Cellular mechanisms governing cross-presentation of exogenous antigens

Phagocytosis: the convoluted way from nutrition to adaptive immunity

Phagocytosis, the process by which cells internalize large particulate materials from their milieu and sequester them in phagosomes, plays a role in a variety of cell functions ranging from nutrition in ameba to innate and adaptive immunity in mammals. Recent findings revealed unexpected characteristics of phagosomes, highlighting how this complex organelle may have evolved, from Dictyostelium to human, to become a key player in our ability to mount an efficient immune response against a variety of intracellular pathogens.

Viral modulation of antigen presentation: manipulation of cellular targets in the ER and beyond

Viruses that establish long-term infections in their hosts have evolved a number of methods to interfere with the activities of the innate and adaptive immune systems. Control of viral infections is achieved in part through the action of cytotoxic T lymphocytes (CTLs) that recognize cytosolically derived antigenic peptides in the context of class I major histocompatibility complex (MHC) molecules. Viral replication within host cells produces abundant proteinaceous fodder for proteasomal digestion and display by class I MHC products. Tactics that disrupt antigen-presentation pathways and prevent the display of peptides to CD8(+) CTLs have been favored during the course of host-virus co-evolution. Viral immunoevasins exploit diverse cellular processes to interfere with host antiviral functions. The study of such viral factors has uncovered novel host proteins that assist these viral factors in their task and that themselves perform important cellular functions. Here, we focus on viral immunoevasins that, together with their cellular targets, interfere with antigen-presentation pathways. In particular, we emphasize the intersection of the cellular quality-control machinery in the endoplasmic reticulum with the herpesvirus proteins that have co-opted it.

Complexity, contradictions, and conundrums: studying post-proteasomal proteolysis in HLA class I antigen presentation

The vast majority of the peptides produced during protein degradation by the cytosolic proteasome-ubiquitin system are consecutively hydrolyzed to single amino acids by multiple cytosolic peptidases preferring intermediate length or short substrates. The small fraction of peptides surviving the aggressive cytosolic environment can be recruited for presentation by major histocompatibility complex (MHC) class I molecules. However, such peptides may frequently have to be adapted to the strict MHC class I-binding requirements by one or several N-terminal-trimming steps. A recent model proposes that an initial step, in which peptides of 15 or more residues are shortened by cytosolic tripeptidylpeptidase II, is followed by additional trimming by cytosolic or endoplasmic reticulum (ER) aminopeptidases. In humans, at least two ER resident aminopeptidases, ERAP1 and ERAP2, contribute to trimming of human leukocyte antigen class I ligands. These interferon-gamma-regulated metallopeptidases show distinct substrate preferences and may have to act in a concerted fashion to remove some complex or longer N-terminal extensions and to trim the full spectrum of precursor peptides. This task is likely facilitated by the formation of presumably heterodimeric ERAP1-2 complexes. RNA interference experiments suggest that both enzymes are important for normal antigen presentation, but precise determination of the extent and the cellular context of their requirement will be left to future experimentation.

Induction of CD8+ T cell responses by Yersinia vaccine carrier strains

Yersinia enterocolitica employs a type III secretion system (TTSS) to target virulence factors (e.g. YopE) into the cytosol of the host cells. We utilized the TTSS to introduce a recombinant antigen directly into the cytosol of host cells and to investigate the potential of Y. enterocolitica and Y. pseudotuberculosis as live carrier for vaccines. The model antigen ovalbumin (Ova) was fused to defined secretion or translocation domains of the Yersinia effector protein YopE and introduced into attenuated mutant strains of Y. enterocolitica and Y. pseudotuberculosis. In vitro experiments showed secretion and translocation of YopE-Ova hybrid proteins into host cells. To investigate the resulting immune responses, mice expressing transgenic Ova-specific T cell receptors were used. Both Y. enterocolitica and Y. pseudotuberculosis mutants induced efficaciously Ova-specific CD8+ T cell responses. The translocation domain of YopE was required for induction of CD8+ T cell responses in vivo, but not for T cell responses induced in vitro. The in vivo frequency of Ova-specific splenic T cells was up to six-fold higher in mice immunized with YopE-Ova-translocating Y. enterocolitica/Y. pseudotuberculosis mutants than in control mice. The Ova-specific T cells were shown to produce high amounts of IFN-gamma. We did not observe significant Ova-specific CD4+ T cell or antibody responses upon vaccination with either of the strains. In conclusion, Yersinia live carrier vaccine strains are suitable to target antigens into the MHC class I pathway and stimulate CD8+ T cell responses and thus, might be useful in vaccine approaches against intracellular pathogens.

