Identification of macaque dendritic cell precursors in blood and tissue reveals their dysregulation in early SIV infection

Distinct dendritic cell (DC) subsets play important roles in shaping immune responses. Circulating DC precursors (pre-DCs) are more susceptible to HIV infection in vitro, which may explain the inefficiency of immune responses against HIV. However, the interplay between HIV and pre-DC is not defined in vivo. We identify human pre-DC equivalents in the cynomolgus macaque and then analyze their dynamics during simian immunodeficiency virus (SIV) infection to illustrate a sharp decrease of blood pre-DCs in early SIV infection and accumulation in lymph nodes (LNs), where they neglect to upregulate CD83/CD86 or MHC-II. Additionally, SIV infection attenuates the capacity of stimulated LN pre-DCs to produce IL-12p40. Analysis of HIV cohorts provides correlation between costimulatory molecule expression on pre-DCs and T cell activation in spontaneous HIV controllers. These findings pinpoint certain dynamics and functional changes of pre-DCs during SIV infection, providing a deeper understanding of immune dysregulation mechanisms elicited in people living with HIV.

A candidate nanoparticle vaccine comprised of multiple epitopes of the African swine fever virus elicits a robust immune response

The African swine fever (ASF) pandemics pose a significant threat to the global swine industry, and the development of safe and effective vaccines is a daunting but necessary challenge. The level and persistence of immunity are very important for the effectiveness of the vaccine. Targeting antigens to antigen presenting cells (APCs) can greatly enhance immunogenicity. In this study, we developed a self-assembled nano-ASFV vaccine candidate (NanoFVax) targeting DCs, by covalently coupling the self-assembled 24-mer ferritin with the dominant B and T cell epitopes of the highly immunogenic ASFV antigen (p72, CD2v, pB602L and p30) and fused with the chemokine receptor XCL1 (a DC targeting molecule) through the SpyTag/SpyCatcher protein ligase system. Compared to monomeric protein, the nanoparticle vaccines can induce a more robust T-cell response, and the high-level antibody response against ASFV can last for more than 231 days. Therefore, the NanoFVax is a novel and promising vaccine candidate for ASFV.

XCR1+ DCs are critical for T cell-mediated immunotherapy of chronic viral infections

The contribution of cross-presenting XCR1+ dendritic cells (DCs) and SIRPα+ DCs in maintaining T cell function during exhaustion and immunotherapeutic interventions of chronic infections remains poorly characterized. Using the mouse model of chronic LCMV infection, we found that XCR1+ DCs are more resistant to infection and highly activated compared with SIRPα+ DCs. Exploiting XCR1+ DCs via Flt3L-mediated expansion or XCR1-targeted vaccination notably reinvigorates CD8+ T cells and improves virus control. Upon PD-L1 blockade, XCR1+ DCs are not required for the proliferative burst of progenitor exhausted CD8+ T (TPEX) cells but are indispensable to sustain the functionality of exhausted CD8+ T (TEX) cells. Combining anti-PD-L1 therapy with increased frequency of XCR1+ DCs improves functionality of TPEX and TEX subsets, while increase of SIRPα+ DCs dampened their proliferation. Together, this demonstrates that XCR1+ DCs are crucial for the success of checkpoint inhibitor-based therapies through differential activation of exhausted CD8+ T cell subsets.

Effective innate immune response in natural HIV-1 controllers. Can mimicking lead to novel preventive and cure strategies against HIV-1?

PURPOSE OF REVIEW

HIV-1 controller individuals represents a model that can be useful for the development of novel vaccines and therapies. Initial studies pointed to the involvement of improved adaptive immunity, however, new emerging evidence suggests the contribution of innate cells to effective antiviral responses in spontaneous controllers. Therefore, understanding the alterations on innate cell subsets might be crucial to develop new effective therapeutic strategies.

RECENT FINDINGS

Among different innate immune cells, dendritic cell (DC) and natural killer (NK) cell are essential for effective antiviral responses. DC from controllers display improved innate detection of HIV-1 transcripts, higher induction of interferons, higher antigen presenting capacities and increased metabolism and higher capacities to induce polyfunctional CD8+ T-cell responses. Such properties have been mimicked by Toll-like receptor ligands and applied to DC-based immunotherapies in humans and in animal models. NK cells from controllers display higher expression of activating receptors promoting increased antibody-dependent cellular cytotoxicity (ADCC) and natural cytotoxicity activities. Neutralizing antibodies in combination with interleukin-15 superagonist or interferon-α can increase ADCC and cytotoxicity in NK cells from HIV-1 progressors.

SUMMARY

Mimicking DC and NK cell innate profiles in controllers has become a promising strategy to step forward a novel efficient immunotherapy against the HIV-1 infection.

