Efficient in vivo presentation of Listeria monocytogenes- derived CD4 and CD8 T cell epitopes in the absence of IFN-gamma

de la Fuente I. Weighted lambda superstrings applied to vaccine design

We generalize the notion of λ-superstrings, presented in a previous paper, to the notion of weighted λ-superstrings. This generalization entails an important improvement in the applications to vaccine designs, as it allows epitopes to be weighted by their immunogenicities. Motivated by these potential applications of constructing short weighted λ-superstrings to vaccine design, we approach this problem in two ways. First, we formalize the problem as a combinatorial optimization problem (in fact, as two polynomially equivalent problems) and develop an integer programming (IP) formulation for solving it optimally. Second, we describe a model that also takes into account good pairwise alignments of the obtained superstring with the input strings, and present a genetic algorithm that solves the problem approximately. We apply both algorithms to a set of 169 strings corresponding to the Nef protein taken from patiens infected with HIV-1. In the IP-based algorithm, we take the epitopes and the estimation of the immunogenicities from databases of experimental epitopes. In the genetic algorithm we take as candidate epitopes all 9-mers present in the 169 strings and estimate their immunogenicities using a public bioinformatics tool. Finally, we used several bioinformatic tools to evaluate the properties of the candidates generated by our method, which indicated that we can score high immunogenic λ-superstrings that at the same time present similar conformations to the Nef virus proteins.

Identification and characterisation of T-cell epitopes for incorporation into dendritic cell-delivered Listeria vaccines

Dendritic cells loaded with antigenic peptides, because of their safety and robust immune stimulation, would be ideal for induction of immunity to protect against listeriosis. However, there is no currently accepted method to predict which peptides derived from the Listeria proteome might confer protection. While elution of peptides from MHC molecules after Listeria infection yields high-affinity immune-dominant epitopes, these individual epitopes did not reliably confer Listeria protection. Instead we applied bioinformatic predictions of MHC class I and II epitopes to generate antigenic peptides that were then formulated with Advax™, a novel polysaccharide particulate adjuvant able to enhance cross-presentation prior to being screened for their ability to induce protective T-cell responses. A combination of at least four intermediate strength MHC-I binding epitopes and one weak MHC-II binding epitope when expressed in a single peptide sequence and formulated with Advax adjuvant induced a potent T-cell response and high TNF-α and IL-12 production by dendritic cells resulting in robust listeriosis protection in susceptible mice. This T-cell vaccine approach might be useful for the design of vaccines to protect against listeriosis or other intracellular infections.

A gold glyco-nanoparticle carrying a Listeriolysin O peptide and formulated with Advax™ delta inulin adjuvant induces robust T-cell protection against listeria infection

In the search for an effective vaccine against the human pathogen, Listeria monocytogenes (Listeria), gold glyconanoparticles (GNP) loaded with a listeriolysin O peptide LLO91-99 (GNP-LLO) were used to immunise mice, initially using a dendritic cell (DC) vaccine approach, but subsequently using a standard parenteral immunisation approach. To enhance vaccine immunogenicity a novel polysaccharide adjuvant based on delta inulin (Advax™) was also co-formulated with the GNP vaccine. Confirming previous results, DC loaded in vitro with GNP-LLO provided better protection against listeriosis than DC loaded in vitro using free LLO peptide. The immunogenicity of GNP-LLO loaded DC vaccines was further increased by addition of Advax™ adjuvant. However, as DC vaccines are expensive and impracticable for prophylactic use, we next asked whether the same GNP-LLO antigen could be used to directly target DC in vivo. Immunisation of mice with GNP-LLO plus Advax™ adjuvant induced LLO-specific T-cell immunity and protection against Listeria challenge. Protection correlated with an increased frequency of splenic CD4(+) and CD8(+) T cells, NK cells and CD8α(+) DC, and Th1 cytokine production (IL-12, IFN-γ, TNF-α, and MCP-1), post-challenge. Enhanced T-cell epitope recruitment post-challenge was seen in the groups that received Advax™ adjuvant. Immunisation with GNP-LLO91-99 plus Advax™ adjuvant provided equally robust Listeria protection as the best DC vaccine strategy but without the complexity and cost, making this a highly promising strategy for development of a prophylactic vaccine against listeriosis.

