Induction of mucosal and systemic responses against human immunodeficiency virus type 1 glycoprotein 120 in mice after oral immunization with a single dose of a Salmonella-HIV vector

Live-attenuatedSalmonellaas a prototype vaccine vector for passenger immunogens in humans: are we there yet?

It has been nearly 20 years since the first Phase I clinical trial of a live-attenuated bacterial vaccine was created by recombinant DNA methods, opening the door to the use of these organisms as mucosal delivery vehicles for passenger antigens. Over this time, a number of animal studies have indicated the feasibility of this approach. These include studies showing that bacteria can deliver antigens expressed by the bacterium itself and that bacteria can deliver DNA vaccines to be expressed in target eukaryotic cells. Concomitant studies have identified a number of attenuating mutations that render the bacterial vectors both safe and immunogenic in humans. Both avenues of research indicate the significant promise of this approach to mucosal vaccine development; however, this promise remains largely unrealized at the level of human clinical trials. This review sketches the history of this problem and points toward possible solutions using Salmonella vaccine vectors as the prototypes.

Recombinant Salmonella Bacteria Vectoring HIV/AIDS Vaccines

HIV/AIDS is an important public health problem globally. An affordable, easy-to-deliver and protective HIV vaccine is therefore required to curb the pandemic from spreading further. Recombinant Salmonella bacteria can be harnessed to vector HIV antigens or DNA vaccines to the immune system for induction of specific protective immunity. These are capable of activating the innate, humoral and cellular immune responses at both mucosal and systemic compartments. Several studies have already demonstrated the utility of live recombinant Salmonella in delivering expressed foreign antigens as well as DNA vaccines to the host immune system. This review gives an overview of the studies in which recombinant Salmonella bacteria were used to vector HIV/AIDS antigens and DNA vaccines. Most of the recombinant Salmonella-based HIV/AIDS vaccines developed so far have only been tested in animals (mainly mice) and are yet to reach human trials.

SAFETY AND ETHICAL CONSIDERATION OF AIDS VACCINE

It has been demonstrated that HIV-1 gp120 resembles several important properties of immunoglobulins allowing it strong influence on the human immune system, especially through induction of the deceptive imprinting and deregulation of the immune network. On the other hand there are many unanswered questions concerning properties and control of the genetically modified viruses and bacteria used as vectors in AIDS vaccines. This situation opens a serious question about the safety of vectored AIDS vaccine and the ethics of their trials in humans.

Oral attenuated Salmonella enterica serovar Typhimurium vaccine expressing codon-optimized HIV type 1 Gag enhanced intestinal immunity in mice

Oral immunization is a safe and easily applicable route to induce mucosal immunity to HIV infection. We examined the ability of oral attenuated Salmonella typhimurium (ST) vaccine expressing Gag for the efficiency of generating Gag-specific mucosal IgA and CD8+ T cell responses in intestinal lymphoid tissues. By optimizing the codon of HIV-1 gag to the preferred codon bias of Salmonella, the expression of Gag in Salmonella was dramatically improved. The oral ST-Gag vaccine by itself was not so powerful and induces little Gag-specific CD8+ T cell responses in the intestine. Nevertheless, we found that it potentiates otherwise weak intestinal CD8+ T cell responses in nasally primed mice with Gag p24 and cholera toxin adjuvant. Thus, the oral delivery of Salmonella expressing Gag would be utilized in combination with other parenteral vaccine to direct and strengthen intestinal HIV-specific CTL responses.

