Major histocompatibility complex class II expression and hemagglutinin subtype influence the infectivity of type A influenza virus for respiratory dendritic cells

Dendritic cells (DC) play a key role in antiviral immunity, functioning both as innate effector cells in early phases of the immune response and subsequently as antigen-presenting cells that activate the adaptive immune response. In the murine respiratory tract, there are several respiratory dendritic cell (RDC) subsets, including CD103(+) DC, CD11b(hi) DC, monocyte/macrophage DC, and plasmacytoid DC. However, little is known about the interaction between these tissue-resident RDC and viruses that are encountered during natural infection in the respiratory tract. Here, we show both in vitro and in vivo that the susceptibility of murine RDC to infection with type A influenza virus varies with the level of MHC class II expression by RDC and with the virus strain. Both CD103(+) and CD11b(hi) RDC, which express the highest basal level of major histocompatibility complex (MHC) class II, are highly susceptible to infection by type A influenza virus. However, efficient infection is restricted to type A influenza virus strains of the H2N2 subtype. Furthermore, enhanced infectivity by viruses of the H2N2 subtype is linked to expression of the I-E MHC class II locus product. These results suggest a potential novel role for MHC class II molecules in influenza virus infection and pathogenesis in the respiratory tract.     

Respiratory Dendritic Cell Subsets Differ in Their Capacity to Support the Induction of Virus-Specific Cytotoxic CD8+ T Cell Responses

Dendritic cells located at the body surfaces, e.g. skin, respiratory and gastrointestinal tract, play an essential role in the induction of adaptive immune responses to pathogens and inert antigens present at these surfaces. In the respiratory tract, multiple subsets of dendritic cells (RDC) have been identified in both the normal and inflamed lungs. While the importance of RDC in antigen transport from the inflamed or infected respiratory tract to the lymph nodes draining this site is well recognized, the contribution of individual RDC subsets to this process and the precise role of migrant RDC within the lymph nodes in antigen presentation to T cells is not clear. In this report, we demonstrate that two distinct subsets of migrant RDC - exhibiting the CD103⁺ and CD11b^hi phenotype, respectively - are the primary DC presenting antigen to naïve CD4⁺ and CD8⁺ T lymphocytes in the draining nodes in response to respiratory influenza virus infection. Furthermore, the migrant CD103⁺ RDC subset preferentially drives efficient proliferation and differentiation of naive CD8⁺ T cells responding to infection into effector cells, and only the CD103⁺ RDC subset can present to naïve CD8⁺ T cells non-infectious viral vaccine introduced into the respiratory tract. These results identify CD103⁺ and CD11b^hi RDC as critical regulators of the adaptive immune response to respiratory tract infection and potential targets in the design of mucosal vaccines.     

Regulation of T helper cell differentiation by respiratory tract dendritic cells.

Studies on dendritic cells (DC) of the respiratory and gastric mucosae have identified an extensive network of cells that represent the predominant antigen-presenting cell type at these sites. Under steady-state conditions, respiratory tract DC (RTDC) are specialized for antigen uptake and spontaneously migrate to local lymph nodes, although in vivo transfer studies have shown that the T-cell priming activity of these cells is restricted to low-level, Th2-skewed responses. Following exposure to inflammatory stimuli, the migration of RTDC to lymph nodes is accelerated and is associated with a rapid and dramatic increase in the ability of these cells to induce both Th1- and Th2-dependent immunity. Under normal circumstances, however, responsiveness of epithelial RTDC to maturation stimuli is regulated by locally produced micro-environmental factors, including pro-inflammatory cytokines, reactive oxygen species and prostanoids. These studies have led to a greater understanding of airway DC function and their role in T helper cell differentiation and provide the basis for future studies to determine the role of the cells in the aetiology and pathogenesis of respiratory immunoinflammatory disorders.     