Cellular vaccines in listeriosis: role of the Listeria antigen GAPDH

The use of live Listeria-based vaccines carries serious difficulties when administrated to immunocompromised individuals. However, cellular carriers have the advantage of inducing multivalent innate immunity as well as cell-mediated immune responses, constituting novel and secure vaccine strategies in listeriosis. Here, we compare the protective efficacy of dendritic cells (DCs) and macrophages and their safety. We examined the immune response of these vaccine vectors using two Listeria antigens, listeriolysin O (LLO) and glyceraldehyde-3-phosphate-dehydrogenase (GAPDH), and several epitopes such as the LLO peptides, LLO_189−201 and LLO₉₁₋₉₉ and the GAPDH peptide, GAPDH₁₋₂₂. We discarded macrophages as safe vaccine vectors because they show anti-Listeria protection but also high cytotoxicity. DCs loaded with GAPDH₁₋₂₂ peptide conferred higher protection and security against listeriosis than the widely explored LLO₉₁₋₉₉ peptide. Anti-Listeria protection was related to the changes in DC maturation caused by these epitopes, with high production of interleukin-12 as well as significant levels of other Th1 cytokines such as monocyte chemotactic protein-1, tumor necrosis factor-α, and interferon-γ, and with the induction of GAPDH₁₋₂₂-specific CD4⁺ and CD8⁺ immune responses. This is believed to be the first study to explore the use of a novel GAPDH antigen as a potential DC-based vaccine candidate for listeriosis, whose efficiency appears to highlight the relevance of vaccine designs containing multiple CD4⁺ and CD8⁺ epitopes.

Probing CD8 T cell responses with Listeria monocytogenes infection

CD8 T cells play a critical role in the control and eradication of intracellular pathogens. Increased understanding of CD8 T cell biology provides insight that can be translated into improved vaccination strategies. The intracellular bacterium, Listeria monocytogenes, has been used as a model organism to study every phase of the CD8 T cell response to intracellular bacterial infection. Infection of laboratory mice with L. monocytogenes has provided insight into the factors that are involved in primary T cell responses, memory CD8 T cell generation, maintenance, functionality, and diversification following repeated pathogenic challenges. In this review, we will focus on work from our laboratories utilizing the murine model of L. monocytogenes to investigate the characteristics of CD8 T cell responses to infection. This model has profoundly advanced our understanding of the CD8 T cell response to infection and is likely to continue to provide invaluable basic insights that can be translated into the development of effective vaccination strategies to protect against pathogens.

Suppression of cell-mediated immunity following recognition of phagosome-confined bacteria

Listeria monocytogenes is a facultative intracellular pathogen capable of inducing a robust cell-mediated immune response to sub-lethal infection. The capacity of L. monocytogenes to escape from the phagosome and enter the host cell cytosol is paramount for the induction of long-lived CD8 T cell–mediated protective immunity. Here, we show that the impaired T cell response to L. monocytogenes confined within a phagosome is not merely a consequence of inefficient antigen presentation, but is the result of direct suppression of the adaptive response. This suppression limited not only the adaptive response to vacuole-confined L. monocytogenes, but negated the response to bacteria within the cytosol. Co-infection with phagosome-confined and cytosolic L. monocytogenes prevented the generation of acquired immunity and limited expansion of antigen-specific T cells relative to the cytosolic L. monocytogenes strain alone. Bacteria confined to a phagosome suppressed the production of pro-inflammatory cytokines and led to the rapid MyD88-dependent production of IL-10. Blockade of the IL-10 receptor or the absence of MyD88 during primary infection restored protective immunity. Our studies demonstrate that the presence of microbes within a phagosome can directly impact the innate and adaptive immune response by antagonizing the signaling pathways necessary for inflammation and the generation of protective CD8 T cells.

