The endogenous pathway of MHC class II antigen presentation

Endogenous Antigen Presentation of MHC Class II Epitopes through Non-Autophagic Pathways

Antigenic peptides presented by major histocompatibility complex (MHC) class II molecules are generally derived from exogenous proteins acquired by antigen presenting cells. However, in some circumstances, MHC class II molecules can present intracellular proteins expressed within the antigen-presenting cells. There are several described pathways by which endogenous antigens are degraded and gain access to MHC class II molecules. These include autophagy and other non-autophagic pathways; the latter category includes the MHC class I-like pathways, heat shock protein 90-mediated pathways, and internalization from the plasma membrane. This review will summarize and discuss the non-autophagic pathways.

Assessment of the frequency of regulatory T cells (CD4+CD25+CD127-) in children with hemophilia A: relation to factor VIII inhibitors and disease severity

A rapidly growing evidence showed that regulatory T cells (Tregs) play a crucial role in tolerance to coagulation factors and may be involved in the pathogenesis of inhibitor formation in patients with hemophilia. We determined the percentage of Tregs (CD4CD25CD127) in 45 children with hemophilia A compared with 45 healthy controls, and assessed their relation to the clinical characteristics of patients and factor VIII (FVIII) inhibitors. Patients were studied stressing on frequency of bleeding attacks, joint pain, history of viral hepatitis, and the received therapy (FVIII precipitate/cryotherapy). FVIII activity and FVIII inhibitors were assessed with flow cytometric analysis of CD4CD25CD127 Tregs. According to residual FVIII activity levels, 30 patients (66.7%) had mild/moderate hemophilia A, whereas 15 (33.3%) patients had severe hemophilia A. The frequency of Tregs was significantly lower among all patients with hemophilia A compared with controls (2.59 ± 1.1 versus 3.73 ± 1.12%; P = 0.002). Tregs were significantly decreased among patients with FVIII inhibitors compared with the inhibitor-negative group (P < 0.001). Patients with hematuria or severe hemophilia A had lower Tregs levels than those without (P = 0.34 and P = 0.011, respectively). A significant positive correlation was found between the percentage of Tregs and FVIII among hemophilia A patients. ROC curve analysis revealed that the cut-off value of Tregs at 1.91% could differentiate patients with and without FVIII inhibitors, with a sensitivity of 100% and a specificity of 91.3%. We suggest that alteration in the frequency of Tregs in young patients with hemophilia A may contribute to inhibitor formation and disease severity.

Toward a Network Model of MHC Class II-Restricted Antigen Processing

The standard model of Major Histocompatibility Complex class II (MHCII)-restricted antigen processing depicts a straightforward, linear pathway: internalized antigens are converted into peptides that load in a chaperone dependent manner onto nascent MHCII in the late endosome, the complexes subsequently trafficking to the cell surface for recognition by CD4(+) T cells (TCD4+). Several variations on this theme, both moderate and radical, have come to light but these alternatives have remained peripheral, the conventional pathway generally presumed to be the primary driver of TCD4+ responses. Here we continue to press for the conceptual repositioning of these alternatives toward the center while proposing that MHCII processing be thought of less in terms of discrete pathways and more in terms of a network whose major and minor conduits are variable depending upon many factors, including the epitope, the nature of the antigen, the source of the antigen, and the identity of the antigen-presenting cell.

Development of a cytokine-modified allogeneic whole cell pancreatic cancer vaccine

Management of patients with pancreatic cancer is a multidisciplinary approach that presents enormous challenges to the clinician. Overall 5-year survival for all patients remains <3%. Symptoms of early pancreas cancer are nonspecific. As such, only a fraction of patients are candidates for surgery. While surgical resection provides the only curative option, most patients will develop tumor recurrence and die of their disease. To date, the clinical benefits of chemotherapy and radiation therapy have been important but have led to modest improvements. Tumor vaccines have the potential to specifically target the needle of pancreas cancer cells amidst the haystack of normal tissue. The discovery of pancreas tumor-specific antigens and the subsequent ability to harness this technology has become an area of intense interest for tumor immunologists and clinicians alike. Without knowledge of specific antigen targets, the whole tumor cell represents the best source of immunizing antigens. This chapter will focus on the development of whole tumor cell vaccine strategies for pancreas cancer.

Major histocompatibility complex class II+ invariant chain negative breast cancer cells present unique peptides that activate tumor-specific T cells from breast cancer patients

The major histocompatibility complex (MHC) class II-associated Invariant chain (Ii) is present in professional antigen presenting cells where it regulates peptide loading onto MHC class II molecules and the peptidome presented to CD4+ T lymphocytes. Because Ii prevents peptide loading in neutral subcellular compartments, we reasoned that Ii- cells may present peptides not presented by Ii+ cells. Based on the hypothesis that patients are tolerant to MHC II-restricted tumor peptides presented by Ii+ cells, but will not be tolerant to novel peptides presented by Ii- cells, we generated MHC II vaccines to activate cancer patients' T cells. The vaccines are Ii- tumor cells expressing syngeneic HLA-DR and the costimulatory molecule CD80. We used liquid chromatography coupled with mass spectrometry to sequence MHC II-restricted peptides from Ii+ and Ii- MCF10 human breast cancer cells transfected with HLA-DR7 or the MHC Class II transactivator CIITA to determine if Ii- cells present novel peptides. Ii expression was induced in the HLA-DR7 transfectants by transfection of Ii, and inhibited in the CIITA transfectants by RNA interference. Peptides were analyzed and binding affinity predicted by artificial neural net analysis. HLA-DR7-restricted peptides from Ii- and Ii+ cells do not differ in size or in subcellular location of their source proteins; however, a subset of HLA-DR7-restricted peptides of Ii- cells are not presented by Ii+ cells, and are derived from source proteins not used by Ii+ cells. Peptides from Ii- cells with the highest predicted HLA-DR7 binding affinity were synthesized, and activated tumor-specific HLA-DR7+ human T cells from healthy donors and breast cancer patients, demonstrating that the MS-identified peptides are bonafide tumor antigens. These results demonstrate that Ii regulates the repertoire of tumor peptides presented by MHC class II+ breast cancer cells and identify novel immunogenic MHC II-restricted peptides that are potential therapeutic reagents for cancer patients.

