Alternative processing of endogenous or exogenous antigens extends the immunogenic, H-2 class I-restricted peptide repertoire

Strategy to Configure Multi-epitope Recombinant Immunogens with Weightage on Proinflamatory Response using SARS-CoV-2 Spike Glycoprotein (S-protein) and RNA-dependent RNA Polymerase (RdRp) as Model Targets

Development of a suitable recombinant peptide vaccine against pathogens requires designing of effective immunogenic polypeptide taking various aspects and complexity of immune-response into consideration. Implementing SARS-CoV-2 spike glycoprotein (S-protein) and RNA-dependent RNA polymerase (RdRp) as model targets, in this study, we outline and assess a strategy for in silico recombinant vaccine designing. After mapping the linear B-cell epitopes and MHC1-binding T-cell epitopes six epitopes were sorted from each of the proteins on the basis of extent of residue-conservancy among three types of coronaviruses namely SARS-CoV-2, SARS-CoV and MERS-CoV. Each of the selected epitopes were profiled for their pro-inflammatory potential through molecular docking analysis with surface bound Toll-like receptors, namely TLR2, TLR4 and TLR5. Based on a custom scoring function, the epitopes were ranked for highest and least pro-inflammatory potential. Segments of Spike and RdRp harboring such epitopes were combined using linkers to design immunogenic recombinant polypeptide. Antigenicity and allergenicity of each of the combination was scored; and the best fitting one was docked against TLR2, TLR4 and TLR5 for assessing pro-inflammatory potential. Codon optimization and in silico cloning in expression vector indicated that the designed peptide can be satisfactorily expressed in bacteria, reinforcing the viability of the strategy in identification and designing of potential immunogens.

Hepatitis B virus surface antigen as delivery vector can enhance Chlamydia trachomatis MOMP multi-epitope immune response in mice

Chlamydia trachomatis is the leading cause of sexually transmitted infections worldwide. There is currently no commercially available vaccine against C. trachomatis. Major outer membrane protein (MOMP) of C. trachomatis is considered to be an ideal candidate for prophylactic vaccine. We designed a MOMP multi-epitope containing T- and B-cell epitope-rich peptides and developed hepatitis B surface antigen (HBsAg) as antigen delivery vehicle. In order to study the immunogenicity and efficacy of the candidate vaccine in a murine model of chlamydial genital infection, we engineered a recombinant plasmid expressing HBsAg and MOMP multi-epitope genes. Results of reverse transcription polymerase chain reaction and immunofluorescence assay revealed successful expression of the recombinant HBsAg/MOMP multi-epitope gene at both the transcription and translation levels. Intramuscular administration in mice was able to elicit not only antibodies against Chlamydia and HBsAg but also cytotoxic T lymphocyte activity against Chlamydia. In addition, mice inoculated with the rHBsAg were highly resistant to C. trachomatis genital infection. The rHBsAg DNA with MOMP multi-epitope appended at the C terminus of the HBsAg stimulated a stronger immune response and protective response than that appended at the N terminus. Together, our results suggested that use of a recombinant HBsAg encoding the MOMP multi-epitope could be a powerful approach to developing a safe and immunogenic C. trachomatis vaccine.

Impact of the Immunogen Nature on the Immune Response against the Major HBV Antigens in an HBsAg and HLA-humanized Transgenic Mouse Model

Hepatitis B chronic carriage remains as a major public health problem. Protein and DNA vaccines are now widely used in therapeutic vaccine candidates. Although, the hepatitis B surface antigen (HBsAg) based vaccines have been largely studied, candidates comprising both HBsAg and core (HBcAg) either protein- or DNA-based approaches deserve further immunological characterization.

In the present study, a repeated dose administration schedule for protein or DNA immunogens was conducted in order to characterize the resulting immune response in a humanized and HBsAg-tolerized setting. A novel transgenic (Tg) mice that express the HBsAg, human MHC class I (HLA-A*0201) and MHC class II (HLA-DRB1*01) molecules and devoid of endogenous murine class I and II molecules was used as a model of HBV chronic carrier. Mice were immunized by subcutaneous (protein) or intramuscular (DNA) routes and the humoral and cellular responses were evaluated.

