Major histocompatibility complex class II presentation of cell-associated antigen is mediated by CD8alpha+ dendritic cells in vivo

Intestinal cDC1s provide IL-12 dependent and independent functions required for CD4 + T cell-mediated resistance to Cryptosporidium

Cryptosporidium is an enteric pathogen that is a prominent cause of diarrheal disease. Control of this infection requires CD4 + T cells, though the processes that lead to T cell-mediated resistance have been difficult to assess. Here, Cryptosporidium parasites that express MHCII-restricted model antigens were generated to dissect the early events that influence CD4 + T cell priming and effector function. These studies highlight that parasite-specific CD4 + T cells are primed in the draining mesenteric lymph node (mesLN) and differentiate into Th1 cells in the gut, where they mediate IFN-γ-dependent control of the infection. Although type 1 conventional dendritic cells (cDC1s) were not required for initial priming of CD4 + T cells, cDC1s were required for CD4 + T cell expansion and gut homing. cDC1s were also a major source of IL-12 that was not required for priming but promoted full differentiation of CD4 + T cells and local production of IFN-γ. Together, these studies reveal distinct roles for cDC1s in shaping CD4 + T cell responses to enteric infection: first to drive early expansion in the mesLN and second to drive effector responses in the gut.

Summary

Cryptosporidium parasites that express model antigens were generated to dissect how parasite-specific CD4 + T cells are primed and mediate effector functions required to control this enteric pathogen. cDC1s produced IL-12p40 and were required for early expansion and gut homing of CD4 + T cells. However, IL-12p40 was only required for the development of Th1 CD4 + T cell effector function in the gut.

Emerging strategies in targeting tumor-resident myeloid cells for cancer immunotherapy

Immune checkpoint inhibitors targeting programmed cell death protein 1, programmed death-ligand 1, and cytotoxic T-lymphocyte-associated protein 4 provide deep and durable treatment responses which have revolutionized oncology. However, despite over 40% of cancer patients being eligible to receive immunotherapy, only 12% of patients gain benefit. A key to understanding what differentiates treatment response from non-response is better defining the role of the innate immune system in anti-tumor immunity and immune tolerance. Teleologically, myeloid cells, including macrophages, dendritic cells, monocytes, and neutrophils, initiate a response to invading pathogens and tissue repair after pathogen clearance is successfully accomplished. However, in the tumor microenvironment (TME), these innate cells are hijacked by the tumor cells and are imprinted to furthering tumor propagation and dissemination. Major advancements have been made in the field, especially related to the heterogeneity of myeloid cells and their function in the TME at the single cell level, a topic that has been highlighted by several recent international meetings including the 2021 China Cancer Immunotherapy workshop in Beijing. Here, we provide an up-to-date summary of the mechanisms by which major myeloid cells in the TME facilitate immunosuppression, enable tumor growth, foster tumor plasticity, and confer therapeutic resistance. We discuss ongoing strategies targeting the myeloid compartment in the preclinical and clinical settings which include: (1) altering myeloid cell composition within the TME; (2) functional blockade of immune-suppressive myeloid cells; (3) reprogramming myeloid cells to acquire pro-inflammatory properties; (4) modulating myeloid cells via cytokines; (5) myeloid cell therapies; and (6) emerging targets such as Siglec-15, TREM2, MARCO, LILRB2, and CLEVER-1. There is a significant promise that myeloid cell-based immunotherapy will help advance immuno-oncology in years to come.

Dendritic cells in cancer immunology

The clinical success of immune checkpoint therapy (ICT) has produced explosive growth in tumor immunology research because ICT was discovered through basic studies of immune regulation. Much of the current translational efforts are aimed at enhancing ICT by identifying therapeutic targets that synergize with CTLA4 or PD1/PD-L1 blockade and are solidly developed on the basis of currently accepted principles. Expanding these principles through continuous basic research may help broaden translational efforts. With this mindset, we focused this review on three threads of basic research directly relating to mechanisms underlying ICT. Specifically, this review covers three aspects of dendritic cell (DC) biology connected with antitumor immune responses but are not specifically oriented toward therapeutic use. First, we review recent advances in the development of the cDC1 subset of DCs, identifying important features distinguishing these cells from other types of DCs. Second, we review the antigen-processing pathway called cross-presentation, which was discovered in the mid-1970s and remains an enigma. This pathway serves an essential in vivo function unique to cDC1s and may be both a physiologic bottleneck and therapeutic target. Finally, we review the longstanding field of helper cells and the related area of DC licensing, in which CD4 T cells influence the strength or quality of CD8 T cell responses. Each topic is connected with ICT in some manner but is also a fundamental aspect of cell-mediated immunity directed toward intracellular pathogens.

cDC1 prime and are licensed by CD4+ T cells to induce anti-tumour immunity

Conventional type 1 dendritic cells (cDC1)1 are thought to perform antigen cross-presentation, which is required to prime CD8+ T cells2,3, whereas cDC2 are specialized for priming CD4+ T cells4,5. CD4+ T cells are also considered to help CD8+ T cell responses through a variety of mechanisms6-11, including a process whereby CD4+ T cells 'license' cDC1 for CD8+ T cell priming12. However, this model has not been directly tested in vivo or in the setting of help-dependent tumour rejection. Here we generated an Xcr1Cre mouse strain to evaluate the cellular interactions that mediate tumour rejection in a model requiring CD4+ and CD8+ T cells. As expected, tumour rejection required cDC1 and CD8+ T cell priming required the expression of major histocompatibility class I molecules by cDC1. Unexpectedly, early priming of CD4+ T cells against tumour-derived antigens also required cDC1, and this was not simply because they transport antigens to lymph nodes for processing by cDC2, as selective deletion of major histocompatibility class II molecules in cDC1 also prevented early CD4+ T cell priming. Furthermore, deletion of either major histocompatibility class II or CD40 in cDC1 impaired tumour rejection, consistent with a role for cognate CD4+ T cell interactions and CD40 signalling in cDC1 licensing. Finally, CD40 signalling in cDC1 was critical not only for CD8+ T cell priming, but also for initial CD4+ T cell activation. Thus, in the setting of tumour-derived antigens, cDC1 function as an autonomous platform capable of antigen processing and priming for both CD4+ and CD8+ T cells and of the direct orchestration of their cross-talk that is required for optimal anti-tumour immunity.