Profiling of differentially expressed genes in haemophilia A with inhibitor

Inhibitor development is the most significant complication in the therapy of haemophilia A (HA) patients. In spite of many studies, not much is known regarding the mechanism underlying inhibitor development. To understand the mechanism, we analysed profiles of differentially expressed genes (DEGs) between inhibitor and non-inhibitor HA via a microarray technique. Twenty unrelated Korean HAs were studied: 11 were non-inhibitor and nine were HA with inhibitor (≥5 BU mL(-1)). Microarray analysis was conducted using a Human Ref-8 expression Beadchip system (Illumina) and the data were analysed using Beadstudio software. We identified 545 DEGs in inhibitor HA as compared with the non-inhibitor patients; 384 genes were up-regulated and 161 genes were down-regulated. Among them, 75 genes whose expressions were altered by at least two-fold (>+2 or <-2) were selected and classified via the PANTHER classification method. The expressions of signal transduction and immunity-related genes differed significantly in the two groups. For validation of the DEGs, semi-quantitative RT-PCR (semi-qRT-PCR) was conducted with the six selected DEGs. The results corresponded to the microarray data, with the exception of one gene. We also examined the expression of the genes associated with the antigen presentation process via real-time PCR. The average levels of IL10, CTLA4 and TNFα slightly reduced, whereas that of IFNγ increased in the inhibitor HA group. We are currently unable to explain whether this phenomenon is a function of the inhibitor-inducing factor or is an epiphenomenon of antibody production. Nevertheless, our results provide a possible explanation for inhibitor development.

HLA-DR-presented peptide repertoires derived from human monocyte-derived dendritic cells pulsed with blood coagulation factor VIII

Activation of T-helper cells is dependent upon the appropriate presentation of antigen-derived peptides on MHC class II molecules expressed on antigen presenting cells. In the current study we explored the repertoire of peptides presented on MHC class II molecules on human monocyte derived dendritic cells (moDCs) from four HLA-typed healthy donors. MHC class II-bound peptides could be routinely recovered from small cultures containing 5 × 10(6) cells. A fraction of the identified peptides were derived from proteins localized in the plasma membrane, endosomes, and lysosomes, but the majority of peptides that were presented on MHC class II originate from other organelles. Subsequently, we studied the antigen-specific peptide repertoire after endocytosis of a soluble antigen. Blood coagulation factor VIII (FVIII) was chosen as the antigen since our current knowledge on MHC class II presented peptides derived from this immunogenic therapeutic protein is limited. Analysis of the total repertoire of MHC class II-associated peptides revealed that per individual sample 20-50 FVIII-derived peptides were presented on FVIII-pulsed moDCs. Repertoires of FVIII-derived peptides eluted from moDCs derived from a panel of four HLA typed donors revealed that some MHC class II-presented FVIII peptides were presented by multiple donors, whereas the presentation of other FVIII peptides was donor-specific. In total 32 different core peptides were presented on FVIII-pulsed moDCs from four HLA-typed donors. Together our findings provide an unbiased approach to identify peptides that are presented by MHC class II on antigen-loaded moDCs from individual donors.

'Nodophagy': New crossroads in Crohn disease pathogenesis

Autophagy is a homeostatic pathway that processes and recycles damaged organelles and other cytoplasmic contents. While studies have implicated autophagy in the immune response to infection, the understanding of how the autophagic machinery specifically targets intracellular pathogens has remained elusive. Two recent studies have uncovered an autophagy-mediated immune response to bacteria through their detection by Nod receptors. In particular, Nod1 and Nod2 recruit the autophagic protein ATG16L1 to the plasma membrane at the bacterial entry site to promote an autophagy-dependent elimination of bacteria. In addition, Nod2 and ATG16L1 synergize to initiate an adaptive immune response to bacterial invasion by enhancing major histocompatibility complex (MHC) class II antigen presentation. These findings link two Crohn disease-associated susceptibility genes and reveal that cells expressing the risk-associated variants of ATG16L1 are defective in autophagy-mediated bacterial handling and antigen presentation. This could lead to bacterial persistence and contribute to the pathogenesis of the disease.

T cell detection of a B-cell tropic virus infection: newly-synthesised versus mature viral proteins as antigen sources for CD4 and CD8 epitope display

Viruses that naturally infect cells expressing both MHC I and MHC II molecules render themselves potentially visible to both CD8⁺ and CD4⁺ T cells through the de novo expression of viral antigens. Here we use one such pathogen, the B-lymphotropic Epstein-Barr virus (EBV), to examine the kinetics of these processes in the virally-infected cell, comparing newly synthesised polypeptides versus the mature protein pool as viral antigen sources for MHC I- and MHC II-restricted presentation. EBV-transformed B cell lines were established in which the expression of two cognate EBV antigens, EBNA1 and EBNA3B, could be induced and then completely suppressed by doxycycline-regulation. These cells were used as targets for CD8⁺ and CD4⁺ T cell clones to a range of EBNA1 and EBNA3B epitopes. For both antigens, when synthesis was induced, CD8 epitope display rose quickly to near maximum within 24 h, well before steady state levels of mature protein had been reached, whereas CD4 epitope presentation was delayed by 36–48 h and rose only slowly thereafter. When antigen expression was suppressed, despite the persistence of mature protein, CD8 epitope display fell rapidly at rates similar to that seen for the MHC I/epitope half-life in peptide pulse-chase experiments. By contrast, CD4 epitope display persisted for many days and, following peptide stripping, recovered well on cells in the absence of new antigen synthesis. We infer that, in virally-infected MHC I/II-positive cells, newly-synthesised polypeptides are the dominant source of antigen feeding the MHC I pathway, whereas the MHC II pathway is fed by the mature protein pool. Hence, newly-infected cells are rapidly visible only to the CD8 response; by contrast, latent infections, in which viral gene expression has been extinguished yet viral proteins persist, will remain visible to CD4⁺ T cells.

Many viruses infect cells in which both the MHC I and MHC II pathways of antigen presentation are active, and so viral proteins expressed in those cells may be presented as MHC I-peptide complexes to CD8⁺ T cells and as MHC II-peptide complexes to CD4⁺ T cells. Here we study these processes in a model system involving Epstein-Barr virus-infected human B lymphocytes (MHC I/II-positive) where viral antigen expression can be induced or suppressed at will, and antigen presentation tracked with specific CD8⁺ and CD4⁺ T cell clones. In this system, we find that the MHC I pathway is entirely fed by newly-synthesised polypeptides, whereas the MHC II pathway depends upon antigen supplied from the mature protein pool. Hence, while only CD8⁺ T cells can rapidly recognise new infections, only CD4⁺ T cells will recognise latent infections in which viral gene expression is extinguished yet a pool of viral antigens remains.

