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.

The dendritic cell (DC) system of antigen-presenting cells controls immunity and tolerance. DCs initiate and regulate immune responses in a manner that depends on signals they receive from microbes and their cellular environment. They allow the immune system to make qualitatively distinct responses against different microbial infections. DCs are composed of subsets that express different microbial receptors and express different surface molecules and cytokines. Our studies lead us to propose that interstitial (dermal) DCs preferentially activate humoral immunity, whereas Langerhans cells preferentially induce cellular immunity. Alterations of the DC system result in diseases such as autoimmunity, allergy, and cancer. Conversely, DCs can be exploited for vaccination, and novel vaccines that directly target DCs in vivo are being designed.

Although plasmacytoid dendritic cells (pDCs) respond to virus replication in a non-specific fashion by producing large amounts of type I interferon, a rapid, direct role of pDCs in activating antiviral lymphocytes is less clear. Here we showed that pDCs possess the capacity to rapidly initiate antigenspecific antiviral CD8 + T cell responses. Following virus exposure, pDCs efficiently and rapidly internalized exogenous viral antigens and then presented those antigens on major histocompatibility complex (MHC) class I to CD8 + T cells. Processing of exogenous antigen occurred within endocytic organelles and did not require transit of antigen to the cytosol. Intracellular stores of MHC class I partially colocalized with transferrin receptor and internalized transferrin in endosomes, suggesting that such recycling endosomes are sites of peptide loading onto MHC class I or peptide transit. These data demonstrate that pDCs utilize ready-made stores of MHC class I to rapidly present exogenous antigen to CD8 + T cells.The high rate of viral replication represents a considerable challenge for the immune response. Human DCs can be classified into two major cell subsets, myeloid and plasmacytoid DCs, both of which are critical for the initiation of viral immune responses 1,2 . The presentation of viral peptides to CD4 + and CD8 + T cells is mediated by major histocompatibility complex (MHC) class II molecules and class I, respectively. DC maturation leads to a variety of changes including, activation induced antigen processing and increased surface expression of MHC class I and class II 3,4 . Exogenous antigen presentation on MHC class II molecules by myeloid DC (mDCs) is a rapid and coordinated process 5 . Pools of pre-synthesized MHC class II are stored in late endosomal and lysosomal compartments, which are loaded in an activationdependent manner and translocated to the cell surface 6-10 . NIH Public Access RESULTS pDCs drive CD8 + T cell proliferationHealthy donor pDCs and mDCs were purified from blood by negative depletion followed by direct cell sorting for mDCs (HLA-DR high CD11c high) and pDCs (HLA-DR high CD123 high). Their APC function was assessed by their ability to induce the proliferation of allogeneic (allo) CD4 + and CD8 + T cells. pDCs demonstrated no allo-stimulatory capacity for either CD4 + or CD8 + T cells, while mDCs showed low stimulatory capacity (data not shown). Upon activation with influenza virus, both pDCs and mDCs induced strong proliferation of allo-CD4 + and CD8 + T cells. Influenza virus-treated pDCs induced stronger allo-proliferation of CD8 + T cells when compared to their myeloid counterpart (Fig. 1a,b, upper panels). Reactivation of the pDC-expanded CD8 + T cells with anti-CD3 and anti-CD28 led to high levels of IFN-γ secretion, indicating the acquisition of effector function (data not shown). In contrast, influenza virus-treated mDCs induced a higher proliferation of allo-CD4 + T cells (Fig. 1a,b, bottom panels) indicating that the difference in CD8 + T cell proliferation wa...

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...