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

A live attenuated DIVA vaccine affords protection against Listeria monocytogenes challenge in sheep

Listeria monocytogenes (Lm) is a deadly foodborne pathogen that comprises 14 serotypes, among which, serotype 4b Lm is the primary cause of listeriosis outbreaks in humans and animals. Here, we evaluated the safety, immunogenicity, and protective efficacy of a serotype 4b vaccine candidate Lm NTSNΔactA/plcB/orfX in sheep. The infection dynamics, clinical features, and pathological observation verified that the triple genes deletion strain has adequate safety for sheep. Moreover, NTSNΔactA/plcB/orfX significantly stimulated humoral immune response and 78% protection against lethal wild-type strain challenge. Notably, the attenuated vaccine could differentiate infected and vaccinated animals (DIVA) via serology determination of the antibody against listeriolysin O (LLO, encoded by hly) and phosphatidylinositol-specific phospholipase C (PI-PLC, encoded by plcB). These data suggest that the serotype 4b vaccine candidate has high efficacy, safety, and DIVA characteristics, and may be used to prevent Lm infection in sheep, which provides a theoretical basis for its future application in livestock and poultry breeding.

Listeria-vectored multi-antigenic tuberculosis vaccine protects C57BL/6 and BALB/c mice and guinea pigs against Mycobacterium tuberculosis challenge

Mycobacterium tuberculosis (Mtb) infects one-third of the world's population and is a leading cause of death from a single infectious agent. New TB vaccines are urgently needed to augment immunity conferred by the current modestly protective BCG vaccine. We have developed live attenuated recombinant Listeria monocytogenes (rLm)-vectored TB vaccines expressing five [Mpt64/23.5-EsxH/TB10.4-EsxA/ESAT6-EsxB/CFP10-Ag85B/r30] (rLmMtb5Ag) or nine (additionally EsxN-PPE68-EspA-TB8.4) immunoprotective Mtb antigens (rLmMtb9Ag) and evaluated them for safety, immunogenicity and efficacy as standalone vaccines in two mouse models and an outbred guinea pig model. In immunogenicity studies, rLmMtb5Ag administered subcutaneously induces significantly enhanced antigen-specific CD4+ and CD8+ T-cell responses in C57BL/6 and BALB/c mice, and rLmMtb9Ag induces antigen-specific CD4+ and CD8+ T-cell proliferation in guinea pigs. In efficacy studies, both rLmMtb5Ag and rLmMtb9Ag are safe and protect C57BL/6 and BALB/c mice and guinea pigs against aerosol challenge with highly virulent Mtb. Hence, multi-antigenic rLm vaccines hold promise as new vaccines against TB.

Fast-acting γδ T-cell subpopulation and protective immunity against infections

Unique Vγ2Vδ2 (Vγ9Vδ2) T cells existing only in human and non-human primates, account for the majority of circulating γδ T cells in human adults. Vγ2Vδ2 T cells are the sole γδ T-cell subpopulation capable of recognizing the microbial (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) produced by selected pathogens during infections. Recent seminal studies in non-human primate models have demonstrated that the unique HMBPP-specific Vγ2Vδ2 T cells are fast-acting, multi-functional, and protective during infections. This article reviews the recent seminal observations of Vγ2Vδ2 T cells in protective mechanisms against tuberculosis and other infections.

Immunization of Vγ2Vδ2 T cells programs sustained effector memory responses that control tuberculosis in nonhuman primates

Despite the urgent need for a better tuberculosis (TB) vaccine, relevant protective mechanisms remain unknown. We previously defined protective phosphoantigen (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP)–specific Vγ2Vδ2 T cells as a unique subset in primates, and, here, we immunized them selectively for protection against TB. A single respiratory vaccination of macaques with attenuated HMBPP-producing Listeria monocytogenes (Lm ΔactA prfA*), but not an HMBPP-lacking ΔgcpE Listeria strain, expanded Vγ2Vδ2 T cells, elicited Th1-like Vγ2Vδ2 T cell responses, and reduced TB infection/pathology after moderate-dose TB challenge. Such protection correlated with rapid memory-like, Th1-like Vγ2Vδ2 T cell responses, the presence of tissue-resident Vγ2Vδ2 T effectors coproducing IFN-γ/perforin and inhibiting intracellular Mycobacterium tuberculosis growth, and enhanced CD4⁺/CD8⁺ T cell responses. These findings establish a concept incorporating immunization of human Vγ2Vδ2 T cells for TB vaccine development.