Impact of major histocompatibility complex class II polymorphisms on Iberian red deer parasitism and life history traits

Host genetic diversity plays an important role in buffering populations against pathogens. We characterized the allelic diversity at the second exon of the b (DRB-2) chain of the major histocompatibility complex class II (MHC-II) locus in a population of Iberian red deer (Cervus elaphus hispanicus) and its impact on parasitism by macroparasites, on a microparasite causing tuberculosis, and on relevant life history traits (spleen size and body condition). No DRB-2 haplotype conferred general resistance or susceptibility against all parasites. However, specific significant correlations were found between some DRB-2 haplotypes and specific parasites. We also detected associations between DRB-2 haplotypes and body condition and spleen size after controlling for body size, sex and age. Our results evidenced a functional significance of MHC-II genes in the defence of Iberian red deer against parasites. These results also support a role of MHC-II as a fitness-enhancing genetic element which can be mediated by parasite effects on life traits with a genetic basis. We conclude that MHC immunogenetic studies may assess management decisions in Iberian red deer because (i) loss of genetic diversity may lead to increased disease occurrence, and (ii) MHC genes are ecologically relevant since they underlie host infection rates and life history traits.

Uveal melanoma cell-based vaccines express MHC II molecules that traffic via the endocytic and secretory pathways and activate CD8+ cytotoxic, tumor-specific T cells

We are exploring cell-based vaccines as a treatment for the 50% of patients with large primary uveal melanomas who develop lethal metastatic disease. MHC II uveal melanoma vaccines are MHC class I(+) uveal melanoma cells transduced with CD80 genes and MHC II genes syngeneic to the recipient. Previous studies demonstrated that the vaccines activate tumor-specific CD4(+) T cells from patients with metastatic uveal melanoma. We have hypothesized that vaccine potency is due to the absence of the MHC II-associated invariant chain (Ii). In the absence of Ii, newly synthesized MHC II molecules traffic intracellularly via a non-traditional pathway where they encounter and bind novel tumor peptides. Using confocal microscopy, we now confirm this hypothesis and demonstrate that MHC II molecules are present in both the endosomal and secretory pathways in vaccine cells. We also demonstrate that uveal melanoma MHC II vaccines activate uveal melanoma-specific, cytolytic CD8(+) T cells that do not lyse normal fibroblasts or other tumor cells. Surprisingly, the CD8(+) T cells are cytolytic for HLA-A syngeneic and MHC I-mismatched uveal melanomas. Collectively, these studies demonstrate that MHC II uveal melanoma vaccines are potent activators of tumor-specific CD4(+) and CD8(+) T cells and suggest that the non-conventional intracellular trafficking pattern of MHC II may contribute to their enhanced immunogenicity. Since MHC I compatibility is unnecessary for the activation of cytolytic CD8(+) T cells, the vaccines could be used in uveal melanoma patients without regard to MHC I genotype.

Infection of HLA-DR1 transgenic mice with a human isolate of influenza a virus (H1N1) primes a diverse CD4 T-cell repertoire that includes CD4 T cells with heterosubtypic cross-reactivity to avian (H5N1) influenza virus

The specificity of the CD4 T-cell immune response to influenza virus is influenced by the genetic complexity of the virus and periodic encounters with variant subtypes and strains. In order to understand what controls CD4 T-cell reactivity to influenza virus proteins and how the influenza virus-specific memory compartment is shaped over time, it is first necessary to understand the diversity of the primary CD4 T-cell response. In the study reported here, we have used an unbiased approach to evaluate the peptide specificity of CD4 T cells elicited after live influenza virus infection. We have focused on four viral proteins that have distinct intracellular distributions in infected cells, hemagglutinin (HA), neuraminidase (NA), nucleoprotein, and the NS1 protein, which is expressed in infected cells but excluded from virion particles. Our studies revealed an extensive diversity of influenza virus-specific CD4 T cells that includes T cells for each viral protein and for the unexpected immunogenicity of the NS1 protein. Due to the recent concern about pandemic avian influenza virus and because CD4 T cells specific for HA and NA may be particularly useful for promoting the production of neutralizing antibody to influenza virus, we have also evaluated the ability of HA- and NA-specific CD4 T cells elicited by a circulating H1N1 strain to cross-react with related sequences found in an avian H5N1 virus and find substantial cross-reactivity, suggesting that seasonal vaccines may help promote protection against avian influenza virus.

Understanding the focused CD4 T cell response to antigen and pathogenic organisms

Immunodominance is a term that reflects the final, very limited peptide specificity of T cells that are elicited during an immune response. Recent experiments in our laboratory compel us to propose a new paradigm for the control of immunodominance in CD4 T cell responses, stating that immunodominance is peptide-intrinsic and is dictated by the off-rate of peptides from MHC class II molecules. Our studies have revealed that persistence of peptide:class II complexes both predicts and controls CD4 T cell immunodominance and that this parameter can be rationally manipulated to either promote or eliminate immune responses. Mechanistically, we have determined that DM editing in APC is a key event that is influenced by the kinetic stability of class II:peptide complexes and that differential persistence of complexes also impacts the expansion phase of the immune response. These studies have important implications for rational vaccine design and for understanding the immunological mechanisms that limit the specificity of CD4 T cell responses.

Endogenous presentation of a nuclear antigen on MHC class II by autophagy in the absence of CRM1-mediated nuclear export

Accumulating evidence suggests that intracellular antigens are endogenously presented on MHC class II, but it is still unknown whether antigens within different subcellular compartments are presented with similar efficiency, and via the same or different pathways. We have previously shown that endogenous MHC class II presentation of the cytosolic bacterial antigen neomycin phosphotransferase II (NeoR) is mediated by autophagy. Here, we addressed whether secluding NeoR from this cytoplasmic pathway by directing the protein into the cell nucleus (NucNeoR) would affect antigen presentation. Unexpectedly, NucNeoR was presented at least as efficiently as the cytosolic version of the antigen. Furthermore, presentation of NucNeoR was also dependent on autophagocytosis and lysosomal processing, indicating that both antigens were presented via the same pathway. Inhibition of CRM1-mediated nuclear export did not impede antigen presentation, indicating that NucNeoR gained access to this autophagy-dependent MHC class II presentation pathway by a CRM1-independent route. Thus, this endogenous presentation pathway broadens the spectrum of intracellular antigens surveyed by CD4(+) T cells by efficiently sampling cytoplasmic as well as nuclear antigens.

EBV latent membrane proteins (LMPs) 1 and 2 as immunotherapeutic targets: LMP-specific CD4+ cytotoxic T cell recognition of EBV-transformed B cell lines

The EBV-latent membrane proteins (LMPs) 1 and 2 are among only three viral proteins expressed in EBV-associated Hodgkin's lymphoma and nasopharyngeal carcinoma. Since these tumors are HLA class I and class II-positive, the LMPs could serve as both CD8+ and CD4+ T cell targets. In contrast to CD8 responses, very little is known about CD4 responses to LMPs. In this study, we describe CD4+ T cell clones defining four LMP1- and three LMP2-derived peptide epitopes and their restricting alleles. All clones produced Th1-like cytokines in response to peptide and most killed peptide-loaded target cells by perforin-mediated lysis. Although clones to different epitopes showed different functional avidities in peptide titration assays, avidity per se was a poor predictor of the ability to recognize naturally infected B lymphoblastoid cell lines (LCLs) expressing LMPs at physiologic levels. Some epitopes, particularly within LMP1, consistently mediated strong LCL recognition detectable in cytokine release, cytotoxicity, and outgrowth inhibition assays. Using cyclosporin A to selectively block cytokine release, we found that CD4+ T cell cytotoxicity is the key effector of LCL outgrowth control. We therefore infer that cytotoxic CD4+ T cells to a subset of LMP epitopes could have therapeutic potential against LMP-expressing tumors.