Central role of dendritic cells in shaping the adaptive immune response during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract disease in young children. Premature infants, immunocompromised individuals and the elderly exhibit the highest risk for the development of severe RSV-induced disease. Murine studies demonstrate that CD8 T cells mediate RSV clearance from the lungs. Murine studies also indicate that the host immune response contributes to RSV-induced morbidity as T-cell depletion prevents the development of disease despite sustained viral replication. Dendritic cells (DCs) play a central role in the induction of the RSV-specific adaptive immune response. Following RSV infection, lung-resident DCs acquire viral antigens, migrate to the lung-draining lymph nodes and initiate the T-cell response. This article focuses on data generated from both in vitro DC infection studies and RSV mouse models that together have advanced our understanding of how RSV infection modulates DC function and the subsequent impact on the adaptive immune response.     

Dendritic cell dysfunction in cancer: a mechanism for immunosuppression

Several reports have demonstrated that tumours are not intrinsically resistant to the immune response. However, neoplasias commonly fail to initiate and maintain adequate immunity. A number of factors have been implicated in causing the failure, including aberrant antigen processing by tumour cells, anergy or deletion of T cells, and recruitment of inhibitory/regulatory cell types. It has been suggested that dysfunction of dendritic cells (DC) induced by the tumour is one of the critical mechanisms to escape immune surveillance. As a minor subset of leucocytes, DC are the key APC for initiating immune responses. DC are poised at the boundaries of the periphery and the inner tissues, sampling antigens of diverse origin. Following their encounter with antigen or danger signals, DC migrate to lymph nodes, where they activate effector cells essential for tumour clearance. Although the DC system is highly heterogeneous, the differentiation and function of DC populations is largely regulated by exogenous factors. Malignancies appear to exploit this by producing a plethora of immunosuppressive factors capable of affecting DC, thus exerting systemic effects on immune function. This review examines recent findings on the effects of tumour-derived factors inducing DC dysfunction and in particular examines the findings on alteration of DC differentiation, maturation and longevity as a potent mechanism for immune suppression in cancer.     

Perfluorocarbon particle size influences magnetic resonance signal and immunological properties of dendritic cells

The development of cellular tracking by fluorine ((19)F) magnetic resonance imaging (MRI) has introduced a number of advantages for following immune cell therapies in vivo. These include improved signal selectivity and a possibility to correlate cells labeled with fluorine-rich particles with conventional anatomic proton ((1)H) imaging. While the optimization of the cellular labeling method is clearly important, the impact of labeling on cellular dynamics should be kept in mind. We show by (19)F MR spectroscopy (MRS) that the efficiency in labeling cells of the murine immune system (dendritic cells) by perfluoro-15-crown-5-ether (PFCE) particles increases with increasing particle size (560>365>245>130 nm). Dendritic cells (DC) are professional antigen presenting cells and with respect to impact of PFCE particles on DC function, we observed that markers of maturation for these cells (CD80, CD86) were also significantly elevated following labeling with larger PFCE particles (560 nm). When labeled with these larger particles that also gave an optimal signal in MRS, DC presented whole antigen more robustly to CD8+ T cells than control cells. Our data suggest that increasing particle size is one important feature for optimizing cell labeling by PFCE particles, but may also present possible pitfalls such as alteration of the immunological status of these cells. Therefore depending on the clinical scenario in which the (19)F-labeled cellular vaccines will be applied (cancer, autoimmune disease, transplantation), it will be interesting to monitor the fate of these cells in vivo in the relevant preclinical mouse models.     

Changes in dendritic cell migration and activation during SIV infection suggest a role in initial viral spread and eventual immunosuppression

Dendritic cells (DC) serve an essential function in linking the innate and acquired immune responses to antigen. Peripheral DC acquire antigen and migrate to draining lymph nodes, where they localize to the T cell-rich paracortex and function as potent antigen presenting cells. We examined the effects of human immunodeficiency virus (HIV) infection on DC function in vivo using the rhesus macaque/simian immunodeficiency virus (SIV) model. Our data show that during acute SIV infection, Langerhans cell density is reduced in skin and activated DC are increased in proportion in lymph nodes, whereas during AIDS, DC migration from skin and activation within lymph nodes are suppressed. These findings suggest that changes in DC function at different times during the course of infection may serve to promote virus dissemination and persistence: early during infection, DC mobilization may facilitate virus spread to susceptible lymph node T cell populations, whereas depressed DC function during advanced infection could promote generalized immunosuppression.     

