Trypanosoma cruzi infection from the view of CD8+ T cell immunity--an infection model for developing T cell vaccine

Chagas' disease is caused by Trypanosoma cruzi (T. cruzi) which was once prevalent in Central and South America. Although the recent success in Triatoma vector control has made the disease being possibly "extinct" in the near future, the development of effective preventive and therapeutic vaccines is still necessary to prevent the resurgence of the neglected infection. In addition to the importance for containing the disease, T. cruzi infection presents unique features for elucidating hosts' immune responses against intracellular infectious agents. Due to its biological capacity for invading into principally any types of cells and for causing systemic infection which damages particularly muscle and neural cells, T cell immunity is critical for resolving its infection. Although T cell-mediated immune responses have been, so far, extensively investigated in viral and bacterial infections, parasitic infection such as malaria has presented epoch-making discovery in T cell immunity. Recent advances in the analyses of T cell-mediated immune responses against T. cruzi infection now make this infectious disease potentially more suitable for detecting subtle immunological changes in hosts' immune defense upon modifying immune system. The current review focuses on the usefulness of T. cruzi infection as a model for developing effective CD8(+) T cell-mediated vaccine against intracellular infectious agents.     

DNA vaccines

Within the last decade bacterial plasmids encoding foreign antigens have revolutionized vaccine design. Although no DNA vaccine has yet been approved for routine human or veterinary use, the potential of this vaccine modality has been demonstrated in experimental animal models. Plasmid DNA vaccination has shown efficacy against viral, bacterial and parasitic infections, modulated the effects of autoimmune and allergic diseases and induced control over cancer progression. With a better understanding of the basic immune mechanisms that govern induction of protective or curative immune responses, plasmid DNA vaccines and their mode of delivery are continuously being optimized. Because of the simplicity and versatility of these vaccines, various routes and modes of delivery are possible to engage the desired immune responses. These may be T or B effector cell responses able to eliminate infectious agents or transformed cells. DNA vaccines may also induce an immunoregulatory/modulatory or immunosuppressive (tolerizing) response that interferes with the differentiation, expansion or effector functions of B and T cells. In this sense a DNA vaccine may be thought of as a 'negative' vaccine. Pre-clinical and initial small-scale clinical trials have shown DNA vaccines in either of these modes to be safe and well tolerated. Although DNA vaccines induce significant immune responses in small animal trials their efficacy in humans has so far been less promising thus necessitating additional optimizations of this novel vaccine approach.     

Immunology of BVDV vaccines

Providing acquired immune protection against infection with bovine viral diarrhea viruses (BVDV) is challenging due to the heterogeneity that exists among BVDV strains and the ability of the virus to infect the fetus and establish persistent infections. Both modified live and killed vaccines have been shown to be efficacious under controlled conditions. Both humoral and cellular immune responses are protective. Following natural infection or vaccination with a modified live vaccine, the majority of the B cell response (as measured by serum antibodies) is directed against the viral proteins E2 and NS2/3, with minor responses against the Erns and E1 proteins. Vaccination with killed vaccines results in serum antibodies directed mainly at the E2 protein. It appears that the major neutralizing epitopes are conformational and are located within the N-terminal half of the E2 protein. While it is thought that the E2 and NS2/3 proteins induce protective T cell responses, these epitopes have not been mapped. Prevention of fetal infections requires T and B cell response levels that approach sterilizing immunity. The heterogeneity that exists among circulating BVDV strains, works against establishing such immunity. Vaccination, while not 100% effective in every individual animal, is effective at the herd level.     