IL-6-STAT3 Controls Intracellular MHC Class II αβ Dimer Level through Cathepsin S Activity in Dendritic Cells

We found IL-6-STAT3 pathway suppresses MHC class II (MHCII) expression on dendritic cells (DCs) and attenuates T cell activation. Here, we showed that IL-6-STAT3 signaling reduced intracellular MHCII alphabeta dimmer, Ii, and H2-DM levels in DCs. IL-6-mediated STAT3 activation decreased cystatin C level, an endogenous inhibitor of cathepsins, and enhanced cathepsin activities. Importantly, cathepsin S inhibitors blocked reduction of MHCII alphabeta dimer, Ii, and H2-DM in the IL-6-treated DCs. Overexpression of cystatin C suppressed IL-6-STAT3-mediated increase of cathepsin S activity and reduction of MHCII alphabeta dimer, Ii, and H2-DM levels in DCs. Cathepsin S overexpression in DCs decreased intracellular MHCII alphabeta dimer, Ii, and H2-DM levels, LPS-mediated surface expression of MHCII and suppressed CD4(+) T cell activation. IL-6-gp130-STAT3 signaling in vivo decreased cystatin C expression and MHCII alphabeta dimer level in DCs. Thus, IL-6-STAT3-mediated increase of cathepsin S activity reduces the MHCII alphabeta dimer, Ii, and H2-DM levels in DCs, and suppresses CD4(+) T cell-mediated immune responses.

The structure and location of SIMP/STT3B account for its prominent imprint on the MHC I immunopeptidome

Proteins show drastic discrepancies in their contribution to the collection of self-peptides that shape the repertoire of CD8 T cells (MHC I self-immunopeptidome). To decipher why selected proteins are the foremost sources of MHC I-associated self-peptides, we chose to study SIMP/STT3B because this protein generates very high amounts of MHC I-associated peptides in mice and humans. We show that the endoplasmic reticulum (ER)-associated degradation pathway and MHC I processing intersect at SIMP/STT3B. Relevant key features of SIMP/STT3B are its lysine-rich region, its propensity to misfold and its location in the ER membrane in close proximity to the immunoproteasome. Moreover, we show that coupling to SIMP/STT3B can be used to foster MHC I presentation of a selected peptide, here the ovalbumin peptide SIINFEKL. These data yield novel insights into relations between the cell proteome and the MHC I immunopeptidome. They suggest that the contribution of a given protein to the MHC I immunopeptidome results from the interplay of at least three factors: the presence of degrons (degradation signals), the tendency of the protein to misfold and its subcellular localization. Furthermore, they indicate that substrates of the ER-associated degradation pathway may have a prominent imprint on the MHC I self-immunopeptidome.

PHAGOCYTOSIS: At the Crossroads of Innate and Adaptive Immunity

Phagocytosis, the process by which cells engulf large particles, requires a substantial contribution of membranes. Recent studies have revealed that intracellular compartments, including endocytic organelles and the endoplasmic reticulum (ER), can engage in fusion events with the plasma membrane at the sites of nascent phagosomes. The finding that ER proteins are delivered to phagosomes, where degraded peptides are loaded onto major histocompatibility complex (MHC) class II molecules, has significantly enhanced our understanding of the immune functions associated with these organelles. Although it is well known that pathogens are killed in phagosomes, the contribution of ER proteins to phagosomes has provided a novel pathway for the loading of exogenous peptides onto MHC class I molecules, a process known as cross-presentation. Thus, phagocytosis has evolved from a nutritional function in unicellular organisms to play key roles in both innate and adaptive immunity in vertebrates.

Information sharing and collateral damage

Recent studies have revealed a novel mechanism of antigen cross-presentation by intercellular peptide transfer through gap junctions, which provides new insight about how the immune system recognizes and destroys viruses and tumor cells. At the site of infection or tumorigenesis, gap junctions provide an "information-sharing" channel through which viral- and tumor-derived peptides are transferred to professional antigen-presenting cells (APCs), leading to naïve T-cell stimulation. Similarly, gap-junction-mediated peptide transfer from infected or malignant cells to neighboring cells incurs "collateral damage" by cytotoxic T cells, thereby limiting the spread of viruses or the progression of tumors.

CD8+-T-cell-dependent control of Trypanosoma cruzi infection in a highly susceptible mouse strain after immunization with recombinant proteins based on amastigote surface protein 2

We previously described that DNA vaccination with the gene encoding amastigote surface protein 2 (ASP-2) protects approximately 65% of highly susceptible A/Sn mice against the lethal Trypanosoma cruzi infection. Here, we explored the possibility that bacterial recombinant proteins of ASP-2 could be used to improve the efficacy of vaccinations. Initially, we compared the protective efficacy of vaccination regimens using either a plasmid DNA, a recombinant protein, or both sequentially (DNA priming and protein boosting). Survival after the challenge was not statistically different among the three mouse groups and ranged from 53.5 to 75%. The fact that immunization with a recombinant protein alone induced protective immunity revealed the possibility that this strategy could be pursued for vaccination. We investigated this possibility by using six different recombinant proteins representing distinct portions of ASP-2. The vaccination of mice with glutathione S-transferase fusion proteins representing amino acids 261 to 500 or 261 to 380 of ASP-2 in the presence of the adjuvants alum and CpG oligodeoxynucleotide 1826 provided remarkable immunity, consistently protecting 100% of the A/Sn mice. Immunity was completely reversed by the in vivo depletion of CD8(+) T cells, but not CD4(+) T cells, and was associated with the presence of CD8(+) T cells specific for an epitope located between amino acids 320 and 327 of ASP-2. We concluded that a relatively simple formulation consisting of a recombinant protein with a selected portion of ASP-2, alum, and CpG oligodeoxynucleotide 1826 might be used to cross-prime strong CD8(+)-T-cell-dependent protective immunity against T. cruzi infection.

