The geometry of synthetic peptide-based immunogens affects the efficiency of T cell stimulation by professional antigen-presenting cells

Geometry of a TLR2-Agonist-Based Adjuvant Can Affect the Resulting Antigen-Specific Immune Response

Targeted delivery of otherwise nonimmunogenic antigens to Toll-like receptors (TLRs) expressed on dendritic cells (DCs) has proven to be an effective means of improving immunogenicity. For this purpose, we have used a branched cationic lipopeptide, R₄Pam₂Cys, which is an agonist for TLR2 and enables electrostatic association with antigen for this purpose. Here, we compare the immunological properties of ovalbumin formulated with different geometrical configurations of R₄Pam₂Cys. Our results demonstrate that notwithstanding the presence of the same adjuvant, branched forms of R₄Pam₂Cys are more effective at inducing immune responses than are linear geometries. CD8⁺ T-cell-mediated responses are particularly improved, resulting in significantly higher levels of antigen-specific cytokine secretion and cytolysis of antigen-bearing target cells in vivo. The results correlate with the ability of branched R₄Pam₂Cys conformations to encourage higher levels of DC maturation and facilitate superior antigen uptake, leading to increased production of proinflammatory cytokines. These differences are not attributable to particle size because both branched and linear lipopeptides associate with antigen-forming complexes of similar size, but rather the ability of branched lipopeptides to induce more efficient TLR2-mediated cell signaling. Branched lipopeptides are also more resistant to trypsin-mediated proteolysis, suggesting greater stability than their linear counterparts. The branched lipopeptide facilitates presentation of antigen more efficiently to CD8⁺ T cells, resulting in rapid cell division and upregulation of early cell surface activation markers. These results as well as cognate recognition of Pam₂Cys by TLR2 indicate that the adjuvant's efficiency is also dependent on its geometry.

Memory cytolytic T-lymphocytes: induction, regulation and implications for vaccine design

The design of vaccines that protect against intracellular infections or cancer remains a challenge. In many cases, immunity depends on the development of antigen-specific memory CD8+ T-cells that can express cytokines and kill antigen-bearing cells when they encounter the pathogen or tumor. Here, the authors review current understanding of the signals and cells that lead to memory CD8+ T-cell differentiation, the relationship between the primary CD8+ T-cell response and the memory response and the regulation of memory CD8+ T-cell survival and function. The implications of this new knowledge for vaccine design are discussed, and recent progress in the development of lipidated peptide vaccines as a promising approach for vaccination against intracellular infections and cancer is reviewed.

A modular approach to assembly of totally synthetic self-adjuvanting lipopeptide-based vaccines allows conformational epitope building

The technology described here allows the chemical synthesis of vaccines requiring correctly folded epitopes and that contain difficult or long peptide sequences. The final self-adjuvanting product promotes strong humoral and/or cell-mediated immunity. A module containing common components of the vaccine (T helper cell epitope and the adjuvanting lipid moiety S-[2,3-bis(palmitoyloxy)propyl]cysteine) was assembled to enable a plug and play approach to vaccine assembly. The inclusion within the module of a chemical group with chemical properties complementary and orthogonal to a chemical group present in the target epitope allowed chemoselective ligation of the two vaccine components. The heat-stable enterotoxin of enterotoxigenic Escherichia coli that requires strict conformational integrity for biological activity and the reproductive hormone luteinizing hormone-releasing hormone were used as the target epitopes for the antibody vaccines. An epitope from the acid polymerase of influenza virus was used to assemble a CD8(+) T cell vaccine. Evaluation of each vaccine candidate in animals demonstrated the feasibility of the approach and that the type of immune response required, viz. antibody or cytotoxic T lymphocyte, dictates the nature of the chemical linkage between the module and target epitope. The use of a thioether bond between the module and target epitope had little or no adverse effect on antibody responses, whereas the use of a disulfide bond between the module and target epitope almost completely abrogated the antibody response. In contrast, better cytotoxic T lymphocyte responses were obtained when a disulfide bond was used.