Replication-deficient mutant Herpes Simplex Virus-1 targets professional antigen presenting cells and induces efficient CD4+ T helper responses

Both neutralizing antibodies and cytotoxic T-cells are necessary to control a viral infection. However, vigorous T helper responses are essential for their elicitation and maintenance. Here we show that a recombinant replication-deficient Herpes Simplex Virus (HSV)-1 vector encoding the Human Immunodeficiency Virus (HIV)-1 matrix protein p17 (T0-p17) was capable of infecting professional antigen presenting cells (APCs) in vitro and in vivo. The injection of T0-p17 in the mouse dermis generated a strong p17-specific CD4+ T helper response preceding both p17-specific humoral and effector T cell responses. Moreover, we show that T0-p17 infection did not interfere with the endogenous processing of the transgene encoded antigen, since infected APCs were able to evoke a strong recall response in vitro. Our results demonstrate that replication-deficient HSV vectors can be appealing candidates for the development of vaccines able to trigger T helper responses.

Antigen-loading compartments for major histocompatibility complex class II molecules continuously receive input from autophagosomes

Major histocompatibility complex (MHC) class II molecules present products of lysosomal proteolysis to CD4(+) T cells. Although extracellular antigen uptake is considered to be the main source of MHC class II ligands, a few intracellular antigens have been described to gain access to MHC class II loading after macroautophagy. However, the general relevance and efficacy of this pathway is unknown. Here we demonstrated constitutive autophagosome formation in MHC class II-positive cells, including dendritic, B, and epithelial cells. The autophagosomes continuously fuse with multivesicular MHC class II-loading compartments. This pathway was of functional relevance, because targeting of the influenza matrix protein 1 to autophagosomes via fusion to the autophagosome-associated protein Atg8/LC3 led to strongly enhanced MHC class II presentation to CD4(+) T cell clones. We suggest that macroautophagy constitutively and efficiently delivers cytosolic proteins for MHC class II presentation and can be harnessed for improved helper T cell stimulation.

Genetic risk factors for inhibitors to factors VIII and IX

The formation of alloantibodies against factor VIII (FVIII) or factor IX (FIX) is the most severe complication of replacement therapy in patients with haemophilia. In the last decade, genetic factors have been shown to constitute a decisive risk determinant for the development of inhibitors. In severe haemophilia A and B, mutations that result in an absent or truncated FVIII/FIX protein are associated with a 20-80% risk of inhibitor formation. In mild to moderate haemophilia, missense mutations represent the main mutation type, with an inhibitor prevalence of 5%. These patients synthesize some endogenous, although non-functional protein that is sufficient to induce immune tolerance. However, in patients with missense mutations clustered in the A2 and C2 domains (C1/C2 junction), the risk of inhibitor formation is fourfold greater than in patients with mutations outside this region, indicating that inhibitor prevalence in missense mutations is also dependent on localization of the mutation. Recently, a significant association between inhibitor formation and polymorphisms in genes coding for cytokines (IL-10) and other immunoregulatory factors (TNF-alpha) has been shown. These genetic factors constitute the individual genetic risk profile of a haemophilic patient. This risk is imprinted and fixed; however, environmental factors such as treatment schedule may increase or decrease the inhibitor risk in an individual patient. Improved understanding of these complex interactions may lead to the development of preventive measures to minimize inhibitor formation.

Chaperone-mediated autophagy in aging and disease

Different mechanisms target intracellular components for their degradation into lysosomes through what is known as autophagy. In mammals, three main forms of autophagy have been described: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA). CMA is the only autophagic pathway that allows selective degradation of soluble proteins in lysosomes. In contrast to the other mammalian forms of autophagy, CMA does not require vesicle formation or major changes in the lysosomal membrane. Instead, substrate proteins directly cross the lysosomal membrane to reach the lumen, where they are rapidly degraded. The substrate proteins are targeted to the lysosomal membrane by recognition of a targeting motif (a KFERQ-like motif), by a chaperone complex, consisting of hsc70 and its cochaperones, in the cytoplasm. Once at the lysosomal membrane, the protein interacts with a lysosomal receptor for this pathway, lysosomal associated membrane protein type 2A (LAMP-2A), and it is translocated across the membrane into the lysosomal lumen assisted by a lysosome resident chaperone. These two characteristics--selectivity and direct substrate translocation--determine the particular role of CMA in different physiological and pathological conditions. In this chapter, we cover current findings on the molecular mechanisms for CMA and the possible pathophysiological relevance of this selective lysosomal degradation.

Enhanced antigen-specific antibody production following polyplex-based DNA vaccination via the intradermal route in mice

DNA vaccination is an attractive approach with various advantages over conventional vaccination. The present study was undertaken to examine whether polyplex-based DNA vaccination could be used to modulate immune responses by plasmid DNA (pDNA). Methylated bovine serum albumin (mBSA) was used as a model of a cationic macromolecular carrier of pDNA encoding obalbumin (OVA) and the effects of polyplex formation of pDNA with mBSA on the antigen-specific immune responses were examined. Anti-OVA IgG antibody production was significantly increased following intradermal immunization with the polyplex compared with naked pDNA, although the induction of cytotoxic T lymphocyte activity was lowered by polyplex formation. We also demonstrated that the disposition and gene expression of pDNA following intradermal injection could be manipulated by polyplex formation. Intriguingly, we also found that the migration of dendritic cells to the injected site could be induced by polyplex formation probably due to a high level of tumor necrosis factor alpha production from the keratinocytes treated with mBSA/pDNA complexeses. Thus, the present study has demonstrated that the immune responses could be biased towards a Th2-type response by polyplex-based DNA vaccination through manipulation of not only pDNA disposition but also dendritic cell migration.

Immune surveillance of intracellular pathogens via autophagy

MHC class II molecules are thought to present peptides derived from extracellular proteins to CD4+ T cells, which are important mediators of adaptive immunity to infections. In contrast, autophagy delivers constitutively cytosolic material for lysosomal degradation and has so far been recognized as an efficient mechanism of innate immunity against bacteria and viruses. Recent studies, however, link these two pathways and suggest that intracellular cytosolic and nuclear antigens are processed for MHC class II presentation after autophagy.