Targeting Xcr1 on Dendritic Cells Rapidly Induce Th1-Associated Immune Responses That Contribute to Protection Against Influenza Infection

Targeting antigen to conventional dendritic cells (cDCs) can improve antigen-specific immune responses and additionally be used to influence the polarization of the immune responses. However, the mechanisms by which this is achieved are less clear. To improve our understanding, we here evaluate molecular and cellular requirements for CD4⁺ T cell and antibody polarization after immunization with Xcl1-fusion vaccines that specifically target cDC1s. Xcl1-fusion vaccines induced an IgG2a/IgG2b-dominated antibody response and rapid polarization of Th1 cells both in vitro and in vivo. For comparison, we included fliC-fusion vaccines that almost exclusively induced IgG1, despite inducing a more mixed polarization of T cells. Th1 polarization and IgG2a induction with Xcl1-fusion vaccines required IL-12 secretion but were nevertheless maintained in BATF3^-/- mice which lack IL-12-secreting migratory DCs. Interestingly, induction of IgG2a-dominated responses was highly dependent on the early kinetics of Th1 induction and was important for optimal protection in an influenza infection model. Early Th1 induction was dominant, since a combined Xcl1- and fliC-fusion vaccine induced IgG2a/IgG2b polarized antibody responses similar to Xcl1-fusion vaccines alone. In summary, our results demonstrate that targeting antigen to Xcr1⁺ cDC1s is an efficient strategy for enhancing IgG2a antibody responses through rapid Th1 induction, which can be utilized for improved vaccine design.

Intranasal delivery of a cDC1 targeted influenza vaccine with poly(I:C) enhances T cell responses and protects against influenza infection

Targeting antigens to dendritic cells represent a promising method for enhancing immune responses against specific antigens. However, many studies have focused on systemic delivery (intravenous or intraperitoneally) of targeted antigen, approaches that are not easily transferable to humans. Here we evaluate the efficacy of an influenza vaccine targeting Xcr1⁺ cDC1 administered by intranasal immunization. Intranasal delivery of antigen fused to the chemokine Xcl1, the ligand of Xcr1, resulted in specific uptake by lung CD103⁺ cDC1. Interestingly, intranasal immunization with influenza A/PR/8/34 haemagglutinin (HA) fused to Xcl1, formulated with poly(I:C), resulted in enhanced induction of antigen-specific IFNγ⁺ CD4⁺ and IFNγ⁺ CD8⁺ T cell responses in lung compared non-targeted anti-NIP-HA (αNIP-HA). Induction of antibody responses was, however, similar in Xcl1-HA and αNIP-HA immunized mice, but significantly higher than in mice immunized with monomeric HA. Both Xcl1-HA and αNIP-HA vaccines induced full protection when mice were challenged with a lethal dose of influenza PR8 virus, reflecting the strong induction of HA-specific antibodies. Our results demonstrate that i.n. delivery of Xcl1-HA is a promising vaccine strategy for enhancing T cell responses in addition to inducing strong antibody responses.

Comprehensive Phenotyping of Human Dendritic Cells and Monocytes

Professional antigen-presenting cells (APCs), which include dendritic cells (DCs) and monocytes are essential for inducing and steering adaptive T-cell responses. Recent technological developments in single-cell analysis have significantly advanced our understanding of APC subset heterogeneity. To accurately resolve this functional diversity and to account for tissue-specific adaptation, novel phenotyping markers have been described more recently. While some of these largely overlap with traditionally used markers, more fine-grained phenotyping might be essential during inflammatory settings, where the traditional distinction between monocytes and dendritic cells has become blurred. Within this phenotype report, we provide a concise overview of traditional and recently described markers for the phenotyping of DCs and monocytes in the human system.

Genetic diversity of chemokine XCL1 and its receptor XCR1 in murine rodents

The chemokine ligand XCL1 plays critical roles in immune responses with diverse physiological and pathological implications through interactions with a cognate G protein-coupled receptor XCR1. To shed insight into their versatile nature, we analyzed genetic variations of XCL1 and XCR1 in murine rodents, including commonly-used model organisms Mus musculus (house mouse) and Rattus norvegicus (Norway rat). Our results showed that adaptive selection has contributed to the genetic diversification of these proteins in murine lineage. Moreover, in both M. musculus and R. norvegicus, the chemokine and its receptor exhibit similar signs of selective sweeps resulting from positive selection. In light of currently available structural and interaction information for chemokines and their receptors, the similarity of XCL1/XCR1 evolutionary patterns among murine species and the parallels of their evolutionary footprints within individual species suggest that interplay could exist between the adaptively selected changes, or between the domains on which the identified changes are located, and consequently preserve the physiological interaction of XCL1 and XCR1.

