Langerin+ CD8alpha+ dendritic cells are critical for cross-priming and IL-12 production in response to systemic antigens

Preclinical evaluation of therapeutic vaccines for chronic hepatitis B that stimulate antiviral activities of T cells and NKT cells

Background & Aims

Liver diseases resulting from chronic HBV infection are a significant cause of morbidity and mortality. Vaccines that elicit T-cell responses capable of controlling the virus represent a treatment strategy with potential for long-term effects. Here, we evaluated vaccines that induce the activity of type I natural killer T (NKT) cells to limit viral replication and license stimulation of conventional antiviral T-cells.

Methods

Vaccines were prepared by conjugating peptide epitopes to an NKT-cell agonist to promote co-delivery to antigen-presenting cells, encouraging NKT-cell licensing and stimulation of T cells. Activity of the conjugate vaccines was assessed in transgenic mice expressing the complete HBV genome, administered intravenously to maximise access to NKT cell-rich tissues.

Results

The vaccines induced only limited antiviral activity in unmanipulated transgenic hosts, likely attributable to NKT-cell activation as T-cell tolerance to viral antigens is strong. However, in a model of chronic hepatitis B involving transfer of naive HBcAg-specific CD8+ T cells into the transgenic mice, which typically results in specific T-cell dysfunction without virus control, vaccines containing the targeted HBcAg epitope induced prolonged antiviral activity because of qualitatively improved T-cell stimulation. In a step towards a clinical product, vaccines were prepared using synthetic long peptides covering clusters of known HLA-binding epitopes and shown to be immunogenic in HLA transgenic mice. Predictions based on HLA distribution suggest a product containing three selected SLP-based vaccines could give >90 % worldwide coverage, with an average of 3.38 epitopes targeted per individual.

Conclusions

The novel vaccines described show promise for further clinical development as a treatment for chronic hepatitis B.

Impact and Implications

Although there are effective prophylactic vaccines for HBV infection, it is estimated that 350-400 million people worldwide have chronic hepatitis B, putting these individuals at significant risk of life-threatening liver diseases. Therapeutic vaccination aimed at activating or boosting HBV-specific T-cell responses holds potential as a strategy for treating chronic infection, but has so far met with limited success. Here, we show that a glycolipid-peptide conjugate vaccine designed to coordinate activity of type I NKT cells alongside conventional antiviral T cells has antiviral activity in a mouse model of chronic infection. It is anticipated that a product based on a combination of three such conjugates, each prepared using long peptides covering clusters of known HLA-binding epitopes, could be developed further as a treatment for chronic hepatitis B with broad global HLA coverage.

Clec12A, CD301b, and FcγRIIB/III define the heterogeneity of murine DC2s and DC3s

Over the last decade, multiple studies have investigated the heterogeneity of murine conventional dendritic cells type 2 (cDC2s). However, their phenotypic similarity with monocytes and macrophages renders their clear identification challenging. By creating a protein atlas utilizing multiparameter flow cytometry, we show that ESAM+ cDC2s are a specialized feature of the spleen strongly differing in their proteome from other cDC2s. In contrast, all other tissues are populated by Clec12A+ cDC2s or Clec12A- cDC2s (high or low for Fcγ receptors, C-type lectin receptors, and CD11b, respectively), rendering Clec12A+ cDC2s classical sentinels. Further, expression analysis of CD301b, Clec12A, and FcγRIIB/III provides a conserved definition of cDC2 heterogeneity, including the discovery of putative FcγRIIB/III+ DC3s across tissues. Finally, our data reveal that cell identity (ontogeny) dictates the proteome that is further fine-tuned by the tissue environment on macrophages and dendritic cells (DCs), while monocytes and plasmacytoid DCs (pDCs) display subset intrinsic default settings.

ADAM10 and Neprilysin level decreases in immune cells of mice bearing metastatic breast carcinoma: Possible role in cancer inflammatory response

OBJECTIVE AND DESIGN

ADAM10 and Neprilysin, proteases, play critical role in inflammatory disease, however their role in cancer immune response is not clear. We here evaluated changes in immune response using an experimental model for breast cancer.

MATERIAL AND METHOD

Highly metastatic breast cancer cells (4T1-derived) were injected orthotopically (mammary-pad of Balb-c mice) to induce tumors. Changes in enzyme level and activity as well as alterations in inflammatory cytokine release in the presence or absence of ADAM10 and NEP activity was determined using specific inhibitors and recombinant proteins. Cytokine response was evaluated using mix leucocyte cultures obtained from control and tumor-bearing mice. ANOVA with Dunnett's posttest was used for statistical analysis.

RESULTS

ADAM10 and NEP expression was decreased markedly in lymph nodes and spleens of tumor-bearing mice. ADAM10 activity was reduced together with apparent alterations of ADAM10 processing. ADAM10 and NEP activity decreased TNF-α, IL-6 and IFN-ɣ secretion. Suppression of these inflammatory cytokines were more prominent in cultures obtained from control mice demonstrating counteracting factors that are exist in tumor-bearing mice.

CONCLUSION

Loss of ADAM10 and NEP activity in immune cells during breast cancer metastasis might be one of the main factors involved in induction of chronic inflammation by tumors.

The coenzyme A precursor pantethine enhances antitumor immunity in sarcoma

The tumor microenvironment is a dynamic network of stromal, cancer, and immune cells that interact and compete for resources. We have previously identified the Vanin1 pathway as a tumor suppressor of sarcoma development via vitamin B5 and coenzyme A regeneration. Using an aggressive sarcoma cell line that lacks Vnn1 expression, we showed that the administration of pantethine, a vitamin B5 precursor, attenuates tumor growth in immunocompetent but not nude mice. Pantethine boosts antitumor immunity, including the polarization of myeloid and dendritic cells towards enhanced IFNγ-driven antigen presentation pathways and improved the development of hypermetabolic effector CD8+ T cells endowed with potential antitumor activity. At later stages of treatment, the effect of pantethine was limited by the development of immune cell exhaustion. Nevertheless, its activity was comparable with that of anti-PD1 treatment in sensitive tumors. In humans, VNN1 expression correlates with improved survival and immune cell infiltration in soft-tissue sarcomas, but not in osteosarcomas. Pantethine could be a potential therapeutic immunoadjuvant for the development of antitumor immunity.

