Effect of intragastric and intraperitoneal immunisation with attenuated and wild-type LACK-expressing Listeria monocytogenes on control of murine Leishmania major infection

Live-Attenuated Bacterial Vectors for Delivery of Mucosal Vaccines, DNA Vaccines, and Cancer Immunotherapy

Vaccines save millions of lives each year from various life-threatening infectious diseases, and there are more than 20 vaccines currently licensed for human use worldwide. Moreover, in recent decades immunotherapy has become the mainstream therapy, which highlights the tremendous potential of immune response mediators, including vaccines for prevention and treatment of various forms of cancer. However, despite the tremendous advances in microbiology and immunology, there are several vaccine preventable diseases which still lack effective vaccines. Classically, weakened forms (attenuated) of pathogenic microbes were used as vaccines. Although the attenuated microbes induce effective immune response, a significant risk of reversion to pathogenic forms remains. While in the twenty-first century, with the advent of genetic engineering, microbes can be tailored with desired properties.

In this review, I have focused on the use of genetically modified bacteria for the delivery of vaccine antigens. More specifically, the live-attenuated bacteria, derived from pathogenic bacteria, possess many features that make them highly suitable vectors for the delivery of vaccine antigens. Bacteria can theoretically express any heterologous gene or can deliver mammalian expression vectors harboring vaccine antigens (DNA vaccines). These properties of live-attenuated microbes are being harnessed to make vaccines against several infectious and noninfectious diseases. In this regard, I have described the desired features of live-attenuated bacterial vectors and the mechanisms of immune responses manifested by live-attenuated bacterial vectors. Interestingly anaerobic bacteria are naturally attracted to tumors, which make them suitable vehicles to deliver tumor-associated antigens thus I have discussed important studies investigating the role of bacterial vectors in immunotherapy. Finally, I have provided important discussion on novel approaches for improvement and tailoring of live-attenuated bacterial vectors for the generation of desired immune responses.

A suicidal strain of Listeria monocytogenes is effective as a DNA vaccine delivery system for oral administration

In this study we determined the in vivo activity of model ovalbumin vaccines delivered by direct intramuscular delivery of plasmid DNA or oral delivery using a recombinant suicidal Listeria monocytogenes strain (rsΔ2). In a previous report we described how rsΔ2 is capable of delivering luciferase, as protein or DNA, in vitro, into non-dividing intestinal epithelial cells (Kuo et al., 2009). This is achieved by engineering a dual expression shuttle vector, pDuLX-Luc, that replicates in E. coli and rsΔ2 and drives gene expression from the Listeria promoter (Phly) as well as the eukaryotic cytomegalovirus promoter (CMV), thereby delivering both protein and plasmid DNA to the cell cytoplasm. For the current in vivo study rsΔ2 containing pDuLX-OVA was used to deliver both ovalbumin protein and the mammalian expression plasmid by the oral route. Controls were used to investigate the activity of this system versus positive and negative controls, as well as quantifying activity against direct intramuscular injection of expression plasmids. Oral administration of rsΔ2(pDuLX-OVA) produced significant titres of antibody and was effective at inducing targeted T-cell lysis (approximately 30% lysis relative to an experimental positive control, intravenous OVA-coated splenocytes+lipopolysaccharide). Intramuscular injection of plasmids pDuLX-OVA or p3L-OVA (which lacks the prokaryotic promoter) also produced significant CTL-mediated cell lysis. The delivery of the negative control rsΔ2 (pDuLX-Luc) confirmed that the observed activity was induced specifically by the ovalbumin vaccination. The data suggest that the oral activity of rsΔ2(pDuLX-OVA) is explained by delivery of OVA protein, expressed in rsΔ2 from the prokaryotic promoter present in pDuLX-OVA, but transfection of mammalian cells in vivo may also play a role. Antibody titres were also produced by oral delivery (in rsΔ2) of the p3L-OVA plasmid in which does not include a prokaryotic promoter.