Endocytosis Deficient Murine Xcl1-Fusion Vaccine Enhances Protective Antibody Responses in Mice

Targeting antigen to surface receptors on dendritic cells (DCs) can improve antibody response against subunit vaccines. We have previously observed that human XCL1-fusion vaccines target murine Xcr1⁺ DCs without actively inducing endocytosis of the antigen, resulting in enhanced antibody responses in mice. However, the use of foreign chemokines for targeting is undesirable when translating this observation to human or veterinary medicine due to potential cross-reactive responses against the endogenous chemokine. Here we have identified a mutant version of murine Xcl1, labeled Xcl1(Δ1) owing to removal of a conserved valine in position 1 of the mature chemokine, that retains specific binding to Xcr1⁺ DCs without inducing endocytosis of the receptor. DNA immunization with Xcl1(Δ1) conjugated to influenza hemagglutinin (HA) induced improved antibody responses, with higher end point titers of IgG compared to WT Xcl1-HA. The Xcl1(Δ1) fusion vaccine also resulted in an increased number of HA reactive germinal center B cells with higher avidity toward the antigen, and serum transfer experiments show that Xcl1(Δ1)-HA induced antibody responses provided better protection against influenza infection as compared to WT Xcl1-HA. In summary, our observations indicate that targeting antigen to Xcr1⁺ DCs in an endocytosis deficient manner enhances antibody responses. This effect was obtained by introducing a single mutation to Xcl1, suggesting our strategy may easily be translated to human or veterinary vaccine settings.

Early SIV and HIV infection promotes the LILRB2/MHC-I inhibitory axis in cDCs

Classical dendritic cells (cDCs) play a pivotal role in the early events that tip the immune response toward persistence or viral control. In vitro studies indicate that HIV infection induces the dysregulation of cDCs through binding of the LILRB2 inhibitory receptor to its MHC-I ligands and the strength of this interaction was proposed to drive disease progression. However, the dynamics of the LILRB2/MHC-I inhibitory axis in cDCs during early immune responses against HIV are yet unknown. Here, we show that early HIV-1 infection induces a strong and simultaneous increase of LILRB2 and MHC-I expression on the surface of blood cDCs. We further characterized the early dynamics of LILRB2 and MHC-I expression by showing that SIVmac251 infection of macaques promotes coordinated up-regulation of LILRB2 and MHC-I on cDCs and monocytes/macrophages, from blood and lymph nodes. Orientation towards the LILRB2/MHC-I inhibitory axis starts from the first days of infection and is transiently induced in the entire cDC population in acute phase. Analysis of the factors involved indicates that HIV-1 replication, TLR7/8 triggering, and treatment by IL-10 or type I IFNs increase LILRB2 expression. Finally, enhancement of the LILRB2/MHC-I inhibitory axis is specific to HIV-1 and SIVmac251 infections, as expression of LILRB2 on cDCs decreased in naturally controlled chikungunya virus infection of macaques. Altogether, our data reveal a unique up-regulation of LILRB2 and its MHC-I ligands on cDCs in the early phase of SIV/HIV infection, which may account for immune dysregulation at a critical stage of the anti-viral response.

The levels of DNGR-1 and its ligand-bearing cells were altered after human and simian immunodeficiency virus infection

Dendritic cell NK lectin Group Receptor-1 (DNGR-1), also known as C-type lectin domain family 9, member A (CLEC9A), is a member of C-type lectin receptor superfamily expressed primarily on dendritic cells (DC) that excel in cross-presentation of exogenous antigens. To find out whether and how it is affected in human immunodeficiency virus infections or acquired immunodeficiency syndromes (HIV/AIDS), DNGR-1 expression and DNGR-1-binding cells in simian/human immunodeficiency virus (SHIV) and simian immunodeficiency virus (SIV)-infected rhesus macaques and antiretroviral therapy (ART)-treated AIDS patients were examined by real-time RT-PCR, flow cytometry, and confocal microscopy. DNGR-1 expression was observed in both lymphoid and non-lymphoid tissues including gut-associated lymphoid tissues (GALT) of rhesus macaques. DNGR-1 mRNA levels were significantly reduced in the blood while significantly elevated in the GALT of SHIV/SIV-infected rhesus macaques. DNGR-1 transcription levels were also significantly reduced in the blood of ART-treated AIDS patients irrespective of viral status. White blood cells with exposed DNGR-1 ligands were significantly increased in ART-treated AIDS patients, while significantly decreased in SIV-infected rhesus macaques. These data indicate that DNGR-1 expression, and by extension, the function of cross-presentation of antigens associated with dead/damaged cells might be compromised in HIV/SIV infection, which might play a role in HIV/AIDS pathogenesis and should be taken into consideration in therapeutic AIDS vaccine development.

Prime and Boost Vaccination Elicit a Distinct Innate Myeloid Cell Immune Response

Understanding the innate immune response to vaccination is critical in vaccine design. Here, we studied blood innate myeloid cells after first and second immunization of cynomolgus macaques with the modified vaccinia virus Ankara. The inflammation at the injection site was moderate and resolved faster after the boost. The blood concentration of inflammation markers increased after both injections but was lower after the boost. The numbers of neutrophils, monocytes, and dendritic cells were transiently affected by vaccination, but without any major difference between prime and boost. However, phenotyping deeper those cells with mass cytometry unveiled their high phenotypic diversity with subsets responding differently after each injection, some enriched only after the primary injection and others only after the boost. Actually, the composition in subphenotype already differed just before the boost as compared to just before the prime. Multivariate analysis identified the key features that contributed to these differences. Cell subpopulations best characterizing the post-boost response were more activated, with a stronger expression of markers involved in phagocytosis, antigen presentation, costimulation, chemotaxis, and inflammation. This study revisits innate immunity by demonstrating that, like adaptive immunity, innate myeloid responses differ after one or two immunizations.

