Endogenous antigen presentation by MHC class II molecules

Construction and evaluation of DNA vaccine encoding Ebola virus glycoprotein fused with lysosome-associated membrane protein

Ebola virus (EBOV) of the genus Ebolavirus belongs to the family Filoviridae, which cause disease in both humans and non-human primates. Zaire Ebola virus accounts for the highest fatality rate, reaching 90%. Considering that EBOV has a high infection and fatality rate, the development of a highly effective vaccine has become a top public health priority. Glycoprotein (GP) plays a critical role during infection and protective immune responses. Herein, we developed an EBOV GP recombinant DNA vaccine that targets the major histocompatibility complex (MHC) class II compartment by fusing with lysosomal-associated membrane protein 1 (LAMP1). Through lysosome trafficking and antigen presentation transferring, the LAMP1 targeting strategy successfully improved both humoral and cellular EBOV-GP-specific immune responses. After three consecutive immunizations, the serum antibody titers, especially the neutralizing activity of mice immunized with the pVAX-LAMP/GPEBO vaccine were significantly higher than those of the other groups. Antigen-specific T cells showed positive activity against three dominant peptides, EAAVSHLTTLATIST, IGEWAFWETKKNLTR, and ELRTFSILNRKAIDF, with high affinity for MHC class II molecules predicted by IEDB-recommended. Preliminary safety observation denied histological alterations. DNA vaccine candidate pVAX-LAMP/GPEBO shows promise against Ebola epidemic and further evaluation is guaranteed.

Innate and adaptive immune responses toward nanomedicines

Since the commercialization of the first liposomes used for drug delivery, Doxil/Caelyx® and Myocet®, tremendous progress has been made in understanding interactions between nanomedicines and biological systems. Fundamental work at the interface of engineering and medicine has allowed nanomedicines to deliver therapeutic small molecules and nucleic acids more efficiently. While nanomedicines are used in oncology for immunotherapy or to deliver combinations of cytotoxics, the clinical successes of gene silencing approaches like patisiran lipid complexes (Onpattro®) have paved the way for a variety of therapies beyond cancer. In parallel, the global severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has highlighted the potential of mRNA vaccines to develop immunization strategies at unprecedented speed. To rationally design therapeutic and vaccines, chemists, materials scientists, and drug delivery experts need to better understand how nanotechnologies interact with the immune system. This review presents a comprehensive overview of the innate and adaptative immune systems and emphasizes the intricate mechanisms through which nanomedicines interact with these biological functions.

Viral vectors as vaccine carriers

This chapter reviews the performance of viral vectors based on adenoviruses or adeno-associated virus as vaccine carriers for infectious diseases. Replication-defective adenovirus vectors based on multiple human or non-human serotypes have consistently induced potent transgene product-specific B and T cell responses and are increasingly being explored in human clinical trials. The immunogenicity of most vectors based on adeno-associated virus vectors has been poor with the exception of a recently described hybrid vector from rhesus macaques that due to its ability to induce potent responses in mice warrant further investigation.

Differential capacity of human interleukin-4 and interferon-α monocyte-derived dendritic cells for cross-presentation of free versus cell-associated antigen

Dendritic cells (DC) vaccination is a potent therapeutic approach for inducing tumor-directed immunity, but challenges remain. One of the particular interest is the induction of an immune response targeting multiple (unknown) tumor-associated antigens (TAA), which requires a polyvalent source of TAA. Previously, we described the preferred use of apoptotic cell-derived blebs over the larger apoptotic cell remnants, as a source of TAA for both in situ loading of skin-resident DC and in vitro loading of monocyte-derived DC (MoDC). Recent reports suggest that MoDC cultured in the presence of GM-CSF supplemented with IFNα (IFNα MoDC), as compared to IL-4 (IL-4 MoDC), have an increased capacity to cross-present antigen to CD8(+) T cells. As culture conditions, maturation methods and antigen sources differ between the conducted studies, we analyzed the functional differences between IL-4 MoDC and IFNα MoDC, loaded with blebs, in a head-to-head comparison using commonly used protocols. Our data show that both MoDC types are potent (cross-) primers of CD8(+) T cells. Whereas IFNα MoDC were more potent in their capacity to cross-present a 25-mer MART-1 synthetic long peptide (SLP) to a MART-1aa26-35 recognizing CD8(+) T cell line, IL-4 MoDC proved more potent cross-primers of antigen-specific CD8(+) T cells when loaded with blebs. The latter is likely due to the observed greater capacity of IL-4 MoDC to ingest apoptotic blebs. In conclusion, our data indicate the use of IFNα MoDC over IL-4 MoDC in the context of DC vaccination with SLP, whereas IL-4 MoDC are preferred for vaccination with bleb-derived antigens.