Dendritic cells, DRiPs, and DALIS in the control of antigen processing

The study of the cell biology of antigen processing and presentation has greatly contributed to our understanding of the immune response. The work of many immunologically inclined cell biologists has also permitted new insights into cellular mechanisms shared by many cell types. Here are described recent and particularly exciting findings on the regulation of major histocompatibility complex class I-restricted presentation in dendritic cells and their contribution to the deciphering of the cellular response to pathogen detection.

Mechanisms of MHC class I-restricted antigen processing and cross-presentation

In this review, we discuss recent data from our laboratory that address two aspects of major histocompatibility complex (MHC) class I-restricted antigen processing. First, we consider the nature of the peptide-loading complex, which is the assembly of proteins in the endoplasmic reticulum (ER) into which newly synthesized MHC class I-beta(2) microglobulin (beta(2)m) heterodimers are incorporated, and the mechanisms involved in MHC class I assembly and peptide loading that are facilitated by the peptide-loading complex. Second, we discuss mechanisms of cross-presentation, the phenomenon whereby extracellular and luminal protein antigens can be processed by antigen-presenting cells, particularly dendritic cells, and presented by MHC class I molecules to CD8(+) T cells. The focus of the discussion is mainly on the human MHC class I system.

Accessory molecules in the assembly of major histocompatibility complex class I/peptide complexes: how essential are they for CD8+ T-cell immune responses?

Assembly of major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum is a highly coordinated process that results in abundant class I/peptide complexes at the cell surface for recognition by CD8(+) T cells and natural killer cells. During the assembly process, a number of chaperones and accessory molecules, such as transporter associated with antigen processing, tapasin, ER60, and calreticulin, assist newly synthesized class I molecules to facilitate loading of antigenic peptides and to optimize the repertoire of surface class I/peptide complexes. This review focuses on the relative importance of these accessory molecules for CD8(+) T-cell responses in vivo and discusses reasons that may help explain why some CD8(+) T-cell responses develop normally in mice deficient in components of class I assembly, despite impaired antigen presentation.

Compartmentalization of class II antigen presentation: contribution of cytoplasmic and endosomal processing

During antigen processing, peptides are generated and displayed in the context of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APCs) to modulate immune responses to foreign antigens and guide self-tolerance. Exogenous and cytoplasmic antigens are processed by distinct routes within APCs to yield class II ligands. Exogenous antigens are internalized, processed, and bound to class II molecules within endosomal and lysosomal compartments of APCs. Studies reviewed here demonstrate the importance of reduction in regulating exogenous antigen presentation. The differential expression of a gamma-interferon-inducible lysosomal thiol reductase in professional APCs and melanomas is discussed in the context of tumor immune evasion. Cytoplasmic autoantigens, by contrast, are degraded by the proteasome and other enzymes in the cytosol, with the resulting peptides translocating to endosomal and lysosomal compartments for intersection with class II molecules. Processing and editing of these antigenic peptides within endosomes and lysosomes may be critical in regulating their display via class II proteins. Multiple pathways may regulate the transit of cytosolic peptides to class II molecules. The role of lysosome-associated membrane protein-2a and heat-shock cognate protein 70 in promoting cytoplasmic peptide presentation by MHC class II molecules is discussed.

alpha v beta3-dependent cross-presentation of matrix metalloproteinase-2 by melanoma cells gives rise to a new tumor antigen

A large array of antigens that are recognized by tumor-specific T cells has been identified and shown to be generated through various processes. We describe a new mechanism underlying T cell recognition of melanoma cells, which involves the generation of a major histocompatibility complex class I–restricted epitope after tumor-mediated uptake and processing of an extracellular protein—a process referred to as cross-presentation—which is believed to be restricted to immune cells. We show that melanoma cells cross-present, in an αvβ3-dependent manner, an antigen derived from secreted matrix metalloproteinase–2 (MMP-2) to human leukocyte antigen A*0201-restricted T cells. Because MMP-2 activity is critical for melanoma progression, the MMP-2 peptide should be cross-presented by most progressing melanomas and represents a unique antigen for vaccine therapy of these tumors.

Viral evasion of the MHC class I antigen-processing machinery

In their adaptation to the immune system in vertebrates, viruses have been forced to evolve elaborate strategies for evading the host's immune response. To ensure life-long persistence in the host, herpes viruses, adenoviruses and retroviruses have exploited multiple cellular pathways for their purpose, including the class I antigen-processing machinery. Attractive and prominent targets for viral attacks are the proteasome complex, the transporter associated with antigen processing, and MHC class I molecules. This review briefly outlines the different mechanisms of viral interference with the antigen-presentation pathway.