Alterations in B Cell Compartment Correlate with Poor Neutralization Response and Disease Progression in HIV-1 Infected Children

Several B cell defects are reported in HIV-1 infected individuals including variation in B cell subsets, polyclonal B cell activation and exhaustion, with broadly neutralizing antibodies elicited in less than 10–20% of the infected population. HIV-1 disease progression is faster in children than adults. B Lymphocyte Stimulator (BLyS), expressed on dendritic cells (DCs), is a key regulator of B cell homeostasis. Understanding how DCs influence B cell phenotype and functionality (viral neutralization), thereby HIV-1 disease outcome in infected children, is important to develop interventional strategies for restoration of B cell function. In this study, a total of 38 vertically transmitted HIV-1 infected antiretroviral therapy (ART) naïve children and 25 seronegative controls were recruited. Based on the CD4 counts and years post-infection, infected children were categorized as long-term non-progressors (LTNPs) (n = 20) and progressors (n = 18). Eight of these progressors were followed up at 6–12 months post-ART. Percentages (%) of DCs, B cell subsets, and expression of BLyS on DCs were analyzed by flow-cytometry. Plasma levels of B cell growth factors were measured by ELISA and viral neutralization activity was determined using TZM-bl assay. Lower (%) of myeloid DCs (mDCs), plasmacytoid DCs, and high expression of BLyS on mDCs were observed in HIV-1 infected progressors than seronegative controls. Progressors showed lower % of naive B cells, resting memory B cells and higher % of mature activated, tissue-like memory B cells as compared to seronegative controls. Higher plasma levels of IL-4, IL-6, IL-10, and IgA were observed in progressors vs. seronegative controls. Plasma levels of IgG were high in progressors and in LTNPs than seronegative controls, suggesting persistence of hypergammaglobulinemia at all stages of disease. High plasma levels of BLyS in progressors positively correlated with poor viral neutralizing activity. Interestingly on follow up, treatment naïve progressors, post-ART showed increase in resting memory B cells along with reduction in plasma BLyS levels that correlated with improvement in viral neutralization. This is the first study to demonstrate that reduction in plasma BLyS levels correlates with restoration of B cell function, in terms of viral neutralization in HIV-1-infected children.

Molecular cloning and characterization of DNGR-1 in rhesus macaques

DC, NK lectin group receptor-1 (DNGR-1), also known as C-type lectin domain family 9 member A (CLEC9A), is a promising target for immunological therapeutics and vaccination against tumors and viruses. However, little is known about its property in rhesus macaques. In this study, we cloned rhesus macaque DNGR-1 cDNA, and found that its coding region could encode a 241-amino acid polypeptide with 91.7% sequence identity and similar antigenicity to that of humans. Both free and cell surface rhesus macaque DNGR-1 expressed in vitro could bind to apoptotic/dead cells induced by serum deprivation or freeze-thaw, and to pyroptotic cells stimulated with PMA and LPS. We also demonstrated that rhesus macaque DNGR-1 mRNA was present in all the examined tissues, with the highest in lymph nodes, spleen, blood, and thymus. The expression of DNGR-1 that is highly similar to that of humans warranted the usefulness of rhesus macaques in testing human therapeutics and vaccines targeting DNGR-1, especially those for HIV/AIDS.

From dendritic cells to B cells dysfunctions during HIV-1 infection: T follicular helper cells at the crossroads

T follicular helper (Tfh) cells are essential for B-cell differentiation and the subsequent antibody responses. Their numbers and functions are altered during human and simian immunodeficiency virus (HIV/SIV) infections. In lymphoid tissues, Tfh cells are present in germinal centre, where they are the main source of replicative HIV-1 and represent a major reservoir. Paradoxically, Tfh cell numbers are increased in chronically infected individuals. Understanding the fate of Tfh cells in the course of HIV-1 infection is essential for the design of efficient strategies toward a protective HIV vaccine or a cure. The purpose of this review is to summarize the recent advance in our understanding of Tfh cell dynamics during HIV/SIV infection. In particular, to explore the possible causes of their expansion in lymphoid tissues by discussing the impact of HIV-1 infection on dendritic cells, to identify the molecular players rendering Tfh cells highly susceptible to HIV-1 infection, and to consider the contribution of regulatory follicular T cells in shaping Tfh cell functions.