Immunodominance in CD4 T-cell responses: implications for immune responses to influenza virus and for vaccine design

CD4 T cells play a primary role in regulating immune responses to pathogenic organisms and to vaccines. Antigen-specific CD4 T cells provide cognate help to B cells, a requisite event for immunoglobulin switch and affinity maturation of B cells that produce neutralizing antibodies and also provide help to cytotoxic CD8 T cells, critical for their expansion and persistence as memory cells. Finally, CD4 T cells may participate directly in pathogen clearance via cell-mediated cytotoxicity or through production of cytokines. Understanding the role of CD4 T-cell immunity to viruses and other pathogens, as well as evaluation of the efficacy of vaccines, requires insight into the specificity of CD4 T cells. This review focuses on the events within antigen-presenting cells that focus CD4 T cells toward a limited number of peptide antigens within the pathogen or vaccine. The molecular events are discussed in light of the special challenges that the influenza virus poses, owing to the high degree of genetic variability, unpredictable pathogenicity and the repeated encounters that human populations face with this highly infectious pathogenic organism.

The utility and limitations of current Web-available algorithms to predict peptides recognized by CD4 T cells in response to pathogen infection

The ability to track CD4 T cells elicited in response to pathogen infection or vaccination is critical because of the role these cells play in protective immunity. Coupled with advances in genome sequencing of pathogenic organisms, there is considerable appeal for implementation of computer-based algorithms to predict peptides that bind to the class II molecules, forming the complex recognized by CD4 T cells. Despite recent progress in this area, there is a paucity of data regarding the success of these algorithms in identifying actual pathogen-derived epitopes. In this study, we sought to rigorously evaluate the performance of multiple Web-available algorithms by comparing their predictions with our results--obtained by purely empirical methods for epitope discovery in influenza that used overlapping peptides and cytokine ELISPOTs--for three independent class II molecules. We analyzed the data in different ways, trying to anticipate how an investigator might use these computational tools for epitope discovery. We come to the conclusion that currently available algorithms can indeed facilitate epitope discovery, but all shared a high degree of false-positive and false-negative predictions. Therefore, efficiencies were low. We also found dramatic disparities among algorithms and between predicted IC(50) values and true dissociation rates of peptide-MHC class II complexes. We suggest that improved success of predictive algorithms will depend less on changes in computational methods or increased data sets and more on changes in parameters used to "train" the algorithms that factor in elements of T cell repertoire and peptide acquisition by class II molecules.

Urothelial antigen-specific CD4+ T cells function as direct effector cells and induce bladder autoimmune inflammation independent of CD8+ T cells

The role of CD4(+) T cells in bladder autoimmune inflammation has not been identified because of the lack of a proper animal model. We investigated CD4(+) T-cell responses to bladder urothelial ovalbumin (OVA), a model self-antigen (Ag), in transgenic URO-OVA mice. The expression of bladder urothelial OVA rendered mice unresponsive to OVA and resulted in quick clearance of Ag-specific CD4(+) T cells. Adoptive transfer of naive OVA-specific CD4(+) T cells led to exogenous T-cell proliferation, activation, and bladder infiltration but no inflammatory induction. In contrast, adoptive transfer of preactivated OVA-specific CD4(+) T cells induced bladder inflammation. Studies further demonstrated that CD4(+) T cells induced bladder inflammation in URO-OVA mice depleted of CD8(+) T cells or deficient in the recombinase activating gene-1 (Rag-1(-/-)). These results indicate that urothelial Ag-specific CD4(+) T cells can function as direct effector cells to induce bladder autoimmune inflammation independent of CD8(+) T cells.