Tuberculosis (TB) remains a leading killer among infectious diseases, and a better TB vaccine is urgently needed. The critical components and mechanisms of vaccine-induced protection against Mycobacterium tuberculosis (Mtb) remain incompletely defined. Our previous studies demonstrate that Vγ2Vδ2 T cells specific for (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) phosphoantigen are unique in primates as multifunctional effectors of immune protection against TB infection. Here, we selectively immunized Vγ2Vδ2 T cells and assessed the effect on infection in a rhesus TB model. A single respiratory vaccination of macaques with an HMBPP-producing attenuated Listeria monocytogenes (Lm ΔactA prfA*) caused prolonged expansion of HMBPP-specific Vγ2Vδ2 T cells in circulating and pulmonary compartments. This did not occur in animals similarly immunized with an Lm ΔgcpE strain, which did not produce HMBPP. Lm ΔactA prfA* vaccination elicited increases in Th1-like Vγ2Vδ2 T cells in the airway, and induced containment of TB infection after pulmonary challenge. The selective immunization of Vγ2Vδ2 T cells reduced lung pathology and mycobacterial dissemination to extrapulmonary organs. Vaccine effects coincided with the fast-acting memory-like response of Th1-like Vγ2Vδ2 T cells and tissue-resident Vγ2Vδ2 effector T cells that produced both IFN-γ and perforin and inhibited intracellular Mtb growth. Furthermore, selective immunization of Vγ2Vδ2 T cells enabled CD4⁺ and CD8⁺ T cells to mount earlier pulmonary Th1 responses to TB challenge. Our findings show that selective immunization of Vγ2Vδ2 T cells can elicit fast-acting and durable memory-like responses that amplify responses of other T cell subsets, and provide an approach to creating more effective TB vaccines.

Single vector platform vaccine protects against lethal respiratory challenge with Tier 1 select agents of anthrax, plague, and tularemia

Bacillus anthracis, Yersinia pestis, and Francisella tularensis are the causative agents of Tier 1 Select Agents anthrax, plague, and tularemia, respectively. Currently, there are no licensed vaccines against plague and tularemia and the licensed anthrax vaccine is suboptimal. Here we report F. tularensis LVS ΔcapB (Live Vaccine Strain with a deletion in capB)- and attenuated multi-deletional Listeria monocytogenes (Lm)-vectored vaccines against all three aforementioned pathogens. We show that LVS ΔcapB- and Lm-vectored vaccines express recombinant B. anthracis, Y. pestis, and F. tularensis immunoprotective antigens in broth and in macrophage-like cells and are non-toxic in mice. Homologous priming-boosting with the LVS ΔcapB-vectored vaccines induces potent antigen-specific humoral and T-cell-mediated immune responses and potent protective immunity against lethal respiratory challenge with all three pathogens. Protection against anthrax was far superior to that obtained with the licensed AVA vaccine and protection against tularemia was comparable to or greater than that obtained with the toxic and unlicensed LVS vaccine. Heterologous priming-boosting with LVS ΔcapB- and Lm-vectored B. anthracis and Y. pestis vaccines also induced potent protective immunity against lethal respiratory challenge with B. anthracis and Y. pestis. The single vaccine platform, especially the LVS ΔcapB-vectored vaccine platform, can be extended readily to other pathogens.