Boosting Live Malaria Vaccine with Cytomegalovirus Vector Can Prolong Immunity through Innate and Adaptive Mechanisms

Vaccines to persistent parasite infections have been challenging, and current iterations lack long-term protection. Cytomegalovirus (CMV) chronic vaccine vectors drive protection against SIV, tuberculosis and liver-stage malaria correlated with antigen-specific CD8 T cells with a Tem phenotype. This phenotype is likely driven by a combination of antigen-specific and innate adjuvanting effects of the vector, though these mechanisms are less well understood. Sterilizing immunity from live Plasmodium chabaudi vaccination lasts less than 200 days. While P. chabaudi-specific antibody levels remain stable after vaccination, the decay of parasite-specific T cells correlates with loss of challenge protection. Therefore, we enlisted murine CMV as a booster strategy to prolong T cell responses against malaria. To study induced T cell responses, we included P. chabaudi MSP-1 epitope B5 (MCMV-B5). We found that MCMV vector alone significantly protected against a challenge P. chabaudi infection 40-60 days later, and that MCMV-B5 was able to make B5-specific Teff, in addition to previously-reported Tem, that survive to the challenge timepoint. Used as a booster, MCMV-B5 prolonged protection from heterologous infection beyond day 200, and increased B5 TCR Tg T cell numbers, including both a highly-differentiated Tem phenotype and Teff, both previously reported to protect. B5 epitope expression was responsible for maintenance of Th1 and Tfh B5 T cells. In addition, the MCMV vector had adjuvant properties, contributing non-specifically through prolonged stimulation of IFN-γ. In vivo neutralization of IFN-γ, but not IL-12 and IL-18, late in the course of MCMV, led to loss of the adjuvant effect. Mechanistically, sustained IFN-γ from MCMV increased CD8α+ dendritic cell numbers, and led to increased IL-12 production upon Plasmodium challenge. In addition, neutralization of IFN-γ before challenge reduced the polyclonal Teff response to challenge. Our findings suggest that, as protective epitopes are defined, an MCMV vectored booster can prolong protection through the innate effects of IFN-γ.

Allogeneic CAR Invariant Natural Killer T Cells Exert Potent Antitumor Effects through Host CD8 T-Cell Cross-Priming

PURPOSE

The development of allogeneic chimeric antigen receptor (CAR) T-cell therapies for off-the-shelf use is a major goal that faces two main immunologic challenges, namely the risk of graft-versus-host disease (GvHD) induction by the transferred cells and the rejection by the host immune system limiting their persistence. In this work we assessed the direct and indirect antitumor effect of allogeneic CAR-engineered invariant natural killer T (iNKT) cells, a cell population without GvHD-induction potential that displays immunomodulatory properties.

EXPERIMENTAL DESIGN

After assessing murine CAR iNKT cells direct antitumor effects in vitro and in vivo, we employed an immunocompetent mouse model of B-cell lymphoma to assess the interaction between allogeneic CAR iNKT cells and endogenous immune cells.

RESULTS

We demonstrate that allogeneic CAR iNKT cells exerted potent direct and indirect antitumor activity when administered across major MHC barriers by inducing tumor-specific antitumor immunity through host CD8 T-cell cross-priming.

CONCLUSIONS

In addition to their known direct cytotoxic effect, allogeneic CAR iNKT cells induce host CD8 T-cell antitumor responses, resulting in a potent antitumor effect lasting longer than the physical persistence of the allogeneic cells. The utilization of off-the-shelf allogeneic CAR iNKT cells could meet significant unmet needs in the clinic.

Posttranslational modifications by ADAM10 shape myeloid antigen-presenting cell homeostasis in the splenic marginal zone

The spleen contains phenotypically and functionally distinct conventional dendritic cell (cDC) subpopulations, termed cDC1 and cDC2, which each can be divided into several smaller and less well-characterized subsets. Despite advances in understanding the complexity of cDC ontogeny by transcriptional programming, the significance of posttranslational modifications in controlling tissue-specific cDC subset immunobiology remains elusive. Here, we identified the cell-surface-expressed A-disintegrin-and-metalloproteinase 10 (ADAM10) as an essential regulator of cDC1 and cDC2 homeostasis in the splenic marginal zone (MZ). Mice with a CD11c-specific deletion of ADAM10 (ADAM10^ΔCD11c) exhibited a complete loss of splenic ESAM^hi cDC2A because ADAM10 regulated the commitment, differentiation, and survival of these cells. The major pathways controlled by ADAM10 in ESAM^hi cDC2A are Notch, signaling pathways involved in cell proliferation and survival (e.g., mTOR, PI3K/AKT, and EIF2 signaling), and EBI2-mediated localization within the MZ. In addition, we discovered that ADAM10 is a molecular switch regulating cDC2 subset heterogeneity in the spleen, as the disappearance of ESAM^hi cDC2A in ADAM10^ΔCD11c mice was compensated for by the emergence of a Clec12a⁺ cDC2B subset closely resembling cDC2 generally found in peripheral lymph nodes. Moreover, in ADAM10^ΔCD11c mice, terminal differentiation of cDC1 was abrogated, resulting in severely reduced splenic Langerin⁺ cDC1 numbers. Next to the disturbed splenic cDC compartment, ADAM10 deficiency on CD11c⁺ cells led to an increase in marginal metallophilic macrophage (MMM) numbers. In conclusion, our data identify ADAM10 as a molecular hub on both cDC and MMM regulating their transcriptional programming, turnover, homeostasis, and ability to shape the anatomical niche of the MZ.

Revisiting Current Concepts on the Tolerogenicity of Steady-State Dendritic Cell Subsets and Their Maturation Stages

The original concept stated that immature dendritic cells (DC) act tolerogenically whereas mature DC behave strictly immunogenically. Meanwhile, it is also accepted that phenotypically mature stages of all conventional DC subsets can promote tolerance as steady-state migratory DC by transporting self-antigens to lymph nodes to exert unique functions on regulatory T cells. We propose that in vivo 1) there is little evidence for a tolerogenic function of immature DC during steady state such as CD4 T cell anergy induction, 2) all tolerance as steady-state migratory DC undergo common as well as subset-specific molecular changes, and 3) these changes differ by quantitative and qualitative markers from immunogenic DC, which allows one to clearly distinguish tolerogenic from immunogenic migratory DC.

Inflammasomes in dendritic cells: Friend or foe?

Inflammasomes are cytosolic multiprotein complexes that crucially contribute to host defense against pathogens but are also involved in the pathogenesis of autoinflammatory diseases. Inflammasome formation leads to activation of effector caspases (caspase-1, 4, 5, or 11), the proteolytic maturation of IL-1β and IL-18 as well as cleavage of the pore-forming protein Gasdermin D. Dendritic cells are major regulators of immune responses as they bridge innate and adaptive immunity. We here summarize the current knowledge on inflammasome expression and formation in murine bone marrow-, human monocyte-derived as well as murine and human primary dendritic cells. Further, we discuss both, the beneficial and detrimental, involvement of inflammasome activation in dendritic cells in cancer, infections, and autoimmune diseases. As inflammasome activation is typically accompanied by Gasdermin d-mediated pyroptosis, which is an inflammatory form of programmed cell death, inflammasome formation in dendritic cells seems ill-advised. Therefore, we propose that hyperactivation, which is inflammasome activation without the induction of pyroptosis, may be a general model of inflammasome activation in dendritic cells to enhance Th1, Th17 as well as cytotoxic T cell responses.

Regulation of the Migration of Distinct Dendritic Cell Subsets

Dendritic cells (DCs), a class of antigen-presenting cells, are widely present in tissues and apparatuses of the body, and their ability to migrate is key for the initiation of immune activation and tolerogenic immune responses. The importance of DCs migration for their differentiation, phenotypic states, and immunologic functions has attracted widespread attention. In this review, we discussed and compared the chemokines, membrane molecules, and migration patterns of conventional DCs, plasmocytoid DCs, and recently proposed DC subgroups. We also review the promoters and inhibitors that affect DCs migration, including the hypoxia microenvironment, tumor microenvironment, inflammatory factors, and pathogenic microorganisms. Further understanding of the migration mechanisms and regulatory factors of DC subgroups provides new insights for the treatment of diseases, such as infection, tumors, and vaccine preparation.