The cell-mediated immune response induced by plasmid encoding bovine herpesvirus 1 glycoprotein B is enhanced by plasmid encoding IL-12 when delivered intramuscularly or by gene gun, but not after intradermal injection

Bovine herpesvirus 1 (BHV-1) causes respiratory and genital infections in cattle. Previously we demonstrated that a DNA vaccine encoding a truncated, secreted form of BHV-1 glycoprotein B (tgB) induces cytotoxic T lymphocyte (CTL) responses in C3H mice. In this study we investigated the potential of interleukin 12 (IL-12) to further enhance the CTL response. C3H mice were immunized with a plasmid encoding tgB or with plasmids encoding tgB and murine IL-12. When the plasmid encoding tgB was delivered intramuscularly or epidermally by a gene gun, co-administration with IL-12 plasmid stimulated the synthesis of more IgG2a, the production of higher levels of IFN-gamma, and more effective killing by CTLs. In contrast, after intradermal delivery no effect of co-administration of IL-12 encoding plasmid was observed. Further investigation suggested that antigen and IL-12 need to be expressed in the draining lymph nodes, where IL-12 can have a direct effect on T cells.

Partially assembled K99 fimbriae are required for protection

Antibodies to K99 fimbriae afford protection to F5+ bovine enterotoxigenic Escherichia coli (ETEC). Previous studies show that murine dams immunized with Salmonella vaccine vectors stably expressing K99 fimbriae confer protection to ETEC-challenged neonatal pups. To begin to address adaptation of the K99 scaffold to display heterologous B- and T-cell epitopes, studies were conducted to determine how much of the assembled K99 fimbria is required to maintain protective immunity. Sequential deletions in the K99 gene clusters were made, resulting in diminished localization of the K99 fimbrial subunit in the outer membrane. As placement of the K99 fimbrial subunit became progressively contained within the vaccine vector, diminished immunoglobulin A (IgA) and IgG1 antibody titers, as well as diminished Th2-type cytokine responses, were observed in orally immunized mice. Deletion of fanGH, which greatly reduced the export of the fimbrial subunit to the outer membrane, showed only partial reduction in protective immunity. By contrast, deletion of fanDEFGH, which also reduced the export of the fimbrial subunit to the outer membrane but retained more subunit in the cytoplasm, resulted in protective immunity being dramatically reduced. Thus, these studies showed that retention of K99 fimbrial subunit as native fimbriae or with the deletion of fanGH is sufficient to confer protection.

Immunogenicity of DNA vaccines that direct the coincident expression of the 120 kDa glycoprotein of human immunodeficiency virus and the catalytic domain of cholera toxin

Passive antibody studies unequivocally demonstrate that sterilizing immunity against lentiviruses is obtainable through humoral mechanisms. In this regard, DNA vaccines represent an inexpensive alternative to subunit vaccine for mass vaccination programs designed to induce such responses to human immunodeficiency virus type I (HIV-1). At present, however, this vaccine modality has proven relatively ineffective at inducing humoral responses. In this report, we describe the immunogenicity of DNA vaccines that direct the coincident expression of the cholera toxin catalytic domain (CTA1) with that of the human immunodeficiency virus type I gp120 through genes either encoded in individual plasmids or in a single dicistronic plasmid. In BALB/cJ mice, coincident expression of CTA1 in either a separate plasmid or in the dicistronic plasmid in the DNA vaccines induced serum IgG responses to gp120 that were at least 1000-fold greater, and remained elevated longer than, the analogous responses in mice vaccinated with a DNA vaccine that expressed gp120 alone. In addition, mice vaccinated with CTA1 and gp120 produced significantly more gp120-specific IFN-gamma ELISPOTs than mice vaccinated with the gp120 DNA vaccine. Combined, these data show that the adjuvant properties of cholera toxin can be harnessed in DNA vaccine modalities.

A review of vaccines for HIV prevention

HIV/AIDS has become the most devastating pandemic in recorded history. It has killed 40 million people in the last 20 years and the World Health Organisation estimated that at least 14,000 new infections occurred daily in 2001. There will be up to 100 million new infections in the next 10 years (for current updates, visit http://www.unaids.org/epidemic_update/). Most HIV infections occur in the developing world, and the adverse social and economic impact of the HIV/AIDS pandemic, particularly in the developing world, is unprecedented. Highly active antiretroviral therapy (HAART) has had significant effects on HIV/AIDS in the developed world. The drugs have acted to prolong survival, reduce the viral load, and to alleviate suffering. However, the incidence of side effects and resistance is high and the drugs are unaffordable and unavailable in the developing world. HAART regimens are difficult to comply with. Public health efforts to modify the behaviour, attitude and culture that accelerate the spread of HIV/AIDS have had only modest success. There is urgent need for a prophylactic and/or therapeutic HIV vaccine. This is a review of the obstacles and current trends in HIV vaccine development.