Protein or DNA immunization induced humoral immune responses against both HBsAg and HBcAg. The systematic analysis of epitopes that activate CD4+ and CD8+ T lymphocytes confirmed the accuracy of the model. Cellular immune responses were detected differing in their nature. CD8 T-cell responses were induced mostly after DNA immunization while CD4 T-cell responses were predominant in protein based immunizations. In addition, the intensity of HLA-A2-restricted CD8+ T cell responses was reduced in Tg mice expressing HBsAg when compared to control Tg mice.

In conclusion, our results indicate that cellular immune responses necessary for the development of protective immunity can be achieved by DNA or protein immunization. However, important differences in their nature arise when immunogens are administered several times.

How to cite this article: Mancini-Bourgine M, Guillen G, Michel ML, Aguilar JC. Impact of the Immunogen Nature on the Immune Response against the Major HBV Antigens in an HBsAg and HLA-humanized Transgenic Mouse Model. Euroasian J Hepato-Gastroenterol 2014;4(1):36-44.

Exosome release from infected dendritic cells: a clue for a fast spread of prions in the periphery?

Prion diseases are incurable transmissible neurological disorders. In many natural and experimental prion diseases, infectious prions can be detected in the lymphoreticular system (LRS) long before they reach the brain where they cause a fatal rapidly progressive degeneration. Although major cell types that contribute to prion accumulation have been identified, the mode of prion dissemination in the LRS remains elusive. Recent evidence of a remarkably fast splenic prion accumulation after peripheral infection of mice, resulting in high prion titers in dendritic cells (DCs) and a release of prions from infected DCs via exosomes suggest that intercellular dissemination may contribute to rapid prion colonization in the LRS. A vast body of evidence from retroviral infections shows that DCs and other antigen-presenting cells (APCs) share viral antigens by intercellular transfer to warrant immunity against viruses if APCs remain uninfected. Evolved to adapt the immune response to evading pathogens, these pathways may constitute a portal for unimpeded prion dissemination owing to the tolerance of the immune system against host-encoded prion protein. In this review we summarize current paradigms for antigen-sharing pathways which may be relevant to better understand dissemination of rogue neurotoxic proteins.

Proteolytic antigens interfere with endosome/lysosome fusion in epithelial cells

The airway epithelial barrier function is important in maintaining the homeostasis in the body. A number of airway disorders are associated with the epithelial barrier dysfunction. The present study aims to elucidate a possible mechanism by which the proteolytic allergens compromise the epithelial barrier function. The airway epithelial cell line, RPMI 2650 cells (Rp cells) and kidney epithelial cell line, MDCK cells, were cultured to be monolayers and used as an in vitro epithelial barrier model. House dust mite antigen, Der P1 (Der) was used as an antigen that has the proteolytic property. The epithelial barrier permeability and transepithelial resistance (TER) were used as the indicators of epithelial barrier function. Both epithelial cell lines could endocytose Der in the culture. Some of the Der was transported across the epithelial barrier to the basal chambers of the Transwells without affecting the TER. The endocytic Der could suppress the expression of ubiquitin E3 lases A20 and further interfered with the fusion of endosome/lysosome in the epithelial cells. Mite antigen, Der, can interfere with the fusion of endosome/lysosome in epithelial cells to induce the epithelial barrier dysfunction.

Exogenous, TAP-independent lysosomal presentation of a respiratory syncytial virus CTL epitope

Respiratory syncytial virus causes lower respiratory tract infections in infancy and old age, affecting also immunocompromised patients. The viral fusion protein is an important vaccine candidate eliciting antibody and cell-mediated immune responses. CD8(+) cytotoxic T lymphocytes (CTLs) are known to have a role in both lung pathology and viral clearance. In BALB/c mice, the fusion protein epitope F249-258 is presented to CTLs by the murine major histocompatibility complex (MHC) class I molecule K(d). In cells infected with recombinant vaccinia viruses encoding the fusion protein, F249-258 is presented by MHC class I molecules through pathways that are independent of the transporters associated with antigen processing (TAP). We have now found that F249-258 can be generated from non-infectious virus from an exogenous source. Antigen processing follows a lysosomal pathway that appears to require autophagy. As a practical consequence, inactivated virus suffices for in vivo priming of virus-specific CTLs.