Overview of vaccines

This article lists the vaccines current available for the control of both viral and bacterial infections. They may be attenuated live or inactivated whole microorganisms, or subunit preparations. Many more are in the pipeline and increasing attention is being given to establishing their safety before registration. Following the earlier eradication of smallpox, good progress is now being made toward the global eradication of poliomyelitis and a new program to eliminate measles from the Americas has begun. A variety of new approaches to vaccine development is now available. The hepatitis B virus surface antigen, made by DNA-transfected yeast or mammalian cells, is the basis of the first genetically engineered vaccine. Early in the 21st century, new vaccines based on oligopeptides, recombinant live viral or bacterial vectors (often existing live vaccines), or recombinant DNA plasmids are likely to be registered for human use. The efficacy of vaccines depends on the immune responses generated, and the recent substantial increase in our understanding of the mammalian immune system now offers great opportunities for manipulation to best obtain desired responses. These include mixing vaccine formulations to maximize immune responses, and combining vaccines to simplify their administration. Despite these advances, some persisting infections, such as those caused by HIV, plasmodia, and mycobacteria, still pose a great challenge to vaccine developers.     

T-cell vaccination alters the course of murine herpesvirus 68 infection and the establishment of viral latency in mice

Diseases caused by gammaherpesviruses such as Epstein-Barr virus are a major health concern, and there is significant interest in developing vaccines against this class of viral infections. However, the requirements for effective control of gammaherpesvirus infection are only poorly understood. The recent development of the murine herpesvirus MHV-68 model provides an experimental tool to dissect the immune response to gammaherpesvirus infections. In this study, we investigated the impact of priming T cells specific for class I- and class II-restricted epitopes on the acute phase of the infection and the subsequent establishment of latency and infectious mononucleosis. The data show that vaccination with either major histocompatibility complex class I- or class II-restricted T-cell epitopes derived from lytic cycle proteins significantly reduced lung viral titers during the acute infection. Moreover, the peak level of latently infected spleen cells was significantly reduced following vaccination with immunodominant CD8(+) T-cell epitopes. However, this vaccination approach did not prevent the long-term establishment of latency or the development of the infectious mononucleosis-like syndrome in infected mice. Thus, the virus is able to establish latency efficiently despite strong immunological control of the lytic infection.     

An infectious aetiology of sudden infant death syndrome

Due attention has been given to infectious agents and immune responses to infection in sudden infant death syndrome (SIDS). It has been acknowledged that the pathological, epidemiological and genotypic findings in SIDS infants suggest an infectious aetiology possibly being potentiated by immunoregulatory polymorphisms, however, the cause of SIDS is a mystery and remains open to debate. Consistent pathological findings are seen which display similarities to the pathogenesis of toxaemic shock and/or sepsis. The major risk factors for SIDS parallel those for increased colonization and serious bacterial infections and the natural variation in the incidence of SIDS cases is typical of an infectious disease. The roles played by viral infection, immunoregulatory genes and suspected bacterial species are discussed herein.     

Acquired immune response to oncogenic human papillomavirus associated with prophylactic cervical cancer vaccines

Human papillomavirus (HPV) is a common infection among women and a necessary cause of cervical cancer. Oncogenic HPV types infecting the anogenital tract have the potential to induce natural immunity, but at present we do not clearly understand the natural history of infection in humans and the mechanisms by which the virus can evade the host immune response. Natural acquired immune responses against HPV may be involved in the clearance of infection, but persistent infection with oncogenic virus types leads to the development of precancerous lesions and cancer. B cell responses are important for viral neutralization, but antibody responses in patients with cervical cancer are poor. Prophylactic vaccines targeting oncogenic virus types associated with cervical cancer have the potential to prevent up to 80% of cervical cancers by targeting HPV types 16 and 18. Clinical data show that prophylactic vaccines are effective in inducing antibody responses and in preventing persistent infection with HPV, as well as the subsequent development of high-grade cervical intraepithelial neoplasia. This article reviews the known data regarding natural immune responses to HPV and those developed by prophylactic vaccination.     

Cowpea mosaic virus as a vaccine carrier of heterologous antigens

The plant virus, cowpea mosaic virus (CPMV), has been developed as an expression and presentation system to display antigenic epitopes derived from a number of vaccine targets including infectious disease agents and tumors. These chimeric virus particles (CVPs) could represent a cost-effective and safe alternative to live replicating virus and bacterial vaccines. A number of CVPs have now been generated and their immunogenicity examined in a number of animal species. This review details the humoral and cellular immune responses generated by these CVPs following both parenteral and mucosal delivery and highlights the potential of CVPs to elicit protective immunity from both viral and bacterial infection.     