Little is understood about how the immune system distinguishes between pathogenic and non-pathogenic microbes. Limiting or preventing infections by intracellular pathogens requires the activation of innate immunity and the consequent generation of effector and memory T cells, which recognize and kill infected cells. Investigators are currently testing attenuated versions of pathogenic microbes as vaccines in an attempt to generate pathogen-specific T cells without causing disease. Unfortunately, attenuated microbes often fail to elicit long-lived protective immunity. We hypothesized that attenuated bacterial vaccines do not immunize because they fail to activate a stimulatory arm of host innate immune receptors. However, we found that these attenuated bacterial vaccines are not simply prevented from activating immunity, but rather generate a negative signal that inhibits the desired immune response. These studies may explain why the addition of an adjuvant to ineffective vaccines does not necessarily improve immunogenicity. Furthermore, these studies provide a framework for the development of attenuated vaccines that do not inhibit the desired immune responses.

Fine-tuning the safety and immunogenicity of Listeria monocytogenes-based neonatal vaccine platforms

We have developed virulence-attenuated strains of Listeria monocytogenes (Lm) that can be used as safe yet effective vaccine carriers for neonatal vaccination. Here we compare the vaccine efficacy of Lm based vaccine carrier candidates after only a single immunization in murine neonates and adults: Lm Delta(trpS actA) based strains that express and secrete multiple copies of the model antigen ovalbumin (OVA) either under the control of a phagosomal (P(hly)) or cytosolic (P(actA))-driven listerial promoter. While both strains induced high levels of antigen-specific primary and secondary CD8 and CD4 T cell responses, both neonatal and adult mice immunized with the phagosomal driven strain were significantly better protected against wildtype Lm challenge as compared to the naïve control group than mice immunized with the cytosolic driven strains. Interestingly, only neonatal mice immunized with the phagosomal driven strains generated high IgG antibody responses against OVA. Our phagosomal driven Lm-based vaccine platform presents the broadest (cellular & humoral response) and most efficient (highly protective) vaccine platform for neonatal vaccination yet described.

KBMA Listeria monocytogenes is an effective vector for DC-mediated induction of antitumor immunity

Vaccine strategies that utilize human DCs to enhance antitumor immunity have yet to realize their full potential. Approaches that optimally target a spectrum of antigens to DCs are urgently needed. Here we report the development of a platform for loading DCs with antigen. It is based on killed but metabolically active (KBMA) recombinant Listeria monocytogenes and facilitates both antigen delivery and maturation of human DCs. Highly attenuated KBMA L. monocytogenes were engineered to express an epitope of the melanoma-associated antigen MelanA/Mart-1 that is recognized by human CD8+ T cells when presented by the MHC class I molecule HLA-A*0201. The engineered KBMA L. monocytogenes induced human DC upregulation of costimulatory molecules and secretion of pro-Th1 cytokines and type I interferons, leading to effective priming of Mart-1-specific human CD8+ T cells and lysis of patient-derived melanoma cells. KBMA L. monocytogenes expressing full-length NY-ESO-1 protein, another melanoma-associated antigen, delivered the antigen for presentation by MHC class I and class II molecules independent of the MHC haplotype of the DC donor. A mouse therapeutic tumor model was used to show that KBMA L. monocytogenes efficiently targeted APCs in vivo to induce protective antitumor responses. Together, our data demonstrate that KBMA L. monocytogenes may be a powerful platform that can both deliver recombinant antigen to DCs for presentation and provide a potent DC-maturation stimulus, making it a potential cancer vaccine candidate.