Liposomal Nanovaccine Containing α-Galactosylceramide and Ganglioside GM3 Stimulates Robust CD8+ T Cell Responses via CD169+ Macrophages and cDC1

Successful anti-cancer vaccines aim to prime and reinvigorate cytotoxic T cells and should therefore comprise a potent antigen and adjuvant. Antigen targeting to splenic CD169⁺ macrophages was shown to induce robust CD8⁺ T cell responses via antigen transfer to cDC1. Interestingly, CD169⁺ macrophages can also activate type I natural killer T-cells (NKT). NKT activation via ligands such as α-galactosylceramide (αGC) serve as natural adjuvants through dendritic cell activation. Here, we incorporated ganglioside GM3 and αGC in ovalbumin (OVA) protein-containing liposomes to achieve both CD169⁺ targeting and superior DC activation. The systemic delivery of GM3-αGC-OVA liposomes resulted in specific uptake by splenic CD169⁺ macrophages, stimulated strong IFNγ production by NKT and NK cells and coincided with the maturation of cDC1 and significant IL-12 production. Strikingly, superior induction of OVA-specific CD8⁺ T cells was detected after immunization with GM3-αGC-OVA liposomes. CD8⁺ T cell activation, but not B cell activation, was dependent on CD169⁺ macrophages and cDC1, while activation of NKT and NK cells were partially mediated by cDC1. In summary, GM3-αGC antigen-containing liposomes are a potent vaccination platform that promotes the interaction between different immune cell populations, resulting in strong adaptive immunity and therefore emerge as a promising anti-cancer vaccination strategy.

DCs-based therapies: potential strategies in severe SARS-CoV-2 infection

Pneumonia caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is spreading globally. There have been strenuous efforts to reveal the mechanisms that the host defends itself against invasion by this virus. The immune system could play a crucial role in virus infection. Dendritic cell as sentinel of the immune system plays an irreplaceable role. Dendritic cells-based therapeutic approach may be a potential strategy for SARS-CoV-2 infection. In this review, the characteristics of coronavirus are described briefly. We focus on the essential functions of dendritic cell in severe SARS-CoV-2 infection. Basis of treatment based dendritic cells to combat coronavirus infections is summarized. Finally, we propose that the combination of DCs based vaccine and other therapy is worth further study.

Harnessing the Complete Repertoire of Conventional Dendritic Cell Functions for Cancer Immunotherapy

The onset of checkpoint inhibition revolutionized the treatment of cancer. However, studies from the last decade suggested that the sole enhancement of T cell functionality might not suffice to fight malignancies in all individuals. Dendritic cells (DCs) are not only part of the innate immune system, but also generals of adaptive immunity and they orchestrate the de novo induction of tolerogenic and immunogenic T cell responses. Thus, combinatorial approaches addressing DCs and T cells in parallel represent an attractive strategy to achieve higher response rates across patients. However, this requires profound knowledge about the dynamic interplay of DCs, T cells, other immune and tumor cells. Here, we summarize the DC subsets present in mice and men and highlight conserved and divergent characteristics between different subsets and species. Thereby, we supply a resource of the molecular players involved in key functional features of DCs ranging from their sentinel function, the translation of the sensed environment at the DC:T cell interface to the resulting specialized T cell effector modules, as well as the influence of the tumor microenvironment on the DC function. As of today, mostly monocyte derived dendritic cells (moDCs) are used in autologous cell therapies after tumor antigen loading. While showing encouraging results in a fraction of patients, the overall clinical response rate is still not optimal. By disentangling the general aspects of DC biology, we provide rationales for the design of next generation DC vaccines enabling to exploit and manipulate the described pathways for the purpose of cancer immunotherapy in vivo. Finally, we discuss how DC-based vaccines might synergize with checkpoint inhibition in the treatment of malignant diseases.

Understanding and Modulating Immunity With Cell Reprogramming

Cell reprogramming concepts have been classically developed in the fields of developmental and stem cell biology and are currently being explored for regenerative medicine, given its potential to generate desired cell types for replacement therapy. Cell fate can be experimentally reversed or modified by enforced expression of lineage specific transcription factors leading to pluripotency or attainment of another somatic cell type identity. The possibility to reprogram fibroblasts into induced dendritic cells (DC) competent for antigen presentation creates a paradigm shift for understanding and modulating the immune system with direct cell reprogramming. PU.1, IRF8, and BATF3 were identified as sufficient and necessary to impose DC fate in unrelated cell types, taking advantage of Clec9a, a C-type lectin receptor with restricted expression in conventional DC type 1. The identification of such minimal gene regulatory networks helps to elucidate the molecular mechanisms governing development and lineage heterogeneity along the hematopoietic hierarchy. Furthermore, the generation of patient-tailored reprogrammed immune cells provides new and exciting tools for the expanding field of cancer immunotherapy. Here, we summarize cell reprogramming concepts and experimental approaches, review current knowledge at the intersection of cell reprogramming with hematopoiesis, and propose how cell fate engineering can be merged to immunology, opening new opportunities to understand the immune system in health and disease.

Transcriptional control of dendritic cell development and functions

Dendritic cells (DCs) are major regulators of adaptive immunity, as they are not only capable to induce efficient immune responses, but are also crucial to maintain peripheral tolerance and thereby inhibit autoimmune reactions. DCs bridge the innate and the adaptive immune system by presenting peptides of self and foreign antigens as peptide MHC complexes to T cells. These properties render DCs as interesting target cells for immunomodulatory therapies in cancer, but also autoimmune diseases. Several subsets of DCs with special properties and functions have been described. Recent achievements in understanding transcriptional programs on single cell level, together with the generation of new murine models targeting specific DC subsets, advanced our current understanding of DC development and function. Thus, DCs arise from precursor cells in the bone marrow with distinct progenitor cell populations splitting the monocyte populations and macrophage populations from the DC lineage, which upon lineage commitment can be separated into conventional cDC1, cDC2, and plasmacytoid DCs (pDCs). The DC populations harbor intrinsic programs enabling them to react for specific pathogens in dependency on the DC subset, and thereby orchestrate T cell immune responses. Similarities, but also varieties, between human and murine DC subpopulations are challenging, and will require further investigation of human specimens under consideration of the influence of the tissue micromilieu and DC subset localization in the future.