Transfer of antigen from human B cells to dendritic cells

The cooperation of B lymphocytes with other antigen presenting cells (APCs) is often necessary in the efficient processing and presentation of antigen. Herein, we describe a mechanism by which B cells physically interact with dendritic cells (DCs) resulting in the transfer of B cell receptor (BCR)-enriched antigen to these APCs. Antigen transfer involves direct contact between the two cells followed by the capture of B cell derived membrane and intracellular components. Strikingly, DCs acquire greater amounts of antigen by transfer from B cells than by endocytosis of free antigen. Blocking scavenger receptor A, a DC surface receptor involved in membrane acquisition, abrogates these events. We propose that antigen transfer from B cells to DCs results in a more focused immunologic response due to the selective editing of Ag by the BCR.

Peripheral CD4(+) T-cell tolerance is induced in vivo by rare antigen-bearing B cells in follicular, marginal zone, and B-1 subsets

B cells are efficient APCs when they internalize antigen via BCR-mediated uptake. Adoptively transferred antigen-presenting B cells can induce T-cell tolerance to foreign and self antigens; however, it is unknown whether endogenous B cells presenting self-peptides interact with naïve T cells and contribute to peripheral T-cell self-tolerance. Moreover, the relative abilities of mature B-cell subsets to induce T-cell tolerance have not been examined. To address these questions, we created a new mouse model wherein a very small fraction of B cells expresses an antigen transgene that cannot be transferred to other APCs. We limited antigen expression to follicular, marginal zone, or B-1 B-cell subsets and found that small numbers of each subset interacted with naïve antigen-specific T cells. Although antigen expressed by B-1 B cells induced the most T-cell division, divided T cells subsequently disappeared from secondary lymphoid tissues. Independent of which B-cell subset presented antigen, the remaining T cells were rendered hypo-responsive, and this effect was not associated with Foxp3 expression. Our data show that physiologically relevant proportions of B cells can mediate peripheral T-cell tolerance, and suggest that the mechanisms of tolerance induction might differ among follicular, marginal zone, and B-1 B-cell subsets.

Antigen cross-presentation: extending recent laboratory findings to therapeutic intervention

The initiation of adaptive immune responses requires antigen presentation to lymphocytes. In particular, dendritic cells (DCs) are equipped with specialized machinery that promote effective display of peptide/major histocompatibility complexes (MHC), rendering them the most potent stimulators of naive T lymphocytes. Antigen cross-presentation to CD8(+) T cells is an important mechanism for the development of specific cytotoxic T lymphocyte (CTL) responses against tumours and viruses that do not infect antigen-presenting cells. Here, we review recent findings concerning antigen cross-presentation to CD8(+) T lymphocytes. Specific subtypes of DCs in the mouse have been defined as being especially endowed for antigen cross-presentation, and a human homologue of these DCs has recently been described. DC vaccination strategies for the prevention and treatment of human diseases have been under investigation in recent years, but have not generally reached satisfying results. We here provide an overview of new findings in antigen cross-presentation research and how they can be used for development of the next generation of human DC vaccines.

Monoclonal antibodies directed to CD20 and HLA-DR can elicit homotypic adhesion followed by lysosome-mediated cell death in human lymphoma and leukemia cells

mAbs are becoming increasingly utilized in the treatment of lymphoid disorders. Although Fc-FcgammaR interactions are thought to account for much of their therapeutic effect, this does not explain why certain mAb specificities are more potent than others. An additional effector mechanism underlying the action of some mAbs is the direct induction of cell death. Previously, we demonstrated that certain CD20-specific mAbs (which we termed type II mAbs) evoke a nonapoptotic mode of cell death that appears to be linked with the induction of homotypic adhesion. Here, we reveal that peripheral relocalization of actin is critical for the adhesion and cell death induced by both the type II CD20-specific mAb tositumomab and an HLA-DR-specific mAb in both human lymphoma cell lines and primary chronic lymphocytic leukemia cells. The cell death elicited was rapid, nonapoptotic, nonautophagic, and dependent on the integrity of plasma membrane cholesterol and activation of the V-type ATPase. This cytoplasmic cell death involved lysosomes, which swelled and then dispersed their contents, including cathepsin B, into the cytoplasm and surrounding environment. The resulting loss of plasma membrane integrity occurred independently of caspases and was not controlled by Bcl-2. These experiments provide what we believe to be new insights into the mechanisms by which 2 clinically relevant mAbs elicit cell death and show that this homotypic adhesion-related cell death occurs through a lysosome-dependent pathway.

Editing antigen presentation: antigen transfer between human B lymphocytes and macrophages mediated by class A scavenger receptors

B lymphocytes can function independently as efficient APCs. However, our previous studies demonstrate that both dendritic cells and macrophages are necessary to propagate immune responses initiated by B cell APCs. This finding led us to identify a process in mice whereby Ag-specific B cells transfer Ag to other APCs. In this study, we report the ability and mechanism by which human B lymphocytes can transfer BCR-captured Ag to macrophages. The transfer of Ag involves direct contact between the two cells followed by the capture of B cell-derived membrane and/or intracellular components by the macrophage. These events are abrogated by blocking scavenger receptor A, a receptor involved in the exchange of membrane between APCs. Macrophages acquire greater amounts of Ag in the presence of specific B cells than in their absence. This mechanism allows B cells to amplify or edit the immune response to specific Ag by transferring BCR-captured Ag to other professional APCs, thereby increasing the frequency of its presentation. Ag transfer may perpetuate chronic autoimmune responses to specific self-proteins and help explain the efficacy of B cell-directed therapies in human disease.