Activation of the Listeria monocytogenes Virulence Program by a Reducing Environment

Upon entry into the host cell cytosol, the facultative intracellular pathogen Listeria monocytogenes coordinates the expression of numerous essential virulence factors by allosteric binding of glutathione (GSH) to the Crp-Fnr family transcriptional regulator PrfA. Here, we report that robust virulence gene expression can be recapitulated by growing bacteria in a synthetic medium containing GSH or other chemical reducing agents. Bacteria grown under these conditions were 45-fold more virulent in an acute murine infection model and conferred greater immunity to a subsequent lethal challenge than bacteria grown in conventional media. During cultivation in vitro, PrfA activation was completely dependent on the intracellular levels of GSH, as a glutathione synthase mutant (ΔgshF) was activated by exogenous GSH but not reducing agents. PrfA activation was repressed in a synthetic medium supplemented with oligopeptides, but the repression was relieved by stimulation of the stringent response. These data suggest that cytosolic L. monocytogenes interprets a combination of metabolic and redox cues as a signal to initiate robust virulence gene expression in vivo.

Intracellular pathogens are responsible for much of the worldwide morbidity and mortality from infectious diseases. These pathogens have evolved various strategies to proliferate within individual cells of the host and avoid the host immune response. Through cellular invasion or the use of specialized secretion machinery, all intracellular pathogens must access the host cell cytosol to establish their replicative niches. Determining how these pathogens sense and respond to the intracellular compartment to establish a successful infection is critical to our basic understanding of the pathogenesis of each organism and for the rational design of therapeutic interventions. Listeria monocytogenes is a model intracellular pathogen with robust in vitro and in vivo infection models. Studies of the host-sensing and downstream signaling mechanisms evolved by L. monocytogenes often describe themes of pathogenesis that are broadly applicable to less tractable pathogens. Here, we describe how bacteria use external redox states as a cue to activate virulence.

Listeria-Vectored Vaccine Expressing the Mycobacterium tuberculosis 30-Kilodalton Major Secretory Protein via the Constitutively Active prfA* Regulon Boosts Mycobacterium bovis BCG Efficacy against Tuberculosis

A potent vaccine against tuberculosis, one of the world's deadliest diseases, is needed to enhance the immunity of people worldwide, most of whom have been vaccinated with the partially effective Mycobacterium bovis BCG vaccine. Here we investigate novel live attenuated recombinant Listeria monocytogenes (rLm) vaccines expressing the Mycobacterium tuberculosis 30-kDa major secretory protein (r30/antigen 85B [Ag85B]) (rLm30) as heterologous booster vaccines in animals primed with BCG. Using three attenuated L. monocytogenes vectors, L. monocytogenes ΔactA (LmI), L. monocytogenes ΔactA ΔinlB (LmII), and L. monocytogenes ΔactA ΔinlB prfA* (LmIII), we constructed five rLm30 vaccine candidates expressing r30 linked in frame to the L. monocytogenes listeriolysin O signal sequence and driven by the hly promoter (h30) or linked in frame to the ActA N-terminal 100 amino acids and driven by the actA promoter (a30). All five rLm30 vaccines secreted r30 in broth and macrophages; while rLm30 expressing r30 via a constitutively active prfA* regulon (rLmIII/a30) expressed the largest amount of r30 in broth culture, all five rLm30 vaccines expressed equivalent amounts of r30 in infected macrophages. In comparative studies, boosting of BCG-immunized mice with rLmIII/a30 induced the strongest antigen-specific T-cell responses, including splenic and lung polyfunctional CD4⁺ T cells expressing the three cytokines interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-2 (IL-2) (P < 0.001) and splenic and lung CD8⁺ T cells expressing IFN-γ (P < 0.0001). In mice and guinea pigs, the rLmIII/a30 and rLmI/h30 vaccines were generally more potent booster vaccines than r30 with an adjuvant and a recombinant adenovirus vaccine expressing r30. In a setting in which BCG alone was highly immunoprotective, boosting of mice with rLmIII/a30, the most potent of the vaccines, significantly enhanced protection against aerosolized M. tuberculosis (P < 0.01).

Antigen Delivery System II

This chapter reviews papers mostly written since 2005 that report results using live attenuated bacterial vectors to deliver after administration through mucosal surfaces, protective antigens, and DNA vaccines, encoding protective antigens to induce immune responses and/or protective immunity to pathogens that colonize on or invade through mucosal surfaces. Papers that report use of such vaccine vector systems for parenteral vaccination or to deal with nonmucosal pathogens or do not address induction of mucosal antibody and/or cellular immune responses are not reviewed.