Respiratory syncytial virus infection of monocyte-derived dendritic cells decreases their capacity to activate CD4 T cells

Respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract infections in children, the elderly, and immune-compromised individuals. CD4 and CD8 T cells play a crucial role in the elimination of RSV from the infected lung, but T cell memory is not sufficient to completely prevent reinfections. The nature of the adaptive immune response depends on innate immune reactions initiated after interaction of invading pathogens with host APCs. For respiratory pathogens myeloid dendritic cell (DC) precursors that are located underneath the epithelial cell layer lining the airways may play a crucial role in primary activation of T cells and regulating their functional potential. In this study, we investigated the role of human monocyte-derived DC in RSV infection. We showed that monocyte-derived DC can be productively infected, which results in maturation of the DC judged by the up-regulation of CD80, CD83, CD86, and HLA class II molecules. However, RSV infection of DC caused impaired CD4 T cell activation characterized by a lower T cell proliferation and ablation of cytokine production in activated T cells. The suppressive effect was caused by an as yet unidentified soluble factor produced by RSV-infected DC.     

Murine spleen contains a diversity of myeloid and dendritic cells distinct in antigen presenting function

The spleen contains multiple subsets of myeloid and dendritic cells (DC). DC are important antigen presenting cells (APC) which induce and control the adaptive immune response. They are cells specialized for antigen capture, processing and presentation to naïve T cells. However, DC are a heterogeneous population and each subset differs subtly in phenotype, function and location. Similarly, myeloid cell subsets can be distinguished which can also play an important role in the regulation of immunity. This review aims to characterize splenic subsets of DC and myeloid cells to better understand their individual roles in the immune response.     

Preserved MHC-II antigen processing and presentation function in chronic HCV infection

Individuals with chronic HCV infection have impaired response to vaccine, though the etiology remains to be elucidated. Dendritic cells (DC) and monocytes (MN) provide antigen uptake, processing, presentation, and costimulatory functions necessary to achieve optimal immune responses. The integrity of antigen processing and presentation function within these antigen presenting cells (APC) in the setting of HCV infection has been unclear. We used a novel T cell hybridoma system that specifically measures MHC-II antigen processing and presentation function of human APC. Results demonstrate MHC-II antigen processing and presentation function is preserved in both myeloid DC (mDC) and MN in the peripheral blood of chronically HCV-infected individuals, and indicates that an alteration in this function does not likely underlie the defective HCV-infected host response to vaccination.     

Mature dendritic cells infected with herpes simplex virus type 1 exhibit inhibited T-cell stimulatory capacity

Mature dendritic cells (DC) are the most potent antigen-presenting cells within the entire immune system. Interference with the function of these cells therefore constitutes a very powerful mechanism for viruses to escape immune responses. Several members of the Herpesviridae family have provided examples of such escape strategies, including interference with antigen presentation and production of homologous cytokines. In this study we investigated the infection of mature DC with herpes simplex virus type 1 (HSV-1) and the way in which infection alters the phenotype and function of mature DC. Interestingly, the T-cell-stimulatory capacity of these DC was strongly impaired. Furthermore, we demonstrated that HSV-1 leads to the specific degradation of CD83, a cell surface molecule which is specifically upregulated during DC maturation. These data indicate that HSV-1 has developed yet another novel mechanism to escape immune responses.     

Induction of anti-chlamydial mucosal immunity by transcutaneous immunization is enhanced by topical application of GM-CSF

Transcutaneous immunization (TCI) involves the direct application of antigen plus adjuvant to skin, taking advantage of the large numbers of Langerhans cells and other resident skin dendritic cells, that process antigen then migrate to draining lymph nodes where immune responses are initiated. We have used this form of immunization to protect mice against genital tract and respiratory tract chlamydial infection. Protection was associated with local antibody responses in the vagina, uterus and lung as well as strong Th1 responses in the lymph nodes draining the reproductive tract and lungs respectively. In this study we show that topical application of GM-CSF to skin enhances the numbers and activation status of epidermal dendritic cells. Topical application of GM-CSF also increased the immune responses elicited by TCI. GM-CSF supplementation greatly increased cytokine (IFNgamma and IL-4) gene expression in lymph node and splenic cells compared to cells from animals immunized without GM-CSF. IgG responses in serum, uterine lavage and bronchoalveolar lavage and IgA responses in vaginal lavage were also increased by topical application of GM-CSF. The studies show that TCI induces protection against genital and respiratory tract chlamydial infections and that topical application of cytokines such as GM-CSF can enhance TCI-induced antibody and cell-mediated immunity.     