Salmonella vaccines for use in humans: present and future perspectives

In recent years there has been significant progress in the development of attenuated Salmonella enterica serovar Typhi strains as candidate typhoid fever vaccines. In clinical trials these vaccines have been shown to be well tolerated and immunogenic. For example, the attenuated S. enterica var. Typhi strains CVD 908-htrA (aroC aroD htrA), Ty800 (phoP phoQ) and chi4073 (cya crp cdt) are all promising candidate typhoid vaccines. In addition, clinical trials have demonstrated that S. enterica var. Typhi vaccines expressing heterologous antigens, such as the tetanus toxin fragment C, can induce immunity to the expressed antigens in human volunteers. In many cases, the problems associated with expression of antigens in Salmonella have been successfully addressed and the future of Salmonella vaccine development is very promising.

Fimbriated Salmonella enterica serovar typhimurium abates initial inflammatory responses by macrophages

Oral immunization of mice with a Salmonella vaccine expressing colonization factor antigen I (CFA/I) from enterotoxigenic Escherichia coli results in the rapid onset of interleukin-4 (IL-4) and IL-5 production, which explains the observed elevations in mucosal immunoglobulin A (IgA) and serum IgG1 antibodies. In contrast, oral immunization with the Salmonella vector does not result in the production of Th2-type cytokines. To begin to assess why such differences exist between the two strains, it should be noted that in vitro infection of RAW 264.7 macrophages resulted in the absence of nitric oxide (NO) production in cells infected with the Salmonella-CFA/I vaccine. This observation suggests differential proinflammatory cytokine production by these isogenic Salmonella strains. Upon measurement of proinflammatory cytokines, minimal to no tumor necrosis factor alpha (TNF-alpha), IL-1alpha, IL-1beta, or IL-6 was produced by Salmonella-CFA/I-infected RAW 264.7 or peritoneal macrophages, but production was greatly induced in Salmonella vector-infected macrophages. Only minute levels of IL-12 p70 were induced by Salmonella vector-infected macrophages, and none was induced by Salmonella-CFA/I-infected macrophages. The absence of IL-12 was not due to overt increases in production of either IL-12 p40 or IL-10. CFU measurements taken at 8 h postinfection showed no differences in colonization in RAW 264.7 cells infected with either Salmonella construct, but there were differences in peritoneal macrophages. However, after 24 h, the Salmonella vector strain colonized to a greater extent in RAW 264.7 cells than in peritoneal macrophages. Infection of RAW 264.7 cells or peritoneal macrophages with either Salmonella construct showed no difference in macrophage viabilities. This evidence shows that the expression of CFA/I fimbriae alters how macrophages recognize or process salmonellae and prevents the rapid onset of proinflammatory cytokines which is typical during Salmonella infections.

Development of an oral prime-boost strategy to elicit broadly neutralizing antibodies against HIV-1

Given the increasing incidence of HIV-1 infection world-wide, an affordable, effective vaccine is probably the only way that this virus will be contained. Accordingly, our group is developing an oral prime-boost strategy with the primary goal of eliciting broadly neutralizing antibodies against HIV-1 to provide sterilizing immunity for this virus. Our secondary goal is to elicit broadly cross-reactive anti-viral CD8(+) T cells by this strategy to blunt any breakthrough infections that occur after vaccination of individuals who fail to develop sterilizing immunity. This article describes our progress in the use of the live attenuated intracellular bacteria, Salmonella and Shigella, as oral delivery vehicles for DNA vaccines and the development of conformationally constrained HIV-1 Env immunogens that elicit broadly neutralizing antibodies.