Enhancement of protective immune responses induced by Toxoplasma gondii dense granule antigen 7 (GRA7) against toxoplasmosis in mice using a prime-boost vaccination strategy

Effective vaccines against Toxoplasma gondii may contribute to preventing and controlling the spread of toxoplasmosis, which is important for improving outcomes of infections in humans and livestock animals. The dense granule antigen 7 (GRA7) of T. gondii might be an immunodominant antigen for a vaccine candidate. In the present study, a further exploration of its vaccine effect, a heterologous prime-boost vaccination strategy with a recombinant eukaryotic plasmid pEGFP-GRA7 and a recombinant protein GRA7 expressed from a prokaryotic plasmid pET30-GRA7, was performed in BALB/c mice. The data reveal that a DNA prime-protein boost vaccination induces both humoral and cellular immune responses against T. gondii associated with high levels of total IgG, IgG2a isotype and gamma interferon (IFN-γ). Challenge experiments further show that the DNA prime-protein boost vaccination significantly increases survival rate (60%), compared with controls in which all died within 8 days of challenge. Therefore, the DNA prime-protein boost vaccination based on GRA7 might be a promising regimen for further development of an effective vaccine against T. gondii.

Hepatitis B virus surface antigen can activate dendritic cells and modulate T helper type immune response

Hepatitis B virus surface antigen (HBsAg) is a major antigen of hepatitis B virus (HBV). Dendritic cells (DC) of HBV carriers have been reported to exhibit functional impairment. In this study, the role of HBsAg on mice bone marrow-derived dendritic cells and immune responses in vivo was studied. The immune modulatory function of HBsAg was explored by using mice bone marrow-derived dendritic cells in vitro and also by examining an ovalbumin (OVA) specific immune response in vivo. Treatment of dendritic cells with HBsAg resulted in enhanced cell surface expression of cluster of differentiation (CD) 80, CD83, CD86, and major histocompatibility complex (MHC) class II, and enhanced production of interleukin (IL)-12 p40 and IL-12 p70. Treatment of dendritic cells with HBsAg resulted in decreased T cell secretion of IL-5 by OVA stimulation. In addition, the results showed stronger OVA-specific immunoglobulin (Ig) M and weaker IgG responses in mice sera when they had been immunized with OVA and co-injected with HBsAg. It was also found that the mice exhibited significant enhancement of anti-OVA IgG2a antibody (Ab), as well as marked inhibition of IgG1 Ab production. In cellular immune responses, IL-5 production was significantly decreased and interferon (IFN)-γ increased in the group co-injected with HBsAg. On the other hand, the induction of lymphoproliferative response to OVA stimulation in spleen cells was decreased in the HBsAg co-injected group. These results demonstrate that HBsAg can affect the differentiation of T helper (Th) cells, which might provide a strategy for improving its prophylactic and therapeutic efficacy.

Cutting edge: limited specialization of dendritic cell subsets for MHC class II-associated presentation of viral particles

Dendritic cells (DCs) are the most important APC. It was recently reported that there is a dichotomy for Ag presentation by DC subsets; exogenous Ags reach the MHC class I pathway, but not the MHC class II pathway, in CD8(+) DCs, whereas CD8(-) DCs only process Ags for the MHC class II pathway. In this study, we used virus-like particles (VLPs) to show that CD8(+) and CD8(-) DCs efficiently capture and process VLPs for presentation in association with MHC class II in vivo. In contrast, CD8(+) DCs, but not CD8(-) DCs, cross presented VLP-derived peptides. This pattern was changed in an FcgammaR-dependent fashion in the presence of VLP-specific Abs, because under those conditions both DC subsets failed to efficiently cross present. Thus, the presentation of viral particles to CD4(+) T cells is not restricted to distinct DC subsets, whereas the presentation of viral particles to CD8(+) T cells is limited to CD8(+) DCs.