Freeze-Drying Formulations Increased the Adenovirus and Poxvirus Vaccine Storage Times and Antigen Stabilities

Successful vaccines induce specific immune responses and protect against various viral and bacterial infections. Noninactivated vaccines, especially viral vector vaccines such as adenovirus and poxvirus vaccines, dominate the vaccine market because their viral particles are able to replicate and proliferate in vivo and produce lasting immunity in a manner similar to natural infection. One challenge of human and livestock vaccination is vaccine stability related to the antigenicity and infectivity. Freeze-drying is the typical method to maintain virus vaccine stability, while cold chain transportation is required for temperatures about 2 °C–8 °C. The financial and technological resource requirements hinder vaccine distribution in underdeveloped areas. In this study, we developed a freeze-drying formula consisting of bovine serum albumin(BSA), L-glutamic acid(L-Glu), polyethylene glycol(PEG), and dextran(DEX) to improve the thermal stability and activity of viral vaccines, including vaccinia recombinant vaccine(rTTV-OVA) and adenovirus vaccine(Ad5-ENV). We compared a panel of five different formulations(PEG: DEX: BSA: L-GLU = 50:9:0:0(#1), 50:5:4:0(#2), 50:10:9:0(#3),50:0:0:9(#4), and 50:1:0:8(#5), respectively) and optimized the freeze-drying formula for rTTV-OVA and Ad5-ENV. We found that the freeze-drying formulations #2 and #3 could maintain rTTV-OVA infectivity at temperatures of 4 °C and25 °C and that r TTV-OVA immunogenicity was retained during lyophilization. However, formulations #4 and #5 maintained Ad5-ENV infectivity under the same conditions, and Ad5-ENV immunogenicity had maximum retention with freeze-drying formulation #4. In summary, we developed new freeze-drying formulations that increased virus vaccine storage times and retained immunogenicity at an ambient temperature.     

Protective Immunity Induced by Oral Immunization with a Rotavirus DNA Vaccine Encapsulated in Microparticles

DNA vaccines are usually given by intramuscular injection or by gene gun delivery of DNA-coated particles into the epidermis. Induction of mucosal immunity by targeting DNA vaccines to mucosal surfaces may offer advantages, and an oral vaccine could be effective for controlling infections of the gut mucosa. In a murine model, we obtained protective immune responses after oral immunization with a rotavirus VP6 DNA vaccine encapsulated in poly(lactide-coglycolide) (PLG) microparticles. One dose of vaccine given to BALB/c mice elicited both rotavirus-specific serum antibodies and intestinal immunoglobulin A (IgA). After challenge at 12 weeks postimmunization with homologous rotavirus, fecal rotavirus antigen was significantly reduced compared with controls. Earlier and higher fecal rotavirus-specific IgA responses were noted during the peak period of viral shedding, suggesting that protection was due to specific mucosal immune responses. The results that we obtained with PLG-encapsulated rotavirus VP6 DNA are the first to demonstrate protection against an infectious agent elicited after oral administration of a DNA vaccine.     

Viral and bacterial diseases of Atlantic cod Gadus morhua, their prophylaxis and treatment: a review

This review summarises the state of knowledge of both viral and bacterial diseases of Atlantic cod Gadus morhua, and their diagnosis, prophylaxis and treatment. The most important losses have been at the larval and juvenile stages, and vibriosis has long been the most important bacterial disease in cod, with Listonella (Vibrio) anguillarum dominant among pathogenic isolates. Vaccination of cod against pathogens such as L. anguillarum and Aeromonas salmonicida clearly demonstrates that the cod immune system possesses an effective memory and appropriate mechanisms sufficient for protection, at least against some diseases. Well-known viruses such as the nodavirus that causes viral encephalopathy and retinopathy (VER), infectious pancreatic necrosis virus (IPNV) and viral haemorrhagic septicaemia virus (VHSV) have been isolated from Atlantic cod and can be a potential problem under intensive rearing conditions. No commercial vaccines against nodavirus are currently available, whereas vaccines against IPNV infections based upon inactivated virus as well as IPNV recombinant antigens are available. A number of investigations of the pharmacokinetic properties of antibacterial agents in cod and their efficacy in treating bacterial infections have been reviewed.     