The past, present and future of HIV-vaccine development: a critical view

Despite the extensive efforts that have been made to combat AIDS, the global number of HIV-1 infections is still increasing. There is major consent among scientists worldwide, that the development of successful HIV vaccine strategies requires a profound understanding of the epidemiological principles of a viral pandemic, as well as deep insights into the molecular and immunological mechanisms of HIV pathogenesis. This review provides an overview of past and present developments, as well as future aspects of HIV vaccines, and also provides a summary of current clinical trials in man.

Mucosal and systemic HIV-1 Env-specific CD8(+) T-cells develop after intragastric vaccination with a Salmonella Env DNA vaccine vector

CD8(+) T-cell responses provide beneficial antiviral immunity against human immunodeficiency virus 1 (HIV-1). In this study, we show that intragastric vaccination with a Salmonella HIV-1 Env DNA vaccine vector generates Env-specific CD8(+) T-cells, both in mucosal and systemic lymphoid tissue. By contrast, intramuscular vaccination with the Env DNA vaccine alone only induced systemic CD8(+) T-cells. To our knowledge, this is the first report showing both mucosal and systemic CD8(+) T-cell responses following vaccination with a Salmonella vaccine vector. These data suggest that this mode of HIV-1 DNA vaccine delivery will be advantageous over parenterally administered HIV-1 DNA vaccines.

Candidate vaccines for immunotherapy in HIV

In this short article we review the immunopathology of HIV infection and delineate the kinds of immune responses seen in acute infection and in long-term nonprogression that are understood to be necessary if the host is to maintain long-term control of the virus. In this context we discuss the potential promise of therapeutic vaccination in HIV infection.

Vaccination with a Shigella DNA vaccine vector induces antigen-specific CD8(+) T cells and antiviral protective immunity

A prototype Shigella human immunodeficiency virus type 1 (HIV-1) gp120 DNA vaccine vector was constructed and evaluated for immunogenicity in a murine model. For comparative purposes, mice were also vaccinated with a vaccinia virus-env (vaccinia-env) vector or the gp120 DNA vaccine alone. Enumeration of the CD8(+)-T-cell responses to gp120 after vaccination using a gamma interferon enzyme-linked spot assay revealed that a single intranasal dose of the Shigella HIV-1 gp120 DNA vaccine vector elicited a CD8(+) T-cell response to gp120, the magnitude of which was comparable to the sizes of the analogous responses to gp120 that developed in mice vaccinated intraperitoneally with the vaccinia-env vector or intramuscularly with the gp120 DNA vaccine. In addition, a single dose of the Shigella gp120 DNA vaccine vector afforded significant protection against a vaccinia-env challenge. Moreover, the number of vaccinia-env PFU recovered in mice vaccinated intranasally with the Shigella vector was about fivefold less than the number recovered from mice vaccinated intramuscularly with the gp120 DNA vaccine. Since the Shigella vector did not express detectable levels of gp120, this report confirms that Shigella vectors are capable of delivering passenger DNA vaccines to host cells and inducing robust CD8(+) T-cell responses to antigens expressed by the DNA vaccines. Furthermore, to our knowledge, this is the first documentation of antiviral protective immunity following vaccination with a live Shigella DNA vaccine vector.