Rabies DNA vaccine: no impact of MHC class I and class II targeting sequences on immune response and protection against lethal challenge

Rabies is progressive fatal encephalitis. WHO estimates 55,000 rabies deaths and more than 10 million PEP every year world-wide. A variety of cell-culture derived vaccines are available for prophylaxis against rabies. However, their high cost restricts their usage in developing countries, where such cases are most often encountered. This is driving the quest for newer vaccine formulations; DNA vaccines being most promising amongst them. Here, we explored strategies of antigen trafficking to various cellular compartments aiming at improving both humoral and cellular immunity. These strategies include use of signal sequences namely Tissue Plasminogen Activator (TPA), Ubiquitin (UQ) and Lysosomal-Associated Membrane Protein-1 (LAMP-1). TPA, LAMP-1 and their combination were aimed at enhancing the CD4(+) T cell and antibody response. In contrast, the UQ tag was utilized for enhancing CD8(+) response. The potency of modified DNA vaccines assessed by total antibody response, antibody isotypes, cytokine profile, neutralizing antibody titer and protection conferred against in vivo challenge; was enhanced in comparison to native unmodified vaccine, but the response elicited did not pertain to the type of target sequence and the directed arm of immunity. Interestingly, the DNA vaccines that had been designed to generate different type of immune responses yielded in effect similar response. In conclusion, our data indicate that the directing target sequence is not the exclusive deciding factor for type and extent of immune response elicited and emphasizes on the antigen dependence of immune enhancement strategies.

Enhancement of humoral and cellular immunity in mice against Japanese encephalitis virus using a DNA prime-protein boost vaccine strategy

A synthetic multi-epitope gene containing critical epitopes of the Japanese encephalitis virus (JEV) envelope gene was cloned into both prokaryotic and eukaryotic expression vectors. The recombinant plasmid and purified recombinant protein (heterologously expressed in Escherichia coli) were used as immunogens in a mouse model. The results indicate that both the recombinant protein and the DNA vaccine induce humoral and cellular immune responses. Neutralising antibody titres in mice in the pcDNA-TEP plus rEP group increased considerably relative to mice immunised using either pcDNA-TEP or rEP alone (P<0.05). Furthermore, the highest levels of interleukin (IL)-2, interferon-gamma and IL-4 were induced following priming with the DNA vaccine and boosting with the recombinant protein. Together these findings demonstrate that a DNA-recombinant protein prime-boost vaccination strategy can produce high levels of antibody and trigger significant T cell responses in mice, highlighting the potential value of such an approach in the prevention of JEV infection.

Relevance of viral context and diversity of antigen-processing routes for respiratory syncytial virus cytotoxic T-lymphocyte epitopes

Antigen processing of respiratory syncytial virus (RSV) fusion (F) protein epitopes F85-93 and F249-258 presented to cytotoxic T-lymphocytes (CTLs) by the murine major histocompatibility complex (MHC) class I molecule Kd was studied in different viral contexts. Epitope F85-93 was presented through a classical endogenous pathway dependent on the transporters associated with antigen processing (TAP) when the F protein was expressed from either RSV or recombinant vaccinia virus (rVACV). At least in cells infected with rVACV encoding either natural or cytosolic F protein, the proteasome was required for epitope processing. In cells infected with rVACV encoding the natural F protein, an additional endogenous TAP-independent presentation pathway was found for F85-93. In contrast, epitope F249-258 was presented only through TAP-independent pathways, but presentation was brefeldin A sensitive when the F protein was expressed from RSV, or mostly resistant when expressed from rVACV. Therefore, antigen-processing pathways with different mechanisms and subcellular localizations are accessible to individual epitopes presented by the same MHC class I molecule and processed from the same protein but in different viral contexts. This underscores both the diversity of pathways available and the influence of virus infection on presentation of epitopes to CTLs.

Multiple copies of a tumor epitope in a recombinant hepatitis B surface antigen (HBsAg) vaccine enhance CTL responses, but not tumor protection

We propose the replacement of endogenous epitopes with foreign epitopes to exploit the highly immunogenic hepatitis B surface antigen (HBsAg) as a vaccine vector to elicit disease-protective cytotoxic T-lymphocyte (CTL) responses. Locations were defined within the HBsAg gene where replacements of DNA encoding HBsAg epitopes may be made to generate functional recombinant (r) HBsAg DNA vaccines. We demonstrate that rHBsAg DNA vaccines encoding multiple copies of a model tumor epitope from human papillomavirus (HPV) elicit enhanced CTL responses compared to rHBsAg DNA vaccines encoding a single copy. We show that rHBsAg DNA vaccines elicit a marked prophylactic and long-lived therapeutic protection against epitope expressing tumor, although protective efficacy was not improved by increasing the number of copies of the tumor epitope DNA. These results demonstrate the efficacy of HBsAg as a vector for the delivery of foreign CTL epitopes using the epitope replacement strategy, and have implications for rHBsAg vaccine design. The results also have implications for the derivation of a therapeutic vaccine for HPV-associated squamous carcinoma.