The pleiotropic role of autophagy: from protein metabolism to bactericide

Autophagy is in principle a nonselective, bulk degradation system within cells, with a contribution to intracellular protein degradation estimated to be as large as that of the ubiquitin--proteasome system. The primary roles of autophagy are baseline turnover of intracellular proteins and organelles, production of amino acids in nutrient emergency, and regression of retired tissues. These functions guarantee rejuvenation and adaptation to adverse conditions, and even underlie dynamic processes such as development/metamorphosis. In addition, several other roles for autophagy have recently been discovered, such as presentation of endogenous antigens and degradation of invasive bacteria. This review will discuss the biological significance of autophagy from yeast to higher eukaryotes.

Tumor cells transduced with the MHC class II Transactivator and CD80 activate tumor-specific CD4+ T cells whether or not they are silenced for invariant chain

The specificity and potency of the immune system make immunotherapy a potential strategy for the treatment of cancer. To exploit this potential, we have developed cell-based cancer vaccines consisting of tumor cells expressing syngeneic MHC class II and costimulatory molecules. The vaccines mediate tumor regression in mice and activate human CD4+ T cells in vitro. Previous vaccines were generated by transducing MHC II negative tumor cells with a single HLA-DR allele. Because expression of multiple MHC II alleles would facilitate presentation of a broader repertoire of tumor antigens, we have now transduced tumor cells with the MHC class II transactivator (CIITA), a regulatory gene that coordinately increases expression of all MHC II alleles. Previous studies in mice indicated that coexpression of the MHC II accessory molecule invariant chain (Ii) inhibited presentation of endogenously synthesized tumor antigens and reduced vaccine efficacy. To determine if Ii expression affects presentation of MHC class II-restricted endogenously synthesized tumor antigens in human tumor cells, HLA-DR-MCF10 breast cancer cells were transduced with the CIITA, CD80 costimulatory molecule gene, and with or without small interfering RNAs (siRNA) specific for Ii. Ii expression is silenced >95% in CIITA/CD80/siRNA transductants; down-regulation of Ii does not affect HLA-DR expression or stability; and Ii(+) and Ii(-) transductants activate human CD4+ T cells to DRB1*0701-restricted HER-2/neu epitopes. Therefore, tumor cells transduced with the CIITA, CD80, and with or without Ii siRNA present endogenously synthesized tumor antigens and are potential vaccines for activating tumor-specific CD4+ T cells.

Optimization of CD4+ T lymphocyte response to human cytomegalovirus nuclear IE1 protein through modifications of both size and cellular localization

We have previously reported that the CD4+ T lymphocyte response against nuclear human CMV IE1 protein depends in part on endogenous MHC class II presentation. To optimize presentation by HLA-DR of the nuclear IE1 protein and increase the response by CD4+ T cells, we have constructed two different adenovirus vectors containing mutant versions of IE1, containing a HLA-DR3 epitope, fused to GFP. The first construct consisted of a sequence of 46 aa encoded by exon 4, called GFP-IE1 (86-131). The second construct consisted of the whole IE1 mutated on exon 4 nuclear localization signals, identified in this study, and deleted of already known exon 2 nuclear localization signals (GFP-IE1M). Both of these IE1 vectors expressed proteins with cytoplasmic localization, as evidenced by GFP expression, as opposed to control GFP-IE1, which was nuclear. GFP-IE1 (86-131) induced IE1-specific CD4+ T cell clone response that was >30-fold more potent than that against GFP-IE1 and GFP-IE1M. The CD4+ T cell response was due to endogenous presentation followed by exogenous presentation at later time points. Presentation was dependent on both proteasome and acidic compartments. GFP-IE1 (86-131) was rapidly degraded by the APC, which may account for better presentation. Our data show potentiation of the CD4+ T cell response to a specific epitope through shortening and relocation of an otherwise nuclear protein and suggest applications in vaccination.

Antigen Presentation by Human Uterine Epithelial Cells to Autologous T Cells

PROBLEM

Epithelial cells, as sentinels of immune protection in the endometrium, use innate immune mechanisms to protect against infection from pathogenic microbes. Our goal in this study was to assess the ability of human uterine epithelial cells to present antigen to cells of the adaptive immune system.

METHOD OF STUDY

Highly purified preparations of uterine epithelial cells from 11 patients were assessed for their ability to present tetanus toxoid (TT) to autologous T cells. Leukocyte contamination in the epithelial cell preparations was numerically and functionally determined. Using confocal microscopy, epithelial cells were tested for the expression of CD40 and CD1d.

RESULTS

Purified preparations of endometrial epithelial cells isolated from every patient presented TT recall antigen to autologous T cells. Leukocyte contamination of epithelial cell preparations was insignificant. Uterine epithelial cells express CD40 and CD1d.

CONCLUSION

Antigen presentation is an additional aspect of uterine epithelial cell function in maintaining women's health.

Cellular immune responses in hemophilia: why do inhibitors develop in some, but not all hemophiliacs?

Advances in molecular immunology over the past two decades permit a better understanding of why antibodies develop to peptide antigens like factor VIII and the events that lead to the development of these antibodies. Two important variables that are critical in antibody formation are (i) the molecular defect in FVIII and the consequences of that defect on translation and protein production, and (ii) the major histocompatibility complex (MHC) molecules which bind specific peptide sequences and present those peptides to CD4 T lymphocytes to initiate the cellular cascade leading to B-cell stimulation and differentiation, and ultimately to antibody formation. Inhibitors develop in hemophilia because transfused FVIII can be seen as a foreign protein and elicits an immune response in much the same way that any other foreign protein might elicit an immune response. However, not all hemophiliacs generate an immune response, either because they do not recognize FVIII as foreign or because their MHC phenotype is such that a cellular immune response is not initiated. In this model, it is the combination of molecular defect and MHC phenotype that determines inhibitor formation. The interplay of these two variables in the context of why some but not all hemophiliacs develop antibodies after treatment with replacement factor is reviewed.

MHC immunoevasins: protecting the pathogen reservoir in infection

Alteration of antigen recognition by T cells as result of insufficient major histocompatibility complex (MHC)-dependent antigen-presenting function has been observed in many cases of infections, particularly in in vitro systems. To hide themselves from an efficient immune response, pathogens may act on MHC-related functions at three levels: (i) by limiting the number of potential antigens that can be presented to naïve T cells; (ii) by synthesizing proteins which directly affect MHC cell-surface expression; and (iii) by altering the normal intracellular pathway of peptide loading on MHC. Here, we review examples of pathogens' action on each single step of MHC function and we suggest that the result of these often synergistic actions is both a limitation of the priming of naïve T cells and, more importantly, a protection of the pathogen's reservoir from the attack of primed T cells. The above mechanisms may also generate a skewing effect on immune effector mechanisms, which helps preserving the reservoir of infection from sterilization by the immune system.