Functional macroautophagy induction by influenza A virus without a contribution to major histocompatibility complex class II-restricted presentation

Major histocompatibility complex (MHC) class II-presented peptides can be derived from both exogenous (extracellular) and endogenous (biosynthesized) sources of antigen. Although several endogenous antigen-processing pathways have been reported, little is known about their relative contributions to global CD4(+) T cell responses against complex antigens. Using influenza virus for this purpose, we assessed the role of macroautophagy, a process in which cytosolic proteins are delivered to the lysosome by de novo vesicle formation and membrane fusion. Influenza infection triggered productive macroautophagy, and autophagy-dependent presentation was readily observed with model antigens that naturally traffic to the autophagosome. Furthermore, treatments that enhance or inhibit macroautophagy modulated the level of presentation from these model antigens. However, validated enzyme-linked immunospot (ELISpot) assays of influenza-specific CD4(+) T cells from infected mice using a variety of antigen-presenting cells, including primary dendritic cells, revealed no detectable macroautophagy-dependent component. In contrast, the contribution of proteasome-dependent endogenous antigen processing to the global influenza CD4(+) response was readily appreciated. The contribution of macroautophagy to the MHC class II-restricted response may vary depending upon the pathogen.

Enhancement of DNA vaccine potency by antigen linkage to IFN-γ-inducible protein-10

DNA vaccines have emerged as an attractive approach to generate antigen-specific T-cell immune response. Nevertheless, the potency of DNA vaccines still needs to be improved for cancer immunotherapy. In this study, we explored whether functional linkage of a Th1-polarizing chemokine, IP-10, to a model tumor antigen, human papillomavirus type 16 (HPV-16) E7, enhanced DNA vaccine potency. IP-10 linkage changed the location of E7 from the nucleus to the endoplasmic reticulum and led to the secretion of functionally chemoattractive chimeric IP-10/E7 protein. In addition, this linkage drastically enhanced the endogenous processing of E7 antigen through MHC class I. More importantly, we found that C57BL/6 mice intradermally vaccinated with IP-10/E7 DNA exhibited a dramatic increase in the number of E7-specific CD4(+) Th1 T-cells and CD8(+) T-cells and, consequently, were strongly resistant over the long term to E7-expressing tumors compared to mice vaccinated with wild-type E7 DNA. Thus, because of the increase in tumor antigen-specific T-cell immune responses obtained through both enhanced antigen presentation and chemoattraction, vaccination with DNA encoding IP-10 linked to a tumor antigen holds great promise for treating tumors.

Recipient B cells are not required for graft-versus-host disease induction

Recipient antigen presenting cells (APCs) are required for CD8-mediated graft-versus-host disease (GVHD), and have an important and nonredundant role in CD4-mediated GVHD in mouse major histocompatibility complex-matched allogeneic bone marrow transplantation (alloBMT). However, the precise roles of specific recipient APCs-dendritic cells, macrophages, and B cells-are not well defined. If recipient B cells are important APCs they could be depleted with rituximab, an anti-CD20 monoclonal antibody. On the other hand, B cells can downregulate T cell responses, and consequently, B cell depletion could exacerbate GVHD. Patients with B cell lymphomas undergo allogeneic hematopoietic stem cell transplantation (alloSCT) and many are B-cell-deficient because of prior rituximab. We therefore studied the role of recipient B cells in major histocompatibility complex-matched murine models of CD8- and CD4-mediated GVHD by using recipients genetically deficient in B cells and with antibody-mediated depletion of host B cells. In both CD4- and CD8-dependent models, B cell-deficient recipients developed clinical and pathologic GVHD. However, although CD8-mediated GVHD was clinically less severe in hosts genetically deficient in B cells, it was unaffected in anti-CD20-treated recipients. These data indicate that recipient B cells are not important initiators of GVHD, and that efforts to prevent GVHD by APC depletion should focus on other APC subsets.

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.

Molecular cloning and characterization of MHC class I- and II-restricted tumor antigens recognized by T cells

T cells play a central role in cancer immunosurveillance, autoimmune, and infectious diseases. Identification of MHC class I- and II-restricted T cell peptides is a critical step for the development of effective vaccines against cancer and infectious diseases. This unit describes a cDNA expression system and a genetic targeting expression system for the cloning of genes encoding for MHC class I- and II-restricted antigens recognized by antigen-specific CD8(+) and CD4(+) T cells.

