Progress towards the use of Listeria monocytogenes as a live bacterial vaccine vector for the delivery of HIV antigens

Identification and evaluation of a panel of strong constitutive promoters in Listeria monocytogenes for improving the expression of foreign antigens

Attenuated Listeria monocytogenes could be a potential vaccine vector for the immunotherapy of tumors or pathogens. However, the lack of reliable promoters has limited its ability to express foreign antigens. In the present study, 21 promoters were identified from Listeria monocytogenes through RNA-seq analysis under two pH conditions of pH 7.4 and pH 5.5. Based on the constructed fluorescence report system, 7 constitutive promoters exhibited higher strength than P_help (1.8-fold to 5.4-fold), a previously reported strong promoter. Furthermore, the selected 5 constitutive promoters exhibited higher UreB production activity than P_help (1.1-fold to 8.3-fold). Notably, a well-characterized constitutive promoter P₁₈ was found with the highest activity of fluorescence intensity and UreB production. In summary, the study provides a panel of strong constitutive promoters for Listeria monocytogenes and offers a theoretical basis for mining constitutive promoters in other organisms. KEY POINTS: • Twenty-one promoters were identified from L. monocytogenes through RNA-seq. • Fluorescent tracer of L. monocytogenes (P₁₈) was performed in vitro and in vivo. • A well-characterized constitutive promoter P₁₈ could improve the expression level of a foreign antigen UreB in L. monocytogenes.

Listeria Monocytogenes: A Model Pathogen Continues to Refine Our Knowledge of the CD8 T Cell Response

Listeria monocytogenes (Lm) infection induces robust CD8 T cell responses, which play a critical role in resolving Lm during primary infection and provide protective immunity to re-infections. Comprehensive studies have been conducted to delineate the CD8 T cell response after Lm infection. In this review, the generation of the CD8 T cell response to Lm infection will be discussed. The role of dendritic cell subsets in acquiring and presenting Lm antigens to CD8 T cells and the events that occur during T cell priming and activation will be addressed. CD8 T cell expansion, differentiation and contraction as well as the signals that regulate these processes during Lm infection will be explored. Finally, the formation of memory CD8 T cell subsets in the circulation and in the intestine will be analyzed. Recently, the study of CD8 T cell responses to Lm infection has begun to shift focus from the intravenous infection model to a natural oral infection model as the humanized mouse and murinized Lm have become readily available. Recent findings in the generation of CD8 T cell responses to oral infection using murinized Lm will be explored throughout the review. Finally, CD8 T cell-mediated protective immunity against Lm infection and the use of Lm as a vaccine vector for cancer immunotherapy will be highlighted. Overall, this review will provide detailed knowledge on the biology of CD8 T cell responses after Lm infection that may shed light on improving rational vaccine design.

Mobilising a global response to hepatitis: Lessons learned from the HIV movement

Hepatitis caused by hepatitis B and C virus is increasingly becoming a significant global health threat, with widespread prevalence that may have severe disease and economic impacts in the future. Yet, preventative measures are not implemented universally and high costs of medicines limits treatment efforts. The global response to HIV/AIDS faced similar issues, but overcame them through a global movement that brought attention to the crisis and ultimately resulted in the creation and implementation of and access to better tools for HIV prevention and treatment. This also included effective policies and programmes behind and supporting the movement. Such could be done for hepatitis, specifically using lessons from the HIV response. Here, we will discuss the current and potentially severe future burden of hepatitis globally, the challenges in addressing this epidemic, and how principles applied from the global HIV response can facilitate a successful and similar hepatitis movement.

HIV/AIDS vaccines for Africa: scientific opportunities, challenges and strategies

More than decades have already elapsed since human immunodeficiency virus (HIV) was identified as the causative agent of acquired immunodeficiency syndrome (AIDS). The HIV has since spread to all parts of the world with devastating effects. In sub-saharan Africa, the HIV/AIDS epidemic has reached unprecedented proportions. Safe, effective and affordable HIV/AIDS vaccines for Africans are therefore urgently needed to contain this public health problem. Although, there are challenges, there are also scientific opportunities and strategies that can be exploited in the development of HIV/AIDS vaccines for Africa. The recent RV144 Phase III trial in Thailand has demonstrated that it is possible to develop a vaccine that can potentially elicit modest protective immunity against HIV infection. The main objective of this review is to outline the key scientific opportunities, challenges and strategies in HIV/AIDS vaccine development in Africa.