Progress in research on C-chemokine XCL1

XCL1, also known as lymphotactin, is the only known member of the C-type-chemokine family, which is produced mainly by CD8⁺ T cells and natural killer cells. XCL1 has a unique amino acid sequence feature and two interchangeable conformations, which makes XCL1 different from other chemokines in structure and function. The XCL1-specific receptor, XCR1, is a member of the G-protein-coupled receptor family and plays an important role in the negative selection of T cells in the thymus and in the initiation of cross-antigen presentation and mediation of cytotoxic immune responses. XCL1 can regulate the balance of the immune system and maintain intestinal immune homeostasis, and it is involved in a variety of diseases such as autoimmune diseases, nephritis, tuberculosis and human immunodeficiency virus infection. In recent years, the selective expression of XCR1 on CD8⁺ DCs with strong cross antigen-presention ability has been proved, which has led to studies using XCL1 for mucosal immunization, antitumor immunotherapy and targeted vaccine development.

The Identification and Distribution of Cattle XCR1 and XCL1 among Peripheral Blood Cells: New Insights into the Design of Dendritic Cells Targeted Veterinary Vaccine

The chemokine (C motif) receptor 1 (XCR1) and its ligandXCL1 have been intensively studied in the mouse and human immune systems. Here, we determined the molecular characteristics of cattle XCR1 and XCL1 and their distribution among peripheral blood cells. Cattle XCR1 mRNA expression was mainly restricted to CD26+CADM1+CD205+MHCII+CD11b- cells in blood that were otherwise lineage marker negative (lin-); these represented a subset of classic dendritic cells (DCs), not plasmacytoid DCs. Some of these DCs expressed CD11a, CD44, CD80 and CD86, but they did not express CD4, CD8, CD163 or CD172a. Cattle XCL1 was expressed in quiescent NK cells and in activated CD8+ T cells. Cattle XCR1+ DCs migrated chemotactically in response to mouse, but not to human, XCL1. The distribution characters of cattle XCR1 and XCL1 suggested a vital role in regulation of acquired immune responses and indicated a potential for a DC targeted veterinary vaccine in cattle using XCL1 fused antigens.

Chronic Type I IFN Is Sufficient To Promote Immunosuppression through Accumulation of Myeloid-Derived Suppressor Cells

Failure of the immune system to eradicate viruses results in chronic viral infections, which are associated with increased susceptibility to secondary infections. Pathogenic HIV or lymphocytic choriomeningitis virus chronic infections display a persistent type I IFN signature. In chronic lymphocytic choriomeningitis virus infection, blockade of type I IFN signaling partially restores antiviral responses. In a mouse model, we tested whether chronic administration of type I IFN, at doses mimicking chronic viral infection, induced immunosuppression. Chronic exposure of mice to IFN-α alone was sufficient to strongly suppress specific CD8⁺ T cells responses to subsequent vaccinia virus infection. It resulted in the accumulation of Ly6C^hi monocytes. These monocytes were similar, phenotypically and functionally, to the myeloid-derived suppressor cells found in cancer because they exerted a potent suppression on CD8⁺ T cell responses in vitro. They acted at least partly through the l-arginine pathway. In vivo, their elimination restored antiviral CD8⁺ T cell responses. Our work provides a specific mechanism accounting for the role of IFN-α in immunosuppression and predicts that type I IFN modulation will be pivotal to cure human chronic infections, cancer, or autoimmune diseases.

Expanding the tools for identifying mononuclear phagocyte subsets in swine: Reagents to porcine CD11c and XCR1

Pig is a domestic species of major importance in the agro-economy and in biomedical research. Mononuclear phagocytes (MNP) are organized in subsets with specialized roles in the orchestration of the immune response and new tools are awaited to improve MNP subset identification in the pig. We cloned pig CD11c cDNA and generated a monoclonal antibody to pig CD11c which showed a pattern of expression by blood and skin MNP subsets similar to humans. We also developed a porcine XCL1-mCherry dimer which specifically reacted with the XCR1-expressing dendritic cell subset of the type 1 lineage in blood and skin. These original reagents will allow the efficient identification of pig MNP subsets to study their role in physiological and pathological processes and also to target these cells in novel intervention and vaccine strategies for veterinary applications and preclinical evaluations.

Unsupervised High-Dimensional Analysis Aligns Dendritic Cells across Tissues and Species

Dendritic cells (DCs) are professional antigen-presenting cells that hold great therapeutic potential. Multiple DC subsets have been described, and it remains challenging to align them across tissues and species to analyze their function in the absence of macrophage contamination. Here, we provide and validate a universal toolbox for the automated identification of DCs through unsupervised analysis of conventional flow cytometry and mass cytometry data obtained from multiple mouse, macaque, and human tissues. The use of a minimal set of lineage-imprinted markers was sufficient to subdivide DCs into conventional type 1 (cDC1s), conventional type 2 (cDC2s), and plasmacytoid DCs (pDCs) across tissues and species. This way, a large number of additional markers can still be used to further characterize the heterogeneity of DCs across tissues and during inflammation. This framework represents the way forward to a universal, high-throughput, and standardized analysis of DC populations from mutant mice and human patients.