In vivo antigen stability affects DNA vaccine immunogenicity

The factors that determine the immunogenicity of Ags encoded by viral vaccines or DNA vaccines in vivo are largely unknown. Depending on whether T cell induction occurs via direct presentation of vaccine-encoded epitopes or via one of the different proposed pathways for Ag cross-presentation, the effect of intracellular Ag stability on immunogenicity may possibly vary. However, the influence of Ag stability on CD8(+) T cell induction has not been addressed in clinically relevant vaccine models, nor has the accumulation of vaccine-encoded Ags been monitored in vivo. In this study, we describe the relationship between in vivo Ag stability and immunogenicity of DNA vaccine-encoded Ags. We show that in vivo accumulation of DNA vaccine-encoded Ags is required for the efficient induction of CD8(+) T cell responses. These data suggest that many of the currently used transgene designs in DNA vaccination trials may be suboptimal, and that one should either use pathogen-derived or tumor-associated Ags that are intrinsically stable, or should increase the stability of vaccine-encoded Ags by genetic engineering.

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.

Differential immunogenicity of a DNA vaccine containing the Streptococcus mutans wall-associated protein A gene versus that containing a truncated derivative antigen A lacking in the hydrophobic carboxyterminal region

Two plasmid DNA constructs were obtained by cloning separately into the eukaryotic expression vector pcDNA3.1/V5-His-TOPO the wall-associated protein A (wapA) gene of Streptococcus mutans GS-5 or its truncated derivative antigen A (agA) gene encoding a known candidate antigen for dental caries vaccine. The immunogenicity of the two constructs, designated pcDNA-wapA and pcDNA-agA, was compared by intranasal immunization of two groups of mice using the cationic DMRIE-C (1,2-dimyristyloxypropyl-3-dimethylhydroxy ethyl ammonium bromide-cholesterol) as an adjuvant. Immunization with pcDNA-wapA or pcDNA- agA resulted in specific salivary IgA and systemic IgG antibodies to the target antigens after two doses given at 3-week intervals. Higher salivary IgA level was observed in the mice immunized with the pcDNA-wapA vaccine compared to those immunized with the pcDNA-agA vaccine. Furthermore, anti-WapA antibody inhibited S. mutans sucrose-dependent adherence suggesting a potential protection against S. mutans colonization of the tooth, while anti-AgA had no significant effect. Indeed, prediction and analysis of protein epitopes showed that WapA contains highly promiscuous MHC-II binding motifs in addition to those found in AgA. Immunodot assay confirmed that WapA bound biotin-labeled dextran, whereas AgA did not. These data indicated that full-length WapA is a better candidate vaccine antigen than the soluble AgA, which is truncated in the hydrophobic membrane and wall-spanning region.

The relationship between immunodominance, DM editing, and the kinetic stability of MHC class II:peptide complexes

Immunodominance refers to the restricted antigen specificity of T cells detected in the immune response after immunization with complex antigens. Despite the presence of many potential peptide epitopes within these immunogens, the elicited T-cell response apparently focuses on a very limited number of peptides. Over the last two decades, a number of distinct explanations have been put forth to explain this very restricted specificity of T cells, many of which suggest that endosomal antigen processing restricts the array of peptides available to recruit CD4 T cells. In this review, we present evidence from our laboratory that suggest that immunodominance in CD4 T-cell responses is primarily due to an intrinsic property of the peptide:class II complexes. The intrinsic kinetic stability of peptide:class II complexes controls DM editing within the antigen-presenting cells and thus the initial epitope density on priming dendritic cells. Additionally, we hypothesize that peptides that possess high kinetic stability interactions with class II molecules display persistence at the cell surface over time and will more efficiently promote T-cell signaling and differentiation than competing, lower-stability peptides contained within the antigen. We discuss this model in the context of the existing data in the field of immunodominance.

Human autologous tumor-specific T-cell responses induced by liposomal delivery of a lymphoma antigen

PURPOSE

The idiotype (Id) of the immunoglobulin on a given B-cell malignancy is a clonal marker that can serve as a tumor-specific antigen. We developed a novel vaccine formulation by incorporating Id protein with liposomal lymphokine that was more potent than a prototype, carrier-conjugated Id protein vaccine in preclinical studies. In the present study, we evaluated the safety and immunogenicity of this vaccine in follicular lymphoma patients.