Targeting of antigens to B cells augments antigen-specific T-cell responses and breaks immune tolerance to tumor-associated antigen MUC1

B cells are antibody (Ab)-secreting cells as well as potent antigen (Ag)-presenting cells that prime T-cell activation, which evokes great interest in their use for vaccine development. Here, we targeted ovalbumin (OVA) to B cells via CD19 and found that a single low dose of anti-CD19-OVA conjugates, but not isotype mAb-OVA, stimulated augmented CD4 and CD8 T-cell proliferation and expansion. Administration of TLR9 agonist CpG could significantly enhance long-term T-cell survival. Similar results were obtained when the tumor-associated Ag MUC1 was delivered to B cells. MUC1 transgenic (Tg) mice were previously found to lack effective T-cell help and produce low-titer of anti-MUC1 Abs after vaccination. Targeting MUC1 to B cells elicited high titer of anti-MUC1 Abs with different isotypes, predominantly IgG2a and IgG2b, in MUC1 Tg mice. The isotype switching of anti-MUC1 Ab was CD4 dependent. In addition, IFN-gamma-producing CD8 T cells and in vivo cytolytic activity were significantly increased in these mice. The mice also showed significant resistance to MUC1(+) lymphoma cell challenge both in the prophylactic and therapeutic settings. We conclude that Ags targeting to B cells stimulate CD4 and CD8 T-cell responses as well as Th-dependent humoral immune responses.

Tumor therapy in mice via antigen targeting to a novel, DC-restricted C-type lectin

The mouse CD8alpha+ DC subset excels at cross-presentation of antigen, which can elicit robust CTL responses. A receptor allowing specific antigen targeting to this subset and its equivalent in humans would therefore be useful for the induction of antitumor CTLs. Here, we have characterized a C-type lectin of the NK cell receptor group that we named DC, NK lectin group receptor-1 (DNGR-1). DNGR-1 was found to be expressed in mice at high levels by CD8+ DCs and at low levels by plasmacytoid DCs but not by other hematopoietic cells. Human DNGR-1 was also restricted in expression to a small subset of blood DCs that bear similarities to mouse CD8alpha+ DCs. The selective expression pattern and observed endocytic activity of DNGR-1 suggested that it could be used for antigen targeting to DCs. Consistent with this notion, antigen epitopes covalently coupled to an antibody specific for mouse DNGR-1 were selectively cross-presented by CD8alpha+ DCs in vivo and, when given with adjuvants, induced potent CTL responses. When the antigens corresponded to tumor-expressed peptides, treatment with the antibody conjugate and adjuvant could prevent development or mediate eradication of B16 melanoma lung pseudometastases. We conclude that DNGR-1 is a novel, highly specific marker of mouse and human DC subsets that can be exploited for CTL cross-priming and tumor therapy.

Nramp1 expression by dendritic cells modulates inflammatory responses during Salmonella Typhimurium infection

Host resistance against Salmonella enterica serovar Typhimurium (S. Typhimurium) is mediated by natural resistance-associated macrophage protein 1 (Nramp1/Slc11a1). Nramp1 is critical to host defence, as mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. Despite this crucial role, the mechanisms underlying Nramp1's protective effects are unclear. Dendritic cells (DCs) that sample the intestinal lumen are among the first cells encountered by S. Typhimurium following oral infection and act as a conduit for S. Typhimurium to cross the intestinal epithelial barrier. We report that DCs, including intestinal, splenic and bone marrow-derived DCs (BMDCs), express Nramp1 protein. In the small intestine, Nramp1 expression is greater in a subset of DCs (CD11c(+)CD103(-)) characterized by the elevated expression of pro-inflammatory cytokines in response to bacterial products. While Nramp1 expression did not affect S. Typhimurium replication in BMDCs, infected Nramp1+/+ BMDCs and intestinal CD11c(+)CD103(-) DCs secreted more inflammatory cytokines (IL-6, IL-12 and TNF-alpha) than Nramp1-/-, suggesting that Nramp1 expression may promote a more rapid inflammatory response following infection. Collectively, these findings reveal a new role for DCs and Nramp1 in modulating the host inflammatory response to S. Typhimurium.

Bystander B cells rapidly acquire antigen receptors from activated B cells by membrane transfer

The B cell antigen receptor (BCR) efficiently facilitates the capture and processing of a specific antigen for presentation on MHC class II molecules to antigen-specific CD4(+) T cells (1). Despite this, the majority of B cells are thought to play only a limited role in CD4(+) T cell activation because BCRs are clonotypically expressed. Here, we show, however, that activated B cells can, both in vitro and in vivo, rapidly donate their BCR to bystander B cells, a process that is mediated by direct membrane transfer between adjacent B cells and is amplified by the interaction of the BCR with a specific antigen. This results in a dramatic expansion in the number of antigen-binding B cells in vivo, with the transferred BCR endowing recipient B cells with the ability to present a specific antigen to antigen-specific CD4(+) T cells.

Multiple dendritic cell populations activate CD4+ T cells after viral stimulation

Dendritic cells (DC) are a heterogeneous cell population that bridge the innate and adaptive immune systems. CD8α DC play a prominent, and sometimes exclusive, role in driving amplification of CD8⁺ T cells during a viral infection. Whether this reliance on a single subset of DC also applies for CD4⁺ T cell activation is unknown. We used a direct ex vivo antigen presentation assay to probe the capacity of flow cytometrically purified DC populations to drive amplification of CD4⁺ and CD8⁺ T cells following infection with influenza virus by different routes. This study examined the contributions of non-CD8α DC populations in the amplification of CD8⁺ and CD4⁺ T cells in cutaneous and systemic influenza viral infections. We confirmed that in vivo, effective immune responses for CD8⁺ T cells are dominated by presentation of antigen by CD8α DC but can involve non-CD8α DC. In contrast, CD4⁺ T cell responses relied more heavily on the contributions of dermal DC migrating from peripheral lymphoid tissues following cutaneous infection, and CD4 DC in the spleen after systemic infection. CD4⁺ T cell priming by DC subsets that is dependent upon the route of administration raises the possibility that vaccination approaches could be tailored to prime helper T cell immunity.

Lymphocytes genetically modified to express tumor antigens target DCs in vivo and induce antitumor immunity

The exploitation of the physiologic processing and presenting machinery of DCs by in vivo loading of tumor-associated antigens may improve the immunogenic potential and clinical efficacy of DC-based cancer vaccines. Here we show that lymphocytes genetically modified to express self/tumor antigens, acting as antigen carriers, efficiently target DCs in vivo in tumor-bearing mice. The infusion of tyrosinase-related protein 2-transduced (TRP-2-transduced) lymphocytes induced the establishment of protective immunity and long-term memory in tumor-bearing mice. Analysis of the mechanism responsible for the induction of such an immune response allowed us to demonstrate that cross-presentation of the antigen mediated by the CD11c(+)CD8alpha(+) DC subset had occurred. Furthermore, we demonstrated in vivo and in vitro that DCs had undergone activation upon phagocytosis of genetically modified lymphocytes, a process mediated by a cell-to-cell contact mechanism independent of CD40 triggering. Targeting and activation of secondary lymphoid organ-resident DCs endowed antigen-specific T cells with full effector functions, which ultimately increased tumor growth control and animal survival in a therapeutic tumor setting. We conclude that the use of transduced lymphocytes represents an efficient method for the in vivo loading of tumor-associated antigens on DCs.