Semi-mature IL-12 secreting dendritic cells present exogenous antigen to trigger cytolytic immune responses

Dendritic cells (DC) are candidates for antigen-presenting cells that present exogenous antigen on MHC class I molecules to cytotoxic T lymphocytes (CTL), a process referred to as cross-priming. We triggered interleukin (IL)-12 release from DC, which was limited to the first day after maturation induction, by a combination of lipopolysaccharide (LPS) and interferon (IFN)-gamma. To stimulate T lymphocytes, we used soluble protein derived from lysis of Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCL) or ovalbumin loaded onto DC. Co-culture was initiated 2-6 or 48 h after maturation corresponding to "semi-mature" actively IL-12-secreting type 1 DC (sm-DC1) or a "fully mature" DC1 that had lost the ability to release IL-12 (fm-DC1), respectively. IL-12-secreting sm-DC1 but not fm-DC1 efficiently triggered cytolytic activity in autologous T lymphocytes. The combination of IL-1beta, IL-6, TNF-alpha, and prostaglandin E2 generated type 2 DC that did not secrete IL-12 (DC2) and could not prime T-cell cytolytic activity. However, supplementation of cultures using DC2 with IL-12 resulted in CTL activity while the presence of anti-IL-12 monoclonal antibodies in cultures using IL-12 secreting sm-DC1 suppressed CTL activity. Thus, actively IL-12-secreting sm-DC1 are necessary and sufficient for the antigen-specific expansion of CTL in response to exogenously provided soluble antigen.

No life without death—apoptosis as prerequisite for T cell activation

The orchestrated death of infected cells is key to our understanding of CD8 T cell activation against pathogens. Most intracellular bacteria including Mycobacterium tuberculosis, the etiologic agent of tuberculosis, remain enclosed in phagosomes of infected macrophages. CD8 T cells play a critical role in defense of infection and recognize antigens originating from the cytosol presented by MHC-I molecules. Since mycobacteria do not gain access to the cytosolic MHC-I presentation pathway, the fundamental question as to how CD8 T cells encounter mycobacterial antigens remains to be solved. In this review, we focus on solutions for this enigma and describe the detour pathway of T cell activation. Mycobacteria induce cell death of infected macrophages which thereby leave a last message by releasing apoptotic vesicles. Subsequently, these antigen-containing entities are engulfed by dendritic cells which process the mycobacterial cargo for efficient antigen presentation and CD8 T cell activation. Since the dying infected cell is the origin of a protective T cell response destined to preserve life and individuality, the detour pathway represents an altruistic principle at a cellular level which corresponds to the macroscopic world where death is the precondition to perpetuate the living.

Survival of Salmonella enterica serovar Typhimurium within late endosomal-lysosomal compartments of B lymphocytes is associated with the inability to use the vacuolar alternative major histocompatibility complex class I antigen-processing pathway

Gamma interferon (IFN-gamma)-activated macrophages use an alternative processing mechanism to present Salmonella antigens to CD8(+) T lymphocytes. This pathway involves processing of antigen in a vacuolar compartment followed by secretion and loading of antigenic peptides to major histocompatibility complex class I (MHC-I) molecules on macrophage cell surface and bystander cells. In this study, we have shown that B lymphocytes are not able to process Salmonella antigens using this alternative pathway. This is due to differences in Salmonella enterica serovar Typhimurium-containing vacuoles (SCV) when comparing late endosomal-lysosomal processing compartments in B lymphocytes to those in macrophages. The IFN-gamma-activated IC21 macrophage cell line and A-20 B-cell line were infected with live or dead Salmonella enterica serovar Typhimurium. The SCV in B cells were in a late endosomal-lysosomal compartment, whereas SCV in macrophages were remodeled to a non-characteristic late endosomal-lysosomal compartment over time. Despite the difference in SCV within macrophages and B lymphocytes, S. enterica serovar Typhimurium survives more efficiently within the IFN-gamma-activated B cells than in activated macrophage cell lines. Similar results were found during in vivo acute infection. We determined that a lack of remodeling of late endosomal-lysosomal compartments by live Salmonella infection in B lymphocytes is associated with the inability to use the alternative MHC-I antigen-processing pathway, providing a survival advantage to the bacterium. Our data also suggest that the B lymphocyte late endosome-lysosome environment allows the expression of Salmonella virulence mechanisms favoring B lymphocytes in addition to macrophages and dendritic cells as a reservoir during in vivo infection.

Autoreactive CD8+ T cells in multiple sclerosis: a new target for therapy?

Multiple sclerosis afflicts more than 1 million individuals worldwide and is widely considered to be an autoimmune disease. Traditionally, CD4(+) T helper cells have almost exclusively been held responsible for its immunopathogenesis, partly because certain MHC class II alleles clearly predispose for developing multiple sclerosis and also, because of their importance in inducing experimental autoimmune encephalomyelitis (EAE), the animal model for multiple sclerosis. However, several strategies that target CD4(+) T cells beneficially in EAE have failed to ameliorate disease activity in multiple sclerosis, and some have even triggered exacerbations. Recently, the potential importance of CD8(+) T cells has begun to emerge. Physiologically, CD8(+) T cells are essential for detecting and eliminating abnormal cells, whether infected or neoplastic. In multiple sclerosis, genetic associations with MHC class I alleles have now been established, and CD8(+) as well as CD4(+) T cells have been found to invade and clonally expand in inflammatory central nervous system plaques. Recent animal models induced by CD8(+) T cells show interesting similarities to multiple sclerosis, in particular, in lesion distribution (more inflammation in the brain relative to the spinal cord), although not all of the features of the human disease are recapitulated. Here we outline the arguments for a possible role for CD8(+) T cells, a lymphocyte subset that has long been underrated in multiple sclerosis and should now be considered in new therapeutic approaches.