EXPERIMENTAL DESIGN

Ten patients with advanced-stage follicular lymphoma were treated with five doses of this second generation vaccine after chemotherapy-induced clinical remission. All patients were evaluated for cellular and humoral immune responses.

RESULTS

Autologous tumor and Id-specific type I cytokine responses were induced by vaccination in 10 and 9 patients, respectively. Antitumor immune responses were mediated by both CD4+ and CD8+ T cells, were human lymphocyte antigen class I and II associated, and persisted 18 months beyond the completion of vaccination. Specific anti-Id antibody responses were detected in four patients. After a median follow-up of 50 months, 6 of the 10 patients remain in continuous first complete remission.

CONCLUSIONS

This first clinical report of a liposomal cancer vaccine demonstrates that liposomal delivery is safe, induces sustained tumor-specific CD4+ and CD8+ T-cell responses in lymphoma patients, and may serve as a model for vaccine development against other human cancers and infectious pathogens.

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.

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.

A modified human ELISPOT assay to detect specific responses to primary tumor cell targets

Background

The desired outcome of cancer vaccination is to induce a potent T cell response which can specifically recognize and eliminate autologous tumor cells in vivo. Accordingly, immunological assays that demonstrate recognition of native tumor cells (tumor-specific) may be more clinically relevant than assays that demonstrate recognition of tumor protein or peptide (antigen-specific).

Methods

Towards this goal, we adapted the IFN-γ ELISPOT assay to measure immune responses against autologous primary tumor cells in vaccinated cancer patients. As a model system to develop the assay, we utilized peripheral blood mononuclear cells (PBMC) directly isolated from follicular lymphoma patients vaccinated with tumor-derived idiotype protein.

Results

After optimizing several variables, we demonstrated that the modified IFN-γ ELISPOT assay could be used to reliably and reproducibly determine the tumor-reactive T cell frequency in the PBMC of these patients. The precursor frequency of tumor-reactive T cells was significantly higher in the postvaccine PBMC, compared with prevaccine samples in all patients tested. Furthermore, the specificity of these T cells was established by the lack of reactivity against autologous normal B cells.

Conclusions

These results demonstrate the feasibility of quantitating tumor-specific T cell responses when autologous, primary tumor cells are available as targets.

Synthesis, conformation and T-helper cell stimulation of an O-linked glycopeptide epitope containing extended carbohydrate side-chains

To answer the question whether or not T cells to immunodominant protein fragments recognize glycosylated antigens, we synthesized a series of glycopeptides corresponding to peptide 31D, a major T-helper cell epitope of the rabies virus nucleoprotein. Thr4 of the epitope is known to allow mono- or disaccharide side-chain substitutions in either alpha- or beta-anomeric configuration without interfering with MHC-binding. To model naturally occurring glycoprotein fragments that carry extended sugar chains, we prepared Fmoc-Ser/Thr-OPfp building blocks containing alpha- and beta-linked linear tri- and heptasaccharides. Peptide 31D was synthesized with the complex carbohydrates attached to Thr4, and the T-helper cell activity of the glycopeptides was determined. Addition of alpha-linked carbohydrates, that mimic most of the natural O-linked glycoproteins, resulted in a major drop in the T-cell stimulatory ability in a sugar length-dependent manner. In contrast, the cytosolic glycoprotein mimicking beta-linked glycopeptides retained their T-cell stimulatory activity, with the trisaccharide-containing analogue being almost as potent as the unglycosylated peptide. When the peptides were preincubated with diluted human serum, all peptides lost their ability to stimulate the 9C5.D8-H hybridoma. These findings indicated that (i) in contrast to cytosolic glycosylation, incorporation of long O-linked carbohydrates into T-helper cell epitopes abrogates the antigenicity of these protein fragments, and (ii) glycosylation is not a viable alternative to improve the immunogenic properties of subunit peptide vaccines. Glycosylation with all four carbohydrate moieties similarly destroyed the inducible alpha-helical structure of peptide 31D as detected by CD, indicating that the differences in the T-cell activity were not due to different peptide conformations.