Development of immunotherapeutic strategies for HIV-1

In the majority of untreated patients, HIV-1 infection presents as a progressive disease of the immune system. Recent studies indicate that immune responses can be induced in HIV-1 infected individuals, leading to some immune control of virus replication. Such immune responses are also observed in small numbers of untreated HIV-1 infected long-term non-progressor (LTNP) patients, as well as in other viral infections (including those with human herpes viruses). Emerging novel technologies, animal studies and detailed immunological studies have proven invaluable in defining the immune responses that are associated with a favourable clinical outcome. Central effector and regulatory cells are HIV-1-specific CD8+ cytotoxic T-lymphocytes (CTL) and CD4+ helper T-lymphocytes respectively. Fully functional antigen-presenting cells (APC) are also essential in all stages of HIV-1 infection and possibly some (but not all) antibody responses contribute to beneficial immunity. The availability of combination anti-retroviral drug therapy, which successfully controls viraemia, has enabled a beneficial outcome in many HIV-1 infected individuals. Since no chronically HIV-1 infected patient has been shown to eradicate virus, novel approaches utilising therapeutic immunisation and various cytokines to manipulate immune responses and to induce and steer immunity towards a desired phenotype are required. There is a clear rationale for immunotherapeutic intervention in chronic progressive HIV-1 infection, which forms the foundation for novel approaches aimed at inducing and maintaining immune control. Here we review the immunopathogenesis of HIV-1 infection and discuss the promises of therapeutic immunisation and immunotherapy in general and their potential in the treatment of chronic HIV-1 disease.

Impaired mucosal immunity in L-selectin-deficient mice orally immunized with a Salmonella vaccine vector

Lymphocyte trafficking in the gastrointestinal tract is primarily mediated by interactions with the mucosal addressin cell adhesion molecule 1 and its lymphocyte ligand, alpha(4)beta(7), and partly by L-selectin (L-Sel) interactions with peripheral node addressin coexpressed on some mucosal addressin cell adhesion molecule 1. We inquired whether intestinal responses in mice lacking L-Sel would be enhanced. L-Sel-deficient (L-Sel(-/-)) mice were orally immunized with either Salmonella vaccine vector or Salmonella vector-expressing colonization factor Ag I (CFA/I) from enterotoxigenic Escherichia coli. In L-Sel(-/-) mice, mucosal IgA anti-CFA/I fimbrial responses were greatly reduced, and systemic IgG2a anti-CFA/I fimbrial responses were 26-fold greater compared with C57BL/6 (L-Sel(+/+)) mice. L-Sel(-/-) Peyer's patch (PP) CD4(+) Th cells revealed IFN-gamma-dominated responses and an unprecedented absence of IL-4, whereas the expected mixed Th cell phenotype developed in L-Sel(+/+) mice. PP CD4(+) Th cell anti-Salmonella responses were nearly nonexistent in L-Sel(-/-) mice immunized with either Salmonella vaccine. Splenic CD4(+) Th cell anti-Salmonella responses were reduced but did show cytokine production in Ag restimulation assays. Increased colonization of PP and spleen was noted only with the Salmonella vector in L-Sel(-/-) mice, resulting in increased splenomegaly, suggesting that the Salmonella-CFA/I vaccine was not as infectious or that the presence of the fimbriae improved clearance, possibly because of reduced neutrophil recruitment. However, sufficient anti-Salmonella immunity was induced, because Salmonella vector-immunized L-Sel(-/-) mice showed complete protection against wild-type Salmonella challenge, unlike L-Sel(+/+) mice. This evidence shows that L-Sel is important for development of mucosal immunity, and absence of L-Sel is protective against salmonellosis.

