Vaccine molecules targeting Xcr1 on cross‐presenting DCs induce protective CD8+ T‐cell responses against influenza virus

Targeting antigens to cross‐presenting dendritic cells (DCs) is a promising method for enhancing CD8⁺ T‐cell responses. However, expression patterns of surface receptors often vary between species, making it difficult to relate observations in mice to other animals. Recent studies have indicated that the chemokine receptor Xcr1 is selectively expressed on cross‐presenting murine CD8α⁺ DCs, and that the expression is conserved on homologous DC subsets in humans (CD141⁺ DCs), sheep (CD26⁺ DCs), and macaques (CADM1⁺ DCs). We therefore tested if targeting antigens to Xcr1 on cross‐presenting DCs using antigen fused to Xcl1, the only known ligand for Xcr1, could enhance immune responses. Bivalent Xcl1 fused to model antigens specifically bound CD8α⁺ DCs and increased proliferation of antigen‐specific T cells. DNA vaccines encoding dimeric Xcl1‐hemagglutinin (HA) fusion proteins induced cytotoxic CD8⁺ T‐cell responses, and mediated full protection against a lethal challenge with influenza A virus. In addition to enhanced CD8⁺ T‐cell responses, targeting of antigen to Xcr1 induced CD4⁺ Th1 responses and highly selective production of IgG2a antibodies. In conclusion, targeting of dimeric fusion vaccine molecules to CD8α⁺ DCs using Xcl1 represents a novel and promising method for induction of protective CD8⁺ T‐cell responses.     

Triggering of TLR‐3, ‐4, NOD2, and DC‐SIGN reduces viral replication and increases T‐cell activation capacity of HIV‐infected human dendritic cells

A variety of signals influence the capacity of dendritic cells (DCs) to mount potent antiviral cytotoxic T‐cell (CTL) responses. In particular, innate immune sensing by pathogen recognition receptors, such as TLR and C‐type lectines, influences DC biology and affects their susceptibility to HIV infection. Yet, whether the combined effects of PPRs triggering and HIV infection influence HIV‐specific (HS) CTL responses remain enigmatic. Here, we dissect the impact of innate immune sensing by pathogen recognition receptors on DC maturation, HIV infection, and on the quality of HS CTL activation. Remarkably, ligand‐driven triggering of TLR‐3, ‐4, NOD2, and DC‐SIGN, despite reducing viral replication, markedly increased the capacity of infected DCs to stimulate HS CTLs. This was exemplified by the diversity and the quantity of cytokines produced by HS CTLs primed by these DCs. Infecting DCs with viruses harboring members of the APOBEC family of antiviral factors enhanced the antigen‐presenting skills of infected DCs. Our results highlight the tight interplay between innate and adaptive immunity and may help develop innovative immunotherapies against viral infections.     

DC‐induced CD8+ T‐cell response is inhibited by MHC class II‐dependent DX5+CD4+ Treg

CD4⁺ T cells are important for CD8⁺ T‐cell priming by providing cognate signals for DC maturation. We analyzed the capacity of CD4⁺ T cells to influence CD8⁺ T‐cell responses induced by activated DC. Surprisingly, mice depleted for CD4⁺ cells were able to generate stronger antigen‐specific CD8⁺ T‐cell responses after DC vaccination than non‐depleted mice. The same observation was made when mice were vaccinated with MHC class II^?/? DC, indicating the presence of a MHC class II‐dependent CD4⁺ T‐cell population inhibiting CD8⁺ T‐cell responses. Recently we described the expansion of DX5⁺CD4⁺ T cells, a T‐cell population displaying immune regulatory properties, upon vaccination with DC. Intriguingly, we now observe an inverse correlation between CD8⁺ T‐cell induction and expansion of DX5⁺CD4⁺ T cells as the latter cells did not expand after vaccination with MHC class II^?/? DC. In vitro, DX5⁺CD4⁺ T cells were able to limit proliferation, modulate cytokine production and induce Foxp3⁺ expression in OVA‐specific CD8⁺ T cells. Together, our data show an inhibitory role of CD4⁺ T cells on the induction of CD8⁺ T‐cell responses by activated DC and indicate the involvement of DX5⁺CD4⁺, but not CD4⁺CD25⁺, T cells in this process.     