Human immunodeficiency virus-1 vaccine design: where do we go now?

Numerous human immunodeficiency virus (HIV)-1 vaccines have been developed over the last three decades, but to date an effective HIV-1 vaccine that can be used for prophylactic or therapeutic purposes in humans has not been identified. The failures and limited successes of HIV-1 vaccines have highlighted the gaps in our knowledge with regard to fundamental immunity against HIV-1 and have provided insights for vaccine strategies that may be implemented for designing more effective HIV-1 vaccines in the future. Recent studies have shown that robust mucosal immunity, high avidity and polyfunctional T cells, and broadly neutralizing antibodies are important factors governing the induction of protective immunity against HIV-1. Furthermore, optimization of vaccine delivery methods for DNA or live viral vector-based vaccines, elucidating the immune responses of individuals who remain resistant to HIV-1 infections and also understanding the core immune responses mediating protection against simian immunodeficiency viruses (SIV) and HIV-1 in animal models following vaccination, are key aspects to be regarded for designing more effective HIV-1 vaccines in the future.     

An Attenuated Herpes Simplex Virus Type 1 (HSV1) Encoding the HIV-1 Tat Protein Protects Mice from a Deadly Mucosal HSV1 Challenge

Herpes simplex virus types 1 and 2 (HSV1 and HSV2) are common infectious agents in both industrialized and developing countries. They cause recurrent asymptomatic and/or symptomatic infections, and life-threatening diseases and death in newborns and immunocompromised patients. Current treatment for HSV relies on antiviral medications, which can halt the symptomatic diseases but cannot prevent the shedding that occurs in asymptomatic patients or, consequently, the spread of the viruses. Therefore, prevention rather than treatment of HSV infections has long been an area of intense research, but thus far effective anti-HSV vaccines still remain elusive. One of the key hurdles to overcome in anti-HSV vaccine development is the identification and effective use of strategies that promote the emergence of Th1-type immune responses against a wide range of epitopes involved in the control of viral replication. Since the HIV1 Tat protein has several immunomodulatory activities and increases CTL recognition of dominant and subdominant epitopes of heterologous antigens, we generated and assayed a recombinant attenuated replication-competent HSV1 vector containing the tat gene (HSV1-Tat). In this proof-of-concept study we show that immunization with this vector conferred protection in 100% of mice challenged intravaginally with a lethal dose of wild-type HSV1. We demonstrate that the presence of Tat within the recombinant virus increased and broadened Th1-like and CTL responses against HSV-derived T-cell epitopes and elicited in most immunized mice detectable IgG responses. In sharp contrast, a similarly attenuated HSV1 recombinant vector without Tat (HSV1-LacZ), induced low and different T cell responses, no measurable antibody responses and did not protect mice against the wild-type HSV1 challenge. These findings strongly suggest that recombinant HSV1 vectors expressing Tat merit further investigation for their potential to prevent and/or contain HSV1 infection and dissemination.     

Biotechnology approaches to produce potent,self-adjuvanting antigen-adjuvant fusion protein subunit vaccines