Synthesis of a multivalent, multiepitope vaccine construct

A major challenge to contemporary peptide chemistry is to reproduce highly complex active sites or complexes of native proteins. In addition to the need for the combination of different peptide fragments, true protein mimics designed for therapeutic use frequently require the incorporation of multiple copies of given active domains in a biologically relevant spatial arrangement. Perhaps the best examples for this line of drug design are subunit vaccine candidates against extracellular domains of ion-channel proteins. One of our earlier constructs containing four copies of the ectodomain of the M2 protein of influenza virus together with two independent T-helper cell epitopes induced protective antibody production in mice. Here I describe an improved synthesis of the M2-based multiepitope and multivalent peptide construct. In general, the synthetic strategy outlined here can serve as a model for the orthogonal N-terminal and side-chain-protecting scheme during the preparation of large, complex peptides or small, engineered proteins.

Recombinant peptide replicates immunogenicity of synthetic linear peptide chimera for use as pre-erythrocytic stage malaria vaccine

Synthetic linear peptide chimeras (LPCs(cys+)) show promise as delivery platforms for malaria subunit vaccines. Maximal immune response to LPCs(cys+) in rodent malaria models depends upon formation of cross-linkages to generate homopolymers, presenting challenges for vaccine production. To replicate the immunogenicity of LPCs(cys+) using a recombinant approach, we designed a recombinant LPC (rLPC) based on Plasmodium yoelii circumsporozoite protein-specific sequences of 208 amino acids consisting of four LPC subunits in series. BALB/c or CAF1/J mice were immunized with synthetic or recombinant LPCs. Antibody concentrations, cytokine production and protection against challenge were compared. Recombinant peptide replicated the robust, high avidity antibody responses obtained with the synthetic linear peptide chimera. After in vitro stimulation spleen cells from mice immunized with rLPC or synthetic LPC(cys+) produced gamma interferon and IL-4 suggesting the efficient priming of T cells. Immunization of mice with either recombinant or synthetic LPC(cys+) provided comparable protection against experimental challenge with P. yoelii sporozoites. Recombinant LPCs reproduced the immunogenicity of synthetic LPC(cys+) without requiring polymerization, improving prospects for use as malaria vaccines.

Branched and linear lipopeptide vaccines have different effects on primary CD4+ and CD8+ T-cell activation but induce similar tumor-protective memory CD8+ T-cell responses

We compared murine T-cell responses to synthetic lipopeptide vaccines in which the TLR2 ligand Pam(2)Cys was attached to co-linear CD4+ and CD8+ T-cell epitopes of ovalbumin (OVA) in a linear or branched configuration. Mice received OVA-specific transgenic CD8+ and CD4+ T-cells followed by one injection of vaccine. Although the branched lipopeptide was more potent in activating OVA-specific CD4+ and CD8+ T-cells in the primary response, both vaccines induced cytolytic T lymphocytes (CTL) that expressed perforin, granzyme A-C, and IFN-gamma mRNAs and conferred long-term protection of most mice against challenge with OVA-expressing tumor cells. OVA epitope display was reduced in tumors that developed in some mice, suggesting CD8+ T-cell dependent selection.

Comparison of lipopeptide-based immunocontraceptive vaccines containing different lipid groups

We have previously shown that incorporating the lipid moiety dipalmitoyl-S-glyceryl cysteine (Pam2Cys) into peptide structures effectively adjuvants otherwise weak immunogens. In this study lipopeptides based on luteinising hormone releasing hormone (LHRH) as a B cell epitope, [B], were synthesised in tandem with a 17-residue T-helper epitope, [T], derived from the fusion protein of the morbillivirus canine distemper virus. In this way vaccine candidates with the structure [T]-[B] were produced. These peptides were then lipidated with different diacylated moieties. The acyl moieties used were: palmitic acid (C16) to give Pam2Cys, stearic acid (C18) to give Ste2Cys, lauric acid (C12) to give Lau2Cys and octanoic acid (C8) to give Oct2Cys. We compared the immunogenicities of these simple lipopeptides in BALB/c mice by measuring their ability to induce anti-LHRH antibodies and found that immunogenicity was dependent on the length of the alkane chains of the incorporated lipid moieties with the hierarchy C16=C18>C12>C8. The antibody levels elicited by the lipopeptides also correlated with their ability to inhibit the reproductive capability of female mice in fertility trials. Furthermore, the C16 lipopeptide was the most effective in activating dendritic cells, measured by up regulation of surface MHC Class II molecules, and also in activating NF-kappaB in a Toll-like receptor-2 (TLR2)-dependent manner.