Salmonella: immune responses and vaccines

Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Vaccination is an effective tool for the prevention of Salmonella infections. Host resistance to Salmonella relies initially on the production of inflammatory cytokines leading to the infiltration of activated inflammatory cells in the tissues. Thereafter T- and B-cell dependent specific immunity develops allowing the clearance of Salmonella microorganisms from the tissues and the establishment of long-lasting acquired immunity to re-infection. The increased resistance that develops after primary infection/ vaccination requires T-cells cytokines such as IFNgamma TNFalpha and IL12 in addition to opsonising antibody. However for reasons that are not fully understood seroconversion and/or the presence of detectable T-cell memory do not always correlate with the development of acquired resistance to infection.Whole-cell killed vaccines and subunit vaccines are used in the prevention of Salmonella infection in animals and in humans with variable results. A number of early live Salmonella vaccines derived empirically by chemical or u.v. mutagenesis proved to be immunogenic and protective and are still in use despite the need for repeated parenteral administration. Recent progress in the knowledge of the genetics of Salmonella virulence and modern recombinant DNA technology offers the possibility to introduce multiple defined attenuating and irreversible mutations into the bacterial genome. This has recently allowed the development of Salmonella strains devoid of significant side effects but still capable of inducing solid immunity after single oral administration. Live attenuated Salmonella vaccines have been used for the expression of heterologous antigens/proteins that can be successfully delivered to the immune system. Furthermore Salmonella can transfer plasmids encoding foreign antigens under the control of eukaryotic promoters (DNA vaccines) to antigen-presenting cells resulting in targeted delivery of DNA vaccines to these cells. Despite the great recent advances in the development of Salmonella vaccines a large proportion of the work has been conducted in laboratory rodents and more research in other animal species is required.

Vaccines for the control of HIV/AIDS

This review discusses the feasibility of an HIV vaccine and describes the history, efficacy and potential to succeed of old and new vaccine concepts.

Expression of recombinant enterotoxigenic Escherichia coli colonization factor antigen I by Salmonella typhimurium elicits a biphasic T helper cell response

Protective immunity to enterotoxigenic Escherichia coli (ETEC) is antibody (Ab) dependent; however, oral immunization with purified ETEC fimbriae fails to elicit protective immunity as a consequence of antigenic alteration by the gastrointestinal (GI) tract. Unless unaltered ETEC fimbriae can reach the inductive lymphoid tissues of the GI tract, immunity to ETEC cannot be induced. To produce immunity, live vectors, such as Salmonella typhimurium, can effectively target passenger antigens to the inductive lymphoid tissues of the GI tract. By convention, oral immunizations with Salmonella vectors induce CD4(+) T helper (Th) cell responses by gamma interferon (IFN-gamma)-dominated pathways both to the vector and passenger antigen, resulting in serum immunoglobulin G2a (IgG2a) and modest mucosal IgA Ab responses. In the present study, mice orally immunized with a Salmonella vector engineered to stably express ETEC colonization factor antigen I (CFA/I) showed initially elevated serum IgG1 and mucosal IgA anti-CFA/I Ab responses. As expected, mice orally immunized with an E. coli-CFA/I construct elicited poor anti-CFA/I Ab responses. In fact, the addition of cholera toxin during oral E. coli-CFA/I immunization failed to greatly enhance mucosal IgA Ab responses. Seven days after immunization with the Salmonella-CFA/I construct, cytokine-specific ELISPOT showed induction of predominant Th2-type responses in both mucosal and systemic immune compartments supporting the early IgG1 and IgA anti-CFA/I Abs. By 4 weeks, the Th cell response became Th1 cell dominant from the earlier Th2-type responses, as evidenced by increased mucosal and systemic IFN-gamma-producing T cells and a concomitant elevation of serum IgG2a Ab responses. This biphasic response offers an alternative strategy for directing Salmonella vector-induced host immunity along a Th2 cell-dependent pathway, allowing for early promotion of mucosal and systemic Abs.

Antigenicity and immunogenicity of the V3 domain of HIV type 1 glycoprotein 120 expressed on the surface of Streptococcus gordonii

Five different V3 domains of HIV-1 gp120 were expressed on the surface of the gram-positive bacterium Streptococcus gordonii, a model live vector for vaccine delivery. Sera of HIV-1-infected individuals and human monoclonal antibodies specifically recognized the gp120 sequences on the bacterial surface. Recombinant V3 from the reference HIV-1 strain MN was also shown to retain a conformation that allowed reaction with a conformation-specific monoclonal antibody. A V3-specific serum antibody response was detected in mice immunized both by subcutaneous injection and by vaginal colonization. V3-specific IgG2a antibodies, suggestive of a Th1 response, were found in the sera of mice colonized by recombinant bacteria.