Tumor-specific CD4+ T cells from a patient with renal cell carcinoma recognize diverse shared antigens

Murine models for immune-mediated tumor regression have defined an essential role for CD4+ T helper (Th) cells, but the contribution of these cells to antitumoral immune responses in humans remains poorly defined. Here, we investigated the Th cell response against the autologous tumor in a patient with metastasized renal cell carcinoma (RCC) exhibiting objective clinical response to immunotherapy. Peripheral blood T cells of the patient were repeatedly stimulated in vitro using either autologous IFNgamma-treated whole tumor cells or Epstein-Barr virus-immortalized B cells (EBV-B) pulsed with tumor cell lysate. CD4+ T-cell clones recognizing autologous tumor cells but not EBV-B cells were efficiently reactivated and expanded with both types of stimulator cells, establishing the latter as potentially useful for isolating CD4+ T cells reactive against MHC class II-negative tumors. Two T-cell clones from both stimulation protocol were further characterized. The restricting MHC class II molecules were defined by using allogeneic EBV-B cells pulsed with tumor lysate, and the expression pattern of the antigens was examined by analyzing lysates from normal kidney cells, allogeneic RCCs as well as tumors of different histologic origin. Furthermore, the subcellular localization of the antigens recognized by the T-cell clones was examined by fractionating the tumor lysate, and the Th phenotype was determined by assessing the cytokines released after T cell activation. These experiments show that a dual Th1/Th2, MHC class II-restricted T-helper-cell response against diverse shared tumor antigens has been elicited in this patient.

Autophagy promotes MHC class II presentation of peptides from intracellular source proteins

MHC-peptide complexes mediate key functions in adaptive immunity. In a classical view, MHC-I molecules present peptides from intracellular source proteins, whereas MHC-II molecules present antigenic peptides from exogenous and membrane proteins. Nevertheless, substantial crosstalk between these two pathways has been observed. We investigated the influence of autophagy on the MHC-II ligandome and demonstrated that peptide presentation is altered considerably upon induction of autophagy. The presentation of peptides from intracellular and lysosomal source proteins was strongly increased on MHC-II in contrast with peptides from membrane and secreted proteins. In addition, autophagy influenced the MHC-II antigen-processing machinery. Our study illustrates a profound influence of autophagy on the class II peptide repertoire and suggests that this finding has implications for the regulation of CD4(+) T cell-mediated processes.

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.

Distinct roles for donor- and host-derived antigen-presenting cells and costimulatory molecules in murine chronic graft-versus-host disease: requirements depend on target organ

The application of allogeneic stem cell transplantation (alloSCT) is limited by graft-versus-host disease (GVHD). GVHD can be divided into acute and chronic forms that likely have different requirements for initiation and pathogenesis mechanisms. In prior studies we demonstrated that residual host antigen-presenting cells (APCs) were required to initiate acute GVHD (aGVHD) mediated by CD8 T cells. In contrast, here we demonstrate that either donor or host APCs can initiate CD4-mediated GVHD in a model that has features of chronic GVHD (cGVHD). Both donor and host APCs must provide CD80/86-dependent costimulation to elicit maximal cGVHD, and there is no GVHD when both donor and host lack CD80/86. Finally, we were surprised to find that, although either donor or host APCs are sufficient to stimulate skin cGVHD, donor APCs play a dominant role in intestinal cGVHD. Both CD40 and CD80/86 are critical for donor APC function in intestinal cGVHD, but only CD80/86 is required for skin cGVHD. Thus, there are target-tissue–specific differences in APC requirements. These results identify differences in APC requirements between CD8-mediated aGVHD and CD4-mediated cGVHD. They further highlight donor APCs as additional targets for GVHD therapy.

Presentation of Endogenously Synthesized MHC Class II-Restricted Epitopes by MHC Class II Cancer Vaccines Is Independent of Transporter Associated with Ag Processing and the Proteasome

Cell-based vaccines consisting of invariant chain-negative tumor cells transfected with syngeneic MHC class II (MHC II) and costimulatory molecule genes are prophylactic and therapeutic agents for the treatment of murine primary and metastatic cancers. Vaccine efficacy is due to direct presentation of endogenously synthesized, MHC II-restricted tumor peptides to CD4+ T cells. Because the vaccine cells lack invariant chain, we have hypothesized that, unlike professional APC, the peptide-binding groove of newly synthesized MHC II molecules may be accessible to peptides, allowing newly synthesized MHC II molecules to bind peptides that have been generated in the proteasome and transported into the endoplasmic reticulum via the TAP complex. To test this hypothesis, we have compared the Ag presentation activity of multiple clones of TAP-negative and TAP-positive tumor cells transfected with I-Ak genes and the model Ag hen egg white lysozyme targeted to the endoplasmic reticulum or cytoplasm. Absence of TAP does not diminish Ag presentation of three hen egg white lysozyme epitopes. Likewise, cells treated with proteasomal and autophagy inhibitors are as effective APC as untreated cells. In contrast, drugs that block endosome function significantly inhibit Ag presentation. Coculture experiments demonstrate that the vaccine cells do not release endogenously synthesized molecules that are subsequently endocytosed and processed in endosomal compartments. Collectively, these data indicate that vaccine cell presentation of MHC II-restricted endogenously synthesized epitopes occurs via a mechanism independent of the proteasome and TAP complex, and uses a pathway that overlaps with the classical endosomal pathway for presentation of exogenously synthesized molecules.

Processing and presentation of HLA class I and II epitopes by dendritic cells after transfection with in vitro-transcribed MUC1 RNA

RNA transfection of dendritic cells (DCs) was shown to be highly efficient in eliciting CD8+ and CD4+ T-cell responses. However, antigen presentation pathways involved in generation of human leukocyte antigen (HLA) class I and class II peptides have remained elusive. To analyze this we incubated mucin 1 (MUC1) RNA-transfected DCs with compounds known to inhibit HLA class I presentation and used these cells in chromium 51 (51Cr)-release assays. As effectors, we used cytotoxic T lymphocyte (CTL) lines specific for the MUC1 peptides M1.1 and M1.2. We observed that the presentation of HLA-A*02 epitopes is inhibited by brefeldin A and lactacystin. To determine the requirement of a functional transporter associated with antigen processing (TAP), we cotransfected DCs with MUC1 and infected cell peptide 47 (ICP47) RNA. ICP47 could only inhibit the presentation of the M1.1 but not the M1.2 peptide, indicating that this epitope derived from the signal sequence is presented independently of TAP. Cocultivation of MUC1 RNA-transfected DCs with MUC1-specific CD4+ T lymphocytes revealed that the presentation of HLA class II peptides is sensitive to proteasomal inhibitors and brefeldin A. Furthermore, the presentation pathway requires lysosomal and endosomal processing and is mediated by autophagy. Our results demonstrate that the efficient presentation of cytosolic proteins on major histocompatibility complex (MHC) class II combines the proteolytic and lysosomal pathways.