Rh autoantigen presentation to helper T cells in chronic lymphocytic leukemia by malignant B cells

Chronic lymphocytic leukemia (CLL) is frequently associated with autoimmune diseases directed against constituents of the blood, including hemolytic anemia (AIHA). We hypothesized that CLL cells predispose to hematologic autoimmunity by acting as aberrant antigen-presenting cells (APCs). Initially, it was confirmed that all studied patients with AIHA secondary to CLL harbored activated helper T (TH) cells specific for epitopes on the dominant red blood cell (RBC) autoantigens in primary AIHA, the Rh proteins. Rh-specific TH cells were also detected in a number of patients with CLL who, although they did not have AIHA, had low levels of anti-RBC antibody in their sera. Fractionation of putative APC populations from the peripheral blood of patients by negative selection showed that CD5+ CLL cells are the most effective cell type in processing and presenting purified Rh protein to autoreactive TH cells. This ability was confirmed using positively selected CD5+ CLL cells. Thus, our study provides the first evidence for malignant cells driving an autoimmune response by acting as aberrant APCs.

A totally synthetic vaccine of generic structure that targets Toll-like receptor 2 on dendritic cells and promotes antibody or cytotoxic T cell responses

A simple generic peptide-based vaccine structure that targets Toll-like receptor 2-expressing dendritic cells and causes their activation is described. The vaccines are totally synthetic, serve as their own adjuvant, and are composed of (i) a single helper T cell epitope, (ii) a target epitope that is either recognized by CD8+ T cells or B cells, and (iii) a Toll-like receptor 2-targeting lipid moiety, S-[2,3-bis(palmitoyloxy)propyl]cysteine, that is situated between the peptide epitopes to form a branched configuration. The different CD8+ T cell epitopes examined were from (i) influenza virus, (ii) the intracellular bacterium Listeria monocytogenes, and (iii) ovalbumin as a model tumor antigen. Vaccines containing a B cell epitope from gastrin or luteinizing hormone-releasing hormone as a B cell epitope were also examined for their ability to elicit antibody against the parent hormones. Each of the vaccines was capable of inducing either CD8+ T cell or antibody-mediated immune responses. The lipidated vaccines, but not the nonlipidated vaccines, were able to mediate protection against viral or bacterial infection and mediate prophylactic and therapeutic anticancer activity. The two hormone-based vaccines induced high antibody titers, which in the case of luteinizing hormone-releasing hormone resulted in abrogation of reproductive function. These results highlight the utility of simple, totally synthetic, epitope-based vaccines.

DCs as targets for vaccine design

The increasingly stringent requirements laid down by regulatory authorities have brought to an end the largely empirical design of vaccines. Vaccines must now be designed rationally, in order that appropriate immune responses are elicited with few or no side effects. The DC plays a pivotal role in determining the type of immune response that ensues following exposure of the host to an Ag. In this review, we identify some of the features and properties of DCs, and how these properties can be exploited in the design of smart vaccines.

Dissecting the role of peptides in the immune response: theory, practice and the application to vaccine design

Analytical biochemistry and synthetic peptide based chemistry have helped to reveal the pivotal role that peptides play in determining the specificity, magnitude and quality of both humoral (antibody) and cellular (cytotoxic and helper T cell) immune responses. In addition, peptide based technologies are now at the forefront of vaccine design and medical diagnostics. The chemical technologies used to assemble peptides into immunogenic structures have made great strides over the past decade and assembly of highly pure peptides which can be incorporated into high molecular weight species, multimeric and even branched structures together with non-peptidic material is now routine. These structures have a wide range of applications in designer vaccines and diagnostic reagents. Thus the tools of the peptide chemist are exquisitely placed to answer questions about immune recognition and along the way to provide us with new and improved vaccines and diagnostics.