DC‐SIGN expression on podocytes and its role in inflammatory immune response of lupus nephritis

Podocytes, the main target of immune complex, participate actively in the development of glomerular injury as immune cells. Dendritic cell‐specific intercellular adhesion molecule‐3‐grabbing non‐integrin (DC‐SIGN) is an innate immune molecular that has an immune recognition function, and is involved in mediation of cell adhesion and immunoregulation. Here we explored the expression of DC‐SIGN on podocytes and its role in immune and inflammatory responses in lupus nephritis (LN). Expression of DC‐SIGN and immunoglobulin (Ig)G1 was observed in glomeruli of LN patients. DC‐SIGN was co‐expressed with nephrin on podocytes. Accompanied by increased proteinuria of LN mice, DC‐SIGN and IgG1 expressions were observed in the glomeruli from 20 weeks, and the renal function deteriorated up to 24 weeks. Mice with anti‐DC‐SIGN antibody showed reduced proteinuria and remission of renal function. After the podocytes were stimulated by serum of LN mice in vitro, the expression of DC‐SIGN, major histocompatibility complex (MHC) class II and CD80 was up‐regulated, stimulation of T cell proliferation was enhanced and the interferon (IFN)‐γ/interleukin (IL)‐4 ratio increased. However, anti‐DC‐SIGN antibody treatment reversed these events. These results suggested that podocytes in LN can exert DC‐like function through their expression of DC‐SIGN, which may be involved in immune and inflammatory responses of renal tissues. However, blockage of DC‐SIGN can inhibit immune functions of podocytes, which may have preventive and therapeutic effects.     

Induction of peripheral CD4+ T‐cell tolerance and CD8+ T‐cell cross‐tolerance by dendritic cells

DC can present and cross‐present self‐antigens to autoreactive CD4⁺ and CD8⁺ T cells, respectively, and incapacitate them by inducing anergy, deletion or converting them into Treg. In this review, we summarize the recent progress in immune tolerance research, which has been achieved by employing antigen‐ and TCR‐transgenic mice. We cover the numerous discoveries that have furthered our knowledge of the DC subsets and maturation pathways involved in tolerance; the signals, such as CD70, TGF‐β, B7‐H1/PD‐L1, which dictate the decision between immunity and tolerance; and the in vivo role of DC in the maintenance of CD4⁺ T‐cell tolerance and CD8⁺ T‐cell cross‐tolerance.     

CD70 encoded by modified vaccinia virus Ankara enhances CD8 T‐cell‐dependent protective immunity in MHC class II‐deficient mice

The immunological outcome of infections and vaccinations is largely determined during the initial first days in which antigen‐presenting cells instruct T cells to expand and differentiate into effector and memory cells. Besides the essential stimulation of the T‐cell receptor complex a plethora of co‐stimulatory signals not only ensures a proper T‐cell activation but also instils phenotypic and functional characteristics in the T cells appropriate to fight off the invading pathogen. The tumour necrosis factor receptor/ligand pair CD27/CD70 gained a lot of attention because of its key role in regulating T‐cell activation, survival, differentiation and maintenance, especially in the course of viral infections and cancer. We sought to investigate the role of CD70 co‐stimulation for immune responses induced by the vaccine vector modified vaccinia virus Ankara–Bavarian Nordic^® (MVA‐BN^®). Short‐term blockade of CD70 diminished systemic CD8 T‐cell effector and memory responses in mice. The dependence on CD70 became even more apparent in the lungs of MHC class II‐deficient mice. Importantly, genetically encoded CD70 in MVA‐BN^® not only increased CD8 T‐cell responses in wild‐type mice but also substituted for CD4 T‐cell help. MHC class II‐deficient mice that were immunized with recombinant MVA‐CD70 were fully protected against a lethal virus infection, whereas MVA‐BN^®‐immunized mice failed to control the virus. These data are in line with CD70 playing an important role for vaccine‐induced CD8 T‐cell responses and prove the potency of integrating co‐stimulatory molecules into the MVA‐BN^® backbone.     