Traditional vaccination approaches (e.g. live attenuated or killed microorganisms) are among the most effective means to prevent the spread of infectious diseases. These approaches, nevertheless, have failed to yield successful vaccines against many important pathogens. To overcome this problem, methods have been developed to identify microbial components, against which protective immune responses can be elicited. Subunit antigens identified by these approaches enable the production of defined vaccines, with improved safety profiles. However, they are generally poorly immunogenic, necessitating their administration with potent immunostimulatory adjuvants. Since few safe and effective adjuvants are currently used in vaccines approved for human use, with those available displaying poor potency, or an inability to stimulate the types of immune responses required for vaccines against specific diseases (e.g. cytotoxic lymphocytes (CTLs) to treat cancers), the development of new vaccines will be aided by the availability of characterized platforms of new adjuvants, improving our capacity to rationally select adjuvants for different applications. One such approach, involves the addition of microbial components (pathogen-associated molecular patterns; PAMPs), that can stimulate strong immune responses, into subunit vaccine formulations. The conjugation of PAMPs to subunit antigens provides a means to greatly increase vaccine potency, by targeting immunostimulation and antigen to the same antigen presenting cell. Thus, methods that enable the efficient, and inexpensive production of antigen-adjuvant fusions represent an exciting mean to improve immunity towards subunit antigens. Herein we review four protein-based adjuvants (flagellin, bacterial lipoproteins, the extra domain A of fibronectin (EDA), and heat shock proteins (Hsps)), which can be genetically fused to antigens to enable recombinant production of antigen-adjuvant fusion proteins, with a focus on their mechanisms of action, structural or sequence requirements for activity, sequence modifications to enhance their activity or simplify production, adverse effects, and examples of vaccines in preclinical or human clinical trials.     

Therapeutic vaccines against infectious diseases

Therapeutic vaccines against chronic infectious diseases aim at eliciting broad humoral and cellular immune responses against multiple target antigens. Importantly, the development of such vaccines will help to establish surrogate markers of protection in humans and thus will augment the subsequent development of efficient prophylactic vaccines. A combination of synthetic small-molecule drugs and immunotherapeutics is likely to represent a powerful means of controlling chronic infections in the future. Challenges faced in developing therapeutic vaccines include the following: first, overcoming the potential impairment of immune responses due to established infection; second, optimizing schedules of vaccine administration in combination with standard of care chemotherapy; and third, defining what biological and immunological read-outs should be used to infer vaccine efficacy.     

In vivo protective anti-HIV immune responses in non-human primates through DNA immunization

Abstract: An effective immune response involves the specific recognition of and elimination of an infectious organism at multiple levels. In this context DNA immunization can present functional antigenic proteins to the host for recognition by all arms of the immune system, yet provides the opportunity to delete any genes of the infectious organism which code for antigens or pieces of antigens that may have deleterious effects. Our group has developed the use of nucleic acid immunization as a possible method of vaccination against Human immunodeficiency virus type 1 (HIV-1) [1,2,3,10,11,12]. Sera from non-human primates immunized with DNA vectors that express the envelope proteins from HIV-1 contain antibodies specific to the HIV-1 envelope. These sera also neutralize HIV-1 infection in vitro and inhibit cell to cell infection in tissue culture. Analysis of cellular responses is equally encouraging. T cell proliferation as well as cytotoxic T cell lysis of relevant env expressing target cells were observed. In addition, evidence that DNA vaccines are capable of inducing a protective response against live virus was demonstrated using a chimeric SIV/HIV (SHIV) challenge in vaccinated cynomologous macaques. We found that nucleic acid vaccination induced protection from challenge in one out of four immunized cynomolgus macaques and viral load was lower in the vaccinated group of animals versus the control group of animals. These data encouraged us to analyze this vaccination technique in chimpanzees, the most closely related animal species to man. We observed the induction of both cellular and humoral immune responses with a DNA vaccine in chimpanzees. These studies demonstrate the utility of this technology to induce relevant immune responses in primates which may ultimately lead to effective vaccines.     