Unique functions of splenic CD8alpha+ dendritic cells during infection with intracellular pathogens

Deciphering the prerequisites for the induction of protective cytotoxic T cell responses is essential for future development of more effective CD8(+) T cell-based vaccines against infectious diseases and cancer. Since crucial events for CD8(+) T cell priming and differentiation occur during the first contacts of naïve T cells with distinct antigen-presenting cells (APCs), the identification and therapeutic targeting of these 'master' APCs has become a major quest in the field. A decade ago, dendritic cells (DCs) were discovered as potent APCs, as they combine all major features for the initiation of T cell responses: (1) naïve DCs demonstrate high endocytic activity and scan continuously their environment in strategic positions throughout the whole body; (2) after activation (e.g. during pathogen invasion), DCs migrate into T cell zones of their draining lymphatic compartments, meanwhile processing captured antigen and maturing in order to stimulate encountered antigen-specific T cells. During the last years, different subsets of DCs that can be distinguished by specific surface marker expression and effector functions have been identified in mice. Their distinct functional capabilities have led to the concept of work-sharing; "migrating" DCs primarily transport antigens to the lymph node, where a specialized subset of "resident" DCs, defined by the expression of the CD8alphaalpha homodimer (CD8alpha(+) DCs), primes CD8(+) T cells upon antigen cross-presentation. Accordingly, CD8alpha(+) DCs have been found to prime CD8(+) T cells against different viruses as well as intracellular bacteria such as Listeria monocytogenes (L.m.). Recently, L.m. was found to survive specifically in splenic CD8alpha(+) DCs shortly after intravenous infection. Further experiments revealed a more generalized sampling activity of CD8alpha(+) DCs for blood-borne particles. These findings indicate that splenic CD8alpha(+) DCs might unite efficacious antigen-trapping with the licence to prime CD8(+) T cells. This new aspect of DC function could have evolved to guarantee a more rapid antigen-specific response against generalized infections.

Antigen presentation and transfer between B cells and macrophages

B cells play an active role in directing immunity against specific proteins in part because of their capacity to sequester antigen via B cell receptor (BCR). Our prior findings indicate that B cells can initiate an immune response in vivo to self proteins independent of other antigen-presenting cells (APC). However, these studies also demonstrated that both dendritic cells and macrophages are important in the ongoing immune response. The present work illustrates a mechanism by which antigen acquired by B cells through BCR is specifically transferred to other APC, in particular, macrophages. The transfer of antigen is dependent on the specificity of BCR and requires direct contact between the cells, but does not require MHC compatibility between the cells and is independent of the activation state of macrophages. Antigen transfer is functional, in that macrophages, which received B cell derived-antigen, can activate CD4 T cells. Overall, these results define a novel mechanism by which B cells can focus immunity toward a specific antigen and transfer the ability to activate CD4 T cells to other APC.

Intrinsic and cooperative antigen-presenting functions of dendritic-cell subsets in vivo

Dendritic cells (DCs) comprise several subsets, and their roles in the presentation of antigens derived from pathogens, vaccines and self tissues are now beginning to be elucidated. Differences in location, life cycle and intrinsic abilities to capture, process and present antigens on their MHC class I and class II molecules enable each DC subset to have distinct roles in immunity to infection and in the maintenance of self tolerance. Unexpected interactions among DC subsets have also been revealed. These interactions, which allow the integration of the intrinsic abilities of different DC types, enhance the ability of the DC network to respond to multiple scenarios of infection.

Transcriptional targeting of B cells for induction of peripheral CD8 T cell tolerance

Several mechanisms are in place to neutralize autoimmune CD8 T cells by tolerance induction. Developing self-specific CD8 T cells are eliminated in the thymus by Ag-presenting epithelial and dendritic cells (DCs). However, CD8 T cells escaping thymic central tolerance can also be inactivated by tolerance mechanisms in peripheral organs. In contrast to DCs, the role of B cells in generating CD8 T cell tolerance is not well-characterized. To investigate this question in more detail, we transcriptionally targeted Ag to B cells using B cell-specific retroviral vectors in vivo. Although Ag expression could be detected in B cells of thymus, lymph nodes, and spleen, B cells were unable to induce central tolerance of CD8 thymocytes. In contrast, in peripheral organs, we could identify clonal deletion and functional inhibition (anergy) of CD8 T cells as tolerance-inducing mechanisms. Although Ag expressed by B cells was acquired and cross-presented by DCs, B cells were also sufficient to tolerize CD8 T cells directly. These findings suggest exploitation of B cells for Ag-specific immunotherapy of CD8 T cell-mediated autoimmune diseases.

Life cycle, migration and antigen presenting functions of spleen and lymph node dendritic cells: limitations of the Langerhans cells paradigm

The phenotypic and functional studies carried out in recent years on dendritic cells (DC) purified from spleen and lymph nodes has revealed the existence of heterogeneous populations with distinct life cycles, migratory properties and antigen presenting functions. A major subdivision can be made between "tissue derived" DC that migrate to the lymph nodes from peripheral tissues, both in the steady state and in the course of infections, and "blood-derived" DC, which reside in the spleen and lymph nodes throughout their life cycle. These two groups of DC can be subdivided into smaller subsets. The tissue-derived and the blood-derived DC also show fundamental differences in maturational status and antigen presenting capabilities. In this review, we summarize the roles played by the different DC types in the steady state and during pathogen infections, relating those roles to maintenance of peripheral tolerance and the induction of immunity. We point out the caveats of assuming that the DC that collect antigens are the ones involved in their presentation, emphasizing the phenomenon of antigen transfer as an important component of the immune response.