DNA vaccination via RALA nanoparticles in a microneedle delivery system induces a potent immune response against the endogenous prostate cancer stem cell antigen

Castrate resistant prostate cancer (CRPC) remains a major challenge for healthcare professionals. Immunotherapeutic approaches, including DNA vaccination, hold the potential to harness the host's own immune system to mount a cell-mediated, anti-tumour response, capable of clearing disseminated tumour deposits. These anti-cancer vaccines represent a promising strategy for patients with advanced disease, however, to date DNA vaccines have demonstrated limited efficacy in clinical trials, owing to the lack of a suitable DNA delivery system. This study was designed to evaluate the efficacy of a two-tier delivery system incorporating cationic RALA/pDNA nanoparticles (NPs) into a dissolvable microneedle (MN) patch for the purposes of DNA vaccination against prostate cancer. Application of NP-loaded MN patches successfully resulted in endogenous production of the encoded Prostate Stem Cell Antigen (PSCA). Furthermore, immunisation with RALA/pPSCA loaded MNs elicited a tumour-specific immune response against TRAMP-C1 tumours ex vivo. Finally, vaccination with RALA/pPSCA loaded MNs demonstrated anti-tumour activity in both prophylactic and therapeutic prostate cancer models in vivo. This is further evidence that this two-tier MN delivery system is a robust platform for prostate cancer DNA vaccination. STATEMENT OF SIGNIFICANCE: This research describes the development and utilisation of our unique microneedle (MN) DNA delivery system, which enables penetration through the stratum corneum and deposition of the DNA within the highly immunogenic skin layers via a dissolvable MN matrix, and facilitates cellular uptake via complexation of pDNA cargo into nanoparticles (NPs) with the RALA delivery peptide. We report for the first time on using the NP-MN platform to immunise mice with encoded Prostate Stem Cell Antigen (mPSCA) for prostate cancer DNA vaccination. Application of the NP-MN system resulted in local mPSCA expression in vivo. Furthermore, immunisation with the NP-MN system induced a tumour-specific cellular immune response, and inhibited the growth of TRAMP-C1 prostate tumours in both prophylactic and therapeutic challenge models in vivo.

NKT Cell-Driven Enhancement of Antitumor Immunity Induced by Clec9a-Targeted Tailorable Nanoemulsion

Invariant natural killer T (iNKT) cells are a subset of lymphocytes with immune regulatory activity. Their ability to bridge the innate and adaptive immune systems has been studied using the glycolipid ligand α-galactosylceramide (αGC). To better harness the immune adjuvant properties of iNKT cells to enhance priming of antigen-specific CD8⁺ T cells, we encapsulated both αGC and antigen in a Clec9a-targeted nanoemulsion (TNE) to deliver these molecules to cross-presenting CD8⁺ dendritic cells (DC). We demonstrate that, even in the absence of exogenous glycolipid, iNKT cells supported the maturation of CD8α⁺ DCs to drive efficient cross-priming of antigen-specific CD8⁺ T cells upon delivery of Clec9a/OVA-TNE. The addition of αGC to the TNE (Clec9a/OVA/αGC) further enhanced activation of iNKT cells, NK cells, CD8α⁺ DCs, and polyfunctional CD8⁺ T cells. When tested therapeutically against HPVE7-expressing TC-1 tumors, long-term tumor suppression was achieved with a single administration of Clec9a/E7 peptide/αGC TNE. Antitumor activity was correlated with the recruitment of mature DCs, NK cells, and tumor-specific effector CD8⁺ T cells to the tumor-draining lymph node and tumor tissue. Thus, Clec9a-TNE codelivery of CD8⁺ T-cell epitopes with αGC induces alternative helper signals from activated iNKT cells, elicits innate (iNKT, NK) immunity, and enhances antitumor CD8⁺ T-cell responses for control of solid tumors.

Langerin+CD8+ Dendritic Cells in the Splenic Marginal Zone: Not So Marginal After All

Dendritic cells (DC) fulfill an essential sentinel function within the immune system, acting at the interface of innate and adaptive immunity. The DC family, both in mouse and man, shows high functional heterogeneity in order to orchestrate immune responses toward the immense variety of pathogens and other immunological threats. In this review, we focus on the Langerin⁺CD8⁺ DC subpopulation in the spleen. Langerin⁺CD8⁺ DC exhibit a high ability to take up apoptotic/dying cells, and therefore they are essential to prime and shape CD8⁺ T cell responses. Next to the induction of immunity toward blood-borne pathogens, i.e., viruses, these DC are important for the regulation of tolerance toward cell-associated self-antigens. The ontogeny and differentiation pathways of CD8⁺CD103⁺ DC should be further explored to better understand the immunological role of these cells as a prerequisite of their therapeutic application.

Langerin+ CD8α+ Dendritic Cells Drive Early CD8+ T Cell Activation and IL-12 Production During Systemic Bacterial Infection

Bloodstream infections induce considerable morbidity, high mortality, and represent a significant burden of cost in health care; however, our understanding of the immune response to bacteremia is incomplete. Langerin⁺ CD8α⁺ dendritic cells (DCs), residing in the marginal zone of the murine spleen, have the capacity to cross-prime CD8⁺ T cells and produce IL-12, both of which are important components of antimicrobial immunity. Accordingly, we hypothesized that this DC subset may be a key promoter of adaptive immune responses to blood-borne bacterial infections. Utilizing mice that express the diphtheria toxin receptor under control of the langerin promoter, we investigated the impact of depleting langerin⁺ CD8α⁺ DCs in a murine model of intravenous infection with Mycobacterium bovis bacille Calmette–Guerin (BCG). In the absence of langerin⁺ CD8α⁺ DCs, the immune response to blood-borne BCG infection was diminished: bacterial numbers in the spleen increased, serum IL-12p40 decreased, and delayed CD8⁺ T cell activation, proliferation, and IFN-γ production was evident. Our data revealed that langerin⁺ CD8α⁺ DCs play a pivotal role in initiating CD8⁺ T cell responses and IL-12 production in response to bacteremia and may influence the early control of systemic bacterial infections.

Immune barriers of Ebola virus infection

Since its initial emergence in 1976 in northern Democratic Republic of Congo (DRC), Ebola virus (EBOV) has been a global health concern due to its virulence in humans, the mystery surrounding the identity of its host reservoir and the unpredictable nature of Ebola virus disease (EVD) outbreaks. Early after the first clinical descriptions of a disease resembling a 'septic-shock-like syndrome', with coagulation abnormalities and multi-system organ failure, researchers began to evaluate the role of the host immune response in EVD pathophysiology. In this review, we summarize how data gathered during the last 40 years in the laboratory as well as in the field have provided insight into EBOV immunity. From molecular mechanisms involved in EBOV recognition in infected cells, to antigen processing and adaptive immune responses, we discuss current knowledge on the main immune barriers of infection as well as outstanding research questions.

Role of Lymphocyte Subsets in the Immune Response to Primary B Cell-Derived Exosomes

Exosomes are lipid nanovesicles released after fusion of the endosomal limiting membrane with the plasma membrane. In this study, we investigated the requirement for CD4 T cells, B cells, and NK cells to provide help for CD8 T cell-mediated response to B cell-derived exosomes. CTL responses to Ag-loaded exosomes were dependent on host MHC class I, with a critical role for splenic langerin⁺ CD8α⁺ dendritic cells (DCs) in exosomal Ag cross-presentation. In addition, there was an absolute dependence on the presence of CD4 T cells, CD8 T cells, and NK cells, where the loss of any one of these subsets led to a complete loss of CTL response. Interestingly, NK cell depletion experiments demonstrated a critical cutoff point for depletion efficacy, with low-level residual NK cells providing sufficient help to allow optimal CD8 T cell proliferative responses to exosomal protein. Despite the potential role for B cells in the response to B cell-derived exosomal proteins, B cell depletion did not alter the exosome-induced CTL response. Similarly, a possible role for the BCR or circulating Ab in mediating CTL responses to B cell-derived exosomes was ruled out using D_HLMP2A mice, which lack secreted and membrane-bound Ab, yet harbor marginal zone and follicular B cells. In contrast, CTL responses to DC-derived exosomes were significantly inhibited within Ab-deficient D_HLMP2A mice compared with wild-type mice. However, this response was not restored upon serum transfer, implicating a role for the BCR, but not circulating Ab, in DC-derived exosome responses.