“One size fits it all”: translocation of foreign antigens by Yersinia type III secretion system (TTSS) leads to concomitant CD4 and CD8 T-cell priming

Live replicating bacteria expressing heterologous antigens are vaccine candidates that are able to induce complex immune responses. Yersinia pseudotuberculosis employs a type III secretion system for translocation of several virulence factors directly to the cytosol of eukaryotic cells. Mice orally inoculated with an attenuated recombinant Yersinia strain translocating a chimeric Yersinia outer protein E (YopE) molecule reveal high numbers of foreign antigen-specific CD4 and CD8 T cells. Thus, cytosolic display of a single hybrid protein results in concomitant CD4 and CD8 T-cell priming. This "one-size-fits-it-all"-feature of Yersinia-translocated heterologous antigens might be advantageous to mount T-cellular immune responses against complex microbes and tumors.

T cell-based therapies for EBV-associated malignancies

Epstein-Barr virus (EBV) is associated with a number of human malignancies. The cells of these tumours express a range of EBV latent cycle gene products that have the potential to be exploited as targets for T cell-mediated immunological therapies. Considerable progress has been made in developing adoptive T cell transfer for EBV-associated post-transplant lymphoproliferative disease (PTLD) and clinical experience clearly demonstrates that EBV-specific T cell responses can be used to treat this EBV-associated malignancy. Adoptive T cell therapies for other EBV-associated malignancies are less advanced, although encouraging data are starting to emerge. Adoptive T cell transfer, however, does require significant levels of specialist laboratory support. Large-scale treatment of patients in geographical areas with a high prevalence of EBV-associated malignancy is likely to require the development of therapeutic vaccination strategies, a number of which are in development at present. Although it remains to be seen whether long-lasting sterilising immunity to EBV could be achieved, an alternative vaccine-based approach would be to develop a prophylactic vaccine to protect against primary EBV infection.

In vivo analysis of autophagy in response to nutrient starvation using transgenic mice expressing a fluorescent autophagosome marker

Macroautophagy mediates the bulk degradation of cytoplasmic components. It accounts for the degradation of most long-lived proteins: cytoplasmic constituents, including organelles, are sequestered into autophagosomes, which subsequently fuse with lysosomes, where degradation occurs. Although the possible involvement of autophagy in homeostasis, development, cell death, and pathogenesis has been repeatedly pointed out, systematic in vivo analysis has not been performed in mammals, mainly because of a limitation of monitoring methods. To understand where and when autophagy occurs in vivo, we have generated transgenic mice systemically expressing GFP fused to LC3, which is a mammalian homologue of yeast Atg8 (Aut7/Apg8) and serves as a marker protein for autophagosomes. Fluorescence microscopic analyses revealed that autophagy is differently induced by nutrient starvation in most tissues. In some tissues, autophagy even occurs actively without starvation treatments. Our results suggest that the regulation of autophagy is organ dependent and the role of autophagy is not restricted to the starvation response. This transgenic mouse model is a useful tool to study mammalian autophagy.

Inhibition of invariant chain expression in dendritic cells presenting endogenous antigens stimulates CD4+ T-cell responses and tumor immunity

Induction of potent and sustained antiviral or antitumor immunity is dependent on the efficient activation of CD8+ and CD4+ T cells. While dendritic cells constitute a powerful platform for stimulating cellular immunity, presentation of endogenous antigens by dendritic cells transfected with nucleic acid-encoded antigens favors the stimulation of CD8+ T cells over that of CD4+ T cells. A short incubation of mRNA-transfected dendritic cells with antisense oligonucleotides directed against the invariant chain enhances the presentation of mRNA-encoded class II epitopes and activation of CD4+ T-cell responses in vitro and in vivo. Immunization of mice with the antisense oligonucleotide-treated dendritic cells stimulates a more potent and longer lasting CD8+ cytotoxic T-cell (CTL) response and enhances the antitumor efficacy of dendritic cell-based tumor vaccination protocols. Transient inhibition of invariant chain expression represents a simple and general method to enhance the stimulation of CD4+ T-cell responses from endogenous antigens.

Attenuated Yersinia pseudotuberculosis carrier vaccine for simultaneous antigen-specific CD4 and CD8 T-cell induction

Yersinia pseudotuberculosis employs a type III secretion system for targeting of several virulence factors directly to the cytosol of eukaryotic cells. This protein translocation mechanism mediates the ability of Yersinia to resist phagocytosis and is required for sustained extracellular bacterial replication. In the present study, the Yersinia outer protein E (YopE) was used as a carrier molecule for type III-dependent secretion and translocation of listeriolysin O (LLO) from Listeria monocytogenes. In comparison to wild-type Yersinia, an attenuated Y. pseudotuberculosis yopK-null mutant strain hypertranslocates chimeric YopE/LLO into the cytosol of macrophages, resulting in enhanced major histocompatibility complex (MHC) class I-restricted antigen presentation of an LLO-derived CD8 T-cell epitope. Remarkably, T-cell activation assays also revealed a superior ability of translocated over secreted LLO to induce MHC class II-restricted antigen presentation. These in vitro observations were confirmed after immunization of mice with a single dose of the yopK-null mutant strain. Animals orally inoculated with recombinant Yersinia expressing translocated chimeric YopE/LLO revealed high numbers of gamma interferon-producing LLO-specific CD4 and CD8 T cells. For the first time, it is shown that cytosolic antigen display mediated by an extracellular bacterial carrier vaccine results in simultaneous CD4 and CD8 T-cell priming, conferring protection against an intracellular pathogen.

Presentation by recycling MHC class II molecules of an influenza hemagglutinin-derived epitope that is revealed in the early endosome by acidification

We investigated the roles of nascent and recycling MHC class II molecules (MHC II) in the presentation of two well-defined I-E(d)-restricted epitopes that are within distinct regions of the influenza virus hemagglutinin (HA) protein. The site 3 epitope (S3; residues 302-313) lies in the stalk region that unfolds in response to mild acidification, while the site 1 epitope (S1; residues 107-119) is situated in the stable globular domain. In a murine B lymphoma cell line and an I-E(d)-transfected fibroblast cell line, presentation from inactivated virus of S3 is inhibited by primaquine, a compound that prevents recycling of cell surface proteins, including MHC II, while S1 presentation is unaffected. In contrast, brefeldin A, an agent that inhibits exit of proteins from the endoplasmic reticulum, selectively inhibited S1 presentation without affecting S3 presentation, suggesting that S1 presentation requires nascent MHC II. The use of agents that perturb endosomal function revealed a requirement for acidification of internalized viral particles for presentation of both epitopes. Notably, all compounds tested had similar effects on presentation of the two epitopes derived from endogenously synthesized HA. Thus, recycling I-E(d) molecules appear to be crucial for capturing and presenting an epitope that is revealed in mild acidic conditions following the uptake of virions or the synthesis of Ag, while nascent I-E(d) molecules are required for presentation of a second epitope located in a structurally constrained region of the same polypeptide. Viral glycoproteins, such as HA, may have been a major impetus for the evolutionary establishment of this recycling pathway.