CD8α+ Dendritic Cells Are Required for Efficient Entry of Listeria monocytogenes into the Spleen

In addition to their bridging function between innate and adaptive immunity, dendritic cells (DCs) may also contribute to primary resistance against infection. Here we analyzed the role of DCs during infection with Listeria monocytogenes by performing systemic in vivo depletion of these cells. We showed that CD8alpha(+) DCs were crucial for L. monocytogenes spreading and proliferation in the spleen. Efficient and rapid uptake of L. monocytogenes by CD8alpha(+) DCs required the small GTPase Rac1 and is a general characteristic of this DC subpopulation in filtering particles out of the blood. Thus, CD8alpha(+) DCs appear to play an important role for efficient bacterial entry into the spleen, which is of relevance for subsequent immune responses.

Preferential Induction of CD4+ T Cell Responses through In Vivo Targeting of Antigen to Dendritic Cell-Associated C-Type Lectin-1

Targeting of Ags and therapeutics to dendritic cells (DCs) has immense potential for immunotherapy and vaccination. Because DCs are heterogeneous, optimal targeting strategies will require knowledge about functional specialization among DC subpopulations and identification of molecules for targeting appropriate DCs. We characterized the expression of a fungal recognition receptor, DC-associated C-type lectin-1 (Dectin-1), on mouse DC subpopulations and investigated the ability of an anti-Dectin-1 Ab to deliver Ag for the stimulation of immune responses. Dectin-1 was shown to be expressed on CD8alpha-CD4-CD11b+ DCs found in spleen and lymph nodes and dermal DCs present in skin and s.c. lymph nodes. Injection of Ag-anti-Dectin-1 conjugates induced CD4+ and CD8+ T cell and Ab responses at low doses where free Ag failed to elicit a response. Notably, qualitatively different immune responses were generated by targeting Ag to Dectin-1 vs CD205, a molecule expressed on CD8alpha+CD4-CD11b- DCs, dermal DCs, and Langerhans cells. Unlike anti-Dectin-1, anti-CD205 conjugates failed to elicit an Ab response. Moreover, when conjugates were injected i.v., anti-Dectin-1 stimulated a much stronger CD4+ T cell response and a much weaker CD8+ T cell response than anti-CD205. The results reveal Dectin-1 as a potential targeting molecule for immunization and have implications for the specialization of DC subpopulations.

The dominant role of CD8+ dendritic cells in cross-presentation is not dictated by antigen capture

Mouse spleens contain three populations of conventional (CD11c(high)) dendritic cells (DCs) that play distinct functions. The CD8(+) DC are unique in that they can present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. It is unclear whether this special ability is because only the CD8(+) DC can capture the antigens used in cross-presentation assays, or because this is the only DC population that possesses specialized machinery for cross-presentation. To solve this important question we examined the splenic DC subsets for their ability to both present via MHC class II molecules and cross-present via MHC class I using four different forms of the model antigen ovalbumin (OVA). These forms include a cell-associated form, a soluble form, OVA expressed in bacteria, or OVA bound to latex beads. With the exception of bacterial antigen, which was poorly cross-presented by all DC, all antigenic forms were cross-presented much more efficiently by the CD8(+) DC. This pattern could not be attributed simply to a difference in antigen capture because all DC subsets presented the antigen via MHC class II. Indeed, direct assessments of endocytosis showed that CD8(+) and CD8(-) DC captured comparable amounts of soluble and bead-associated antigen, yet only the CD8(+) DC cross-presented these antigenic forms. Our results indicate that cross-presentation requires specialized machinery that is expressed by CD8(+) DC but largely absent from CD8(-) DC. This conclusion has important implications for the design of vaccination strategies based on antigen targeting to DC.

CD8+ recent thymic emigrants home to and efficiently repopulate the small intestine epithelium

Prevailing knowledge dictates that naive alphabeta T cells require activation in lymphoid tissues before differentiating into effector or memory T cells capable of trafficking to nonlymphoid tissues. Here we demonstrate that CD8(+) recent thymic emigrants (RTEs) migrated directly into the small intestine. CCR9, CCL25 and alpha(4)beta(7) integrin were required for gut entry of CD8(+) RTEs. After T cell receptor stimulation, intestinal CD8(+) RTEs proliferated and acquired a surface phenotype resembling that of intraepithelial lymphocytes. CD8(+) RTEs efficiently populated the gut of lymphotoxin-alpha-deficient mice, which lack lymphoid organs. These studies challenge the present understanding of naive alphabeta T cell trafficking and suggest that RTEs may be involved in maintaining a diverse immune repertoire at mucosal surfaces.

ICSBP/IRF-8 differentially regulates antigen uptake during dendritic-cell development and affects antigen presentation to CD4+ T cells

Interferon consensus sequence-binding protein (ICSBP)/interferon regulatory factor 8 (IRF-8) is a transcription factor that plays critical roles in the differentiation of defined dendritic-cell (DC) populations and in the immune response to many pathogens. In this study, we show that splenic DCs (s-DCs) from ICSBP(-/-) mice are markedly defective in their ability to capture and to present exogenous antigens (Ags) to naive CD4(+) T lymphocytes. We found that CD8alpha(+) DCs and, to a lesser extent, CD8alpha(-) DCs from ICSBP(-/-) mice are impaired at internalizing Ags, either through a receptor-mediated pathway or by macropinocytosis, in spite of having a more immature phenotype than their wild-type (WT) counterparts. These features reflected a greatly impaired ability of ICSBP(-/-) s-DCs to present injected soluble ovalbumin (OVA) to OVA-specific CD4(+) T cells in vivo. Conversely, bone marrow (BM)-derived DCs from ICSBP(-/-) mice, in keeping with their immature phenotype, exhibited higher endocytic activity than WT cells. However, Ag-loaded ICSBP(-/-) BM-DCs were defective in priming Ag-specific CD4(+) T lymphocytes and failed to induce a contact hypersensitivity (CHS) response when injected into competent WT hosts. Together, these results indicate that, throughout the developmental program of DCs, ICSBP differentially controls Ag uptake and MHC class II (MHC-II) presentation affecting both functions only in differentiated peripheral DCs.