Dendritic cell-lysosomal-associated membrane protein (LAMP) and LAMP-1-HIV-1 gag chimeras have distinct cellular trafficking pathways and prime T and B cell responses to a diverse repertoire of epitopes

Ag processing is a critical step in defining the repertoire of epitope-specific immune responses. In the present study, HIV-1 p55Gag Ag was synthesized as a DNA plasmid with either lysosomal-associated membrane protein-1 (LAMP/gag) or human dendritic cell-LAMP (DC-LAMP/gag) and used to immunize mice. Analysis of the cellular trafficking of these two chimeras demonstrated that both molecules colocalized with MHC class II molecules but differed in their overall trafficking to endosomal/lysosomal compartments. Following DNA immunization, both chimeras elicited potent Gag-specific T and B cell immune responses in mice but differ markedly in their IL-4 and IgG1/IgG2a responses. The DC-LAMP chimera induced a stronger Th type 1 response. ELISPOT analysis of T cell responses to 122 individual peptides encompassing the entire p55gag sequence (15-aa peptides overlapping by 11 residues) showed that DNA immunization with native gag, LAMP/gag, or DC-LAMP/gag induced responses to identical immunodominant CD4+ and CD8+ peptides. However, LAMP/gag and DC-LAMP/gag plasmids also elicited significant responses to 23 additional cryptic epitopes that were not recognized after immunization with native gag DNA. The three plasmids induced T cell responses to a total of 39 distinct peptide sequences, 13 of which were induced by all three DNA constructs. Individually, DC-LAMP/gag elicited the most diverse response, with a specific T cell response against 35 peptides. In addition, immunization with LAMP/gag and DC-LAMP/gag chimeras also promoted Ab secretion to an increased number of epitopes. These data indicate that LAMP-1 and DC-LAMP Ag chimeras follow different trafficking pathways, induce distinct modulatory immune responses, and are able to present cryptic epitopes.

Hepatitis B surface antigen vector delivers protective cytotoxic T-lymphocyte responses to disease-relevant foreign epitopes

Although hepatitis B surface antigen (HBsAg) per se is highly immunogenic, its use as a vector for the delivery of foreign cytotoxic T-lymphocyte (CTL) epitopes has met with little success because of constraints on HBsAg stability and secretion imposed by the insertion of foreign sequence into critical hydrophobic/amphipathic regions. Using a strategy entailing deletion of DNA encoding HBsAg-specific CTL epitopes and replacement with DNA encoding foreign CTL epitopes, we have derived chimeric HBsAg DNA immunogens which elicited effector and memory CTL responses in vitro, and pathogen- and tumor-protective responses in vivo, when the chimeric HBsAg DNAs were used to immunize mice. We further show that HBsAg DNA recombinant for both respiratory syncytial virus and human papillomavirus CTL epitopes elicited simultaneous responses to both pathogens. These data demonstrate the efficacy of HBsAg DNA as a vector for the delivery of disease-relevant protective CTL responses. They also suggest the applicability of the approach of deriving chimeric HBsAg DNA immunogens simultaneously encoding protective CTL epitopes for multiple diseases. The DNAs we tested formed chimeric HBsAg virus-like particles (VLPs). Thus, our results have implications for the development of vaccination strategies using either chimeric HBsAg DNA or VLP vaccines. HBsAg is the globally administered vaccine for hepatitis B virus infection, inviting its usage as a vector for the delivery of immunogens from other diseases.

Priming of T cells by exogenous antigen cross-presented on MHC class I molecules

Cross-presentation is the process whereby bone-marrow-derived antigen-presenting cells acquire, process and present exogenous antigen as peptides bound to MHC class I molecules to CD8(+) T cells. Professional antigen-presenting cells acquire cell-associated antigen predominantly in the form of protein, then process and present antigenic peptides on their surface MHC class I molecules via several mechanisms and efficiently cross-prime naïve CD8(+) T cells in vivo. This pathway is of considerable interest because it has an important role in the immune surveillance of tissues for pathogens and cancers.