CTL-based cancer preventive/therapeutic vaccines for carcinomas: role of tumour-associated carbohydrate antigens

In this study, we discuss the use of glycopeptides containing tumour-associated carbohydrate antigens (TACA) as preventive vaccines for carcinomas. The results of our recent studies suggest that CD8(+) cytotoxic T cells are capable of recognizing small TACA in a conventional class I MHC-restricted fashion. TACA-specific T-cell receptors are highly degenerate and their fine specificity includes the glycosylated amino acid linker together with the sugar moiety. TF, a disaccharide and Tn, its immediate precursor, are TACA largely expressed in carcinomas that can be successfully used as vaccines when conjugated to designer peptide backbones with optimal binding affinity for class I MHC molecules.

Effective induction of naive and recall T-cell responses by targeting antigen to human dendritic cells via a humanized anti-DC-SIGN antibody

Current dendritic cell (DC)-based vaccines are based on ex vivo-generated autologous DCs loaded with antigen prior to readministration into patients. A more direct and less laborious strategy is to target antigens to DCs in vivo via specific surface receptors. Therefore, we developed a humanized antibody, hD1V1G2/G4 (hD1), directed against the C-type lectin DC-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) to explore its capacity to serve as a target receptor for vaccination purposes. hD1 was cross-linked to a model antigen, keyhole limpet hemocyanin (KLH). We observed that the chimeric antibody-protein complex (hD1-KLH) bound specifically to DC-SIGN and was rapidly internalized and translocated to the lysosomal compartment. To determine the targeting efficiency of hD1-KLH, monocyte-derived DCs and peripheral blood lymphocytes (PBLs) were obtained from patients who had previously been vaccinated with KLH-pulsed DCs. Autologous DCs pulsed with hD1-KLH induced proliferation of patient PBLs at a 100-fold lower concentration than KLH-pulsed DCs. In addition, hD1-KLH-targeted DCs induced proliferation of naive T cells recognizing KLH epitopes in the context of major histocompatibility complex (MHC) classes I and II. We conclude that antibody-mediated targeting of antigen to DCs via DC-SIGN effectively induces antigen-specific naive as well as recall T-cell responses. This identifies DC-SIGN as a promising target molecule for DC-based vaccination strategies.

Lamp-2a facilitates MHC class II presentation of cytoplasmic antigens

Extracellular antigens are internalized and processed before binding MHC class II molecules within endosomal and lysosomal compartments of professional antigen presenting cells (APC) for subsequent presentation to T cells. Yet select cytoplasmic peptides derived from autoantigens also intersect and bind class II molecules via an unknown mechanism. In human B lymphoblasts, inhibition of the peptide transporter associated with antigen processing (TAP) failed to alter class II-restricted cytoplasmic epitope presentation. By contrast, decreased display of cytoplasmic epitopes via class II molecules was observed in cells with diminished expression of the lysosome-associated membrane protein-2 (Lamp-2). Overexpression of Lamp-2 isoform A (Lamp-2a), an established component of chaperone-mediated autophagy, enhanced cytoplasmic autoantigen presentation. Manipulating APC expression of heat shock cognate protein 70 (hsc70), a cofactor for Lamp-2a, also altered cytoplasmic class II peptide presentation. These results demonstrate a novel role for the lysosomal Lamp-2a-hsc70 complex in promoting immunological recognition and antigen presentation.

Phenotypic and functional analysis of EBV-specific memory CD8 cells in SLE

T cell dysfunction has been described in systemic lupus erythematosus (SLE). However, the specific phenotype and function of antigen-specific CD8 cells is less clear. Here we determined phenotype and function of Epstein-Barr virus (EBV)-specific CD8 cells at the single-cell level in SLE. HLA-A2-restricted EBV-BMLF-1-specific CD8 cells were enumerated by flow cytometry using tetramers in SLE and healthy control subjects. Antigen-specific CD8 cells were analyzed for expression of differentiation, activation, proliferation, and anti-apoptotic markers. EBV-specific, other virus-specific (specific against a viral peptide pool consisting of cytomegalovirus, EBV and influenza virus peptides), and mitogen-induced CD8 cell function was assessed by INF-gamma ELISPOT assay. Frequencies of EBV-specific CD8 cells tended to be greater in SLE subjects than in healthy control subjects (p=0.07). While over 10% of EBV-specific CD8 cells were capable of producing IFN-gamma in four out of five healthy control subjects, such proportions of EBV-specific CD8 cells capable of IFN-gamma production were observed in only one out of six SLE subjects (p=0.04). In contrast, viral peptide pool-specific and mitogen-induced IFN-gamma-producing T cell function was intact in SLE subjects. Phenotypic analysis revealed EBV-specific CD8 cells to be in an early to intermediate differentiation and resting memory state in both groups. While EBV-specific CD8 cells are similar in phenotype, their frequency tends to be increased, and function appears to be decreased in SLE. Therefore, an impaired EBV-specific CD8 immune response may exist in SLE, potentially contributing to disease pathogenesis.