Infection of mouse bone marrow-derived dendritic cells with recombinant adenovirus vectors leads to presentation of encoded antigen by both MHC class I and class II molecules-potential benefits in vaccine design

Dendritic cells (DCs) are highly specialised antigen-presenting cells (APCs) that are essential for the initiation and modulation of T cell-mediated immune responses. In order to induce effective CTL responses against most infections and tumours, DCs must prime both CD4(+) and CD8(+) antigen-specific T cells. It is, therefore, important in vaccine design to produce antigen-delivery systems that lead to the simultaneous presentation of multiple histocompatibility complex (MHC) class I- and class II-restricted antigenic peptides by DCs. In this study, the infection of immature mouse bone marrow-derived DCs (BMDCs) with recombinant adenovirus (rAd) vectors led to a marked upregulation of surface costimulatory molecules, IL-12 p40 production and capacity to stimulate both allogeneic and antigen-specific T cells. Furthermore, infection of immature and mature BMDCs with a rAd encoding chicken ovalbumin (OVA) led to presentation of the antigen to TCR-transgenic OVA-specific CD4(+) and CD8(+) T cells. In addition, the activation state of responding CD8(+) T cells was further amplified if they recognised antigen on rAd-transduced BMDCs in the presence of antigen-specific CD4(+) T cells. The results suggest that rAd-encoded OVA protein is secreted by BMDCs, taken up by endocytosis and presented in association with MHC class II molecules for activation of OVA-specific CD4(+) T cells. Consequently, rAd-transduced immature BMDCs become better stimulators of antigen-specific CD4(+) T cells than rAd-infected mature BMDCs. Taken together, these data have important implications for vaccine design, and suggest that infection of immature DCs with rAd encoding MHC class I and class II-restricted T cell epitopes could be an efficient means of inducing effective immune responses.

RNA based vaccines

The recognition that CD8(+) T-cell mediated Th1 immune responses were necessary to produce immunity to intracellular and transformed self pathogens led to intense interest in the delivery of nucleic acids, DNA, or RNA encoding candidate antigens, as vaccines. Antigen presenting cells (APC) encounter most protein and vaccine immunogens as extracellular proteins and, thus, present them on major histocompatibility complex (MHC) class II molecules leading to the activation of CD4(+) T cells. Protein antigens encoded by nucleic acids delivered to dendritic cell (DC) are produced inside the cell and, thus, can stimulate MHC class I mediated activation of CD8(+) T-cell immune responses. Unfortunately, DCs are not readily transfected with DNA (Akbari et al., 1999) resulting in the requirement for high concentrations of DNA and repeated immunizations to achieved immune responses. RNA, on the other hand, is readily taken up and expressed by DC, making it an alternative vaccine candidate. In this article, we will discuss immune responses developed, interactions between APC and RNA that activate and dictate DC activation, and preliminary studies using RNA in vivo and in vitro to develop protective immunity.

Infection of APC by human cytomegalovirus controlled through recognition of endogenous nuclear immediate early protein 1 by specific CD4(+) T lymphocytes

Infections by human CMV are controlled by cellular immune responses. Professional APC such as monocytes and macrophages can be infected in vivo and are considered as a reservoir of virus. However, CMV-specific CD4(+) responses against infected APC have not been reported. To develop a model of CD4-infected APC interaction, we have transfected the U373MG astrocytoma cell line with the class II transactivator (CIITA). Confocal microscopy experiments showed that U373MG-CIITA cells expressed markers characteristic of APC. Functional assays demonstrated that infected U373MG-CIITA APC processed and presented both exogenous and endogenously neosynthesized nuclear immediate early (IE) protein 1 through the MHC class II pathway. More importantly, endogenous presentation of IE1 by infected APC lead to efficient control of CMV infection as revealed by decreased viral titer. Thus, these results describe the endogenous presentation of a nuclear viral protein by the MHC class II pathway and suggest that IE1-specific CD4(+) T cells may play an important role in CMV infection by directly acting against infected APC.

Selective modification of antigen-specific CD4(+) T cells by retroviral-mediated gene transfer and in vitro sensitization with dendritic cells

Adoptive therapy with antigen-specific T cells is a potential treatment against cancers and viral diseases. To establish a system to modify the genes of these cells to increase their effectiveness, we examined whether the combined use of retroviral vector, which only infects dividing cells, and in vitro sensitization of T cells with antigen-loaded dendritic cells (DCs) could selectively modify antigen-specific T cells with a bcl-2 gene. Human CD4(+) T cells were used as target cells. Autologous DCs transfected with genes of hepatitis B virus (HBV) stimulated a specific T cell proliferation. Importantly, these proliferating T cells were selectively transduced by a bcl-2-retrovirus, and CD25(+) T cells isolated from them contained higher levels of integrated provirus. To select bcl-2-transduced, activated T cells, cells were subjected to interleukin-2 (IL-2) withdrawal. In contrast to CD25(-) and mock-infected CD25(+) T cells, 70% of CD25(+) T cells transduced with bcl-2-retrovirus survived IL-2 withdrawal. These surviving T cells were demonstrated to contain integrated bcl-2 provirus and exhibited HBV-specific proliferation and interferon-gamma secretion. In addition, bcl-2 overexpression protected HBV-specific T cells from transforming growth factor (TGF)-beta-induced cell death. These results demonstrate the feasibility of our strategy in the generation of genetically modified antigen-specific CD4(+) T cells and show that bcl-2-transduced antigen-specific T cells survive IL-2 withdrawal and TGF-beta-induced apoptosis.

Diversity in MHC class II antigen presentation

Processing exogenous and endogenous proteins for presentation by major histocompatibility complex (MHC) molecules to T cells is the defining function of antigen-presenting cells (APC) as major regulatory cells in the acquired immune response. MHC class II-restricted antigen presentation to CD4 T cells is achieved by an essentially common pathway that is subject to variation with regard to the location and extent of degradation of protein antigens and the site of peptide binding to MHC class II molecules. These subtle variations reveal a surprising flexibility in the ways a diverse peptide repertoire is displayed on the APC surface. This diversity may have profound consequences for the induction of immunity to infection and tumours, as well as autoimmunity and tolerance.