Immature human DCs efficiently translocate endocytosed antigens into the cytosol for proteasomal processing

Cross-presentation of endocytosed antigen is essential for induction of CD8 effector T cell responses and a hallmark of dendritic cells (DCs). The mode of antigen processing in this context is controversial and some models imply translocation of the antigen from the endosomes into the cytosol. To test this hypothesis we made use of the pro-apoptotic properties of cytochrome c when in the cytosol, and confirmed that it indeed triggered apoptosis of human immature DCs but only at high concentrations. Proteasome inhibitors reduced the required concentration of cytochrome c thousand-fold, indicating that protein translocated into the cytosol is rapidly degraded by proteasomes. Mature DCs were also susceptible to cytochrome c-triggered apoptosis at high concentrations but proteasome inhibitors did not increase their sensitivity. Other cross-presenting cells such as B cells and monocytes were not sensitive to cytochrome c at all, indicating that they do not shuttle internalized antigen into the cytosol. Thus, processing of internalized antigens seems to follow different pathways depending on cell type and, in case of DCs, maturation state. Immature DCs appear to have a unique capacity to shuttle external antigen into the cytosol for proteasomal processing, which could explain their efficiency in antigen cross-presentation.

Herpes Simplex Virus Glycoprotein D Targets a Specific Dendritic Cell Subset and Improves the Performance of Vaccines to Human Papillomavirus-Associated Tumors

Cervical cancer is a major public health problem and one of the leading causes of cancer deaths in women. Virtually all cases of cervical cancer, as well as a growing share of anal and head/neck tumors, are associated with human papillomavirus (HPV) infection. Despite the effectiveness, the available prophylactic vaccines do not benefit women with cervical lesions or cancer. Therefore, the search of new immunotherapeutic approaches to treat HPV-induced tumors is still a priority. The present study characterizes a therapeutic antitumor vaccine based on the genetic fusion of the Herpes simplex virus-1 (HSV-1) glycoprotein D (gD) with the E7 oncoprotein from HPV-16 (gDE7). Two subcutaneous doses of gDE7, admixed with poly (I:C), conferred complete and long-lasting therapeutic antitumor protection on mice previously challenged with tumor cells expressing the HPV-16 oncoproteins. The vaccine induced multifunctional E7-specific CD8⁺ T cells with cytotoxic activity and effector memory phenotype (CD44⁺ CD62L^low). In addition, gDE7 admixed with poly (I:C) vaccination controlled the expansion of tumor-induced regulatory T cells and myeloid-derived suppressor cells. More importantly, gDE7 activated mouse CD11c⁺ CD8α⁺ and human BDCA3⁺ dendritic cells (DC), specialized in antigen cross-presentation to CD8⁺ T cells, under in vitro conditions. These results indicated that the activation of a specific DC population, mediated by gD, improved the antigen-specific immune responses and the therapeutic performance induced by antitumor vaccines. These results open perspectives for the clinical testing of gDE7-based vaccines under the concept of active immunization as a tool for the therapeutic control of cancer. Mol Cancer Ther; 16(9); 1922-33. ©2017 AACR.

Hyperosmolarity impedes the cross-priming competence of dendritic cells in a TRIF-dependent manner

Tissue osmolarity varies among different organs and can be considerably increased under pathologic conditions. Hyperosmolarity has been associated with altered stimulatory properties of immune cells, especially macrophages and dendritic cells. We have recently reported that dendritic cells upon exposure to hypertonic stimuli shift their profile towards a macrophage-M2-like phenotype, resulting in attenuated local alloreactivity during acute kidney graft rejection. Here, we examined how hyperosmotic microenvironment affects the cross-priming capacity of dendritic cells. Using ovalbumin as model antigen, we showed that exposure of dendritic cells to hyperosmolarity strongly inhibits activation of antigen-specific T cells despite enhancement of antigen uptake, processing and presentation. We identified TRIF as key mediator of this phenomenon. Moreover, we detected a hyperosmolarity-triggered, TRIF-dependent clustering of MHCI loaded with the ovalbumin-derived epitope, but not of overall MHCI molecules, providing a possible explanation for a reduced T cell activation. Our findings identify dendritic cells as important players in hyperosmolarity-mediated immune imbalance and provide evidence for a novel pathway of inhibition of antigen specific CD8⁺ T cell response in a hypertonic micromilieu.

Radiation-induced decrease of CD8+ dendritic cells contributes to Th1/Th2 shift

Exposure to ionizing radiation (IR) often reduce the helper T (Th) 1 like function, resulting in a Th1/Th2 imbalance, which could affect the efficacy of cancer radiotherapy. As the most potent antigen presenting cells, dendritic cells (DC) can be divided into several subsets with specialized function. However, there is no literature covering the changes of DC subsets and their roles in immune regulation in response to IR. In the present study, we were aimed to investigate the changes of DC subsets after IR and its relationship with Th1/Th2 immunity. We found a significant decrease of BDCA3+DC in the blood of patients treated with radiotherapy. CD8+DC, a mouse equivalent of human BDCA3+DC, was also found decreased in mice spleen, peripheral blood and lymph node tissues after irradiation. As CD8+DC mainly induce Th1 immunity, we tested the changes of Th1/Th2 response and found that IR caused a repression of Th1 immunity, indicating a possible role of CD8+DC in radiation-induced Th1/Th2 imbalance. We also found that a CD8+DC-inducing cytokine, Fms-like tyrosine kinase 3 ligand (FLT3 ligand), restored CD8+DC and reversed Th1/Th2 shift. And then we found that bone marrow cells from irradiated mice differentiated into less CD8+DC, which was also protected by FLT3 ligand. In conclusion, our data showed that IR induced a decrease of CD8+DC and Th1/Th2 shift, which was reversed by Flt3 ligand treatment, suggesting a novel mechanism for radiation-induced immunosuppression.

Ebola virus infection kinetics in chimeric mice reveal a key role of T cells as barriers for virus dissemination

Ebola virus (EBOV) causes severe systemic disease in humans and non-human primates characterized by high levels of viremia and virus titers in peripheral organs. The natural portals of virus entry are the mucosal surfaces and the skin where macrophages and dendritic cells (DCs) are primary EBOV targets. Due to the migratory properties of DCs, EBOV infection of these cells has been proposed as a necessary step for virus dissemination via draining lymph nodes and blood. Here we utilize chimeric mice with competent hematopoietic-driven immunity, to show that EBOV primarily infects CD11b⁺ DCs in non-lymphoid and lymphoid tissues, but spares the main cross-presenting CD103⁺ DC subset. Furthermore, depletion of CD8 and CD4 T cells resulted in loss of early control of virus replication, viremia and fatal Ebola virus disease (EVD). Thus, our findings point out at T cell function as a key determinant of EVD progress and outcome.