B Lymphocytes Participate in Cross-Presentation of Antigen following Gene Gun Vaccination

Although endocytosed proteins are commonly presented via the class II MHC pathway to stimulate CD4(+) T cells, professional APCs can also cross-present Ags, whereby these exogenous peptides can be complexed with class I MHC for cross-priming of CD8(+) T cells. Whereas the ability of dendritic cells (DCs) to cross-present Ags is well documented, it is not known whether other APCs may also play a role, or what is the relative contribution of cross-priming to the induction of acquired immunity after DNA immunization. In this study, we compared immune responses generated after gene gun vaccination of mice with DNA vaccine plasmids driven by the conventional CMV promoter, the DC-specific CD11c promoter, or the keratinocyte-specific K14 promoter. The CD11c promoter achieved equivalent expression in CD11c(+) DCs in draining lymph nodes over time, as did a conventional CMV-driven plasmid. However, immunization with DC-restricted DNA vaccines failed to generate protective humoral or cellular immunity to model Ags influenza hemagglutinin and OVA, despite the ability of CD11c(+) cells isolated from lymph nodes to stimulate proliferation of Ag-specific T cells directly ex vivo. In contrast, keratinocyte-restricted vaccines elicited comparable T and B cell activity as conventional CMV promoter-driven vaccines, indicating that cross-priming plays a major role in the generation of immune responses after gene gun immunization. Furthermore, parallel studies in B cell-deficient mu-MT mice demonstrated that B lymphocytes, in addition to DCs, mediate cross-priming of Ag-specific T cells. Collectively, these data indicate that broad expression of the immunogen is required for optimal induction of protective acquired immunity.

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.

Fibrocytes are potent stimulators of anti-virus cytotoxic T cells

Fibrocytes (Fb) are a population of circulating leukocytes reported to be capable of presenting antigen to CD4(+) T lymphocytes. In contrast, no information is available about their capacity to stimulate CD8(+) cytolytic T lymphocyte (CTL) responses. To this end, Fb were isolated from porcine blood to investigate their ability to stimulate CTL responses using a classical swine fever virus model. The isolated Fb (referred to as primary Fb) displayed the phenotype previously reported for mouse and human Fb, particularly in terms of the surface proteins necessary for antigen presentation, major histocompatibility complex (MHC) classes I and II, and CD80/86. These primary Fb endocytosed and degraded antigen efficiently. In absence of exogenous stimuli, endocytosis and MHC II expression were lost when the Fb were passaged and cultured. Treatment of such secondary Fb with interferon-gamma (IFN-gamma) restored the MHC II expression. The primary and secondary Fb were capable of stimulating antigen-specific CD4(+) T lymphocytes relating to previous reports. In addition, an efficient stimulation of virus-specific CD8(+)CTL was measured in terms of CD8(+) T cell proliferation, IFN-gamma production, and cytotoxic activity. This was noted even at low Fb/T lymphocyte ratios, at which dendritic cells were less efficient. Although IFN-gamma pretreatment of Fb was not necessary for this function, it could enhance the Fb activity. These results demonstrate that Fb are efficient, accessory cells for the presentation of viral antigen to specific CD8(+) CTL.

Cross-presentation by intercellular peptide transfer through gap junctions

Major histocompatibility complex (MHC) class I molecules present peptides that are derived from endogenous proteins. These antigens can also be transferred to professional antigen-presenting cells in a process called cross-presentation, which precedes initiation of a proper T-cell response; but exactly how they do this is unclear. We tested whether peptides can be transferred directly from the cytoplasm of one cell into the cytoplasm of its neighbour through gap junctions. Here we show that peptides with a relative molecular mass of up to approximately 1,800 diffuse intercellularly through gap junctions unless a three-dimensional structure is imposed. This intercellular peptide transfer causes cytotoxic T-cell recognition of adjacent, innocent bystander cells as well as activated monocytes. Gap-junction-mediated peptide transfer is restricted to a few coupling cells owing to the high cytosolic peptidase activity. We present a mechanism of antigen acquisition for cross-presentation that couples the antigen presentation system of two adjacent cells and is lost in most tumours: gap-junction-mediated intercellular peptide coupling for presentation by bystander MHC class I molecules and transfer to professional antigen presenting cells for cross-priming.

Tails of wonder: endocytic-sorting motifs key for exogenous antigen presentation

Antigen-presenting molecules, including MHC I, II and CD1, have central roles in the induction of T cell-mediated immunity against pathogens and tumors and also in the maintenance of tolerance towards self-antigens. The presentation of exogenously derived peptide and lipid antigens to specific T cells by professional antigen-presenting cells (pAPCs) is an essential part of both processes. Exogenous antigen loading takes place mostly within specialized endocytic and phagocytic compartments of pAPCs and targeting of antigen-presenting molecules to these intracellular compartments is mediated by highly conserved cytoplasmic sorting motifs. Recent data have revealed that the cytoplasmic tails of antigen-presenting molecules, by controlling the access of these molecules to exogenously derived antigens, have a crucially important and largely underappreciated role in the generation of tolerance and T-cell mediated immunity.