The calcineurin phosphatase complex modulates immunogenic B cell responses

A series of signal-directed transitions regulates the development of distinct populations of self-tolerant B cells and ultimately the production of antibody-producing plasma cells. We studied the role of calcineurin/NFAT signaling in B cells by deleting the regulatory b1 subunit of calcineurin specifically in B cells. Follicular (FO) and marginal zone (MZ) B cells develop normally in these mice, but B1 cell numbers are reduced. In vitro, calcineurin b1-deficient B cells have a cell-intrinsic proliferation defect downstream of the B cell receptor. These mice have higher total serum IgM despite the absence of B1 cells and have enhanced T cell-independent-1 responses. Conversely, mice with calcineurin b1-deficient B cells develop larger germinal centers and have reduced plasma cell development and antigen-specific antibody production during T cell-dependent immune responses. By several different criteria, calcineurin is dispensable for B cell tolerance, indicating that this phosphatase complex modulates immunogenic, but not tolerogenic, responses in vivo.

Dendritic cells: key cells for the induction of regulatory T cells?

Even though dendritic cells (DCs) are well known for their capacity to induce immune responses, recent results show that they are also involved in the induction of tolerance. These two contrary effects of otherwise homologous DCs on a developing immune response maybe explainedby different DC developmental stages, i.e., different subsets of DCs may exist and/or different spatial distribution of DCs in the body might influence their function. However, independently from the subtype(s), it is obvious that the ability of DCs to act in a tolerogenic fashion depends on the maturation status, since immature DCs are prone to induce regulatory T cells and hence promote tolerance, whereas mature DCs stimulate effector T cells, facilitating immunity. The means by which DCs convey tolerance are not entirely clear yet, but secretion of suppressive cytokines such as IL-10 and induction of regulatory lymphocytes are involved. In this review we focus on the interaction between DCs and T cells and highlight some mechanisms in the decision-making process of whether immunity or tolerance is induced.

CELL BIOLOGY OF ANTIGEN PROCESSING IN VITRO AND IN VIVO

The conversion of exogenous and endogenous proteins into immunogenic peptides recognized by T lymphocytes involves a series of proteolytic and other enzymatic events culminating in the formation of peptides bound to MHC class I or class II molecules. Although the biochemistry of these events has been studied in detail, only in the past few years has similar information begun to emerge describing the cellular context in which these events take place. This review thus concentrates on the properties of antigen-presenting cells, especially those aspects of their overall organization, regulation, and intracellular transport that both facilitate and modulate the processing of protein antigens. Emphasis is placed on dendritic cells and the specializations that help account for their marked efficiency at antigen processing and presentation both in vitro and, importantly, in vivo. How dendritic cells handle antigens is likely to be as important a determinant of immunogenicity and tolerance as is the nature of the antigens themselves.

Toll-like receptor 3 promotes cross-priming to virus-infected cells

Cross-presentation of cell-associated antigens plays an important role in regulating CD8+ T cell responses to proteins that are not expressed by antigen-presenting cells (APCs). Dendritic cells are the principal cross-presenting APCs in vivo and much progress has been made in elucidating the pathways that allow dendritic cells to capture and process cellular material. However, little is known about the signals that determine whether such presentation ultimately results in a cytotoxic T cell (CTL) response (cross-priming) or in CD8+ T cell inactivation (cross-tolerance). Here we describe a mechanism that promotes cross-priming during viral infections. We show that murine CD8alpha+ dendritic cells are activated by double-stranded (ds)RNA present in virally infected cells but absent from uninfected cells. Dendritic cell activation requires phagocytosis of infected material, followed by signalling through the dsRNA receptor, toll-like receptor 3 (TLR3). Immunization with virus-infected cells or cells containing synthetic dsRNA leads to a striking increase in CTL cross-priming against cell-associated antigens, which is largely dependent on TLR3 expression by antigen-presenting cells. Thus, TLR3 may have evolved to permit cross-priming of CTLs against viruses that do not directly infect dendritic cells.

B-Lymphocytes activated by CD40 ligand induce an antigen-specific anti-tumour immune response by direct and indirect activation of CD8(+) T-cells

In this report, we describe the ability of CD40-ligand (CD40L)-activated, antigen-loaded B-cells to initiate antigen-specific anti-tumour immune responses in vivo. Mice immunized by means of intravenous administration of CD40L-activated B-cells loaded with an MHC class-I-binding peptide, and challenged with a tumour cell line expressing the same class-I epitope, showed a marked delay in tumour growth, compared to non-immunized controls or to mice receiving either freshly isolated B-cells or B-cells activated with lipopolysaccharide or interleukin-4. The ability of CD40L-activated B-cells to induce antigen-specific T-cell activation appeared to be through a combination of cross-presentation of antigen from activated B-cells to resident antigen-presenting cells and direct T-cell activation by the administered B-cells themselves. Immunization with CD40L-activated B-cells may, therefore, represent a means by which to stimulate anti-tumour CD8(+) T-cell responses in vivo.

Regulation of Antigen Presentation and Cross-Presentation in the Dendritic Cell Network: Facts, Hypothesis, and Immunological Implications

Dendritic cells (DCs) are central to the maintenance of immunological tolerance and the initiation and control of immunity. The antigen-presenting properties of DCs enable them to present a sample of self and foreign proteins, contained within an organism at any given time, to the T-cell repertoire. DCs achieve this communication with T cells by displaying antigenic peptides bound to MHC I and MHC II molecules. Here we review the studies carried out over the past 15 years to characterize these antigen presentation mechanisms, emphasizing their significance in relation to DC function in vivo. The life cycles of different DC populations found in vivo are described. Furthermore, we provide a critical assessment of the studies that examine the mechanisms controlling DC MHC class II antigen presentation, which have often reached contradictory conclusions. Finally, we review findings pertaining to the biological mechanisms that enable DCs to present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. Throughout, we highlight what we consider to be major knowledge gaps in the field and speculate on possible directions for future research.

Cross-presentation, dendritic cell subsets, and the generation of immunity to cellular antigens

Cross-presentation involves the uptake and processing of exogenous antigens within the major histocompatibility complex (MHC) class I pathway. This process is primarily performed by dendritic cells (DCs), which are not a single cell type but may be divided into several distinct subsets. Those expressing CD8alpha together with CD205, found primarily in the T-cell areas of the spleen and lymph nodes, are the major subset responsible for cross-presenting cellular antigens. This ability is likely to be important for the generation of cytotoxic T-cell immunity to a variety of antigens, particularly those associated with viral infection, tumorigenesis, and DNA vaccination. At present, it is unclear whether the CD8alpha-expressing DC subset captures antigen directly from target cells or obtains it indirectly from intermediary DCs that traffic from peripheral sites. In this review, we examine the molecular basis for cross-presentation, discuss the role of DC subsets, and examine the contribution of this process to immunity, with some emphasis on DNA vaccination.