CD8+ T Cell Priming by Dendritic Cell Vaccines Requires Antigen Transfer to Endogenous Antigen Presenting Cells

Background

Immunotherapeutic strategies to stimulate anti-tumor immunity are promising approaches for cancer treatment. A major barrier to their success is the immunosuppressive microenvironment of tumors, which inhibits the functions of endogenous dendritic cells (DCs) that are necessary for the generation of anti-tumor CD8+ T cells. To overcome this problem, autologous DCs are generated ex vivo, loaded with tumor antigens, and activated in this non-suppressive environment before administration to patients. However, DC-based vaccines rarely induce tumor regression.

Methodology/Principal Findings

We examined the fate and function of these DCs following their injection using murine models, in order to better understand their interaction with the host immune system. Contrary to previous assumptions, we show that DC vaccines have an insignificant role in directly priming CD8+ T cells, but instead function primarily as vehicles for transferring antigens to endogenous antigen presenting cells, which are responsible for the subsequent activation of T cells.

Conclusions/Significance

This reliance on endogenous immune cells may explain the limited success of current DC vaccines to treat cancer and offers new insight into how these therapies can be improved. Future approaches should focus on creating DC vaccines that are more effective at directly priming T cells, or abrogating the tumor induced suppression of endogenous DCs.     

Dendritic Cells: Migratory Cells that are Attractive

Dendritic cells (DC) are professional antigen presenting cells, playing an important role in the initiation of T- and T cell dependent immune responses. DC are highly mobile cells and the sequential migration of DC in and out of tissues is accompanied by phenotypical as well as functional changes instrumental to their function as sentinels of the immune system. Herein, we will review recent progress in understanding the origin of DC, their migratory behaviour and their capacity to attract and interact with lymphocytes, with emphasis on the chemokine system.     

CD8+ T cell-dependent elimination of dendritic cells in vivo limits the induction of antitumor immunity

The fate of dendritic cells (DC) after they have initiated a T cell immune response is still undefined. We have monitored the migration of DC labeled with a fluorescent tracer and injected s.c. into naive mice or into mice with an ongoing immune response. DC not loaded with Ag were detected in the draining lymph node in excess of 7 days after injection with maximum numbers detectable approximately 40 h after transfer. In contrast, DC that had been loaded with an MHC class I-binding peptide disappeared from the lymph node with kinetics that parallel the known kinetics of activation of CD8+ T cells to effector function. In the presence of high numbers of specific CTL precursors, as in TCR transgenic mice, DC numbers were significantly decreased by 72 h after injection. The rate of DC disappearance was extremely rapid and efficient in recently immunized mice and was slower in "memory" mice in which memory CD8+ cells needed to reacquire effector function before mediating DC elimination. We also show that CTL-mediated clearance of Ag-loaded DC has a notable effect on immune responses in vivo. Ag-specific CD8+ T cells failed to divide in response to Ag presented on a DC if the DC were targets of a pre-existing CTL response. The induction of antitumor immunity by tumor Ag-loaded DC was also impaired. Therefore, CTL-mediated clearance of Ag-loaded DC may serve as a negative feedback mechanism to limit the activity of DC within the lymph node.     

MicroRNAs regulate dendritic cell differentiation and function

MicroRNAs (miRNAs) are an important class of cellular regulators that modulate gene expression and thereby influence cell fate and function. In the immune system, miRNAs act at checkpoints during hematopoietic development and cell subset differentiation, they modulate effector cell function, and they are implicated in the maintenance of homeostasis. Dendritic cells (DCs), the professional APCs involved in the coordination of adaptive immune responses, are also regulated by miRNAs. Some DC-relevant miRNAs, including miR-155 and miR-146a, are shared with other immune cells, whereas others have been newly identified. In this review, we summarize the current understanding of where miRNAs are active during DC development from myeloid precursors and differentiation into specialized subsets, and which miRNAs play roles in DC function.     