Highly immunogenic and totally synthetic lipopeptides as self-adjuvanting immunocontraceptive vaccines

In this study, we describe the synthesis of various lipopeptides based on the sequence of luteinizing hormone-releasing hormone (LHRH) and report on their abilities to induce Abs against this "self" hormone when inoculated into mice in the absence of additional adjuvant. The peptides consisted of a colinear CD4(+) T helper cell epitope from the L chain of influenza virus hemagglutinin and LHRH, which has B cell epitopes but no T cell epitopes present in its sequence. Lipids were attached either at the N terminus or between the T cell epitope and LHRH, in the approximate center of the peptide. The lipopeptide constructs displayed different solubilities and immunological properties that depended not only on the lipid content but also on the position of attachment of the lipids. Some of these constructs were highly immunogenic, inducing high titers of Ab, which were capable of efficiently sterilizing female mice when administered in saline by s.c. or intranasal routes. The most effective vaccines were highly soluble, contained the dipalmitoyl-S-glyceryl cysteine moiety, and had this lipid attached at the center of the molecule. The relative ability of the lipopeptides to induce an Ab response in the absence of external adjuvant was reflected by their ability to up-regulate the surface expression of MHC class II molecules on immature dendritic cells. These results demonstrate that the composition and position within peptide vaccines of self-adjuvanting lipid groups can influence the ability to induce the maturation of dendritic cells and, in turn, the magnitude of the resulting Ab response.

Reactivity and isotype profiling of monoclonal antibodies using multiple antigenic peptides

The characterisation of monoclonal antibodies (MAbs) is essential for the development of assay systems particularly where antigens have been developed using synthetic peptides. Indeed some peptide-carrier conjugates fail to induce immune responses and may not generate antibodies that bind to native protein. As an alternative to peptide-carrier conjugates, multiple antigenic peptides (MAPs) have been used for immunization strategies, but with little regard to the characteristics of the MAbs produced. In this study, we used 3 MAPs of Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) to immunise BALB/c mice. Overall, the polyclonal antibody responses from tail bleeds showed that MAPs evoked B-cell responses. However, on screening 144 hybridomas, 24 MAb supernatants exhibited weak to moderate reactivity in enzyme-linked immunosorbant assay (ELISA) and against cell cytospin preparations (B95.8 and AG876 LCL), respectively. Isotype profiling of hybridoma supernatants also showed that 11 out of 24 were IgM. Further characterization of 6 MAbs in Western blotting showed reactivity to recombinant LMP1 and only one MAb (B28D) showed weak reactivity to the malignant cells (Hodgkin/Reed-Sternberg; HRS cells) of an EBV+ Hodgkin's lymphoma using paraffin-embedded tissue. It is probable that these MAPs failed to augment T-cell help and contributed to the production of low affinity (IgM) antibodies. These observations may be of importance to future immunization strategies, where MAPs are used in the production of monoclonal reagents.

Assembly of synthetic peptide vaccines by chemoselective ligation of epitopes: influence of different chemical linkages and epitope orientations on biological activity

In this paper, we describe the assembly of synthetic peptide vaccines composed of a T helper cell epitope and a B cell epitope that were synthesized separately and then attached using three different chemoselective ligation methods: oxime bond formation, thioether bond formation and disulfide bond formation. The resulting vaccines were tested in animals to investigate their efficacy. We found that thioether bond formation gave the highest yield of material and that the chemistry involved did not adversely affect immunogenicity and biological activity of the peptide vaccine. Ligation of epitopes by oxime bond formation did not diminish biological activity either, but the yields of peptide vaccine were lower than when thioether bond formation was used. The vaccines in which a disulfide bond was used to attach the two epitopes resulted in the lowest yield and produced vaccines that also generated a weaker immune response with sub-optimal biological activity. Connecting the T helper epitope via its N-terminus or its C-terminus to the N-terminus of the B cell epitope had little influence on resulting immunogenicity and biological activity.