Signal transducer and activator of transcription 3 (Stat3) regulates host defense and protects mice against herpes simplex virus-1 (HSV-1) infection

Signal transducer and activator of transcription 3 (STAT3) mediates cellular responses to multiple cytokines, governs gene expression, and regulates the development and activation of immune cells. STAT3 also modulates reactivation of latent herpes simplex virus-1 (HSV-1) in ganglia. However, it is unclear how STAT3 regulates the innate immune response during the early phase of HSV-1 lytic infection. Many cell types critical for the innate immunity are derived from the myeloid lineage. Therefore, in this study, we used myeloid-specific Stat3 knockout mice to investigate the role of STAT3 in the innate immune response against HSV-1. Our results demonstrate that Stat3 knockout bone marrow-derived macrophages (BMMs) expressed decreased levels of interferon-α (IFN-α) and interferon-stimulated genes (ISGs) upon HSV-1 infection. In vivo, knockout mice were more susceptible to HSV-1, as marked by higher viral loads and more significant weight loss. Splenic expression of IFN-α and ISGs was reduced in the absence of STAT3, indicating that STAT3 is required for optimal type I interferon response to HSV-1. Expression of TNF-α and IL-12, cytokines that have been shown to limit HSV-1 replication and pathogenesis, was also significantly lower in knockout mice. Interestingly, Stat3 knockout mice failed to expand the CD8⁺ conventional DC (cDC) population upon HSV-1 infection, and this was accompanied by impaired NK and CD8 T cell activation. Collectively, our data demonstrate that myeloid-specific Stat3 deletion causes defects in multiple aspects of the immune system and that STAT3 has a protective role at the early stage of systemic HSV-1 infection.

In vivo and in vitro analyses of α-galactosylceramide uptake by conventional dendritic cell subsets using its fluorescence-labeled derivative

Conventional dendritic cells (cDCs) present α-galactosylceramide (αGC) to invariant natural killer T (iNKT) cells through CD1d. Among cDC subsets, CD8(+) DCs efficiently induce IFN-γ production in iNKT cells. Using fluorescence-labeled αGC, we showed that CD8(+) DCs incorporated larger amounts of αGC and kept it intact longer than CD8(-) DCs. Histological analyses revealed that Langerin(+)CD8(+) DCs in the splenic marginal zone, which was the unique equipment to capture blood-borne antigens, preferably incorporated αGC, and the depletion of Langerin(+) cells decreased IFN-γ and IL-12 production in response to αGC. Furthermore, splenic Langerin(+)CD8(+) DCs expressed more membrane-bound CXCL16, which possibly anchored iNKT cells in the marginal zone, than CD8(-) DCs. Collectively, it is suggested that the cellular properties and localization of CD8(+) DCs are important for stimulation of iNKT cells by αGC.

Activated NKT Cells Can Condition Different Splenic Dendritic Cell Subsets To Respond More Effectively to TLR Engagement and Enhance Cross-Priming

The function of dendritic cells (DCs) can be modulated through multiple signals, including recognition of pathogen-associated molecular patterns, as well as signals provided by rapidly activated leukocytes in the local environment, such as innate-like T cells. In this article, we addressed the possibility that the roles of different murine DC subsets in cross-priming CD8(+) T cells can change with the nature and timing of activatory stimuli. We show that CD8α(+) DCs play a critical role in cross-priming CD8(+) T cell responses to circulating proteins that enter the spleen in close temporal association with ligands for TLRs and/or compounds that activate NKT cells. However, if NKT cells are activated first, then CD8α(-) DCs become conditioned to respond more vigorously to TLR ligation, and if triggered directly, these cells can also contribute to priming of CD8(+) T cell responses. In fact, the initial activation of NKT cells can condition multiple DC subsets to respond more effectively to TLR ligation, with plasmacytoid DCs making more IFN-α and both CD8α(+) and CD8α(-) DCs manufacturing more IL-12. These results suggest that different DC subsets can contribute to T cell priming if provided appropriately phased activatory stimuli, an observation that could be factored into the design of more effective vaccines.

Dendritic cell-based vaccination in cancer: therapeutic implications emerging from murine models

Dendritic cells (DCs) play a pivotal role in the orchestration of immune responses, and are thus key targets in cancer vaccine design. Since the 2010 FDA approval of the first cancer DC-based vaccine (Sipuleucel-T), there has been a surge of interest in exploiting these cells as a therapeutic option for the treatment of tumors of diverse origin. In spite of the encouraging results obtained in the clinic, many elements of DC-based vaccination strategies need to be optimized. In this context, the use of experimental cancer models can help direct efforts toward an effective vaccine design. This paper reviews recent findings in murine models regarding the antitumoral mechanisms of DC-based vaccination, covering issues related to antigen sources, the use of adjuvants and maturing agents, and the role of DC subsets and their interaction in the initiation of antitumoral immune responses. The summary of such diverse aspects will highlight advantages and drawbacks in the use of murine models, and contribute to the design of successful DC-based translational approaches for cancer treatment.

An autologous leukemia cell vaccine prevents murine acute leukemia relapse after cytarabine treatment

Acute leukemias with adverse prognostic features carry a high relapse rate without allogeneic stem cell transplantation (allo-SCT). Allo-SCT has a high morbidity and is precluded for many patients because of advanced age or comorbidities. Postremission therapies with reduced toxicities are urgently needed. The murine acute leukemia model C1498 was used to study the efficacy of an intravenously administered vaccine consisting of irradiated leukemia cells loaded with the natural killer T (NKT)-cell agonist α-galactosylceramide (α-GalCer). Prophylactically, the vaccine was highly effective at preventing leukemia development through the downstream activities of activated NKT cells, which were dependent on splenic langerin(+)CD8α(+) dendritic cells and which led to stimulation of antileukemia CD4(+) and CD8(+) T cells. However, hosts with established leukemia received no protective benefit from the vaccine, despite inducing NKT-cell activation. Established leukemia was associated with increases in regulatory T cells and myeloid-derived suppressor cells, and the leukemic cells themselves were highly suppressive in vitro. Although this suppressive environment impaired both effector arms of the immune response, CD4(+) T-cell responses were more severely affected. When cytarabine chemotherapy was administered prior to vaccination, all animals in remission posttherapy were protected against rechallenge with viable leukemia cells.

Batf3-independent langerin- CX3CR1- CD8α+ splenic DCs represent a precursor for classical cross-presenting CD8α+ DCs

This study tests the hypothesis that CD8α(+) DCs in the spleen of mice contain an immature precursor for functionally mature, "classical" cross-presenting CD8α(+) DCs. The lymphoid tissues contain a network of phenotypically distinct DCs with unique roles in surveillance and immunity. Splenic CD8α(+) DCs have been shown to exhibit a heightened capacity for phagocytosis of cellular material, secretion of IL-12, and cross-priming of CD8(+) T cells. However, this population can be subdivided further on the basis of expression of both langerin/CD207 and CX(3)CR1. We therefore evaluated the functional capacities of these different subsets. The CX(3)CR1(+) CD8α(+) DC subset does not express langerin and does not exhibit the classical features above. The CX(3)CR1(-) CD8α(+) DC can be divided into langerin-positive and negative populations, both of which express DEC205, Clec9A, and high basal levels of CD86. However, the langerin(+) CX(3)CR1(-) CD8α(+) subset has a superior capacity for acquiring cellular material and producing IL-12 and is more susceptible to activation-induced cell death. Significantly, following purification and adoptive transfer into new hosts, the langerin(-) CX(3)CR1(-) CD8α(+) subset survives longer, up-regulates expression of langerin, and becomes more susceptible to activation-induced cell death. Last, in contrast to langerin(+) CX(3)CR1(-) CD8α(+), the langerin(-) CX(3)CR1(-) CD8α(+) are still present in Batf3(-/-) mice. We conclude that the classical attributes of CD8α(+) DC are confined primarily to the langerin(+) CX(3)CR1(-) CD8α(+) DC population and that the langerin(-) CX(3)CR1(-) subset represents a Batf3-independent precursor to this mature population.