Live bacterial vaccine vectors: an overview

Genetically attenuated microorganisms, pathogens, and some commensal bacteria can be engineered to deliver recombinant heterologous antigens to stimulate the host immune system, while still offering good levels of safety. A key feature of these live vectors is their capacity to stimulate mucosal as well as humoral and/or cellular systemic immunity. This enables the use of different forms of vaccination to prevent pathogen colonization of mucosal tissues, the front door for many infectious agents. Furthermore, delivery of DNA vaccines and immune system stimulatory molecules, such as cytokines, can be achieved using these special carriers, whose adjuvant properties and, sometimes, invasive capacities enhance the immune response. More recently, the unique features and versatility of these vectors have also been exploited to develop anti-cancer vaccines, where tumor-associated antigens, cytokines, and DNA or RNA molecules are delivered. Different strategies and genetic tools are constantly being developed, increasing the antigenic potential of agents delivered by these systems, opening fresh perspectives for the deployment of vehicles for new purposes. Here we summarize the main characteristics of the different types of live bacterial vectors and discuss new applications of these delivery systems in the field of vaccinology.

Display of Eimeria tenella EtMic2 protein on the surface of Saccharomyces cerevisiae as a potential oral vaccine against chicken coccidiosis

S. cerevisiae is generally regarded as safe and benign organism and its surface display system may be used as a unique eukaryotic expression system that is suitable for expressing eukaryotic antigen. In addition to the convenience of vaccine delivery, the yeast cell wall has been shown to enhance the innate immunity when immunized with the yeast live oral vaccine. In the present study, we expressed the chicken coccidian E. tenella EtMic2, a microneme protein, on the surface of the S. cerevisiae and evaluated it as a potential oral vaccine for chicken against E. tenella challenge. The protective efficacy against a homologous challenge was evaluated by body weight gains, lesion scores and fecal oocyst shedding. The results showed that the live oral vaccine can improve weight gains, reduced cecal pathology and lower oocyst fecal shedding compared with non immunized controls. In addition, the yeast oral vaccine could stimulate humoral as well as cell mediate immune responses. These results suggested that EtMic2 displayed on the cell surface of S. cerevisiae could be used as potential live vaccine against chicken coccidiosis.

HIV-1 vaccine-specific responses induced by Listeria vector vaccines are maintained in mice subsequently infected with a model helminth parasite, Schistosoma mansoni

In areas co-endemic for helminth parasites and HIV/AIDS, infants are often administered vaccines prior to infection with immune modulatory helminth parasites. Systemic Th2 biasing and immune suppression caused by helminth infection reduces cell-mediated responses to vaccines such as tetanus toxoid and BCG. Therefore, we asked if infection with helminthes post-vaccination, alters already established vaccine induced immune responses. In our model, mice are vaccinated against HIV-1 Gag using a Listeria vaccine vector (Lm-Gag) in a prime-boost manner, then infected with the human helminth parasite Schistosoma mansoni. This allows us to determine if established vaccine responses are maintained or altered after helminth infection. Our second objective asked if helminth infection post-vaccination alters the recipient's ability to respond to a second boost. Here we compared responses between uninfected mice, schistosome infected mice, and infected mice that were given an anthelminthic, which occurred coincident with the boost or four weeks prior, as well as comparing to un-boosted mice. We report that HIV-1 vaccine-specific responses generated by Listeria vector HIV-1 vaccines are maintained following subsequent chronic schistosome infection, providing further evidence that Listeria vector vaccines induce potent vaccine-specific responses that can withstand helminth infection. We also were able to demonstrate that administration of a second Listeria boost, which markedly enhanced the immune response, was minimally impacted by schistosome infection, or anthelminthic therapy. Surprisingly, we also observed enhanced antibody responses to HIV Gag in vaccinated mice subsequently infected with schistosomes.

Successful vaccination of immune suppressed recipients using Listeria vector HIV-1 vaccines in helminth infected mice

Vaccines for HIV, malaria and TB remain high priorities, especially for sub-Saharan populations. The question is: will vaccines currently in development for these diseases function in populations that have a high prevalence of helminth infection? Infection with helminth parasites causes immune suppression and a CD4+ Th2 skewing of the immune system, thereby impairing Th1-type vaccine efficacy. In this study, we conduct HIV vaccine trials in mice with and without chronic helminth infection to mimic the human vaccine recipient populations in Sub-Saharan Africa and other helminth parasite endemic regions of the world, as there is large overlap in global prevalence for HIV and helminth infection. Here, we demonstrate that Listeria monocytogenes functions as a vaccine vector to drive robust and functional HIV-specific cellular immune responses, irrespective of chronic helminth infection. This observation represents a significant advance in the field of vaccine research and underscores the concept that vaccines in the developmental pipeline should be effective in the target populations.