Augmented humoral and cellular immune responses of a hepatitis B DNA vaccine encoding HBsAg by protein boosting

Several reports have indicated that combinatorial regimens with DNA and protein vaccines can elicit both strong immune responses, to circumvent the limits of each vaccine. Surprisingly little was known on HBV vaccine. Here, we investigated the immunization effects of several regimens in BALB/c mice. The level of total antibody and isotypes of IgG were determined by ELISA. Cellular immune responses were assayed by measuring the production of cytokines and CTL activity after re-stimulation for 7 days in vitro with tumor cells CT26/S stably expressing HBsAg. The efficacy of immunoprotection against the challenge of transplanted CT26/S was also examined. The regimen involving twice priming pVAX(S) encoding HBsAg and once recombinant HBsAg protein (rHBsAg) boosting, induced strong and homogenous antibody responses. This regimen induced significant stronger responses of interleukin-12 and gamma interferon (IFN-gamma) in splenocytes, and elicited stronger CD8+ CTL responses and greater immunopretectional efficacy than those elicited by immunization with rHBsAg or pVAX(S) alone. Our regimen may thus provide a strategy for developing an improved immunization against HBV and many other pathogens.

Cancer immunotherapy and heat-shock proteins: promises and challenges

Recent mechanistic studies on the role of heat-shock proteins (HSPs) to induce innate and adaptive immune responses have resulted in conflicting reports. Whereas some groups reported that HSPs have direct immunological function, others emphasised the endotoxin contamination of HSP preparations and questioned the antigen-specificity of HSP vaccines. The present review will discuss these issues and suggest that HSPs have diverse and distinct immunological functions that could be superimposed on effects resulting from endotoxin contamination or misunderstood by using experimental procedures with inadequate controls. To understand the actual function of HSPs in their interaction with the immune system, methods and procedures need to be optimised and appropriate controls need to be used. These points should also clarify the conflicting findings about HSPs and promote our knowledge about other immuologically important components that may be present in HSP preparations.

Endolysosomal Processing of Exogenous Antigen into Major Histocompatibility Complex Class I-Binding Peptides

An alternative endolysosomal pathway has recently been suggested for the processing of MHC-I-binding peptides, and peptide/MHC-I complexes have been demonstrated in this compartment. However, it remains unclear where in the antigen-presenting cells such peptides are processed, in the endolysosomes themselves or in the proteasomal complex. Here, we have investigated this using monoclonal antibodies specific for the immunodominant SIINFEKL/Kb complex (25-D1) or for the carbohydrate part of Db- or Kb-binding glycopeptides in combination with inhibitors for classical and endolysosomal MHC-I-processing pathways. Alternative processing was detected in both wt and TAP1(-/-) immature DC (iDC) as the expression of SIINFEKL/Kb complexes on the surface of OVA-treated cells in the presence of Brefeldin A (BFA) or lactacystin and their absence in the presence of the lysosomotropic amines ammonium chloride, chloroquine and methylamine. Internalized Db- and Kb-binding glycopeptides, detected with high specificity using an anti-galabiose (Gal2) monoclonal antibody, were found to appear on the cell surface of BFA-treated cells after intracellular MHC-I-binding. Peptide exchange in Kb was demonstrated as the gradual appearance of SIINFEKL/Kb complexes on BFA-treated cells which earlier had been saturated with another Kb-binding peptide. Our data support the presence of a fully functional endolysosomal processing pathway in iDC guided by the chaperone function of MHC-I molecules.

DNA vaccine encoding human immunodeficiency virus-1 Gag, targeted to the major histocompatibility complex II compartment by lysosomal-associated membrane protein, elicits enhanced long-term memory response

Antigen presentation by major histocompatibility complex type II (MHC II) molecules and activation of CD4+ helper T cells are critical for the generation of immunological memory. We previously described a DNA vaccine encoding human immunodeficiency virus-1 p55Gag as a chimera with the lysosome-associated membrane protein (LAMP/gag). The LAMP/gag chimera protein traffics to the MHC II compartment of transfected cells and elicits enhanced immune responses as compared to a DNA vaccine encoding native gag not targeted to the MHC II compartment. We have now investigated the long-term responses of immunized mice and show that the LAMP/gag DNA vaccine promotes long-lasting B cell- and CD4+ and CD8+ T-cell memory responses induced by DNA encoding non-targeted Gag decay rapidly and elicit very low or undetectable levels of gag DNA is sufficient to generate T-cell memory. Following this initial priming immunization with LAMP/gag DNA, booster immunizations with native gag DNA or the LAMP/gag chimera are equally efficient in eliciting B- and T-cell secondary responses, results in accordance with observations that secondary expansion of CD8+ cells in the boost phase does not require additional CD4+ help. These findings underscore the significance of targeting DNA-encoded vaccine antigens to the MHC II processing compartments for induction of long-term immunological memory.