H2-O inhibits presentation of bacterial superantigens, but not endogenous self antigens

H2-O/HLA-DO are MHC class II accessory molecules that modulate exogenous Ag presentation. Most class II accessory molecules are expressed in all professional APC; however, H2-O is only expressed in B cells and medullary thymic epithelial cells. Because B cells present exogenous Ags and superantigens (SAgs), and medullary thymic epithelial cells are specialized APC for self Ags during negative selection in the thymus, we have hypothesized that H2-O might play a role in MHC class II-restricted SAg and self Ag presentation. In this study, we demonstrate that H2-O expression inhibits presentation of the bacterial SAgs staphylococcal enterotoxins A and B to four SAg-reactive T hybridoma cells. In contrast, H2-O has no effect on presentation of endogenous self Ags, as measured by tumorigenicity in vivo and Ag presentation to three self Ag-specific T hybridoma cells. Additional experiments suggest that H2-O inhibits presentation of exogenous Ags by both newly synthesized and recycling MHC class II molecules. These data suggest H2-O may have a physiological role in tolerance induction and SAg-mediated toxic shock.

Tumor cells present MHC class II-restricted nuclear and mitochondrial antigens and are the predominant antigen presenting cells in vivo

MHC class II-restricted tumor Ags presented by class II(+) tumor cells identified to date are derived from proteins expressed in the cytoplasm or plasma membrane of tumor cells. It is unclear whether MHC class II(+) tumor cells present class II-restricted epitopes derived from other intracellular compartments, such as nuclei and/or mitochondria, and whether class II(+) tumor cells directly present Ag in vivo. To address these questions, a model Ag, hen egg lysozyme, was targeted to various subcellular compartments of mouse sarcoma cells, and the resulting cells were tested for presentation of three lysozyme epitopes in vitro and for presentation of nuclear Ag in vivo. In in vitro studies, Ags localized to all tested compartments (nuclei, cytoplasm, mitochondria, and endoplasmic reticulum) are presented in the absence invariant chain and H-2M. Coexpression of invariant chain and H-2M inhibit presentation of some, but not all, of the epitopes. In vivo studies demonstrate that class II(+) tumor cells, and not host-derived cells, are the predominant APC for class II-restricted nuclear Ags. Because class II(+) tumor cells are effective APC in vivo and probably present novel tumor Ag epitopes not presented by host-derived APC, their inclusion in cancer vaccines may enhance activation of tumor-reactive CD4(+) T cells.

MHC class II presentation of endogenous tumor antigen by cellular vaccines depends on the endocytic pathway but not H2-M

We have developed cell-based cancer vaccines that activate anti-tumor immunity by directly presenting endogenously synthesized tumor antigens to CD4+ T helper lymphocytes via MHC class II molecules. The vaccines are non-conventional antigen-presenting cells because they express MHC class II, do not express invariant chain or H-2M, and preferentially present endogenous antigen. To further improve therapeutic efficacy we have studied the intracellular trafficking pathway of MHC class II molecules in the vaccines using endoplasmic reticulumlocalized lysozyme as a model antigen. Experiments using endocytic and cytosolic pathway inhibitors (chloroquine, primaquine, and brefeldin A) and protease inhibitors (lactacystin, LLnL, E64, and leupeptin) indicate antigen presentation depends on the endocytic pathway, although antigen degradation is not mediated by endosomal or proteasomal proteases. Because H2-M facilitates presentation of exogenous antigen via the endocytic pathway, we investigated whether transfection of vaccine cells with H-2M could potentiate endogenous antigen presentation. In contrast to its role in conventional antigen presentation, H-2M had no effect on endogenous antigen presentation by vaccine cells or on vaccine efficacy. These results suggest that antigen/MHC class II complexes in the vaccines may follow a novel route for processing and presentation and may produce a repertoire of class II-restricted peptides different from those presented by professional APC. The therapeutic efficacy of the vaccines, therefore, may reside in their ability to present novel tumor peptides, consequently activating tumor-specific CD4+ T cells that would not otherwise be activated.

Differential antigen-processing pathways of the hepatitis B virus e and core proteins

BACKGROUND & AIMS

Hepatitis B e antigen (HBeAg) and hepatitis B core antigen (HBcAg) seem to play different roles in the induction and regulation of the antiviral immune response, although the two antigens share all major CD4(+) T-cell epitopes, and these epitopes can be processed from both antigens via the exogenous antigen-presenting pathway. The aim of this study was to test the ability of antigen-presenting cells to present epitopes from endogenously synthesized HBcAg/HBeAg on HLA class II molecules.