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A conserved gating strategy aligns dendritic cells (DCs) in mouse and human tissues

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Unsupervised computational analysis of flow cytometry data outperforms manual analysis

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Mass cytometry reveals heterogeneity of DC subsets across mouse and human tissues

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DC activation upon inflammation tracked by automated analysis of mass cytometry

Comparative genomics analysis of mononuclear phagocyte subsets confirms homology between lymphoid tissue-resident and dermal XCR1(+) DCs in mouse and human and distinguishes them from Langerhans cells

Dendritic cells (DC) are mononuclear phagocytes which exhibit a branching (dendritic) morphology and excel at naïve T cell activation. DC encompass several subsets initially identified by their expression of cell surface molecules and later shown to possess distinct functions. DC subset differentiation is orchestrated by transcription factors, growth factors and cytokines. Identifying DC subsets is challenging as very few cell surface molecules are uniquely expressed on any one of these cell populations. There is no standard consensus to identify mononuclear phagocyte subsets; varying antigens are employed depending on the tissue and animal species studied and between laboratories. This has led to confusion in how to accurately define and classify DCs across tissues and between species. Here we report a comparative genomics strategy that enables universal definition of DC and other mononuclear phagocyte subsets across species. We performed a meta-analysis of several public datasets of human and mouse mononuclear phagocyte subsets isolated from blood, spleen, skin or cutaneous lymph nodes, including by using a novel and user friendly software, BubbleGUM, which generates and integrates gene signatures for high throughput gene set enrichment analysis. This analysis demonstrates the equivalence between human and mouse skin XCR1(+) DCs, and between mouse and human Langerhans cells.

Plasmacytoid dendritic cells and myeloid cells differently contribute to B-cell-activating factor belonging to the tumor necrosis factor superfamily overexpression during primary HIV infection

BACKGROUND

After describing heightened levels of circulating B-cell-activating factor belonging to the tumor necrosis factor superfamily (BAFF) as well as changes in B-cell phenotype and functions during acute infection by simian immunodeficiency virus, we wanted to determine whether and by which cells BAFF was over-expressed in primary HIV-infected (PHI) patients.

DESIGN AND METHODS

We simultaneously examined circulating BAFF levels by ELISA and membrane-bound BAFF (mBAFF) expression by flow cytometry in peripheral blood mononuclear cells of healthy donors and PHI patients followed for 6 months. We also examined whether HIV-1 modifies BAFF expression or release in various myeloid cells and plasmacytoid dendritic cells (pDC) in vitro.

RESULTS

Circulating BAFF levels were transiently increased at enrolment. They positively correlated with CXCL10 levels and inversely with B-cell counts. Whereas mBAFF was expressed by most pDC and on a fraction of intermediate monocytes in healthy donors, the frequency of mBAFF cells significantly increased among nonclassical monocytes and CD1c dendritic cells but decreased among pDC in PHI patients. In contrast to myeloid cells, pDC never released BAFF upon stimulation. Their mBAFF expression was enhanced by HIV-1, independently of type I IFN.

CONCLUSION

Our findings reveal that the pattern of BAFF expression by myeloid cells and pDC is altered in PHI patients and constitutes a valuable marker of immune activation whose circulating levels correlate with CXCL10 levels. Due to their homing in different tissue areas, pDC and myeloid cells might target different B-cell subsets through their mBAFF expression or soluble BAFF release.

Expression analysis of surface molecules on human thymic dendritic cells with the 10th HLDA Workshop antibody panel

Dendritic cells (DC) in the thymus have an important role in the establishment of central tolerance by promoting negative selection of autoreactive T cells and regulatory T-cell differentiation. Whereas human DC have recently been studied in various tissues in more detail, thymic DC subsets are still ill-defined. In the present work, we studied the binding of 71 monoclonal antibodies (mAb) submitted to the HLDA10 workshop to human CD123⁺ plasmacytoid DC and the two subsets of conventional DC (cDC, CD141⁺ and CD11b⁺) isolated from thymus tissue of infants undergoing corrective heart surgery. Within the panel, we found mAb binding to thymic pDC and both cDC subsets (for example, anti-Clec12A, TIM-3, Clec4A, CCR5, Axl, FLT3), but most of them additionally reacted with other thymic cell types. MAb directed to CD85h (ILT1) and the C-type lectin Clec7A (now CD369) reacted selectively with both cDC subsets, but not with other cells. Only one mAb directed to CD85g (ILT7) stained thymic pDC in a highly specific manner. Clec9A (DNGR1, now CD370) was the only tested HLDA10 antigen exclusively expressed on thymic CD141⁺ cDC. The present report summarizes all data obtained.