Live-attenuated bacteria as a cancer vaccine vector

In the emerging field of active and specific cancer immunotherapy, strategies using live-attenuated bacterial vectors have matured in terms of academic and industrial development. Different bacterial species can be genetically engineered to deliver antigen to APCs with strong adjuvant effects due to their microbial origin. Proteic or DNA-encoding antigen delivery routes and natural bacterial tropisms might differ among species, permitting different applications. After many academic efforts to resolve safety and efficacy issues, some firms have recently engaged clinical trials with live Listeria or Salmonella spp. We describe here the main technological advances that allowed bacteria to become one of the most promising vectors in cancer immunotherapy.

Bacterial vectors for active immunotherapy reach clinical and industrial stages

Active immunotherapy based on live attenuated bacterial vectors has matured in terms of industrial development and develops through a combination of three phenomena. First, active immunotherapy that stimulates an antigen-specific cytotoxic T-cell immune response has become a reality after several years of work. Second, there is still a need to identify vectors that can deliver antigens to the cytosol of antigen-presenting cells in vivo. Third, the recent progress in the understanding of bacterial lifestyle and in developing genetic engineering tools has enabled the design of bioengineered bugs that are capable of delivering antigens. Here, we review the mechanisms by which clinical bacterial vectors deliver antigens into the cytosol of antigen-presenting cells and summarize the development strategy of the three identified firms in this field.

In vitro properties of a Listeria monocytogenes bacteriophage-resistant mutant predict its efficacy as a live oral vaccine strain

A Listeria monocytogenes glcV mutation precludes the binding of certain listerial phages and produces a profound attenuation characterized by the absence of detectable mutants in the livers and spleens of orally inoculated mice. In vitro, we found that the mutant formed plaques on mouse enterocyte monolayers as efficiently as the parent but the plaques formed were smaller. Intracellular growth rate determinations and examination of infected enterocytes by light and fluorescence microscopy established that the mutant was impaired not in intracellular growth rate but in cell-to-cell spreading. Because this property is shared by other immunogenic mutants (e.g., actA mutants), our glcV mutant was tested for vaccine efficacy. Oral immunization with the mutant and subsequent oral challenge (22 days postvaccination) with the parent revealed a ca. 10,000-fold increase in protection afforded by the mutant compared to sham-vaccinated controls. The glcV mutant did not stimulate innate immunity under the dose and route employed for vaccination, and an infectivity index time course experiment revealed pronounced mutant persistence in Peyer's patches. The immunogenicity of the glcV mutant compared to an isogenic actA mutant reference strain was next tested in an experiment with a challenge given 52 days postvaccination. Both mutant strains showed scant vital organ infectivity and high levels of protection similar to those seen using the glcV mutant in the 22-day postvaccination challenge. Our results indicate that oral administration of a profoundly attenuated listerial mutant can safely elicit solid protective immunity.

Transgenic Plasmodium that expresses HIV-1 Gag elicits immunity and protects mice against vaccinia virus-gag and malarial parasites

Malaria and human immunodeficiency virus type 1 (HIV-1) infection overlap in many regions of the world. Our goal was to determine the feasibility of developing transgenic Plasmodium berghei that expresses HIV-1 Gag, PbGAG, as a conceptual bivalent vaccine against both HIV-1 infection and malaria. Immunization of mice with PbGAG induced specific responses to the HIV-1 Gag. Importantly, mice vaccinated with PbGAG were significantly protected from challenge with vaccinia virus-gag (VV-gag) with an average 30-fold reduction in titer (P<0.05). In addition, mice immunized with PbGAG developed Plasmodium-specific immune responses and the immunized animals were protected from challenges with blood-stage P. berghei NK65 and Plasmodium yoelii 17XL. We demonstrated a novel vaccination strategy that uses a live transgenic protozoan parasite-based bivalent vaccine to immunize mice and confer significant levels of protection against VV-gag and malarial parasite challenges. These observations have important implications for the development of a new form of bivalent vaccine against both HIV-1 and malaria.