The DC‐HIL/syndecan‐4 pathway inhibits human allogeneic T‐cell responses

T‐cell activation is regulated by binding of ligands on APC to corresponding receptors on T cells. In mice, we discovered that binding of DC‐HIL on APC to syndecan‐4 (SD‐4) on activated T cells potently inhibits T‐cell activation. In humans, we now show that DC‐HIL also binds to SD‐4 on activated T cells through recognition of its heparinase‐sensitive saccharide moiety. DC‐HIL blocks anti‐CD3‐induced T‐cell responses, reducing secretion of pro‐inflammatory cytokines and blocking entry into the S phase of the cell cycle. Binding of DC‐HIL phosphorylates SD‐4's intracellular tyrosine and serine residues. Anti‐SD‐4 Ab mimics the ability of DC‐HIL to attenuate anti‐CD3 response more potently than Ab directed against other inhibitory receptors (CTLA‐4 or programmed cell death‐1). Among leukocytes, DC‐HIL is expressed highest by CD14⁺ monocytes and this expression can be upregulated markedly by TGF‐β. Among APC, DC‐HIL is expressed highest by epidermal Langerhans cells, an immature type of dendritic cells. Finally, the level of DC‐HIL expression on CD14⁺ monocytes correlates inversely with allostimulatory capacity, such that treatment with TGF‐β reduced this capacity, whereas knocking down the DC‐HIL gene augmented it. Our findings indicate that the DC‐HIL/SD‐4 pathway can be manipulated to treat T‐cell‐driven disorders in humans.     

IL‐10 promotes homeostatic proliferation of human CD8+ memory T cells and,when produced by CD1c+ DCs,shapes naive CD8+ T‐cell priming

IL‐10 is an anti‐inflammatory cytokine that inhibits maturation and cytokine production of dendritic cells (DCs). Although mature DCs have the unique capacity to prime CD8⁺ CTL, IL‐10 can promote CTL responses. To understand these paradoxic findings, we analyzed the role of IL‐10 produced by human APC subsets in T‐cell responses. IL‐10 production was restricted to CD1c⁺ DCs and CD14⁺ monocytes. Interestingly, it was differentially regulated, since R848 induced IL‐10 in DCs, but inhibited IL‐10 in monocytes. Autocrine IL‐10 had only a weak inhibitory effect on DC maturation, cytokine production, and CTL priming with high‐affinity peptides. Nevertheless, it completely blocked cross‐priming and priming with low‐affinity peptides of a self/tumor‐antigen. IL‐10 also inhibited CD1c⁺ DC‐induced CD4⁺ T‐cell priming and enhanced Foxp3 induction, but was insufficient to induce T‐cell IL‐10 production. CD1c⁺ DC‐derived IL‐10 had also no effect on DC‐induced secondary expansions of memory CTL. However, IL‐15‐driven, TCR‐independent proliferation of memory CTL was enhanced by IL‐10. We conclude that DC‐derived IL‐10 selects high‐affinity CTL upon priming. Moreover, IL‐10 preserves established CTL memory by enhancing IL‐15‐dependent homeostatic proliferation. These combined effects on CTL priming and memory maintenance provide a plausible mechanism how IL‐10 promotes CTL responses in humans.     