Selected Aspects in the Pathogenesis of Autoimmune Diseases

Autoimmune processes can be found in physiological circumstances. However, they are quenched with properly functioning regulatory mechanisms and do not evolve into full-blown autoimmune diseases. Once developed, autoimmune diseases are characterized by signature clinical features, accompanied by sustained cellular and/or humoral immunological abnormalities. Genetic, environmental, and hormonal defects, as well as a quantitative and qualitative impairment of immunoregulatory functions, have been shown in parallel to the relative dominance of proinflammatory Th17 cells in many of these diseases. In this review we focus on the derailed balance between regulatory and Th17 cells in the pathogenesis of autoimmune diseases. Additionally, we depict a cytokine imbalance, which gives rise to a biased T-cell homeostasis. The assessment of Th17/Treg-cell ratio and the simultaneous quantitation of cytokines, may give a useful diagnostic tool in autoimmune diseases. We also depict the multifaceted role of dendritic cells, serving as antigen presenting cells, contributing to the development of the pathognomonic cytokine signature and promote cellular and humoral autoimmune responses. Finally we describe the function and role of extracellular vesicles in particular autoimmune diseases. Targeting these key players of disease progression in patients with autoimmune diseases by immunomodulating therapy may be beneficial in future therapeutic strategies.

A discrete population of IFN λ-expressing BDCA3hi dendritic cells is present in human thymus

Human thymus contains two major subpopulations of dendritic cells (DCs), conventional DCs (cDCs) and plasmacytoid DCs (pDCs), which are mainly involved in central tolerance and also in protecting the thymus against infections. In blood and peripheral organs cDCs include the subpopulation of BDCA3(hi) DCs, considered as equivalents to mouse CD8α(+) DCs. In this study we describe in human thymus the presence of a discrete population of BDCA3(hi) DCs that, like their peripheral counterparts, express CD13, low-intermediate levels of CD11c, CLEC9A, high levels of XCR1, IRF8 and TLR3, and mostly lack the expression of CD11b, CD14 and TLR7. Thymic BDCA3(hi) DCs display immature features with a low expression of costimulatory molecules and HLA-DR, and a low allostimulatory capacity. Also, BDCA3(hi) DCs exhibit a strong response to TLR3 stimulation, producing high levels of interferon (IFN)-λ1 and CXCL10, which indicates that, similarly to thymic pDCs, BDCA3(hi) DCs can have an important role in thymus protection against viral infections.

Identification of skin immune cells in non-human primates

The skin is a valuable target for vaccine delivery because it contains many immune cell populations, notably antigen presenting cells. Skin immune cells have been extensively described in mice and humans but not in non-human primates, which are pertinent models for immunological research in vaccination. The aim of this work was to describe immune cell populations in the epidermis, dermis and skin draining lymph nodes in cynomolgus macaques by a single 12-parameter flow cytometry protocol. Given that skin cells share several markers, we defined a gating strategy to identify accurately immune cells and to limit contamination of one immune cell population by another. The epidermis contained CD1a(+)CD1c(-) Langerhans cells (LCs), CD3(+) T cells and putative NK cells. The dermis contained CD1a(+)CD1c(-) cells, which were similar to LCs, CD1a(+)CD1c(+) dermal dendritic cells (DDCs), CD163(high)CD11b(+) resident macrophages, CD3(+) T cells and putative NK cells. The skin also contained CD66(+) polymorphonuclear cells in some animals. Thus, immune cell populations in the macaque are similar to those in humans despite some differences in phenotype. In skin draining lymph nodes, we identified migratory LCs, CD1a(+)CD1c(+) DDCs and macrophages. The simultaneous identification of these different immune cells with one panel of markers avoids the use of large amounts of precious sample and may improve the understanding of immune mechanisms in the skin after treatment or vaccination.

Defining Mononuclear Phagocyte Subset Homology Across Several Distant Warm-Blooded Vertebrates Through Comparative Transcriptomics

Mononuclear phagocytes are organized in a complex system of ontogenetically and functionally distinct subsets, that has been best described in mouse and to some extent in human. Identification of homologous mononuclear phagocyte subsets in other vertebrate species of biomedical, economic, and environmental interest is needed to improve our knowledge in physiologic and physio-pathologic processes, and to design intervention strategies against a variety of diseases, including zoonotic infections. We developed a streamlined approach combining refined cell sorting and integrated comparative transcriptomics analyses which revealed conservation of the mononuclear phagocyte organization across human, mouse, sheep, pigs and, in some respect, chicken. This strategy should help democratizing the use of omics analyses for the identification and study of cell types across tissues and species. Moreover, we identified conserved gene signatures that enable robust identification and universal definition of these cell types. We identified new evolutionarily conserved gene candidates and gene interaction networks for the molecular regulation of the development or functions of these cell types, as well as conserved surface candidates for refined subset phenotyping throughout species. A phylogenetic analysis revealed that orthologous genes of the conserved signatures exist in teleost fishes and apparently not in Lamprey.