HMBPP-deficient Listeria mutant immunization alters pulmonary/systemic responses, effector functions, and memory polarization of Vγ2Vδ2 T cells

Whereas infection or immunization of humans/primates with microbes coproducing HMBPP/IPP can remarkably activate Vγ2Vδ2 T cells, in vivo studies have not been done to dissect HMBPP- and IPP-driven expansion, pulmonary trafficking, effector functions, and memory polarization of Vγ2Vδ2 T cells. We define these phosphoantigen-host interplays by comparative immunizations of macaques with the HMBPP/IPP-coproducing Listeria ΔactA prfA* and HMBPP-deficient Listeria ΔactA ΔGCPE: prfA* mutant. The HMBPP-deficient ΔGCPE: mutant shows lower ability to expand Vγ2Vδ2 T cells in vitro than the parental HMBPP-producing strain but displays comparably attenuated infectivity or immunogenicity. Respiratory immunization of macaques with the HMBPP-deficient mutant elicits lower pulmonary and systemic responses of Vγ2Vδ2 T cells compared with the HMBPP-producing vaccine strain. Interestingly, HMBPP-deficient mutant reimmunization or boosting elicits enhanced responses of Vγ2Vδ2 T cells, but the magnitude is lower than that by HMBPP-producing listeria. HMBPP-deficient listeria differentiated fewer Vγ2Vδ2 T effector cells capable of coproducing IFN-γ and TNF-α and inhibiting intracellular listeria than HMBPP-producing listeria. Furthermore, HMBPP deficiency in listerial immunization influences memory polarization of Vγ2Vδ2 T cells. Thus, both HMBPP and IPP production in listerial immunization or infection elicit systemic/pulmonary responses and differentiation of Vγ2Vδ2 T cells, but a role for HMBPP is more dominant. Findings may help devise immune intervention.

SHIV antigen immunization alters patterns of immune responses to SHIV/malaria coinfection and protects against life-threatening SHIV-related malaria

Whether vaccination against a virus can protect against more virulent coinfection with the virus and additional pathogen(s) remains poorly characterized. Overlapping endemicity of human immunodeficiency virus (HIV) and malaria suggests that HIV/malaria coinfection frequently complicates acute and chronic HIV infection. Here we showed that vaccination of macaques with recombinant Listeria ΔactA prfA* expressing simian/human immunodeficiency virus (SHIV) gag and env elicited Gag- and Env-specific T-cell responses, and protected against life-threatening SHIV-related malaria after SHIV/Plasmodium fragile coinfection. SHIV antigen immunization reduced peak viremia, resisted SHIV/malaria-induced lymphoid destruction, and blunted coinfection-accelerated decline of CD4(+) T-cell counts after SHIV/malaria coinfection. SHIV antigen immunization also weakened coinfection-driven overreactive proinflammatory interferon-γ (IFNγ) responses and led to developing T helper cell 17/22 (Th17/Th22) responses after SHIV/malaria coinfection. The findings suggest that vaccination against AIDS virus can alter patterns of immune responses to the SHIV/malaria coinfection and protect against life-threatening SHIV-related malaria.

Multieffector-functional immune responses of HMBPP-specific Vγ2Vδ2 T cells in nonhuman primates inoculated with Listeria monocytogenes ΔactA prfA*

Although Listeria monocytogenes can induce systemic infection causing spontaneous abortion, septicemia, and meningitis, studies have not been performed to investigate human anti-L. monocytogenes immune responses, including those of Ag-specific Vγ2Vδ2 T cells, a dominant human γδ T cell subset. L. monocytogenes is the only pathogen known to possess both the mevalonate and non-mevalonate isoprenoid biosynthesis pathways that produce metabolic phosphates or phosphoantigens activating human Vγ2Vδ2 T cells, making it interesting to explore in vivo anti-L. monocytogenes immune responses of Vγ2Vδ2 T cells. In this study, we demonstrated that subclinical systemic L. monocytogenes infection of rhesus macaques via parenteral inoculation or vaccination with an attenuated Listeria strain induced multieffector-functional immune responses of phosphoantigen-specific Vγ2Vδ2 T cells. Subclinical systemic infection and reinfection with attenuated L. monocytogenes uncovered the ability of Vγ2Vδ2 T cells to mount expansion and adaptive or recall-like expansion. Expanded Vγ2Vδ2 T cells could traffic to and accumulate in the pulmonary compartment and intestinal mucosa. Expanded Vγ2Vδ2 T cells could evolve into effector cells producing IFN-γ, TNF-α, IL-4, IL-17, or perforin after L. monocytogenes infection, and some effector Vγ2Vδ2 T cells could coproduce IL-17 and IFN-γ, IL-4 and IFN-γ, or TNF-α and perforin. Surprisingly, in vivo-expanded Vγ2Vδ2 T effector cells in subclinical L. monocytogenes infection could directly lyse L. monocytogenes-infected target cells and inhibit intracellular L. monocytogenes bacteria. Thus, we present the first demonstration, to our knowledge, of multieffector-functional Vγ2Vδ2 T cell responses against L. monocytogenes.