METHODS

Lymphoblastoid cell lines infected with recombinant vaccinia viruses containing various HBcAg or HBeAg constructs and stable transfectants were tested for their ability to stimulate HBcAg/HBeAg-specific CD4(+) T-cell clones.

RESULTS

Only antigen-presenting cells infected with HBeAg constructs but not those infected with HBcAg constructs were able to stimulate HBcAg/HBeAg-specific CD4(+) T-cell clones. T-cell activation by HBeAg constructs was completely inhibited by brefeldin A but not affected by chloroquin. In contrast, T-cell activation by exogenous, recombinant HBcAg was inhibited by chloroquin but not by brefeldin A.

CONCLUSIONS

The findings indicate that processing and HLA class II-associated presentation of endogenously synthesized HBeAg in virus-infected cells, including hepatocytes, may occur. This mechanism may be involved in the regulation of the CD4(+) T-cell response to HBcAg/HBeAg.

Peptides bound to major histocompatibility complex molecules

Peptides are the means by which immune effector T cells recognize and defend against the foreign proteins of pathogens. T cell recognition of these molecules, however, is strictly dependent on peptide binding to the receptor-like molecules of the major histocompatibility complex (MHC) locus. The basic unit of recognition is a trimolecular complex consisting of the T cell antigen receptor, the MHC molecule, and the MHC-bound peptide ligand. The multistep process that culminates in MHC presentation of peptides to T cells begins in the last phases of protein catabolism. While the individual roles of many key molecules involved in peptide presentation have recently been defined, there still remain many questions regarding processing of proteins into MHC-bound peptides. This review summarizes the recent developments in peptide antigen processing for MHC molecules, with focus on how proteins are believed to be sampled and selected for degradation into peptides.

Naturally Processed Tissue- and Differentiation Stage-Specific Autologous Peptides Bound by HLA Class I and II Molecules of Chronic Myeloid Leukemia Blasts

Structural analysis of naturally processed peptides bound to the HLA class I and class II molecules of chronic myeloid leukemia (CML) blast cells was performed to characterize the antigen processing and autoantigen repertoire in this hematopoietic malignancy. Self-peptides derived from the carboxy-terminal end of the breakpoint cluster region (bcr) protein, as well as several differentiation stage- and tissue-specific self-antigens characteristic of early stages of myeloid differentiation, such as c-fes, c-pim, granulocyte-macrophage colony-stimulating factor receptor α chain, proteinase 3, and cathepsin G, were identified. A common characteristic of several of the high copy-number self-peptides identified in this study is the participation of their parent proteins in signal transduction or myeloid effector function. Because bcr-abl junctional peptides bind to a limited number of major histocompatibility complex (MHC) class I alleles, an effective peptide-based immunotherapy strategy for CML requires identification of further tumor-associated or tissue-specific peptide antigens binding to common MHC alleles such as HLA-A2. The differentiation stage- and tissue-specific MHC-bound peptides found in this study, as well as the naturally processed proteins from which they are derived, may represent autoantigens towards which T-cell responses may potentially be developed for immunotherapy of hematopoietic malignancies such as CML.

Naturally Processed Tissue- and Differentiation Stage-Specific Autologous Peptides Bound by HLA Class I and II Molecules of Chronic Myeloid Leukemia Blasts

Structural analysis of naturally processed peptides bound to the HLA class I and class II molecules of chronic myeloid leukemia (CML) blast cells was performed to characterize the antigen processing and autoantigen repertoire in this hematopoietic malignancy. Self-peptides derived from the carboxy-terminal end of the breakpoint cluster region (bcr) protein, as well as several differentiation stage- and tissue-specific self-antigens characteristic of early stages of myeloid differentiation, such as c-fes, c-pim, granulocyte-macrophage colony-stimulating factor receptor α chain, proteinase 3, and cathepsin G, were identified. A common characteristic of several of the high copy-number self-peptides identified in this study is the participation of their parent proteins in signal transduction or myeloid effector function. Because bcr-abl junctional peptides bind to a limited number of major histocompatibility complex (MHC) class I alleles, an effective peptide-based immunotherapy strategy for CML requires identification of further tumor-associated or tissue-specific peptide antigens binding to common MHC alleles such as HLA-A2. The differentiation stage- and tissue-specific MHC-bound peptides found in this study, as well as the naturally processed proteins from which they are derived, may represent autoantigens towards which T-cell responses may potentially be developed for immunotherapy of hematopoietic malignancies such as CML.