Innovative approaches to develop prophylactic and therapeutic vaccines against HIV/AIDS

The acquired immunodeficiency syndrome (AIDS) emerged in the human population in the summer of 1981. According to the latest United Nations estimates, worldwide over 33 million people are infected with human immunodeficiency virus (HIV) and the prevalence rates continue to rise globally. To control the alarming spread of HIV, an urgent need exists for developing a safe and effective vaccine that prevents individuals from becoming infected or progressing to disease. To be effective, an HIV/AIDS vaccine should induce broad and long-lasting humoral and cellular immune responses, at both mucosal and systemic level. However, the nature of protective immune responses remains largely elusive and this represents one of the major roadblocks preventing the development of an effective vaccine. Here we summarize our present understanding of the factors responsible for resistance to infection or control of progression to disease in human and monkey that may be relevant to vaccine development and briefly review recent approaches which are currently being tested in clinical trials. Finally, the rationale and the current status of novel strategies based on nonstructural HIV-1 proteins, such as Tat, Nef and Rev, used alone or in combination with modified structural HIV-1 Env proteins are discussed.

Listeria and Salmonella bacterial vectors of tumor-associated antigens for cancer immunotherapy

This review covers the use of the facultative intracellular bacteria, Listeriamonocytogenes and Salmonella enterica serovar typhimurium as delivery systems for tumor-associated antigens in tumor immunotherapy. Because of their ability to infect and survive in antigen presenting cells, these bacteria have been harnessed to deliver tumor antigens to the immune system both as bacterially expressed proteins and encoded on eukaryotic plasmids. They do this in the context of strong innate immunity, which provides the required stimulus to the immune response to break tolerance against those tumor-associated antigens that bear homology to self. Here we describe differences in the properties of these bacteria as vaccine vectors, a summary of the major therapies they have been applied to and their advancement towards the clinic.

Imaging Listeria monocytogenes infection in vivo

Listeria monocytogenes infection in mice is a highly prolific model of bacterial infection. Several in vivo imaging approaches have been used to study host cell dynamics in response to infection, including bioluminescence imaging, confocal microscopy and two-photon microscopy, The application of in vivo imaging to study transgenic mouse models is providing unprecedented opportunities to test specific molecular mechanistic theories about how the host immune response unfolds. In complementary studies, in vivo imaging can be performed using genetically engineered bacterial mutants to assess the impact of specific virulence factors in host cell invasion and pathogenesis. The purpose of this chapter is to provide a general rationale for why in vivo imaging is important, provide an overview of various techniques highlighting the strengths and weaknesses of each, and provide examples of how various imaging techniques have been used to study Listeria infection. Lastly, our goal is to make the reader aware of the tremendous potential these approaches hold for studying host-pathogen interactions.

Cytotoxic T-lymphocyte antigen 4 blockade augments the T-cell response primed by attenuated Listeria monocytogenes resulting in more rapid clearance of virulent bacterial challenge

Cytotoxic T-lymphocyte antigen 4 (CTLA-4) uniformly suppresses antigen-specific T cells during chronic infection with bacterial, parasitic or viral pathogens. However, the importance of CTLA-4 in controlling the T-cell response during acute infection or after priming with live attenuated vaccine vectors has not been well characterized. Since strategies aimed at blocking CTLA-4 are being actively developed to therapeutically augment T-cell-mediated immunity, the effects of CTLA-4 blockade on T-cell activation during these conditions need to be more clearly defined. We have examined the role of CTLA-4 in a prime-challenge model of acute bacterial infection using both attenuated and virulent strains of the intracellular bacterium Listeria monocytogenes. Although Foxp3(+) CD4(+) T cells are the predominant CTLA-4-expressing cell type in naïve mice, antigen-specific Foxp3(-) CD4(+) cells upregulate CTLA-4 expression after primary L. monocytogenes infection. Blockade of CTLA-4 results in increased numbers of L. monocytogenes-specific CD4 and CD8 T cells after primary infection with attenuated L. monocytogenes, and confers more rapid bacterial clearance after secondary challenge with virulent L. monocytogenes. Accordingly, CTLA-4 plays an important suppressive role in T-cell priming and protective immunity in a prime-challenge model of acute bacterial infection.