Variability of Listeria monocytogenes virulence: a result of the evolution between saprophytism and virulence?

The genus Listeria consists of eight species but only two are pathogenic. Human listeriosis due to Listeria monocytogenes is a foodborne disease. L. monocytogenes is widespread in the environment living as a saprophyte, but is also capable of making the transition into a pathogen following its ingestion by susceptible humans or animals. It is now known that many distinct strains of L. monocytogenes differ in their virulence and epidemic potential. Unfortunately, there is currently no standard definition of virulence levels and no complete comprehensive overview of the evolution of Listeria species and L. monocytogenes strains taking into account the presence of both epidemic and low-virulence strains. This article focuses on the methods and genes allowing us to determine the pathogenic potential of Listeria strains, and the evolution of Listeria virulence. The presence of variable levels of virulence within L. monocytogenes has important consequences on detection of Listeria strains and risk analysis but also on our comprehension of how certain pathogens will behave in a population over evolutionary time.

Intranasal vaccination with the recombinant Listeria monocytogenes ΔactA prfA* mutant elicits robust systemic and pulmonary cellular responses and secretory mucosal IgA

We previously showed that recombinant (r) Listeria monocytogenes carrying ΔactA and a selected prfA* mutation (r-Listeria ΔactA prfA*) secreted >100-fold more immunogen in broth culture than wild-type r-Listeria or r-Listeria ΔactA and elicited much greater cellular and humoral immune responses than r-Listeria ΔactA after intravenous vaccination of mice. Here, we conducted comparative studies evaluating vaccine-elicited immune responses in systemic and mucosal sites after intranasal, intravenous, intraperitoneal, or subcutaneous immunization of mice with r-Listeria ΔactA prfA* vaccine candidates. Intranasal vaccination of mice with r-Listeria ΔactA prfA* vaccine candidates elicited a robust gamma interferon-positive (IFN-γ(+)) cellular response in systemic sites, although intravenous or intraperitoneal immunization was more efficient. Surprisingly, intranasal vaccination elicited an appreciable pulmonary IFN-γ(+) cellular response that was nonstatistically higher than the magnitude induced by the intravenous route but was significantly greater than that elicited by subcutaneous immunization. Furthermore, although intranasal r-Listeria ΔactA prfA* delivery induced poor systemic IgG responses, intranasal vaccination elicited appreciable secretory immunogen-specific IgA titers that were similar to or higher in mucosal fluid than those induced by subcutaneous and intravenous immunizations. Thus, intranasal vaccination with r-Listeria ΔactA prfA* appears to be a useful approach for eliciting robust systemic and pulmonary cellular responses and measurable secretory mucosal IgA titers.

Listeria monocytogenes - from saprophyte to intracellular pathogen

Listeria monocytogenes is a bacterium that lives in the soil as a saprophyte but is capable of making the transition into a pathogen following its ingestion by susceptible humans or animals. Recent studies suggest that L. monocytogenes mediates its saprophyte-to-cytosolic-parasite transition through the careful modulation of the activity of a virulence regulatory protein known as PrfA, using a range of environmental cues that include available carbon sources. In this Progress article we describe the regulation of PrfA and its role in the L. monocytogenes transition from the saprophytic stage to the virulent intracellular stage.