Investigating Evolutionary Conservation of Dendritic Cell Subset Identity and Functions

Dendritic cells (DCs) were initially defined as mononuclear phagocytes with a dendritic morphology and an exquisite efficiency for naïve T-cell activation. DC encompass several subsets initially identified by their expression of specific cell surface molecules and later shown to excel in distinct functions and to develop under the instruction of different transcription factors or cytokines. Very few cell surface molecules are expressed in a specific manner on any immune cell type. Hence, to identify cell types, the sole use of a small number of cell surface markers in classical flow cytometry can be deceiving. Moreover, the markers currently used to define mononuclear phagocyte subsets vary depending on the tissue and animal species studied and even between laboratories. This has led to confusion in the definition of DC subset identity and in their attribution of specific functions. There is a strong need to identify a rigorous and consensus way to define mononuclear phagocyte subsets, with precise guidelines potentially applicable throughout tissues and species. We will discuss the advantages, drawbacks, and complementarities of different methodologies: cell surface phenotyping, ontogeny, functional characterization, and molecular profiling. We will advocate that gene expression profiling is a very rigorous, largely unbiased and accessible method to define the identity of mononuclear phagocyte subsets, which strengthens and refines surface phenotyping. It is uniquely powerful to yield new, experimentally testable, hypotheses on the ontogeny or functions of mononuclear phagocyte subsets, their molecular regulation, and their evolutionary conservation. We propose defining cell populations based on a combination of cell surface phenotyping, expression analysis of hallmark genes, and robust functional assays, in order to reach a consensus and integrate faster the huge but scattered knowledge accumulated by different laboratories on different cell types, organs, and species.

Altered antigen-presenting cells during HIV-1 infection

PURPOSE OF REVIEW

The purpose of this study is to describe the alterations that HIV-1 induces in antigen-presenting cells (APCs), in vitro, ex vivo and in vivo.

RECENT FINDINGS

HIV-1 disarms several arms of the immune system including APCs. We summarize here recent findings on the impact of the virus on APC.

SUMMARY

HIV-1 can invade APC and overall reduce their capacity to present antigens effectively, mostly by reducing their numbers and inducing permanent hyperactivation. This occurs via a combination of alterations; however, the host can counteract, at least in part, some of these defects via restriction factors, autophagy, the production of type I interferon, antiviral cytokines, among others. However, these specific mechanisms of viral evasion from APCs' control lead to a chronic hyperactivation of the immune system implicated in AIDS-related and non-AIDS related pathogenesis. Unfortunately, the current regimens of antiretroviral therapy are unable to dampen sufficiently APC-driven viral-induced immune hyperactivation. Understanding how HIV alters APC will help to tune appropriately both intrinsic immunity and innate immunity, as well as achieve efficient antigen presentation to the adaptive immune system, without inducing a detrimental pervasive hyperactivation of the immune system.

Dendritic cells in humans--from fetus to adult

The human immune system evolves continuously during development from the embryo into the adult, reflecting the ever-changing environment and demands of our body. This ability of our immune system to sense external cues and adapt as we develop is just as important in the early tolerogenic environment of the fetus, as it is in the constantly pathogen-challenged adult. Dendritic cells (DCs), the professional antigen-sensing and antigen-presenting components of the immune system, play a crucial role in this process where they act as sentinels, both initiating and regulating immune responses. Here, we provide an overview of the human immune system in the developing fetus and the adult, with a focus on DC ontogeny and function during these discrete but intimately linked life stages.

Dendritic cells, monocytes and macrophages: a unified nomenclature based on ontogeny

The mononuclear phagocyte system (MPS) has historically been categorized into monocytes, dendritic cells and macrophages on the basis of functional and phenotypical characteristics. However, considering that these characteristics are often overlapping, the distinction between and classification of these cell types has been challenging. In this Opinion article, we propose a unified nomenclature for the MPS. We suggest that these cells can be classified primarily by their ontogeny and secondarily by their location, function and phenotype. We believe that this system permits a more robust classification during both steady-state and inflammatory conditions, with the benefit of spanning different tissues and across species.

Human XCR1+ dendritic cells derived in vitro from CD34+ progenitors closely resemble blood dendritic cells, including their adjuvant responsiveness, contrary to monocyte-derived dendritic cells

Human monocyte-derived dendritic cell (MoDC) have been used in the clinic with moderately encouraging results. Mouse XCR1(+) DC excel at cross-presentation, can be targeted in vivo to induce protective immunity, and share characteristics with XCR1(+) human DC. Assessment of the immunoactivation potential of XCR1(+) human DC is hindered by their paucity in vivo and by their lack of a well-defined in vitro counterpart. We report in this study a protocol generating both XCR1(+) and XCR1(-) human DC in CD34(+) progenitor cultures (CD34-DC). Gene expression profiling, phenotypic characterization, and functional studies demonstrated that XCR1(-) CD34-DC are similar to canonical MoDC, whereas XCR1(+) CD34-DC resemble XCR1(+) blood DC (bDC). XCR1(+) DC were strongly activated by polyinosinic-polycytidylic acid but not LPS, and conversely for MoDC. XCR1(+) DC and MoDC expressed strikingly different patterns of molecules involved in inflammation and in cross-talk with NK or T cells. XCR1(+) CD34-DC but not MoDC efficiently cross-presented a cell-associated Ag upon stimulation by polyinosinic-polycytidylic acid or R848, likewise to what was reported for XCR1(+) bDC. Hence, it is feasible to generate high numbers of bona fide XCR1(+) human DC in vitro as a model to decipher the functions of XCR1(+) bDC and as a potential source of XCR1(+) DC for clinical use.