Mar Naranjo-Gómez

Capture and transfer of HIV-1 particles by mature dendritic cells converges with the exosome-dissemination pathway

Exosomes are secreted cellular vesicles that can be internalized by dendritic cells (DCs), contributing to antigen-specific naive CD4(+) T-cell activation. Here, we demonstrate that human immunodeficiency virus type 1 (HIV-1) can exploit this exosome antigen-dissemination pathway intrinsic to mature DCs (mDCs) for mediating trans-infection of T lymphocytes. Capture of HIV-1, HIV-1 Gag-enhanced green fluorescent protein (eGFP) viral-like particles (VLPs), and exosomes by DCs was up-regulated upon maturation, resulting in localization within a CD81(+) compartment. Uptake of VLPs or exosomes could be inhibited by a challenge with either particle, suggesting that the expression of common determinant(s) on VLP or exosome surface is necessary for internalization by mDCs. Capture by mDCs was insensitive to proteolysis but blocked when virus, VLPs, or exosomes were produced from cells treated with sphingolipid biosynthesis inhibitors that modulate the lipid composition of the budding particles. Finally, VLPs and exosomes captured by mDCs were transmitted to T lymphocytes in an envelope glycoprotein-independent manner, underscoring a new potential viral dissemination pathway.

HIV and mature dendritic cells: Trojan exosomes riding the Trojan horse?

Exosomes are secreted cellular vesicles that can induce specific CD4+ T cell responses in vivo when they interact with competent antigen-presenting cells like mature dendritic cells (mDCs). The Trojan exosome hypothesis proposes that retroviruses can take advantage of the cell-encoded intercellular vesicle traffic and exosome exchange pathway, moving between cells in the absence of fusion events in search of adequate target cells. Here, we discuss recent data supporting this hypothesis, which further explains how DCs can capture and internalize retroviruses like HIV-1 in the absence of fusion events, leading to the productive infection of interacting CD4+ T cells and contributing to viral spread through a mechanism known as trans-infection. We suggest that HIV-1 can exploit an exosome antigen-dissemination pathway intrinsic to mDCs, allowing viral internalization and final trans-infection of CD4+ T cells. In contrast to previous reports that focus on the ability of immature DCs to capture HIV in the mucosa, this review emphasizes the outstanding role that mature DCs could have promoting trans-infection in the lymph node, underscoring a new potential viral dissemination pathway.

Maturation of Blood-Derived Dendritic Cells Enhances Human Immunodeficiency Virus Type 1 Capture and Transmission

ABSTRACT Dendritic cells (DCs) are specialized antigen-presenting cells. However, DCs exposed to human immunodeficiency virus type 1 (HIV-1) are also able to transmit a vigorous cytopathic infection to CD4+ T cells, a process that has been frequently related to the ability of DC-SIGN to bind HIV-1 envelope glycoproteins. The maturation of DCs can increase the efficiency of HIV-1 transmission through trans infection. We aimed to comparatively study the effect of maturation in monocyte-derived DCs (MDDCs) and blood-derived myeloid DCs during the HIV-1 capture process. In vitro capture and transmission of envelope-pseudotyped HIV-1 and its homologous replication-competent virus to susceptible target cells were assessed by p24gag detection, luciferase activity, and both confocal and electron microscopy. Maturation of MDDCs or myeloid DCs enhanced the active capture of HIV-1 in a DC-SIGN- and viral envelope glycoprotein-independent manner, increasing the life span of trapped virus. Moreover, higher viral transmission of mature DCs to CD4+ T cells was highly dependent on active viral capture, a process mediated through cholesterol-enriched domains. Mature DCs concentrated captured virus in a single large vesicle staining for CD81 and CD63 tetraspanins, while immature DCs lacked these structures, suggesting different intracellular trafficking processes. These observations help to explain the greater ability of mature DCs to transfer HIV-1 to T lymphocytes, a process that can potentially contribute to the viral dissemination at lymph nodes in vivo, where viral replication takes place and there is a continuous interaction between susceptible T cells and mature DCs.

Stable antigen-specific T-cell hyporesponsiveness induced by tolerogenic dendritic cells from multiple sclerosis patients.

Multiple sclerosis (MS) is a chronic demyelinating autoimmune disease of the central nervous system. Current therapies decrease the frequency of relapses and limit, to some extent, but do not prevent disease progression. Hence, new therapeutic approaches that modify the natural course of MSneed to be identified. Tolerance induction to self‐antigens using monocyte‐derived dendritic cells (MDDCs) is a promising therapeutic strategy in autoimmunity. In this work, we sought to generate and characterize tolerogenic MDDCs (tolDCs) from relapsing‐remitting (RR) MSpatients, loaded with myelin peptides as specific antigen, with the aim of developing immunotherapeutics for MS. MDDCs were generated from both healthy‐blood donors and RR‐MSpatients, and MDDCmaturation was induced with a proinflammatory cytokine cocktail in the absence or presence of 1α,25‐dihydroxyvitamin‐D3, a tolerogenicity‐inducing agent. tolDCs were generated from monocytes of RR‐MSpatients as efficiently as from monocytes of healthy subjects. The RR‐MStolDCs expressed a stable semimature phenotype and an antiinflammatory profile as compared with untreated MDDCs. Importantly, myelin peptide‐loaded tolDCs induced stable antigen‐specific hyporesponsiveness in myelin‐reactive T cells from RR‐MS patients. These results suggest that myelin peptide‐loaded tolDCs may be a powerful tool for inducing myelin‐specific tolerance in RR‐MS patients.