Oral immunization using live Lactococcus lactis co-expressing LACK and IL-12 protects BALB/c mice against Leishmania major infection

Leishmaniasis is a parasitic disease affecting over 12 million individuals worldwide. Current treatments are laborious, expensive, cause severe side effects, and emerging drug resistance has been reported. While vaccination is the most cost-effective means to control infectious diseases there is no human vaccine currently available against Leishmania infections. Lactococcus lactis is a non-pathogenic, non-colonizing Gram-positive lactic acid bacterium commonly used in the dairy industry. Recently, L. lactis was used for the expression and delivery of biologically active molecules, such as antigens and cytokines, in mice and humans. In this study, we report the generation of L. lactis(alr-) strains solely expressing the protective Leishmania antigen, LACK, in the cytoplasm, secreted or anchored to the bacterial cell wall or co-expressing mouse IL-12. We show that oral immunization using live L. lactis, secreting both LACK and IL-12 was the only regimen that partially protected BALB/c mice against subsequent Leishmania major challenge. This highlights the importance of temporal and physical proximity of the delivered antigen and adjuvant for optimal immune priming by oral immunization since co-administration of L. lactis strains independently expressing secLACK and secIL-12 did not induce protective immunity. Protected animals displayed a delay in footpad swelling, which correlated with a significant reduction of parasite burden. Immunization with the L. lactis strain secreting both LACK and IL-12 induced an antigen-specific mucosal immune response and a LACK-specific T(H)1 immune response in splenocytes and mesenteric lymph node cells. Further, protection in immunized animals correlated with a strong Leishmania-specific T(H)1 immune response post-challenge, detectable in splenocytes and lymph node cells draining the site of infection. This report demonstrates the use of L. lactis as an oral live vaccine against L. major infection in susceptible BALB/c mice. The vaccine strains generated in this study provide the basis for the development of an inexpensive and safe oral live vaccine against the human parasite Leishmania.

Immunization against Leishmania major infection using LACK- and IL-12-expressing Lactococcus lactis induces delay in footpad swelling

BACKGROUND

Leishmania is a mammalian parasite affecting over 12 million individuals worldwide. Current treatments are expensive, cause severe side effects, and emerging drug resistance has been reported. Vaccination is the most cost-effective means to control infectious disease but currently there is no vaccine available against Leishmaniasis. Lactococcus lactis is a non-pathogenic, non-colonizing Gram-positive lactic acid bacterium commonly used in the dairy industry. Recently, L. lactis was used to express biologically active molecules including vaccine antigens and cytokines.

METHODOLOGY/PRINCIPAL FINDINGS

We report the generation of L. lactis strains expressing the protective Leishmania antigen, LACK, in the cytoplasm, secreted or anchored to the bacterial cell wall. L. lactis was also engineered to secrete biologically active single chain mouse IL-12. Subcutaneous immunization with live L. lactis expressing LACK anchored to the cell wall and L. lactis secreting IL-12 significantly delayed footpad swelling in Leishmania major infected BALB/c mice. The delay in footpad swelling correlated with a significant reduction of parasite burden in immunized animals compared to control groups. Immunization with these two L. lactis strains induced antigen-specific multifunctional T(H)1 CD4(+) and CD8(+) T cells and a systemic LACK-specific T(H)1 immune response. Further, protection in immunized animals correlated with a Leishmania-specific T(H)1 immune response post-challenge. L. lactis secreting mouse IL-12 was essential for directing immune responses to LACK towards a protective T(H)1 response.

CONCLUSIONS/SIGNIFICANCE

This report demonstrates the use of L. lactis as a live vaccine against L. major infection in BALB/c mice. The strains generated in this study provide the basis for the development of an inexpensive and safe vaccine against the human parasite Leishmania.

Immunity to Leishmania and the rational search for vaccines against canine leishmaniasis

The control of infection by Leishmania infantum (syn. Leishmania chagasi) in dogs is essential to stop the current spread of zoonotic visceral leishmaniasis. The past few years have seen significant advances in achieving efficient immunization of dogs and, more than ever before, an effective vaccine against canine leishmaniasis can now be considered a feasible goal. This article summarizes experimental data gathered from recent dog trials aimed at identifying immunological mechanisms implicated in protection against canine infection to discuss their potential to serve as quantitative surrogate markers of immunization and, more importantly, its usefulness to evaluate whether the immunity induced by the vaccine candidate is strong enough to protect against canine leishmaniasis.