Cigarette smoke exposure impairs dendritic cell maturation and T cell proliferation in thoracic lymph nodes of mice

Respiratory tract dendritic cells (DCs) are juxtaposed to directly sample inhaled environmental particles. Processing and presentation of these airborne Ags could result in either the development of immunity or tolerance. The purpose of this study was to determine the consequences of cigarette smoke exposure on DC function in mice. We demonstrate that while cigarette smoke exposure decreased the number of DCs in the lungs, Ag-induced DC migration to the regional thoracic lymph nodes was unaffected. However, cigarette smoking suppressed DC maturation within the lymph nodes as demonstrated by reduced cell surface expression of MHC class II and the costimulatory molecules CD80 and CD86. Consequently, DCs from cigarette smoke-exposed animals had a diminished capacity to induce IL-2 production by T cells that was associated with diminished Ag-specific T cell proliferation in vivo. Smoke-induced defects in DC function leading to impaired CD4(+) T cell function could inhibit tumor surveillance and predispose patients with chronic obstructive pulmonary disease to infections and exacerbations.     

Immunity to Salmonella from a dendritic point of view

Dendritic cells (DC) are the key link between innate and adaptive immunity. Features of DC, including their presence at sites of antigen entry, their ability to migrate from peripheral sites to secondary lymphoid organs, and their superior capacity to stimulate naïve T cells places them in this pivotal role in the immune system. DC also produce cytokines, particularly IL‐12, upon antigen encounter and can thus influence the ensuing adaptive immune response. As DC are phagocytic antigen‐presenting cells located at sites exposed to bacterial invaders, studies have been performed to gain insight into the role of DC in combating bacterial infections. Indeed, studies with Salmonella have shown that DC can internalize and process this bacterium for peptide presentation on MHC‐II as well as MHC‐I. DC can also act as bystander antigen‐­presenting cells by presenting Salmonella antigens after internalizing neighbouring cells that have undergone Salmonella‐induced apoptotic death. DC also produce IL‐12 and TNF‐α upon Salmonella encounter. Moreover, studies in a murine infection model have shown that splenic DC increase surface expression of co‐stimulatory molecules during infection, and DC contain intracellular bacteria. In addition, quantitative changes occur in splenic DC numbers in the early stages of oral Salmonella infection, and this is accompanied by redistribution of the defined DC subsets in the spleen of infected mice. DC from Salmonella‐infected mice also produce cytokines and can stimulate bacteria‐specific T cells upon ex vivo co‐culture. In addition, DC may play a role in the traversal of bacteria from the intestinal lumen. Studying the function of DC during Salmonella infection provides insight into the capacity of this sophisticated antigen‐presenting cell to initiate and modulate the immune response to bacteria.     

DC-SIGN and CD150 have distinct roles in transmission of measles virus from dendritic cells to T-lymphocytes

Measles virus (MV) is among the most infectious viruses that affect humans and is transmitted via the respiratory route. In macaques, MV primarily infects lymphocytes and dendritic cells (DCs). Little is known about the initial target cell for MV infection. Since DCs bridge the peripheral mucosal tissues with lymphoid tissues, we hypothesize that DCs are the initial target cells that capture MV in the respiratory tract and transport the virus to the lymphoid tissues where MV is transmitted to lymphocytes. Recently, we have demonstrated that the C-type lectin DC-SIGN interacts with MV and enhances infection of DCs in cis. Using immunofluorescence microscopy, we demonstrate that DC-SIGN+ DCs are abundantly present just below the epithelia of the respiratory tract. DC-SIGN+ DCs efficiently present MV-derived antigens to CD4+ T-lymphocytes after antigen uptake via either CD150 or DC-SIGN in vitro. However, DC-SIGN+ DCs also mediate transmission of MV to CD4+ and CD8+ T-lymphocytes. We distinguished two different transmission routes that were either dependent or independent on direct DC infection. DC-SIGN and CD150 are both involved in direct DC infection and subsequent transmission of de novo synthesized virus. However, DC-SIGN, but not CD150, mediates trans-infection of MV to T-lymphocytes independent of DC infection. Together these data suggest a prominent role for DCs during the initiation, dissemination, and clearance of MV infection.