Enhanced priming of multispecific, murine CD8+ T cell responses by DNA vaccines expressing stress protein-binding polytope peptides

A polytope DNA vaccine (pCI/pt10) was used that encodes within a 106-residue sequence 10-well characterized epitopes binding MHC class I molecules encoded by the K, D, or L locus (of H-2(d), H-2(b), and H-2(k) haplotype mice). The pCI/pt10 DNA vaccine efficiently primed all four K(b)/D(b)-restricted CD8(+) T cell responses in H-2(b) mice, but was deficient in stimulating most CD8(+) T cell responses in H-2(d) mice. Comparing CD8(+) T cell responses elicited with the pCI/pt10 DNA vaccine in L(d+) BALB/c and L(d-) BALB/c(dm2) (dm2) mice revealed that L(d)-restricted CD8(+) T cell responses down-regulated copriming of CD8(+) T cell responses to other epitopes regardless of their restriction or epitope specificity. Although the pt10 vaccine could thus efficiently co prime multispecific CD8(+) T cell responses, this priming was impaired by copriming L(d)-restricted CD8(+) T cell responses. When the pt10 sequence was fused to a 77-residue DnaJ-homologous, heat shock protein 73-binding domain (to generate a 183-residue cT(77)-pt10 fusion protein), expression and immunogenicity (for CD8(+) T cells) of the chimeric Ag were greatly enhanced. Furthermore, priming of multispecific CD8(+) T cell responses was readily elicited even under conditions in which the suppressive, L(d)-dependent immunodominance operated. The expression of polytope vaccines as chimeric peptides that endogenously capture stress proteins during in situ production thus facilitates copriming of CD8(+) T cell populations with a diverse repertoire.

Vaccination strategies. An overview

Existing vaccines have contributed significantly to the reduction of the mortality and morbidity burdens of many infectious diseases. In many instances, however, the development of these vaccines has been empirical. Furthermore, the impressive progress in the field of vaccines has been mainly driven the progress in molecular biology and microbiology more than by the progress in immunobiology. Nevertheless, the new challenges vaccinology is facing nowadays can be approached through a comprehensive understanding of the mechanisms behind the induction and the maintenance of efficacious immune responses triggered by vaccines. The new vaccination strategies aim at exploiting the new knowledge in the field of dendritic cells (DC; and their role in priming immune responses), in the field of antigen processing and presentation for the generation of antigen-specific cytolytic T cells, and in the field of induction and maintenance of immunological memory, with the ultimate goal of developing better vaccines with an enhanced safety and efficacy not only in children and adults, but also at neonatal age.

Multiple proteases process viral antigens for presentation by MHC class I molecules to CD8(+) T lymphocytes

Recognition by CD8(+) cytotoxic T lymphocytes of any intracellular viral protein requires its initial cytosolic proteolytic processing, the translocation of processed peptides to the endoplasmic reticulum via the transporters associated with antigen processing, and their binding to nascent major histocompatibility complex (MHC) class I molecules that then present the antigenic peptides at the infected cell surface. From initial assumptions that the multicatalytic and ubiquitous proteasome is the only protease capable of fully generating peptide ligands for MHC class I molecules, the last few years have seen the identification of a number of alternative proteases that contribute to endogenous antigen processing. Trimming by non-proteasomal proteases of precursor peptides produced by proteasomes is now a well-established fact. In addition, proteases that can process antigens in a fully proteasome-independent fashion have also been identified. The final level of presentation of many viral epitopes is probably the result of interplay between different proteolytic activities. This expands the number of tissues and physiological and pathological situations compatible with antigen presentation, as well as the universe of pathogen-derived sequences available for recognition by CD8(+) T lymphocytes.