Engagement of the B lymphocyte antigen receptor induces presentation of intrinsic immunoglobulin peptides on major histocompatibility complex class II molecules

By means of the clonotypic variable region, the immunoglobulin (Ig) is a tumor-specific antigen on B cell neoplasms. We report that engagement of the B cell antigen receptor (BcR) promotes presentation of peptides derived from the B cell's intrinsic Ig to major histocompatibility complex (MHC) class II-restricted T cells. Thus, anti-Ig endowed normal, ex vivo B lymphocytes from H-2d, Ig constant heavy chain allotype b (IgCHb) mice with the capacity to stimulate an I-Ad-restricted T cell clone which recognizes the gamma 2ab 435-451 allopeptide. The corresponding self gamma 2aa peptide is cryptic and 6000-fold less antigenic than the gamma 2ab allopeptide. Even so, the syngeneic B cell lymphoma A20 which expresses surface(s) IgG2aa, was also recognized by the T cells after BcR ligation. Thus, anti-Ig triggered the disclosure of a cryptic tumor antigen determinant. We propose that autoantigens, by engaging the BcR of self-reactive B cells, induce presentation of intrinsic Ig peptides to which the T helper cell (Th) repertoire is not tolerant. In this way, B cells with anti-self potential may be activated without Th recognition of nominal autoantigen.

Identification of amino acids in the glutamate receptor, GluR3, important for antibody-binding and receptor-specific activation

We reported (Twyman, R. E., Gahring, L. C., Speiss, J., and Rogers, S. W. (1995) Neuron 14, 755-762) that antibodies to a subregion of the glutamate receptor (GluR) subunit GluR3 termed GluR3B (amino acids 372-395), act as highly specific GluR agonists. In this study we produced additional rabbit anti-GluR3B-specific antibodies, ranked them according to their ability to function as GluR agonists and characterized the immunoreactivity using deletion and alanine substitution mutagenesis. These anti-GluR3B antibodies bound to a subset of the residues in GluR3B (amino acids 372-386), of which glutamate 375, valine 378, proline 379, and phenylalanine (Phe) 380 were preferred. The level of GluR activation correlated with the binding of antibody to Phe-380, which suggests that immunoreactivity directed toward Phe-380 is an index for the anti-GluR agonist potential. Since the identity of this residue varies between respective GluR subunits, this suggested that this residue may be important for imparting antibody subunit specificity. To test this possibility, the alanine in GluR1 was converted to a phenylalanine, which extended the subunit specificity from GluR3 to the modified GluR1. We conclude that antibody contacts with key residues in the GluR3B region define a novel GluR subunit-specific agonist binding site and impart subunit-specific immunoreactivity.

Invariant chain and DM edit self-peptide presentation by major histocompatibility complex (MHC) class II molecules

We have studied the consequences of invariant chain (Ii) and DM expression on major histocompatibility complex (MHC) class II function. Ii has a number of discrete functions in the biology of class II, including competitive blocking of peptide binding in the endoplasmic reticulum and enhancing localization in the endocytic compartments. DM is thought to act primarily in endosomes to promote dissociation of the Ii-derived (CLIP) peptide from the class II antigen-binding pocket and subsequent peptide loading. In this study, we have evaluated the functional role of Ii and DM by examining their impact on surface expression of epitopes recognized by a large panel of alloreactive T cells. We find most epitopes studied are influenced by both Ii and DM. Most strikingly, we find that surface expression of a significant fraction of peptide-class II complexes is extinguished, rather than enhanced, by DM expression within the APC. The epitopes antagonized by DM do not appear to be specific for CLIP. Finally, we found that DM was also able to extinguish recognition of a defined peptide derived from the internally synthesized H-2Ld protein. Thus, rather than primarily serving in the removal of CLIP, DM may have a more generalized function of editing the array of peptides that are presented by class II. This editing can be either positive or negative, suggesting that DM plays a specifying role in the display of peptides presented to CD4 T cells.