Targeted delivery of α-galactosylceramide to CD8α+ dendritic cells optimizes type I NKT cell-based antitumor responses

Immunotherapy aiming at enhancing innate and acquired host immunity is a promising approach for cancer treatment. The invariant NKT (iNKT) cell ligand α-galactosylceramide (α-GalCer) holds great promise in cancer therapy, although several concerns limit its use in clinics, including the uncontrolled response it promotes when delivered in a nonvectorized form. Therefore, development of delivery systems to in vivo target immune cells might be a valuable option to optimize iNKT cell-based antitumor responses. Using dendritic cell (DC)-depleted mice, DC transfer experiments, and in vivo active cell targeting, we show that presentation of α-GalCer by DCs not only triggers optimal primary iNKT cell stimulation, but also maintains secondary iNKT cell activation after challenge. Furthermore, targeted delivery of α-GalCer to CD8α(+) DCs, by means of anti-DEC205 decorated nanoparticles, enhances iNKT cell-based transactivation of NK cells, DCs, and γδ T cells. We report that codelivery of α-GalCer and protein Ag to CD8α(+) DCs triggers optimal Ag-specific Ab and cytotoxic CD8(+) T cell responses. Finally, we show that targeting nanoparticles containing α-GalCer and Ag to CD8α(+) DCs promotes potent antitumor responses, both in prophylactic and in therapeutic settings. Our data may have important implications in tumor immunotherapy and vaccine development.

Cis-acting pathways selectively enforce the non-immunogenicity of shed placental antigen for maternal CD8 T cells

Maternal immune tolerance towards the fetus and placenta is thought to be established in part by pathways that attenuate T cell priming to antigens released from the placenta into maternal blood. These pathways remain largely undefined and their existence, at face value, seems incompatible with a mother's need to maintain a functional immune system during pregnancy. A particular conundrum is evident if we consider that maternal antigen presenting cells, activated in order to prime T cells to pathogen-derived antigens, would also have the capacity to prime T cells to co-ingested placental antigens. Here, we address this paradox using a transgenic system in which placental membranes are tagged with a strong surrogate antigen (ovalbumin). We find that although a remarkably large quantity of acellular ovalbumin-containing placental material is released into maternal blood, splenic CD8 T cells in pregnant mice bearing unmanipulated T cell repertoires are not primed to ovalbumin even if the mice are intravenously injected with adjuvants. This failure was largely independent of regulatory T cells, and instead was linked to the intrinsic characteristics of the released material that rendered it selectively non-immunogenic, potentially by sequestering it from CD8α⁺ dendritic cells. The release of ovalbumin-containing placental material into maternal blood thus had no discernable impact on CD8 T cell priming to soluble ovalbumin injected intravenously during pregnancy, nor did it induce long-term tolerance to ovalbumin. Together, these results outline a major pathway governing the maternal immune response to the placenta, and suggest how tolerance to placental antigens can be maintained systemically without being detrimental to host defense.

Dendritic cell subsets in type 1 diabetes: friend or foe?

Type 1 diabetes (T1D) is a T cell mediated autoimmune disease characterized by immune mediated destruction of the insulin-producing β cells in the islets of Langerhans. Dendritic cells (DC) have been implicated in the pathogenesis of T1D and are also used as immunotherapeutic agents. Plasmacytoid (p)DC have been shown to have both protective and pathogenic effects and a newly described merocytic DC population has been shown to break tolerance in the mouse model of T1D, the non-obese diabetic (NOD) mouse. We have used DC populations to prevent the onset of T1D in NOD mice and clinical trials of DC therapy in T1D diabetes have been initiated. In this review we will critically examine the recent published literature on the role of DC subsets in the induction and regulation of the autoimmune response in T1D.

Systemic toll-like receptor ligation and selective killing of dendritic cell subsets fail to dissect priming pathways for anti-vaccinia virus CD8⁺ T cells

CD8⁺ T cell responses can be generated by direct or cross-priming mechanisms, and several mouse models have been used to reveal which of these is the most important pathway for various viruses. Among these models is systemic treatment of mice with a CpG-containing oligodeoxynucleotide (CpG) to mature all dendritic cells (DCs), rendering them incapable of cross-presentation. A second is the use of cytochrome c (cytc) as a selective poison of the subsets of DCs able to cross-present antigen. In this study, using two vaccinia virus (VACV) strains, namely, WR and MVA, we found that the CpG and cytc methods gave conflicting data. Moreover, we show for both strains of VACV that treatment of mice with CpG and cytc inhibited CD8⁺ T cell responses to antigens designed to prime exclusively by direct presentation. Further investigation of the CpG method found that the extent to which priming is inhibited depends on the antigen examined, immunization route, replication ability of the virus, and, crucially, immunization dose. We suggest that greater caution is required when interpreting data using these methods and that priming pathways for antiviral CD8⁺ T cells are not simply separated according to DC subsets or their maturation state.

The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting

Dendritic cells (DCs) form a remarkable cellular network that shapes adaptive immune responses according to peripheral cues. After four decades of research, we now know that DCs arise from a hematopoietic lineage distinct from other leukocytes, establishing the DC system as a unique hematopoietic branch. Recent work has also established that tissue DCs consist of developmentally and functionally distinct subsets that differentially regulate T lymphocyte function. This review discusses major advances in our understanding of the regulation of DC lineage commitment, differentiation, diversification, and function in situ.

The mechanism of splenic invariant NKT cell activation dictates localization in vivo

Invariant NKT (iNKT) cells are glycolipid-specific innate lymphocytes emerging as critical players in the immune response to diverse infections and disease. iNKT cells are activated through cognate interactions with lipid-loaded APCs, by Ag-independent cytokine-mediated signaling pathways, or a combination of both. Although each of these modes of iNKT cell activation plays an important role in directing the humoral and cell-mediated immune response, the spatio-temporal nature of these interactions and the cellular requirements for activation are largely undefined. Combining novel in situ confocal imaging of αGalactosylceramide-loaded CD1d tetramer labeling to localize the endogenous iNKT cell population with cytokine reporter mice, we reveal the choreography of early murine splenic iNKT cell activation across diverse settings of glycolipid immunization and systemic infection with Streptococcus pneumoniae. We find that iNKT cells consolidate in the marginal zone and require dendritic cells lining the splenic marginal zone for activation following administration of cognate glycolipids and during systemic infection but not following exogenous cytokine administration. Although further establishing the importance of cognate iNKT cell interactions with APCs, we also show that noncognate iNKT-dependent mechanisms are sufficient to mediate effector outcomes, such as STAT signaling and dendritic cell licensing throughout the splenic parenchyma. Collectively, these data provide new insight into how iNKT cells may serve as a natural adjuvant in facilitating adaptive immune responses, irrespective of their tissue localization.