Reduced numbers of plasmacytoid dendritic cells in aged blood donors

Dendritic cells (DC) play essential functions in both innate and adaptive immune responses. Peripheral blood DCs are divided into two major subsets, named conventional DC (cDC) and plasmacytoid DC (pDC), which play specific functions in the immune response. The absolute numbers of DCs (and their subsets) in peripheral blood may suffer variations due to physiological or pathological reasons, and therefore the enumeration of DC subsets in blood samples may be of clinical interest. However, to date this enumeration has produced controversial rather than consistent results. Here, using a two-tube platform approach, peripheral blood DCs have been enumerated in samples from healthy blood donors aged 18-65 years old. The results obtained showed a significant age-related decrease in pDC numbers, whilst cDC numbers remained unaltered. The different protocols used to define and enumerate DC subsets from blood samples may account for the controversial results reported before, thus emphasizing the importance of a general consensus to enumerate DCs. Reduced pDC numbers may be related to the higher susceptibility to infection and deficient response to vaccination often observed in aged individuals.

Capture of cell-derived microvesicles (exosomes and apoptotic bodies) by human plasmacytoid dendritic cells

cDCs and pDCs differ in multiple aspects. Among those, antigen capture is a recognized feature of cDCs, whereas pDCs display poor capacity to capture cell‐derived antigens. However, animal models of organ transplantation suggested a role for pDCs in tolerance induction via phagocytosis of donor antigens. In a transplantation setting, microvesicles, such as apoptotic bodies and exosomes secreted by the graft, may be potential sources of alloantigen. Here, we tested the capacity of human pDCs to capture exosomes and apoptotic bodies from Jurkat T cells. Exosomes and apoptotic bodies were indeed captured by pDCs, although required longer times of incubation when compared with the highly endocytic cDCs. In cDCs and pDCs, exosome capture was more efficient than apoptotic bodies. Endocytosis inhibitors clearly impaired exosome capture by cDCs, although this could not be verified in pDCs as a result of cellular toxicity. Functionally, capture of Jurkat‐derived exosomes did not induce nor prevent pDC maturation, and exosome‐loaded pDCs induced T cell proliferation, suggesting a link between capture and presentation. Thus, exosomes and apoptotic bodies may be sources of antigen for human pDCs.

Expression and function of the IL‐2 receptor in activated human plasmacytoid dendritic cells

Human and mouse plasmacytoid dendritic cells (PDC) express IL‐2 mRNA specifically upon TLR stimulation, but not under CD40L stimulation. Even though the expression of the IL‐2R by PDC has been described, the functional implications of this expression remain unknown. Here, we investigated the expression and function of the IL‐2R in activated human PDC. The IL‐2Rα chain, CD25, is expressed in both CpG‐ and CD40L‐activated PDC. CD25 expression is a relatively rapid event, as the receptor was detected 6 h after the initial activation signal. Exogenous IL‐2 added to CD40L‐activated PDC increased the expression of CD25, enhanced the secretion of pro‐inflammatory cytokines and promotes PDC survival. CpG‐activated PDC cultured in the presence of IL‐2R‐blocking monoclonal antibodies showed a reduced expression of pro‐inflammatory cytokines, especially TNF‐α. This reduction was dose and time dependent, suggesting a regulatory role of IL‐2 in TNF secretion that might occur at the post‐transcriptional level. These results indicate that the expression of the IL‐2R is relevant to human PDC activation, and that IL‐2 may be an important auto‐ and/or paracrine factor modulating the activation and survival of PDC. Finally, CD25 expression may be considered as a useful early activation marker for human PDC.

