-PBL-SCID mice with DCs generated after exposure of monocytes to GM-CSF/IFN-α (IFN-DCs) and pulsed with inactivated HIV-1 resulted in a marked induction of human anti–HIV-1 antibodies, which was associated with the detection of anti-HIV neutralizing activity in the serum. This vaccination schedule also promoted the generation of a human CD8+ T cell response against HIV-1, as measured by IFN-γ Elispot analysis. Notably, when the hu-PBL-SCID mice immunized with antigen-pulsed IFN-DCs were infected with HIV-1, inhibition of virus infection was observed as compared with control animals. These results suggest that IFN-DCs pulsed with inactivated HIV-1 can represent a valuable approach of immune intervention in HIV-1–infected patients.