Engineering bacterial vectors for delivery of genes and proteins to antigen-presenting cells

Bacterial vectors offer a biological route to gene and protein delivery with this article featuring delivery to antigen-presenting cells (APCs). Primarily in the context of immune stimulation against infectious disease or cancer, the goal of bacterially mediated delivery is to overcome the hurdles to effective macromolecule delivery. This review will present several bacterial vectors as macromolecule (protein or gene) delivery devices with both innate and acquirable (or engineered) biological features to facilitate delivery to APCs. The review will also present topics related to large-scale manufacture, storage, and distribution that must be considered if the bacterial delivery devices are ever to be used in a global market.

Protection in dogs against visceral leishmaniasis caused by Leishmania infantum is achieved by immunization with a heterologous prime-boost regime using DNA and vaccinia recombinant vectors expressing LACK

A heterologous prime-boost vaccination regime with DNA and recombinant vaccinia virus (rVV) vectors expressing relevant antigens has been shown to enhance specific cellular immune responses and to elicit protection against a variety of pathogens in animal models. In this paper, we describe the effectiveness of the prime-boost strategy by immunizing dogs with a plasmid carrying the gene for the LACK antigen from Leishmania infantum (DNA-LACK) followed by a booster with a rVV containing the same gene (rVV-LACK). Thereafter, animals were challenged with L. infantum to induce visceral leishmaniasis (VL). In the vaccinated dogs as compared with the controls, the outcome of the infection after challenge with a high inoculum (10(8)) of L. infantum stationary promastigotes was assessed by tissue parasite load, specific anti-Leishmania antibody production, cytokine level and development of clinical signs of leishmaniasis. We observed a 60% protection against infection in dogs immunized by DNA-LACK prime/rVV/-LACK boost while two doses of DNA-LACK did not elicit protection against the disease. The interleukin 4 (IL-4), interferon gamma (IFNgamma) and IL-12 (p40 subunit) cytokine mRNA expression profiles in PBMC as well as lymphocyte proliferative response and the IgG2/IgG1 ratios specific for LACK suggest that in vaccinated animals there is triggering of cellular immune responses. This type of DNA/rVV prime/boost immunization approach may have utility against visceral leishmaniasis in dogs.

Haemolysin A and listeriolysin--two vaccine delivery tools for the induction of cell-mediated immunity

Haemolysin A of Escherichia coli and listeriolysin of Listeria monocytogenes represent important bacterial virulence factors. While such cytolysins are usually the reason for morbidity and even mortality, vaccine researchers have turned haemolysin A and listeriolysin into tools for vaccine delivery. Both cytolysins have found widespread application in vaccine research and are highly suitable for the elicitation of cell-mediated immunity. In this paper, we will review vaccine delivery mediated by the haemolysin A secretion system and listeriolysin and will highlight their use in vaccination approaches against protozoan parasites.

Enteral immunization with attenuated recombinant Listeria monocytogenes as a live vaccine vector: organ-dependent dynamics of CD4 T lymphocytes reactive to a Leishmania major tracer epitope

Listeria monocytogenes is considered as a potential live bacterial vector, particularly for the induction of CD8 T cells. The CD4 T-cell immune response triggered after enteral immunization of mice has not yet been thoroughly characterized. The dynamics of gamma interferon (IFN-gamma)- and interleukin-4 (IL-4)-secreting CD4 T cells were analyzed after priming through intragastric delivery of an attenuated delta actA recombinant L. monocytogenes strain expressing the Leishmania major LACK protein; a peptide of this protein, LACK(158-173) peptide (pLACK), is a well-characterized CD4 T-cell target in BALB/c mice. Five compartments were monitored: Peyer's patches, mesenteric lymph nodes (MLN), spleen, liver, and blood. A single intragastric inoculation of delta actA-LACK-LM in BALB/c mice led to colonization of the MLN and spleen at a significant level for at least 3 days. Efficient priming of IFN-gamma-secreting pLACK-reactive CD4 T cells was observed in all tested compartments. Interestingly, IL-4-secreting pLACK-reactive CD4 T cells were detectable at day 6 or 7 only in blood and liver. The absence of translocation of viable bacteria through the intestinal epithelium after further delta actA-LACK-LM inoculations was concomitant with the absence of an increase in the level of IFN-gamma secreted by the MLN, blood, and splenic pLACK-reactive Th1 T cells, although the levels remained significantly above the basal level. No change in this population size was detected in the spleen. However, an increase in the number of intragastric inoculations had a clinical beneficial effect in L. major-infected BALB/c mice. L. monocytogenes thus presents the potential of an efficient vector for induction of CD4 T cells when administered by the enteral route.