Selected prfA* mutations in recombinant attenuated Listeria monocytogenes strains augment expression of foreign immunogens and enhance vaccine-elicited humoral and cellular immune responses

While recombinant Listeria monocytogenes strains can be explored as vaccine candidates, it is important to develop attenuated but highly immunogenic L. monocytogenes vaccine vectors. Here, prfA* mutations selected on the basis of upregulated expression of L. monocytogenes PrfA-dependent genes and proteins were assessed to determine their abilities to augment expression of foreign immunogens in recombinant L. monocytogenes vectors and therefore enhance vaccine-elicited immune responses (a prfA* mutation is a mutation that results in constitutive overexpression of PrfA and PrfA-dependent virulence genes; the asterisk distinguishes the mutation from inactivation or stop mutations). A total of 63 recombinant L. monocytogenes vaccine vectors expressing seven individual viral or bacterial immunogens each in nine different L. monocytogenes strains carrying wild-type prfA or having prfA* mutations were constructed and investigated. Mutations selected on the basis of increased PrfA activation in recombinant L. monocytogenes prfA* vaccine vectors augmented expression of seven individual protein immunogens remarkably. Consistently, prime and boost vaccination studies with mice indicated that the prfA(G155S) mutation in recombinant L. monocytogenes DeltaactA prfA* strains enhanced vaccine-elicited cellular immune responses. Surprisingly, the prfA(G155S) mutation was found to enhance vaccine-elicited humoral immune responses as well. The highly immunogenic recombinant L. monocytogenes DeltaactA prfA* vaccine strains were as attenuated as the recombinant parent L. monocytogenes DeltaactA vaccine vector. Thus, recombinant attenuated L. monocytogenes DeltaactA prfA* vaccine vectors potentially are better antimicrobial and anticancer vaccines.

Protection against simian/human immunodeficiency virus (SHIV) 89.6P in macaques after coimmunization with SHIV antigen and IL-15 plasmid

The cell-mediated immune profile induced by a recombinant DNA vaccine was assessed in the simian/HIV (SHIV) and macaque model. The vaccine strategy included coimmunization of a DNA-based vaccine alone or in combination with an optimized plasmid encoding macaque IL-15 (pmacIL-15). We observed strong induction of vaccine-specific IFN-gamma-producing CD8(+) and CD4(+) effector T cells in the vaccination groups. Animals were subsequently challenged with 89.6p. The vaccine groups were protected from ongoing infection, and the IL-15 covaccinated group showed a more rapidly controlled infection than the group treated with DNA vaccine alone. Lymphocytes isolated from the group covaccinated with pmacIL-15 had higher cellular proliferative responses than lymphocytes isolated from the macaques that received SHIV DNA alone. Vaccine antigen activation of lymphocytes was also studied for a series of immunological molecules. Although mRNA for IFN-gamma was up-regulated after antigen stimulation, the inflammatory molecules IL-8 and MMP-9 were down-regulated. These observed immune profiles are potentially reflective of the ability of the different groups to control SHIV replication. This study demonstrates that an optimized IL-15 immune adjuvant delivered with a DNA vaccine can impact the cellular immune profile in nonhuman primates and lead to enhanced suppression of viral replication.

Macaques co-immunized with SIVgag/pol-HIVenv and IL-12 plasmid have increased cellular responses

BACKGROUND

The cell mediated immune profiles following immunization with a recombinant DNA vaccine was assessed in the simian-human immunodeficiency virus (SHIV) and Macaque model. Earlier work demonstrated increased numbers of antigen specific CD8 and CD4 effector cells able to secrete IFN-gamma.

METHOD

The vaccine strategy included co-immunization of a DNA based vaccine alone or in combination with a macaque IL-12 expressing plasmid (pmacIL12). Antigen activated lymphocytes were studied for activation of a set of immunological molecules.

RESULTS

The current study demonstrates lymphocytes isolated and activated from the group that was immunized with DNA and pmacIL12 had a higher level of IFN-gamma producing cells. We also observed a different immunological profile when comparing the cells isolated from macaques immunized with DNA as compared to those animals that also received pmacIL12.

CONCLUSION

The observed immune profiles are reflective of the co-delivery of pmacIL12 and demonstrates that IL-12 can increase the magnitude and polyfunctionality of the cellular immune response.

Listeria as a vaccine vector

The immunostimulatory characteristics and intracellular niche of Listeria monocytogenes make it uniquely suitable for use as a live bacterial vaccine vector. Preclinical results supporting this idea, and current strategies to induce beneficial cell-mediated immunity to both infectious diseases and cancer with this vector, are discussed in this review.