Inflammatory Flt3l is essential to mobilize dendritic cells and for T cell responses during Plasmodium infection

Innate sensing mechanisms trigger a variety of humoral and cellular events that are essential to adaptive immune responses. Here we describe an innate sensing pathway triggered by Plasmodium infection that regulates dendritic cell homeostasis and adaptive immunity through Flt3 ligand (Flt3l) release. Plasmodium-induced Flt3l release in mice requires Toll-like receptor (TLR) activation and type I interferon (IFN) production. We found that type I IFN supports the upregulation of xanthine dehydrogenase, which metabolizes the xanthine accumulating in infected erythrocytes to uric acid. Uric acid crystals trigger mast cells to release soluble Flt3l from a pre-synthesized membrane-associated precursor. During infection, Flt3l preferentially stimulates expansion of the CD8-α(+) dendritic cell subset or its BDCA3(+) human dendritic cell equivalent and has a substantial impact on the magnitude of T cell activation, mostly in the CD8(+) compartment. Our findings highlight a new mechanism that regulates dendritic cell homeostasis and T cell responses to infection.

Fungal contamination of Hanks solution

Laboratory contamination of clinical specimens submitted for cytological examination is an on-going threat that requires continuous vigilance and quality control. We recently were confronted with a suspected fungal infection in a bone aspirate that was found to derive from contaminated Hanks solution used in cytological preparation. Diff-Quik staining of air-dried cytocentrifuged preparations of Hanks fluid as well as any other suspect laboratory fluid is recommended as a routine quality control procedure.

Experimental models to investigate the function of dendritic cell subsets: challenges and implications

The dendritic cell (DC) lineage is remarkably heterogeneous. It has been postulated that specialized DC subsets have evolved in order to select and support the multitude of possible T cell differentiation pathways. However, defining the function of individual DC subsets has proven remarkably difficult, and DC subset control of key T cell fates such as tolerance, T helper cell commitment and regulatory T cell induction is still not well understood. While the difficulty in assigning unique functions to particular DC subsets may be due to sharing of functions, it may also reflect a lack of appropriate physiological in-vivo models for studying DC function. In this paper we review the limitations associated with many of the current DC models and highlight some of the underlying difficulties involved in studying the function of murine DC subsets.

Vaccination with irradiated tumor cells pulsed with an adjuvant that stimulates NKT cells is an effective treatment for glioma

PURPOSE

The prognosis for patients with glioblastoma multiforme (GBM) remains extremely poor despite recent treatment advances. There is an urgent need to develop novel therapies for this disease.

EXPERIMENTAL DESIGN

We used the implantable GL261 murine glioma model to investigate the therapeutic potential of a vaccine consisting of intravenous injection of irradiated whole tumor cells pulsed with the immuno-adjuvant α-galactosylceramide (α-GalCer).

RESULTS

Vaccine treatment alone was highly effective in a prophylactic setting. In a more stringent therapeutic setting, administration of one dose of vaccine combined with depletion of regulatory T cells (Treg) resulted in 43% long-term survival and the disappearance of mass lesions detected by MRI. Mechanistically, the α-GalCer component was shown to act by stimulating "invariant" natural killer-like T cells (iNKT cells) in a CD1d-restricted manner, which in turn supported the development of a CD4(+) T-cell-mediated adaptive immune response. Pulsing α-GalCer onto tumor cells avoided the profound iNKT cell anergy induced by free α-GalCer. To investigate the potential for clinical application of this vaccine, the number and function of iNKT cells was assessed in patients with GBM and shown to be similar to age-matched healthy volunteers. Furthermore, irradiated GBM tumor cells pulsed with α-GalCer were able to stimulate iNKT cells and augment a T-cell response in vitro.

CONCLUSIONS

Injection of irradiated tumor cells loaded with α-GalCer is a simple procedure that could provide effective immunotherapy for patients with high-grade glioma.

The skin: where malaria infection and the host immune response begin

Infection by malaria parasites begins with the inoculation of sporozoites into the skin of the host. The early events following sporozoite deposition in the dermis are critical for both the establishment of malaria infection and for the induction of protective immune responses. The initial sporozoite inoculum is generally low, and only a small percentage of these sporozoites successfully reach the liver and grow to the next life cycle stage, making this a significant bottleneck for the parasite. Recent studies highlight the importance of sporozoite motility and host cell traversal in dermal exit. Importantly, protective immune responses against sporozoites and liver stages of Plasmodium are induced by dendritic cells in the lymph node draining the skin inoculation site. The cellular, molecular, and immunological events that occur in the skin and associated lymph nodes are the topic of this review.

Expression of XCR1 Characterizes the Batf3-Dependent Lineage of Dendritic Cells Capable of Antigen Cross-Presentation

Cross-presentation of antigen by dendritic cells (DCs) to CD8⁺ T cells is a fundamentally important mechanism in the defense against pathogens and tumors. Due to the lack of an appropriate lineage marker, cross-presenting DCs in the mouse are provisionally classified as “Batf3-IRF-8-Id2-dependent DCs” or as “CD8⁺ DCs” in the spleen, and as “CD103⁺CD11b⁻ DCs” in the periphery. We have now generated a mAb to XCR1, a chemokine receptor which is specifically expressed on CD8⁺ DCs and a subpopulation of double negative DCs in the spleen. Using this antibody, we have determined that only XCR1⁺CD8⁺ (around 80% of CD8⁺ DCs) and their probable precursors, XCR1⁺CD8⁻ DCs, efficiently take up cellular material and excel in antigen cross-presentation. In lymph nodes (LNs) and peripheral tissues, XCR1⁺ DCs largely, but not fully, correspond to CD103⁺CD11b⁻ DCs. Most importantly, we demonstrate that XCR1⁺ DCs in the spleen, LNs, and peripheral tissues are dependent on the growth factor Flt3 ligand and are selectively absent in Batf3-deficient animals. These results provide evidence that expression of XCR1 throughout the body defines the Batf3-dependent lineage of DCs with a special capacity to cross-present antigen. XCR1 thus emerges as the first surface marker characterizing a DC lineage in the mouse and potentially also in the human.

Distinct APCs explain the cytokine bias of α-galactosylceramide variants in vivo

α-Galactosylceramide represents a new class of vaccine adjuvants and immunomodulators that stimulate NKT cells to secrete Th1 and Th2 cytokines. Synthetic variants with short or unsaturated acyl chains exhibit a striking Th2 bias in vivo but no evidence of defect in TCR signaling or stimulation of NKT cells in vitro. Using cd1d1(fl/fl) mice, we demonstrated that distinct APC types explained the cytokine bias in vivo. Whereas NKT stimulation by α-Galactosylceramide required CD1d expression by dendritic cells (DCs), presentation of the Th2 variants was promiscuous and unaffected by DC-specific ablation of CD1d. This DC-independent stimulation failed to activate the feedback loop between DC IL-12 and NK cell IFN-γ, explaining the Th2 bias. Conversely, forced presentation of the Th2 variants by DC induced high IL-12. Thus, lipid structural variations that do not alter TCR recognition can activate distinct Th1 or Th2 cellular networks by changing APC targeting in vivo.