Flt3-ligand, IL-4, GM-CSF, and adherence-mediated isolation of murine lung dendritic cells: assessment of isolation technique on phenotype and function

Lung dendritic cells (DCs) are difficult to study due to their limited quantities and the complexities required for isolation. Although many procedures have been used to overcome this challenge, the effects of isolation techniques on lung DCs have not been reported. The current study shows that freshly isolated DCs (CD11c+) have limited ability to induce proliferation in allogeneic T cells, and are immature as indicated by low cell surface expression of costimulatory molecules compared with liver or splenic DCs. DCs isolated after overnight culture or from mice treated with Flt3L are phenotypically mature and potent stimulators of allogeneic T cells. DCs could not be propagated from lung mononuclear cells in response to IL-4 and GM-CSF. Contrary to data reported for nonpulmonary DCs, expression of CCR6 was decreased on mature lung DCs, and only a subset of mature DCs expressed higher levels of CCR7. Absence of CD8alpha expression indicates that freshly isolated DCs are myeloid-type, whereas mature DCs induced by overnight culture are both "lymphoid" (CD8alpha+) and "myeloid" (CD8alpha-). DCs from mice genetically deficient in CD8alpha expression were strong simulators of allogeneic T cells which was consistent with data showing that CD8alpha- DCs from CD8alpha-sufficient mice are better APCs compared with CD8alpha+ DCs from the same mice. These data show that freshly isolated lung DCs are phenotypically and functionally distinct, and that the isolation technique alters the biology of these cells. Therefore, lung DC phenotype and function must be interpreted relative to the technique used for isolation.

Inhibition of phosphatidylserine recognition heightens the immunogenicity of irradiated lymphoma cells in vivo

Strategies to enhance the immunogenicity of tumors are urgently needed. Although vaccination with irradiated dying lymphoma cells recruits a tumor-specific immune response, its efficiency as immunogen is poor. Annexin V (AxV) binds with high affinity to phosphatidylserine on the surface of apoptotic and necrotic cells and thereby impairs their uptake by macrophages. Here, we report that AxV preferentially targets irradiated lymphoma cells to CD8⁺ dendritic cells for in vivo clearance, elicits the release of proinflammatory cytokines and dramatically enhances the protection elicited against the tumor. The response was endowed with both memory, because protected animals rejected living lymphoma cells after 72 d, and specificity, because vaccinated animals failed to reject unrelated neoplasms. Finally, AxV–coupled irradiated cells induced the regression of growing tumors. These data indicate that endogenous adjuvants that bind to dying tumor cells can be exploited to target tumors for immune rejection.

Liver Dendritic Cells Present Bacterial Antigens and Produce Cytokines uponSalmonellaEncounter

The capacity of murine liver dendritic cells (DC) to present bacterial Ags and produce cytokines after encounter with Salmonella was studied. Freshly isolated, nonparenchymal liver CD11c(+) cells had heterogeneous expression of MHC class II and CD11b and a low level of CD40 and CD86 expression. Characterization of liver DC subsets revealed that CD8alpha(-)CD4(-) double negative cells constituted the majority of liver CD11c(+) ( approximately 85%) with few cells expressing CD8alpha or CD4. Flow cytometry analysis of freshly isolated CD11c(+) cells enriched from the liver and cocultured with Salmonella expressing green fluorescent protein (GFP) showed that CD11c(+) MHC class II(high) cells had a greater capacity to internalize Salmonella relative to CD11c(+) MHC class II(low) cells. Moreover, both CD8alpha(-) and CD8alpha(+) liver DC internalized bacteria with similar efficiency after both in vitro and in vivo infection. CD11c(+) cells enriched from the liver could also process Salmonella for peptide presentation on MHC class I and class II to primary, Ag-specific T cells after internalization requiring actin cytoskeletal rearrangements. Flow cytometry analysis of liver CD11c(+) cells infected with Salmonella expressing GFP showed that both CD8alpha(-) and CD8alpha(+) DC produced IL-12p40 and TNF-alpha. The majority of cytokine-positive cells did not contain bacteria (GFP(-)) whereas only a minor fraction of cytokine-positive cells were GFP(+). Furthermore, only approximately 30-50% of liver DC containing bacteria (GFP(+)) produced cytokines. Thus, liver DC can internalize and process Salmonella for peptide presentation to CD4(+) and CD8(+) T cells and elicit proinflammatory cytokine production upon Salmonella encounter, suggesting that DC in the liver may contribute to immunity against hepatotropic bacteria.

Marginal Zone, but Not Follicular B Cells, Are Potent Activators of Naive CD4 T Cells

The early involvement of marginal zone (MZ) B lymphocytes in T-independent immune responses is well established. In this study we compared the abilities of MZ and follicular (FO) B cells to collaborate with T cells. After immunization with soluble hen egg lysozyme, both MZ and FO B cells captured Ag and migrated to T cell areas in the response to hen egg lysozyme. MZ B cells were far superior to FO B cells in inducing CD4+ T cell expansion both in vitro and in vivo. MZ, but not FO, B cells, after interaction with T cells, differentiated into plasma cells, and in addition they stimulated Ag-specific CD4+ T cells to produce high levels of Th1-like cytokines upon primary stimulation in vitro. These results indicate that MZ B cells rapidly and effectively capture soluble Ag and activate CD4+ T cells to become effector T cells. The enhanced capacity of MZ B cells to prime T cells in this study appeared to be intrinsic to MZ B cells, as both MZ and FO B cell populations express an identical Ag receptor.

Immunity toSalmonellafrom 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-alpha 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.