Differential capability for phagocytosis of apoptotic and necrotic leukemia cells by human peripheral blood dendritic cell subsets

CD11c+ dendritic cells (DC) and plasmacytoid DC (PDC) are the two major DC subsets in human peripheral blood. For the purpose of immunotherapy with DC, it is important to investigate the phagocytosis of killed tumor cells by different DC subsets. Using immature monocyte-derived DC (iMoDC) as reference, we have compared the ability of CD11c+ DC and PDC to phagocytose apoptotic and necrotic K562 leukemia cells. Freshly isolated CD11c+ DC phagocytosed apoptotic and necrotic K562 cells, whereas PDC did not show any evidence of uptake of dead cells. Blocking studies showed that CD36 is importantly involved in uptake of apoptotic and necrotic material. CD91 and CD11c were also involved. In addition, we found that beta5 integrin was expressed on CD11c+ DC but not in its classical association with alphaV. Uptake of apoptotic K562 cells by CD11c+ DC was increased following incubation with granulocyte macrophage-colony stimulating factor (GM-CSF) and interleukin (IL)-4, alone or in combination with transforming growth factor-beta1, to levels comparable with those observed for iMoDC. Phagocytosis of dead cellular material by the GM-CSF/IL-4-treated CD11c+ DC was largely restricted to a subset expressing low levels of human leukocyte antigen-DR and CD83. Thus, the relationship between phagocytosis of antigenic material and expression of maturation-related cell-surface molecules is similar for CD11c+ DC and MoDC. We conclude that CD11c+ DC in peripheral blood are precursor cells, which under the influence of cytokines, differentiate to cells with DC phenotype and function.

Viral immune evasion molecules attack the ER peptide-loading complex and exploit ER-associated degradation pathways

The CD8+ cytotoxic-T-cell response is a potent mechanism that controls intracellular pathogens, including many viruses. To facilitate transmission, viruses often counter this response by inhibiting the cell surface display of virus-derived peptides on MHC class I molecules. More specifically, recent studies have demonstrated that viruses have evolved remarkable mechanisms to inhibit MHC class I expression by interfering with the function of the MHC class I assembly machinery (the peptide-loading complex) in the endoplasmic reticulum and/or by exploiting endoplasmic-reticulum-associated degradation pathways. These viral molecules are proving invaluable as research tools to illuminate the novel features of physiological pathways that are central to normal cell biology. Furthermore, the detailed characterization of such pathways has yielded significant new insights into host-pathogen interplay.

Immunological ignorance of solid tumors

Many peripheral solid tumors such as sarcomas and carcinomas express tumor-specific antigens that can serve as targets for immune effector T cells. Nevertheless, the immune surveillance against clinically manifest carcinomas and sarcomas seems relatively inefficient. Naïve cytotoxic T cells are activated exclusively in secondary lymphoid organs including the spleen and lymph nodes. Tumor antigen might be either cross-presented to naïve cytotoxic T cells by professional antigen-presenting cells (pAPC), or presented directly by tumor cells that migrated to secondary lymphoid organs. Direct priming is quite inefficient during early tumor development because metastasis to lymphoid organs is usually limited to advanced stage diseases. Similarly, the process of cross-priming by pAPC seems to depend on relatively large antigen amounts and on maturation stimuli for dendritic cells, and both requirements may be limiting during initial tumorigenesis. Therefore, the immunosurveillance of solid tumors may fail because they are ignored for too long by the immune system. However, these situations may prove promising for the induction of tumor-specific T cell immunity by vaccination, as the T cell repertoire against these antigens has a naïve phenotype and is not yet affected by tolerance mechanisms.

Identification of a naturally processed cytotoxic CD8 T-cell epitope of coxsackievirus B4, presented by HLA-A2.1 and located in the PEVKEK region of the P2C nonstructural protein

The adaptive immune system generates CD8 cytotoxic T lymphocytes (CTLs) as a major component of the protective response against viruses. Knowledge regarding the nature of the peptide sequences presented by HLA class I molecules and recognized by CTLs is thus important for understanding host-pathogen interactions. In this study, we focused on identification of a CTL epitope generated from coxsackievirus B4 (CVB4), a member of the enterovirus group responsible for several inflammatory diseases in humans and often implicated in the triggering and/or acceleration of the autoimmune disease type 1 diabetes. We identified a 9-mer peptide epitope that can be generated from the P2C nonstructural protein of CVB4 (P2C(1137-1145)) and from whole virus by antigen-presenting cells and presented by HLA-A2.1. This epitope is recognized by effector memory (gamma interferon [IFN-gamma]-producing) CD8 T cells in the peripheral blood at a frequency of responders that suggests that it is a major focus of the anti-CVB4 response. Short-term CD8 T-cell lines generated against P2C(1137-1145) are cytotoxic against peptide-loaded target cells. Of particular interest, the epitope lies within a region of viral homology with the diabetes-related autoantigen, glutamic acid decarboxylase-65 (GAD(65)). However, P2C(1137-1145)-specific cytotoxic T lymphocyte (CTL) lines were not activated to produce IFN-gamma by the GAD(65) peptide homologue and did not show cytotoxic activity in the presence of appropriately labeled targets. These results describe the first CD8 T-cell epitope of CVB4 that will prove useful in the study of CVB4-associated disease.