The PrfA virulence regulon

The PrfA protein, a member of the Crp/Cap-Fnr family of bacterial transcription factors, controls the expression of key virulence determinants of the facultative intracellular pathogen Listeria monocytogenes. Each of the steps of the listerial intracellular infection cycle-host cell invasion, phagosomal escape, cytosolic replication, and direct cell-to-cell spread-is mediated by products of the PrfA regulon. Only 10 of the 2853 genes of the L. monocytogenes EGDe genome have been confirmed as bona fide (directly regulated) members of this regulon, a number surprisingly small given the apparent complexity of listerial intracellular parasitism. PrfA activates transcription by binding as a dimer to a palindromic promoter element of canonical sequence tTAACanntGTtAa, with seven invariant nucleotides (in capitals) and a two-mismatch tolerance. PrfA integrates a number of environmental and bacteria-derived signals to ensure the correct spatio-temporal and niche-adapted expression of the regulon, with maximum induction in the host cell cytosol and repression in the environmental habitat. Regulation operates through changes in PrfA activity-presumably by cofactor-mediated allosteric shift-and concentration, and involves transcriptional, translational and post-translational control mechanisms. There is evidence that PrfA exerts a more global influence on L. monocytogenes physiology via indirect mechanisms.

The protein secretion systems in Listeria: inside out bacterial virulence

Listeria monocytogenes, the etiologic agent of listeriosis, remains a serious public health concern with its frequent occurrence in food coupled with a high mortality rate. The capacity of a bacterium to secrete proteins to or beyond the bacterial cell surface is of crucial importance in the understanding of biofilm formation and bacterial pathogenesis to further develop defensive strategies. Recent findings in protein secretion in Listeria together with the availability of complete genome sequences of several pathogenic L. monocytogenes strains, as well as nonpathogenic Listeria innocua Clip11262, prompted us to summarize the listerial protein secretion systems. Protein secretion would rely essentially on the Sec (Secretion) pathway. The twin-arginine translocation pathway seems encoded in all but one sequenced Listeria. In addition, a functional flagella export apparatus, a fimbrilin-protein exporter, some holins and a WXG100 secretion system are encoded in listerial genomes. This critical review brings new insights into the physiology and virulence of Listeria species.

Listeria monocytogenes as a vector for tumor-associated antigens for cancer immunotherapy

As a facultative intracellular bacterium, Listeria monocytogenes has adapted to live within the cytosol of the host cell. It is actively taken up by antigen-presenting cells through phagocytosis, and as Listeria survive within these cells, it is an ideal vector for the delivery of antigens to be processed and presented through both the class I and II antigen-processing pathways. Once phagocytosed, Listeria produces virulence factors within the phagolysosome of the host cell, which allows it to break out of this organelle and live in the host cytosol. It is possible that these virulence factors can enhance the immunogenicity of tumor-associated antigens, which are poorly immunogenic. Recent progress in the development of this bacterium as a vaccine vector for tumor-associated antigens is discussed in the context of bacterial vectors in general. In several mouse models, Listeria-based vaccines have been demonstrated to be an effective method of influencing tumor growth and eliciting potent antitumor immune responses. Safety issues and the transition of Listeria into human clinical trials will also be discussed in this review.

A DNA prime-oral Listeria boost vaccine in rhesus macaques induces a SIV-specific CD8 T cell mucosal response characterized by high levels of alpha4beta7 integrin and an effector memory phenotype

In this study in Rhesus macaques, we tested whether IL-12 or IL-15 in a DNA prime-oral Listeria boost amplifies the SIV-Gag-specific CD8 mucosal response. SIV-specific CD8 T cells were demonstrated in the peripheral blood (PB) in all test vaccine groups, but not the control group. SIV-Gag-specific CD8 T cells in the PB expressed alpha4beta7 integrin, the gut-homing receptor; a minor subset co-express alphaEbeta7 integrin. SIV-Gag-specific CD8 T cells were also detected in the gut tissue, intraepithelial (IEL) and lamina propria lymphocytes (LPL) of the duodenum and ileum. These cells were characterized by high levels of beta7 integrin expression and a predominance of the effector memory phenotype. Neither Il-12 nor IL-15 amplified the frequency of SIV-specific CD8 T cells in the gut. Thus, the DNA prime-oral Listeria boost strategy induced a mucosal SIV-Gag-specific CD8 T cell response characterized by expression of the alpha4beta7 integrin gut-homing receptor.