Dendritic cells: regulators of alloimmunity and opportunities for tolerance induction

Dendritic cells (DCs) are uniquely well-equipped antigen-presenting cells (APCs) regarded classically as sentinels of the immune response, which induce and regulate T-cell reactivity. They play critical roles in central tolerance and in the maintenance of peripheral tolerance in the normal steady state. Following cell or organ transplantation, DCs present antigen to T cells via the direct or indirect pathways of allorecognition. These functions of DCs set in train the rejection response, but they also serve as potential targets for suppression of alloimmune reactivity and promotion of tolerance induction. Much evidence from various model systems now indicates that DCs can induce specific T-cell tolerance. Although underlying mechanisms have not been fully elucidated, the capacity to induce T-regulatory cells may be an important property of tolerogenic or regulatory DCs. Efforts to generate "designer" DCs with tolerogenic properties in the laboratory using specific cytokines, immunologic or pharmacologic reagents, or genetic engineering approaches have already met with some success. Alternatively, targeting of DCs in vivo (e.g. by infusion of apoptotic allogeneic cells) to take advantage of their inherent tolerogenicity has also demonstrated exciting potential. The remarkable heterogeneity and plasticity of these important APCs present additional challenges to optimizing DC-based therapies that may lead to improved tolerance-enhancing strategies in the clinic.

Antigen presentation to naive CD4 T cells in the lymph node

Although the presentation of peptide-major histocompatibility complex class II (pMHC class II) complexes to CD4 T cells has been studied extensively in vitro, knowledge of this process in vivo is limited. Unlike the in vitro situation, antigen presentation in vivo takes place within a complex microenvironment in which the movements of antigens, antigen-presenting cells (APCs) and T cells are governed by anatomic constraints. Here we review developments in the areas of lymph node architecture, APC subsets and T cell activation that have shed light on how antigen presentation occurs in the lymph nodes.

The immune response modifier and Toll-like receptor 7 agonist S-27609 selectively induces IL-12 and TNF-alpha production in CD11c+CD11b+CD8- dendritic cells

IL-12 and TNF-alpha production by dendritic cells (DCs) is a critical step in the initiation of local inflammation and adaptive immune responses. We show in this study that a small molecule immune response modifier that is a Toll-like receptor 7 (TLR7) agonist induces IL-12 and TNF-alpha production from murine CD11c(+)CD11b(+)CD8(-) DCs, a subset not previously known for this activity. Stimulation of these DCs through TLR7 in vivo induces significant cytokine production even 12 h after initial stimulation, as well as migration of the DC into T cell zones of the lymphoid tissue. In contrast, stimulation through TLR4 and TLR9 induced IL-12 production predominantly from CD8(+) DCs, consistent with previously published data. All TLR stimuli induced the increase in surface expression of the activation markers B7-1, B7-2, and class II in both CD8(+) and CD8(-) DCs, demonstrating that CD8(+) DCs do respond to TLR7-mediated stimuli. To date this is the only known stimuli to induce preferential cytokine production from CD8(-) DCs. Given the efficacy of TLR7 agonists as antiviral agents, the data collectively indicate that stimulation of CD8(-) DCs through TLR7 most likely plays a role in the generation of antiviral immune responses.

Dendritic cells as inducers of antimicrobial immunity in vivo

Models of infection have provided important insight into the function of dendritic cells (DC) in vivo. Several microbial products induce DC maturation via Toll-like receptors, a process that is crucial for the ability of DC to initiate adaptive immune responses. Splenic DC have also been shown to produce IL-12 during infection in vivo. This DC-derived IL-12 might be important to skew T cell responses towards Th1. Microbial infections also induce changes in the DC populations of lymphoid organs, often in a subset-specific manner, manifested as an accumulation and redistribution of DC. Furthermore, data are emerging pointing at an absolute requirement of DC in priming of naïve T cells in vivo.

The role of dendritic cell subsets in selection between tolerance and immunity

Dendritic cells (DC) are considered nature's adjuvants. They are potent stimulators of naive T cells and key inducers of primary immune responses. In recent times it has become clear that they can also play a central role in the development of T cell tolerance. Further complicating our understanding of DC function is the realization that DC can no longer be viewed as a homogeneous cell type. Rather, they exist as a complex mixture of strikingly different cell populations. The mechanisms that drive the conflicting immunological outcomes of tolerance and immunity have been the subject of intense scrutiny in recent years, most recently in terms of how the various DC subsets are involved in these events. Here we review recent experiments that provide insights into how DC subsets control the outcome of T cell activation and in so doing select between immunity and tolerance induction.

Cross-presentation of cell-associated antigens by CD8alpha+ dendritic cells is attributable to their ability to internalize dead cells

In the mouse, cross-presentation is an exclusive property of the CD8alpha+ subset of dendritic cells (DC) but the basis for this selectivity remains unclear. Here we report that splenic CD8alpha+ DC are much superior to other DC subsets in internalizing dying cells in vitro. In contrast, CD8alpha+, CD8alpha- CD4+ and CD8alpha- CD4- DC subsets phagocytose bacteria or latex beads to a similar extent. Although CD8alpha+ DC are better than CD4+ DC at presenting ovalbumin (OVA)-loaded splenocytes to naïve OT-I T lymphocytes, CD4+ DC are better at presenting OVA-expressing Escherichia coli to the same T cells. In both cases, presentation is abrogated by lactacystin. These results show that both splenic CD8alpha+ and CD8alpha- DC can present exogenous antigens on major histocompatibility complex (MHC) class I via a proteasome-dependent pathway and suggest that the specialized cross-presenting function of CD8alpha+ DC is a result of their ability to endocytose dying cells rather than a unique pathway for handling endosomal contents.

The role of dendritic cells during Salmonella infection

One type of phagocytic antigen-presenting cell (APC) - the dendritic cell (DC) - may have specialized functions during infection with the bacterium Salmonella, including a possible role in transporting Salmonella across the intestinal barrier. In addition, changes in the number, localization and cytokine production of CD8alpha+, CD8alpha-CD4+ and CD8alpha-CD4- DC subsets occur during infection. DCs function in stimulating bacteria-specific T cells by direct presentation of Salmonella antigens and as bystander APCs. Studying the function of DCs during Salmonella infection provides insight into the capacity of these sophisticated APCs, which are a key link between innate and adaptive immunity, to initiate and modulate the immune response to a bacterial infection.

Interactions between dead cells and dendritic cells in the induction of antiviral CTL responses

Dendritic cells - professional antigen-presenting cells - are key players for activating adaptive immune responses against viruses. Apoptosis or lytic cell death often accompanies viral infection. Dendritic cells can acquire infected dead or dying cells as exogenous sources of antigens for presentation on MHC class I and II molecules to initiate T cell responses. This pathway of activating T cells may be critical for the development of effective antiviral immunity in vivo.