The Effects of Vaccination and Immunity on Bacterial Infection Dynamics In Vivo

Salmonella enterica infections are a significant global health issue, and development of vaccines against these bacteria requires an improved understanding of how vaccination affects the growth and spread of the bacteria within the host. We have combined in vivo tracking of molecularly tagged bacterial subpopulations with mathematical modelling to gain a novel insight into how different classes of vaccines and branches of the immune response protect against secondary Salmonella enterica infections of the mouse. We have found that a live Salmonella vaccine significantly reduced bacteraemia during a secondary challenge and restrained inter-organ spread of the bacteria in the systemic organs. Further, fitting mechanistic models to the data indicated that live vaccine immunisation enhanced both the bacterial killing in the very early stages of the infection and bacteriostatic control over the first day post-challenge. T-cell immunity induced by this vaccine is not necessary for the enhanced bacteriostasis but is required for subsequent bactericidal clearance of Salmonella in the blood and tissues. Conversely, a non-living vaccine while able to enhance initial blood clearance and killing of virulent secondary challenge bacteria, was unable to alter the subsequent bacterial growth rate in the systemic organs, did not prevent the resurgence of extensive bacteraemia and failed to control the spread of the bacteria in the body.     

Development of Vaccine Delivery Vehicles Based on Lactic Acid Bacteria

Live recombinant bacteria represent attractive antigen delivery systems able to induce both mucosal and systemic immune responses against heterologous antigens. The first live recombinant bacterial vectors developed were derived from attenuated pathogenic microorganisms. In addition to the difficulties often encountered in the construction of stable attenuated mutants of pathogenic organisms, attenuated pathogens may retain a residual virulence level that renders them unsuitable for the vaccination of partially immunocompetent individuals such as infants, the elderly or immunocompromised patients. As an alternative to this strategy, non-pathogenic food-grade lactic acid bacteria (LAB) maybe used as live antigen carriers. This article reviews LAB vaccines constructed using antigens other than tetanus toxin fragment C, against bacterial, viral, and parasitic infective agents, for which protection studies have been performed. The antigens utilized for the development of LAB vaccines are briefly described, along with the efficiency of these systems in protection studies. Moreover, the key factors affecting the performance of these systems are highlighted.     

Host responses from innate to adaptive immunity after vaccination: Molecular and cellular events

The availability of effective vaccines has had the most profound positive effect on improving the quality of public health by preventing infectious diseases. Despite many successful vaccines, there are still old and new emerging pathogens against which there is no vaccine available. A better understanding of how vaccines work for providing protection will help to improve current vaccines as well as to develop effective vaccines against pathogens for which we do not have a proper means to control. Recent studies have focused on innate immunity as the first line of host defense and its role in inducing adaptive immunity; such studies have been an intense area of research, which will reveal the immunological mechanisms how vaccines work for protection. Toll-like receptors (TLRs), a family of receptors for pathogen-associated molecular patterns on cells of the innate immune system, play a critical role in detecting and responding to microbial infections. Importantly, the innate immune system modulates the quantity and quality of longterm T and B cell memory and protective immune responses to pathogens. Limited studies suggest that vaccines which mimic natural infection and/or the structure of pathogens seem to be effective in inducing long-term protective immunity. A better understanding of the similarities and differences of the molecular and cellular events in host responses to vaccination and pathogen infection would enable the rationale for design of novel preventive measures against many challenging pathogens.     

Vaccination against the major infectious diseases

The reputation of vaccination rests on a 200-year-old history of success against major infectious diseases. That success has led to the doctrine of 'for each disease, a vaccine'. Although some diseases have proved frustrating, this doctrine carries considerable truth. However, when one reviews the vaccines now available it is apparent that most successes have been obtained when the microbe has a bacteremic or viremic phase during which it is susceptible to the action of neutralizing antibodies, and before replication in the particular organ to which it is tropic. Poliomyelitis and infections by capsulated bacteria are examples where vaccination has worked efficiently. However, some success has also been achieved against agents replicating on respiratory or gastrointestinal mucosae. Influenza, pertussis and rotavirus vaccines are examples of such agents, against which it has been possible to induce immune responses acting locally as well as systemically. In addition, when bacteria produce disease through exotoxins, purification and chemical or genetic inactivation of those toxins has yielded highly efficacious vaccines. Control of intracellular pathogens has not been achieved, except partly with the BCG vaccine against tuberculosis, and modern efforts are directed towards pathogens against which cellular immune responses are critical. In general, two achievements have been crucial to the success of vaccines: the induction of long-lasting immunological memory in individuals and the stimulation of a herd immunity that enhances control of infectious diseases in populations.