A role for IFN-gamma from antigen-specific CD8+ T cells in protective immunity to Listeria monocytogenes

Whether IFN-gamma contributes to the per-cell protective capacity of memory CD8(+) T cells against Listeria monocytogenes (LM) has not been formally tested. In this study, we generated LM Ag-specific memory CD8(+) T cells via immunization of wild-type (WT) and IFN-gamma-deficient (gamma knockout (GKO)) mice with LM peptide-coated dendritic cells and compared them phenotypically and functionally. Immunization of WT and GKO mice resulted in memory CD8(+) T cells that were similar in number, functional avidity, TCR repertoire use, and memory phenotype. The protective capacity of memory CD8(+) T cells from immunized WT and GKO mice was evaluated after adoptive transfer of equal numbers of WT or GKO cells into naive BALB/c mice followed by LM challenge. The adoptively transferred CD8(+) T cells from GKO donors exhibited a decreased ability to reduce bacterial numbers in the organs of recipient mice when compared with an equivalent number of Ag-matched WT CD8(+) T cells. This deficiency was most evident early (day 3) after infection if a relatively low infectious dose was used; however, transferring fewer memory CD8(+) T cells or increasing the LM challenge dose revealed a more pronounced defect in protective immunity mediated by the CD8(+) T cells from GKO mice. Our studies identified a decrease in Ag-specific target cell lysis in vivo by CD8(+) T cells from GKO mice as the mechanism for the decreased protective immunity after LM challenge. Further studies suggest that the lack of IFN-gamma production by the Ag-specific CD8 T cells themselves diminishes target cell sensitivity to cytolysis, thereby reducing the lytic potency of IFN-gamma-deficient LM-specific memory CD8(+) T cells.

Induction of protective immunity to Listeria monocytogenes in neonates

Neonates suffer unduly from infections and also respond suboptimally to most commonly used vaccines. However, a CD8 T cell response can be elicited in neonates if the Ag is introduced into the cytoplasm of APCs. Listeria monocytogenes (Lm) targets the cytoplasm of APC and is a strong CD8 and CD4 Th1-promoting vaccine vehicle in adult mice. We hypothesized that an attenuated strain of Lm would be safe and induce long-lasting protective immunity, even in neonates. We found that neonatal mice immunized only once with the attenuated strain DeltaactA-Lm developed robust primary and secondary CD8 and CD4 Th1 responses and were fully protected from lethal challenge with virulent wild-type Lm without the need for a booster immunization. Furthermore, DeltaactA-Lm expressing a heterologous recombinant Ag induced a strong CD8 and Th1 memory response to that Ag. Based on these data, we propose that DeltaactA-Lm or derivatives thereof might serve as a vaccine vehicle for neonatal immunization.

Human dendritic cells process and present Listeria antigens for in vitro priming of autologous CD4+ T lymphocytes

The role of human dendritic cells (DC) in the immune response toward intracellularly growing Listeria was analyzed under in vitro conditions using several morphological and functional methods. DC incubated with Listeria innocua and L. monocytogenes, respectively, readily phagocytosed the bacteria. Listeria did not impair viability and immunogenic potential of human DC. Listerial antigens were found to be processed within the lysosomal compartment of DC and colocalized with major histocompatibility complex (MHC) class II molecules, as shown by fluorescence and transmission electron microscopy. DC challenged with apathogenic L. innocua were highly effective in priming autologous naive T cells (mainly CD4+) in vitro. The T cells strongly proliferated in the presence of DC incubated with L. innocua, which could be significantly inhibited by anti-MHC II mAb. L. innocua-primed T cells were also successfully stimulated by DC harboring the pathogenic L. monocytogenes, either the wild-type strain EGD or the p60 reduced mutant strain RIII. From our results, we conclude that human DC infected with nonpathogenic intracellular bacteria are able to efficiently prime naive T cells, which are then suitable for recognition of antigens derived from related virulent bacterial species. This in vitro human model provides an interesting tool for basic research in infectious immunology and possibly for a new immunotherapy.