Selective targeting of antitumor immune responses with engineered live-attenuated Listeria monocytogenes

Improved immunization and ex vivo T-cell culture strategies can generate larger numbers and more potent tumor-specific effector cells than previously possible. Nonetheless, the capacity of these cells to eliminate established tumors is limited by their ability to efficiently enter tumor-bearing organs and mediate their effector function. In the current study, we show that the administration of an engineered organ-homing microbe selectively targets tumor-specific immune responses to metastases within that organ. Specifically, an attenuated Listeria monocytogenes strain, which preferentially infects the liver following systemic administration, dramatically enhances the activity of a cancer vaccine against liver metastases but not metastases in the lung. This enhanced activity results from both local recruitment of innate immune effectors as well as concentration and increased activation of vaccine-induced antitumor T cells within the liver. These findings show a general approach to focus systemic cancer immunotherapies to specific organs bearing tumor metastases by taking advantage of differential tropisms and the proinflammatory nature of microbes.

SIV DNA vaccine co-administered with IL-12 expression plasmid enhances CD8 SIV cellular immune responses in cynomolgus macaques

Current evidence suggests that a strong induced CD8 human immunodeficiency virus type 1 (HIV-1)-specific cell mediated immune response may be an important aspect of an HIV vaccine. The response rates and the magnitude of the CTL responses induced by current DNA vaccines in humans need to be improved and cellular immune responses to DNA vaccines can be enhanced in mice by co-delivering DNA plasmids expressing immune modulators. Two reported to work well in the mouse systems are interleukin (IL)-12 and CD40L. We sought to compare these molecular adjuvants in a primate model system. The cDNA for macaque IL-12 and CD40L were cloned into DNA vectors. Groups of cynomolgus macaques were immunized with 2 mg of plasmid expressing SIVgag alone or in combination with either IL-12 or CD40L. CD40L did not appear to enhance the cellular immune response to SIVgag antigen. However, more robust results were observed in animals co-injected with the IL-12 molecular adjuvant. The IL-12 expanded antigen-specific IFN-gamma positive effector cells as well as granzyme B production. The vaccine immune responses contained both a CD8 component as well a CD4 component. The adjuvanted DNA vaccines illustrate that IL-12 enhances a CD8 vaccine immune response, however, different cellular profiles.

Fusion to Listeriolysin O and delivery by Listeria monocytogenes enhances the immunogenicity of HER-2/neu and reveals subdominant epitopes in the FVB/N mouse

Five overlapping fragments of rat HER-2/neu have been expressed in recombinant Listeria monocytogenes. Each fragment of HER-2/neu is secreted as a fusion protein with a truncated, nonhemolytic form of listeriolysin O (LLO). Lm-LLO-EC1, Lm-LLO-EC2, and Lm-LLO-EC3 overlap the extracellular domain of HER-2/neu, whereas Lm-LLO-IC1 and Lm-LLO-IC2 span the intracellular domain. All five strains controlled the growth of established NT-2 tumors, a rat HER-2/neu-expressing tumor line derived from a spontaneously arising mammary tumor in a FVB/N HER-2/neu-transgenic mouse. The antitumor effect of each of these vaccine constructs was abrogated by the in vivo depletion of CD8(+) T cells, although only one known epitope has been defined previously and is present in Lm-LLO-EC2. Anti-HER-2/neu CTL responses were generated by each of the rLm vaccine constructs. With the use of a panel of 3T3 cell lines expressing overlapping fragments of HER-2/neu, regions of HER-2/neu with potential CD8(+) T cell epitopes have been defined. DNA vaccines expressing either a fragment or full-length HER-2/neu were constructed in LLO-fused and non-LLO-fused forms. CTL analysis of the DNA vaccines revealed a broadening in the regions of HER-2/neu recognizable as targets when the target Ag is fused to LLO. These studies show the efficacy of L. monocytogenes-based HER-2/neu vaccines in a murine model of breast cancer and also that the immunogenicity of self-Ags can be increased by fusion to LLO and delivery by L. monocytogenes revealing subdominant epitopes.