The dendritic cell-derived protein DC-STAMP is highly conserved and localizes to the endoplasmic reticulum

Recently, we described the molecular identification of dendritic cell-specific TrAnsMembrane protein (DC-STAMP), a multimembrane-spanning protein preferentially expressed by human DC (hDC). In this report, we describe the identification and expression profile of the murine homologue of DC-STAMP (mDC-STAMP) as well as the characterization of the DC-STAMP protein. The results demonstrate that mDC-STAMP is over 90% homologous to hDC-STAMP and is also preferentially expressed by DC in vitro and ex vivo. mDC-STAMP expression is enhanced by interleukin-4 and down-regulated upon DC maturation. Analysis of differently tagged DC-STAMP proteins further demonstrates that hDC-STAMP and mDC-STAMP are glycosylated and primarily localize to an intracellular compartment. Applying confocal microscopy and electron microscopy, we demonstrate that hDC-STAMP localizes to the endoplasmic reticulum (ER) in human embryonic kidney 293 cells as well as hDC transduced with an adenovirus encoding hDC-STAMP-green fluorescent protein fusion protein. These data imply that DC-STAMP may exert its effect in the ER.

Access of soluble antigens to the endoplasmic reticulum can explain cross-presentation by dendritic cells

In dendritic cells (DCs), peptides derived from internalized particulate substrates are efficiently cross-presented by major histocompatibility complex (MHC) class I molecules. Exogenous soluble antigens are also presented by DCs but with substantially lower efficiency. Here we show that particulate and soluble antigens use different transport pathways. Particulate antigens have been shown to access peripheral endoplasmic reticulum (ER)-like phagosomes that are competent for cross-presentation, whereas we show here that soluble proteins that escape proteolysis enter the lumen of the ER. From there, they may be translocated into the cytosol by the pathway established for ER-associated degradation and their derived peptides may be transported back into the ER for binding by MHC class I molecules. MHC class I presentation involving the constitutive retrograde transport of soluble proteins to the ER by DCs may facilitate DC tolerance to components of their extracellular environment.

Sidechain biology and the immunogenicity of PDC-E2, the major autoantigen of primary biliary cirrhosis

The E2 component of mitochondrial pyruvate dehydrogenase complex (PDC-E2) is the immunodominant autoantigen of primary biliary cirrhosis. Whereas lipoylation of PDC-E2 is essential for enzymatic activity and predominates under normal conditions, other biochemical systems exist that also target the lysine residue, including acylation of fatty acids or xenobiotics and ubiquitinylation. More importantly, the immunogenicity can be affected by derivatization of the lysine residue, as the recognition of lipoylated PDC-E2 by patient autoantibodies is enhanced compared with octanoylated PDC-E2. Furthermore, our laboratory has shown that various xenobiotic modifications of a peptide representing the immunodominant region of PDC-E2 are immunoreactive against patient sera. The only purported regulatory system that prevents the accumulation of potentially autoreactive PDC-E2 is glutathionylation, in which the lysine-lipoic acid moiety is further modified with glutathione during apoptosis. Interestingly, this system is found in several cell lines, including HeLa, Jurkat, and Caco-2 cells, but not in cholangiocytes and salivary gland epithelial cells, both of which are targets for destruction in primary biliary cirrhosis. Hence, the failure of this or other regulatory system(s) may overwhelm the immune system with immunogenic PDC-E2 that can initiate the breakdown of tolerance in a genetically susceptible individual. In this review the authors survey the data available on the biochemical life of PDC-E2, with particular emphasis on the lysine residue and its known interactions with machinery involved in various posttranslational modifications.

Cytolytic CD8+ T cells recognizing CFP10 are recruited to the lung after Mycobacterium tuberculosis infection

Optimum immunity against Mycobacterium tuberculosis requires both CD4⁺ and CD8⁺ T cells. In contrast with CD4⁺ T cells, few antigens are known that elicit CD8⁺ T cells during infection. CD8⁺ T cells specific for culture filtrate protein-10 (CFP10) are found in purified protein derivative positive donors, suggesting that CFP10 primes CD8⁺ T cells in vivo. Using T cells from M. tuberculosis–infected mice, we identified CFP10 epitopes recognized by CD8⁺ T cells and CD4⁺ T cells. CFP10-specific T cells were detected as early as week 3 after infection and at their peak accounted for up to 30% of CD8⁺ T cells in the lung. IFNγ-producing CD8⁺ and CD4⁺ T cells recognizing CFP10 epitopes were preferentially recruited to the lungs of M. tuberculosis–infected mice. In vivo cytolytic activity of CD8⁺ T cells specific for CFP10 and TB10.3/10.4 proteins was detected in the spleen, pulmonary lymph nodes, and lungs of infected mice. The cytolytic activity persisted long term and could be detected 260 d after infection. This paper highlights the cytolytic function of antigen-specific CD8⁺ T cells elicited by M. tuberculosis infection and demonstrates that large numbers of CFP10-specific cytolytic CD8⁺ T cells are recruited to the lung after M. tuberculosis infection.