Vaccination with an adenoviral vector encoding the tumor antigen directly linked to invariant chain induces potent CD4+ T‐cell‐independent CD8+ T‐cell‐mediated tumor control

Antigen‐specific immunotherapy is an attractive strategy for cancer control. In the context of antiviral vaccines, adenoviral vectors have emerged as a favorable means for immunization. Therefore, we chose a strategy combining use of these vectors with another successful approach, namely linkage of the vaccine antigen to invariant chain (Ii). To evaluate this strategy we used a mouse model, in which an immunodominant epitope (GP33) of the LCMV glycoprotein (GP) represents the tumor‐associated neoantigen. Prophylactic vaccination of C57BL/6 mice with a replication‐deficient human adenovirus 5 vector encoding GP linked to Ii (Ad‐Ii‐GP) resulted in complete protection against GP33‐expressing B16.F10 tumors. Therapeutic vaccination with Ad‐Ii‐GP delayed tumor growth by more than 2 wk compared with sham vaccination. Notably, therapeutic vaccination with the linked vaccine was significantly better than vaccination with adenovirus expressing GP alone (Ad‐GP), or GP and Ii unlinked (Ad‐GP+Ii). Ad‐Ii‐GP‐ induced tumor control depended on an improved generation of the tumor‐associated neoantigen‐specific CD8⁺ T‐cell response and was independent of CD4⁺ T cells. IFN‐γ was shown to be a key player during the tumor degradation. Finally, Ad‐Ii‐GP but not Ad‐GP vaccination can break the immunological non‐reactivity in GP transgenic mice indicating that our vaccine strategy will prove efficient also against endogenous tumor antigens.     

Depletion of CD4+CD25+ regulatory T cells enhances natural killer T cell‐mediated anti‐tumour immunity in a murine mammary breast cancer model

Both invariant natural killer T (NK T) cells and CD4⁺CD25⁺ T regulatory cells (T_regs) regulate the immune system to maintain homeostasis. In a tumour setting, NK T cells activated by α‐galactosylceramide (α‐GalCer) execute anti‐tumour activity by secreting cytokines. By contrast, T_regs intrinsically suppress antigen‐specific immune responses and are often found to be elevated in tumour patients. In this study, we have shown that T_regs regulate NK T cell function negatively in vitro, suggesting a direct interaction between these cell types. In a murine mammary tumour model, we demonstrated that administration of either α‐GalCer or anti‐CD25 antibody alone markedly suppressed tumour formation and pulmonary metastasis, and resulted in an increase in the survival rate up to 44% (from a baseline of 0%). When treatments were combined, depletion of T_regs boosted the anti‐tumour effect of α‐GalCer, and the survival rate jumped to 85%. Our results imply a potential application of combining T_reg cell depletion with α‐GalCer to stimulate NK T cells for cancer therapy.     

Absence of Galectin‐1 accelerates CD8+ T cell‐mediated graft rejection

Galectin‐1 (Gal‐1) is a member of a family of endogenous β‐galactose‐binding proteins with a role in preventing autoimmune diseases and chronic inflammation. In this study, the involvement of Gal‐1 in graft rejection was investigated by using Gal‐1‐deficient mice (Gal‐1?/?). We demonstrate that in the absence of Gal‐1, skin grafts are rejected earlier compared with those of WT mice, and that this is due to the role played by CD8+ T cells in graft rejection. The difference in graft survival observed between Gal‐1?/? and WT mice was explained by both an increase in the percentage of antigen‐specific CD8+ T cells and by preferential secretion of IFN‐γ and IL‐17 by CD8+ T cells in Gal‐1?/? mice compared with WT mice. This study suggests that endogenous expression of Gal‐1 contributes to graft survival. The results obtained from the use of mice deficient in Gal‐1 also confirm a key role for CD8+ T cells in graft rejection.     

CD4+ T‐cell‐mediated anti‐tumor immunity can be uncoupled from autoimmunity via the STAT4/STAT6 signaling axis

Previous reports have suggested that autoimmune sequelae may be an unavoidable consequence of successful immunization against tumor‐associated antigens, which are typically non‐mutated self‐antigens. Using a melanoma model, we demonstrated that CD4⁺ T‐cell‐mediated anti‐tumor immunity and autoimmunity could be separated by modulating the STAT4/STAT6 signaling axis. Our results have revealed an unexpected dichotomy in the effector phase following cancer vaccination where anti‐tumor immunity is mediated via a STAT6 and IL‐4‐dependent pathway, whereas autoimmune pathology is mediated via STAT4 through a mechanism that relies partially on IFN‐γ. Our results offer a possibility to elicit specific anti‐tumor responses without triggering unwanted tissue autoimmune diseases.     