Diepitopic construct of functionally and epitopically complementary peptides enhances immunogenicity, reactivity with glucosyltransferase, and protection from dental caries

Coimmunization with peptide constructs from catalytic (CAT) and glucan-binding (GLU) domains of glucosyltransferase (GTF) of mutans streptococci has resulted in enhanced levels of antibody to the CAT construct and to GTF. We designed and synthesized a diepitopic construct (CAT-GLU) containing two copies of both CAT (B epitope only) and GLU (B and T epitope) peptides. The immunogenicity of this diepitopic construct was compared with that of individual CAT and GLU constructs by immunizing groups of Sprague-Dawley rats subcutaneously in the salivary gland vicinity with the CAT-GLU, CAT, or GLU construct or by treating rats by sham immunization. Levels of serum immunoglobulin G (IgG) antibody to GTF or CAT in the CAT-GLU group were significantly greater than in GLU- or CAT-immunized groups. Immunization with CAT-GLU was compared to coimmunization with a mixture of CAT and GLU in a second rodent experiment under a similar protocol. CAT-GLU immunization resulted in serum IgG and salivary IgA responses to GTF and CAT which were greater than after coimmunization. Immunization with the diepitopic construct and communization with CAT and GLU constructs showed proliferation of T lymphocytes to GTF. Immunization with either the CAT or GLU construct has been shown to elicit significant protection in a rodent dental caries model. Similarly in this study, the enhanced response to GTF after immunization with the CAT-GLU construct resulted in protective effects on dental caries. Therefore, the CAT-GLU diepitopic construct can be a potentially important antigen for a caries vaccine, giving rise to greater immune response than after immunization with CAT, GLU, or a mixture of the two.

Differential effect of CD8(+) and CD8(-) dendritic cells in the stimulation of secondary CD4(+) T cells

Dendritic cells (DC), in their role in initiation of the adaptive immune response, have been extensively studied for their capacity to interact and stimulate naive T cells. Subsets of mature murine DC isolated directly from the spleen have been shown to differ in their ability to induce proliferative responses in both primary CD4(+) and primary CD8(+) T cells; the myeloid-related CD8alpha(-) DC induce a more intense or prolonged proliferation of naive T cells than do the lymphoid-related DC bearing CD8alpha despite similar expression of MHC and co-stimulatory molecules. Here we examine the interaction of these DC subpopulations with T cells already in the activated or memory state which are known to have greater sensitivity to antigen stimulation and bear receptors with increased capacity for signal transduction. We show that influenza virus-specific CD4(+) T cell clones and splenic T cells from peptide-primed animals proliferated in response to antigen presented by separated splenic CD8(-) DC. In contrast, these T cells showed only weak, if any, proliferation in response to CD8(+) DC despite observable cluster formation in the cultures. The differential between the two DC types in inducing proliferation was even more pronounced than previously seen with primary T cells and did not reflect differential longevity of the DC in culture, altered response kinetics or deviation from IL-2 to IL-4 induction with CD8(+) DC, but was related to the levels of IL-2 induced. The deficiency in the CD8(+) DC was not overcome by using infectious virus rather than synthetic peptide as the antigen source. These results show that lymphoid-related CD8(+) splenic DC, despite their mature phenotype, fail to provide appropriate signals to secondary CD4(+) T cells to sustain their proliferation.

Leishmaniasis: current status of vaccine development

Leishmaniae are obligatory intracellular protozoa in mononuclear phagocytes. They cause a spectrum of diseases, ranging in severity from spontaneously healing skin lesions to fatal visceral disease. Worldwide, there are 2 million new cases each year and 1/10 of the world's population is at risk of infection. To date, there are no vaccines against leishmaniasis and control measures rely on chemotherapy to alleviate disease and on vector control to reduce transmission. However, a major vaccine development program aimed initially at cutaneous leishmaniasis is under way. Studies in animal models and humans are evaluating the potential of genetically modified live attenuated vaccines, as well as a variety of recombinant antigens or the DNA encoding them. The program also focuses on new adjuvants, including cytokines, and delivery systems to target the T helper type 1 immune responses required for the elimination of this intracellular organism. The availability, in the near future, of the DNA sequences of the human and Leishmania genomes will extend the vaccine program. New vaccine candidates such as parasite virulence factors will be identified. Host susceptibility genes will be mapped to allow the vaccine to be targeted to the population most in need of protection.