CD8 T cell responses to infectious pathogens

CD8 T cells respond to viral infections but also participate in defense against bacterial and protozoal infections. In the last few years, as new methods to accurately quantify and characterize pathogen-specific CD8 T cells have become available, our understanding of in vivo T cell responses has increased dramatically. Pathogen-specific T cells, once thought to be quite rare following infection, are now known to be present at very high frequencies, particularly in peripheral, nonlymphoid tissues. With the ability to visualize in vivo CD8 T cell responses has come the recognition that T cell expansion is programmed and, to a great extent, independent of antigen concentrations. Comparison of CD8 T cell responses to different pathogens also highlights the intricate relationship between microbially induced innate inflammatory responses and the kinetics, magnitude, and character of long-term T cell responses. This review describes recent progress in some of the major murine models of CD8 T cell-mediated immunity to viral, bacterial, and protozoal infection.

Quantitative studies of CD8+ T-cell responses during microbial infection

MHC class I tetramer staining, intracellular cytokine staining and ELISPOT assays have made it possible to quantify CD8(+) T-cell responses precisely during and following viral and bacterial infection. Although these quantitative methods are by now familiar and trusted components of the immunologist's toolbox, their application to models of microbial infection continues to provide surprising insights into mammalian adaptive immunity. In the past year there have been many exciting new findings on CD8(+) T-cell priming, expansion and memory formation in response to microbial infection.

Dendritic cells as inducers of antimicrobial immunity in vivo

Models of infection have provided important insight into the function of dendritic cells (DC) in vivo. Several microbial products induce DC maturation via Toll-like receptors, a process that is crucial for the ability of DC to initiate adaptive immune responses. Splenic DC have also been shown to produce IL-12 during infection in vivo. This DC-derived IL-12 might be important to skew T cell responses towards Th1. Microbial infections also induce changes in the DC populations of lymphoid organs, often in a subset-specific manner, manifested as an accumulation and redistribution of DC. Furthermore, data are emerging pointing at an absolute requirement of DC in priming of naïve T cells in vivo.

CD8 T cell immunome analysis of Listeria monocytogenes

The identification of T cell epitopes is crucial for the understanding of the host response during infections with pathogenic microorganisms. Generally, the identification of relevant T cell responses is based on the analysis of T cell lines propagated in vitro. We used an ex vivo approach for the analysis of the CD8 T cell response against Listeria monocytogenes that is based upon the fractionation of naturally processed antigenic peptides and subsequent analysis with T cells in an enzyme-linked immunospot (ELISPOT) assay. Our data indicate that the direct ex vivo ELISPOT analysis of peptides extracted from infected tissues represents a versatile and potent test system for the analysis of the CD8 T cell immunome of microorganisms that furthermore requires neither the knowledge of the microbial genome nor of the specificity of responding T cells.

Cross-presentation of Listeria monocytogenes-derived CD4 T cell epitopes

Listeriolysin O (LLO) mediates the evasion of Listeria monocytogenes from the phagolysosome into the cytoplasm of the host cell. The recognition of infected cells by CD4 T cells is thought to be limited by the evasion of bacteria from the phagolysosome and also by the direct LLO-mediated inhibition of CD4 T cell activation. To analyze the influence of these immunoevasive mechanisms on the antilisterial CD4 T cell response, the expansion of L. monocytogenes-specific CD4 and CD8 T cells was monitored in infected mice. It was found that expansion of L. monocytogenes-specific CD4 T cells occurred synchronously with CD8 T cell expansion. The analysis of Ag presentation by macrophages and dendritic cells isolated from spleens of infected mice revealed efficient presentation of L. monocytogenes-derived CD4 T cell epitopes that was not dependent on the actA-mediated intercellular spread of bacteria. The further in vitro Ag presentation analysis revealed that although L. monocytogenes-infected macrophages and dendritic cells were poor presenters of CD4 T cell epitopes, more efficient presentation occurred after cocultivation of noninfected dendritic cells or macrophages with infected cells. These data indicate that the suppressive effect of LLO on the antilisterial CD4 T cell response is maintained only in infected APC and support the hypothesis that cross-priming plays a role in the induction of the strong CD4 T cell response in Listeria-infected mice.