DC‐SIGN reacts with TLR‐4 and regulates inflammatory cytokine expression via NF‐κB activation in renal tubular epithelial cells during acute renal injury

In the pathological process of acute kidney injury (AKI), innate immune receptors are essential in inflammatory response modulation; however, the precise molecular mechanisms are still unclear. Our study sought to demonstrate the inflammatory response mechanisms in renal tubular epithelial cells via Toll‐like receptor‐4 (TLR‐4) and dendritic cell‐specific intercellular adhesion molecule 3‐grabbing non‐integrin 1 (DC‐SIGN) signalling. We found that DC‐SIGN exhibited strong expression in renal tubular epithelial cells of human acute renal injury tissues. DC‐SIGN protein expression was increased significantly when renal tubular epithelial cells were exposed to lipopolysaccharide (LPS) for a short period. Furthermore, DC‐SIGN was involved in the activation of p65 by TLR‐4, which excluded p38 and c‐Jun N‐terminal kinases (JNK). Interleukin (IL)‐6 and tumour necrosis factor (TNF)‐α expression was decreased after DC‐SIGN knock‐down, and LPS induced endogenous interactions and plasma membrane co‐expression between TLR‐4 and DC‐SIGN. These results show that DC‐SIGN and TLR‐4 interactions regulate inflammatory responses in renal tubular epithelial cells and participate in AKI pathogenesis.     

Enhancing immunity prevents virus‐induced T‐cell‐mediated immunopathology in B cell‐deficient mice

Hyper‐activated or deviated immune responses can result in immunopathological diseases. Paradoxically, immunodeficiency represents a frequent cause of such immune‐mediated pathologies. Immunopathological manifestations are commonly treated by immunosuppression, but in situations in which immunodeficiency is the basis of disease development, enhancing immunity may represent an alternative treatment option. Here, we tested this counterintuitive concept in a preclinical model using infection of mice with lymphocytic choriomeningitis virus (LCMV). Firstly, we demonstrate that infection of B‐cell‐deficient (B^?/?) but not of wild‐type (WT) mice with the LCMV strain Docile induced a rapid and fatal CD8⁺ T‐cell‐mediated immunopathological disease. Similar to WT mice, LCMV‐infected B^?/? mice generated a potent, functional LCMV‐specific CD8⁺ T‐cell response but exhibited prolonged viral antigen presentation and increased vascular leakage in liver and lungs. Secondly, we were able to prevent this virus‐induced immunopathology in B^?/? mice by active or passive T‐cell immunizations or by treatment with LCMV‐specific virus neutralizing or non‐neutralizing monoclonal antibodies (mAb). Thus, boosting antiviral immunity did not aggravate immunopathology in this model, but prevented it by decreasing the formation of target structures for damage‐causing CD8⁺ T cells.     

IFN‐γ‐receptor signaling ameliorates transplant vasculopathy through attenuation of CD8+ T‐cell‐mediated injury of vascular endothelial cells

Occlusive transplant vasculopathy (TV) is the major cause for chronic graft rejection. Since endothelial cells (EC) are the first graft cells encountered by activated host lymphocytes, it is important to delineate the molecular mechanisms that coordinate the interaction of EC with activated T cells. Here, the interaction of CD8⁺ T cells with Ag‐presenting EC in vivo was examined using a transgenic heart transplantation model with β‐galactosidase (β‐gal) expression exclusively in EC (Tie2‐LacZ hearts). We found that priming with β‐gal peptide‐loaded DC failed to generate a strong systemic IFN‐γ response, but elicited pronounced TV in both IFN‐γ receptor (IFNGR)‐competent, and ifngr^?/? Tie2‐LacZ hearts. In contrast, stimulation of EC‐specific CD8⁺ T cells with β‐gal‐recombinant mouse cytomegalovirus (MCMV‐LacZ) in recipients of ifngr^+/+ Tie2‐LacZ hearts did not precipitate significant TV. However, MCMV‐LacZ infection of recipients of ifngr^?/? Tie2‐LacZ hearts led to massive activation of β‐gal‐specific CD8 T cells, and led to development of fulminant TV. Further analyses revealed that the strong systemic IFN‐γ “storm” associated with MCMV infection induced upregulation of programmed death‐1 ligand 1 (PD‐L1) on EC, and subsequent attenuation of programmed death‐1 (PD‐1)‐expressing EC‐specific CD8⁺ T cells. Thus, IFNGR signaling in ECs activates a potent peripheral negative feedback circuit that protects vascularized grafts from occlusive TV.     