Ligation of Notch Receptors in Human Conventional and Plasmacytoid Dendritic Cells Differentially Regulates Cytokine and Chemokine Secretion and Modulates Th Cell Polarization

Notch signaling is involved in multiple cellular processes. Recent data also support the prominent role of Notch signaling in the regulation of the immune response. In this study, we analyzed the expression and function of Notch receptors and ligands on both human blood conventional dendritic cells (cDCs) and plasmacytoid DCs (pDCs). The expression and modulation upon TLR activation of Notch molecules partially differed between cDCs and pDCs, but functional involvement of the Notch pathway in both cell types was clearly revealed by specific inhibition using DAPT. Beyond the induction of Notch target genes and modulation of maturation markers, Notch pathway was also involved in a differential secretion of some specific cytokines/chemokines by DC subsets. Whereas Notch ligation induced IL-10 and CCL19 secretion in cDCs, Notch inhibition resulted in a diminished production of these proteins. With regard to pDCs, Notch activation induced TNF-α whereas Notch inhibition significantly abrogated the secretion of CCL19, CXCL9, CXCL10, and TNF-α. Additionally, Notch modulation of DC subsets differentially affected Th polarization of allostimulated T cells. Our results suggest that the Notch pathway may function as an additional mechanism controlling human DC responses, with differential activity on cDCs and pDCs. This control mechanism may ultimately contribute to define the local milieu promoted by these cells under the particular conditions of the immune response.

The alpha(7)beta(0) Isoform of the Complement Regulator C4b-Binding Protein Induces a Semimature, Anti-Inflammatory State in Dendritic Cells

The classical pathway complement regulator C4b-binding protein (C4BP) is composed of two polypeptides (α- and β-chains), which form three plasma oligomers with different subunit compositions (α7β1, α7β0, and α6β1). We show in this article that the C4BP α7β0 isoform (hereafter called C4BP[β−] [C4BP lacking the β-chain]), overexpressed under acute-phase conditions, induces a semimature, tolerogenic state on human monocyte-derived dendritic cells (DCs) activated by a proinflammatory stimulus. C4BP isoforms containing β-chain (α7β1 and α6β1; C4BP[β+]) neither interfered with the normal maturation of DCs nor competed with C4BP(β−) activity on these cells. Immature DCs (iDCs) treated with C4BP(β−) retained high endocytic activity, but, upon LPS treatment, they did not upregulate surface expression of CD83, CD80, and CD86. Transcriptional profiling of these semimature DCs revealed that treatment with C4BP(β−) prevented the induction of IDO and BIC-1, whereas TGF-β1 expression was maintained to the level of iDCs. C4BP(β−)–treated DCs were also unable to release proinflammatory Th1 cytokines (IL-12, TNF-α, IFN-γ, IL-6, IL-8) and, conversely, increased IL-10 secretion. They prevented surface CCR7 overexpression and, accordingly, displayed reduced chemotaxis, being morphologically indistinguishable from iDCs. Moreover, C4BP(β−)-treated DCs failed to enhance allogeneic T cell proliferation, impairing IFN-γ production in these cells and, conversely, promoting CD4+CD127low/negCD25highFoxp3+ T cells. Deletion mutant analysis revealed that the complement control protein-6 domain of the α-chain is necessary for the tolerogenic activity of C4BP(β−). Our data demonstrate a novel anti-inflammatory and immunomodulatory function of the complement regulator C4BP, suggesting a relevant role of the acute-phase C4BP(β−) isoform in a number of pathophysiological conditions and potential applications in autoimmunity and transplantation.

Differential effects of monophosphoryl lipid A and cytokine cocktail as maturation stimuli of immunogenic and tolerogenic dendritic cells for immunotherapy

Immunotherapy using monocyte-derived dendritic cells (MDDC) is increasingly being considered as alternative therapeutic approach in cancer, infectious diseases and also in autoimmunity when patients are not responsive to conventional treatments. In general, generation of MDDC from monocytes is induced in the presence of GM-CSF and IL-4, and a maturation stimulus is added to the culture to obtain mature DCs suitable for therapy. For DC maturation, different combinations of pro-inflammatory mediators and Toll-like receptor ligands have been tested, obtaining DCs that differ in their properties and the type of immune response they promote. Therefore, it is necessary to find an optimal cytokine environment for DC maturation to obtain a cellular product suitable for DC-based immunotherapeutic protocols. In this study, we have evaluated in vitro the effects of different maturation stimuli on the viability, phenotype, cytokine profile, stability and functionality of immunogenic and tolerogenic (1α,25-dihydroxyvitamin D(3)-treated) MDDC. Maturation was induced using the clinical grade TLR4-agonist: monophosphoryl lipid A (LA), compared to the traditional cytokine cocktail (CC; clinical grade TNF-α, IL-1β, PGE2) and a combination of both. Our results showed the combination of CC+LA rendered a potent immunogenic DC population that induced the production of IFN-γ and IL-17 in allogeneic co-cultures, suggesting a Th17 polarization. Moreover, these immunogenic DCs showed a high surface expression of CD83, CD86, HLA-DR and secretion of IL-12p70. When aiming to induce tolerance, using LA to generate mature TolDC did not represent a clear advantage, and the stability and the suppressive capability exhibited by CC-matured TolDC may represent the best option. Altogether, these findings demonstrate the relevance of an appropriate maturation stimulus to rationally modulate the therapeutic potential of DCs in immunotherapy.