Pathways of memory CD8+ T‐cell development

CD8⁺ T‐cell responses must have at least two components, a replicative cell type that proliferates in the secondary lymphoid tissue and that is responsible for clonal expansion, and cytotoxic cells with effector functions that mediate the resolution of the infection in the peripheral tissues. To confer memory, the response must also generate replication‐competent T cells that persist in the absence of antigen after the primary infection is cleared. The current models of memory differentiation differ in regards to whether or not memory CD8⁺ T cells acquire effector functions during their development. In this review we discuss the existing models for memory development and the consequences that the recent finding that memory CD8⁺ T cells may express granzyme B during their development has for them. We propose that memory CD8⁺ T cells represent a self‐renewing population of T cells that may acquire effector functions but that do not lose the naïve‐like attributes of lymphoid homing, antigen‐independent persistence or the capacity for self‐renewal.     

Apoptotic cell capture by DCs induces unexpectedly robust autologous CD4+ T‐cell responses

Apoptotic cells represent an important source of self‐antigens and their engulfment by dendritic cells (DCs) is usually considered to be related to tolerance induction. We report here an unexpectedly high level of human CD4⁺ T‐cell proliferation induced by autologous DCs loaded with autologous apoptotic cells, due to the activation of more than 10% of naive CD4⁺ T cells. This proliferation is not due to an increase in the costimulatory capacity of DCs, but is dependent on apoptotic cell‐associated material processed through an endo‐lysosomal pathway and presented on DC MHC class II molecules. Autologous CD4⁺ T cells stimulated with apoptotic cell‐loaded DCs exhibit suppressive capacities. However, in the presence of bacterial lipopolysaccharide, apoptotic cell‐loaded DCs induce the generation of IL‐17‐producing cells. Thus, apoptotic cell engulfment by DCs may lead to increased autologous responses, initially generating CD4⁺ T cells with suppressive capacities able to differentiate into Th17 cells in the presence of a bacterial danger signal such as LPS.     

Membrane‐bound Dickkopf‐1 in Foxp3+ regulatory T cells suppresses T‐cell‐mediated autoimmune colitis

Induction of tolerance is a key mechanism to maintain or to restore immunological homeostasis. Here we show that Foxp3⁺ regulatory T (Treg) cells use Dickkopf‐1 (DKK‐1) to regulate T‐cell‐mediated tolerance in the T‐cell‐mediated autoimmune colitis model. Treg cells from DKK‐1 hypomorphic doubleridge mice failed to control CD4⁺ T‐cell proliferation, resulting in CD4 T‐cell‐mediated autoimmune colitis. Thymus‐derived Treg cells showed a robust expression of DKK‐1 but not in naive or effector CD4 T cells. DKK‐1 expression in Foxp3⁺ Treg cells was further increased upon T‐cell receptor stimulation in vitro and in vivo. Interestingly, Foxp3⁺ Treg cells expressed DKK‐1 in the cell membrane and the functional inhibition of DKK‐1 using DKK‐1 monoclonal antibody abrogated the suppressor function of Foxp3⁺ Treg cells. DKK‐1 expression was dependent on de novo protein synthesis and regulated by the mitogen‐activated protein kinase pathway but not by the canonical Wnt pathway. Taken together, our results highlight membrane‐bound DKK‐1 as a novel Treg‐derived mediator to maintain immunological tolerance in T‐cell‐mediated autoimmune colitis.