Recombinant Listeria monocytogenes as a live vaccine vehicle for the induction of protective anti-viral cell-mediated immunity

Long-Lasting, Fine-Tuned Anti-Tumor Activity of Recombinant Listeria monocytogenes Vaccine Is Controlled by Pyroptosis and Necroptosis Regulatory and Effector Molecules

One of the main objectives of developing new anti-cancer vaccine strategies is to effectively induce CD8+ T cell-mediated anti-tumor immunity. Live recombinant vectors, notably Listeria monocytogenes, have been shown to elicit a robust in vivo CD8+ T-cell response in preclinical settings. Significantly, it has been demonstrated that Listeria induces inflammatory/immunogenic cell death mechanisms such as pyroptosis and necroptosis in immune cells that favorably control immunological responses. Therefore, we postulated that the host's response to Listeria-based vectors and the subsequent induction of CD8+ T cell-mediated immunity would be compromised by the lack of regulatory or effector molecules involved in pyroptosis or necroptosis. To test our hypothesis, we used recombinant L. monocytogenes carrying the ovalbumin gene (LM.OVA) to vaccinate wild-type (WT), caspase-1/11-/-, gsdmd-/-, ripk3-/-, and mlkl-/- C57Bl/6 mice. We performed an in vivo cytotoxicity assay to assess the efficacy of OVA-specific CD8+ T lymphocytes in eliminating target cells in wild-type and genetically deficient backgrounds. Furthermore, we evaluated the specific anti-tumor immune response in mice inoculated with the B16F0 and B16F0.OVA melanoma cell lines. Our findings demonstrated that while caspase-1/11 and GSDMD deficiencies interfere with the rapid control of LM.OVA infection, neither of the KOs seems to contribute to the early activation of OVA-specific CTL responses. In contrast, the individual deficiency of each one of these proteins positively impacts the generation of long-lasting effector CD8+ T cells.

OCA-B/Pou2af1 is sufficient to promote CD4+ T cell memory and prospectively identifies memory precursors

The molecular mechanisms leading to the establishment of immunological memory are inadequately understood, limiting the development of effective vaccines and durable antitumor immune therapies. Here, we show that ectopic OCA-B expression is sufficient to improve antiviral memory recall responses, while having minimal effects on primary effector responses. At peak viral response, short-lived effector T cell populations are expanded but show increased Gadd45b and Socs2 expression, while memory precursor effector cells show increased expression of Bcl2, Il7r, and Tcf7 on a per-cell basis. Using an OCA-B mCherry reporter mouse line, we observe high OCA-B expression in CD4+ central memory T cells. We show that early in viral infection, endogenously elevated OCA-B expression prospectively identifies memory precursor cells with increased survival capability and memory recall potential. Cumulatively, the results demonstrate that OCA-B is both necessary and sufficient to promote CD4 T cell memory in vivo and can be used to prospectively identify memory precursor cells.

STING controls T cell memory fitness during infection through T cell-intrinsic and IDO-dependent mechanisms

Stimulator of interferon genes (STING) signaling has been extensively studied in inflammatory diseases and cancer, while its role in T cell responses to infection is unclear. Using Listeria monocytogenes strains engineered to induce different levels of c-di-AMP, we found that high STING signals impaired T cell memory upon infection via increased Bim levels and apoptosis. Unexpectedly, reduction of TCR signal strength or T cell-STING expression decreased Bim expression, T cell apoptosis, and recovered T cell memory. We found that TCR signal intensity coupled STING signal strength to the unfolded protein response (UPR) and T cell survival. Under strong STING signaling, Indoleamine-pyrrole 2,3-dioxygenase (IDO) inhibition also reduced apoptosis and led to a recovery of T cell memory in STING sufficient CD8 T cells. Thus, STING signaling regulates CD8 T cell memory fitness through both cell-intrinsic and extrinsic mechanisms. These studies provide insight into how IDO and STING therapies could improve long-term T cell protective immunity.

T Cell-Intrinsic Vitamin A Metabolism and Its Signaling Are Targets for Memory T Cell-Based Cancer Immunotherapy

Memory T cells play an essential role in infectious and tumor immunity. Vitamin A metabolites such as retinoic acid are immune modulators, but the role of vitamin A metabolism in memory T-cell differentiation is unclear. In this study, we identified retinol dehydrogenase 10 (Rdh10), which metabolizes vitamin A to retinal (RAL), as a key molecule for regulating T cell differentiation. T cell-specific Rdh10 deficiency enhanced memory T-cell formation through blocking RAL production in infection model. Epigenetic profiling revealed that retinoic acid receptor (RAR) signaling activated by vitamin A metabolites induced comprehensive epigenetic repression of memory T cell-associated genes, including TCF7, thereby promoting effector T-cell differentiation. Importantly, memory T cells generated by Rdh deficiency and blocking RAR signaling elicited potent anti-tumor responses in adoptive T-cell transfer setting. Thus, T cell differentiation is regulated by vitamin A metabolism and its signaling, which should be novel targets for memory T cell-based cancer immunotherapy.

Beyond Checkpoint Inhibitors: Enhancing Antitumor Immune Response in Lung Cancer

The introduction of immune checkpoint inhibitors has dramatically changed the treatment landscape and improved survival for many patients with thoracic malignancies. Although some patients may experience prolonged survival benefit with immune checkpoint inhibitors, a majority do not experience disease control or benefit, supporting the need for research and development of improved approaches for facilitating immune recognition. Additionally, many patients will experience toxicity with the current approaches to immunotherapy, supporting the need for developing treatment strategies with less risk of adverse events. An extensive array of different strategies are currently under investigation, including novel combinations of checkpoint inhibitors or immunotherapies; novel agents beyond checkpoint inhibitors (e.g., bispecific antibodies, vaccine strategies, cytokine therapies); and different approaches for use of radiation to augment systemic immunotherapy agents. With each strategy, researchers are evaluating the potential for augmenting antitumor responses and ensuring more sustained antitumor effects. This article highlights areas of active research, reviewing the rationale for different investigative strategies, as well as currently available clinical data.

CD4+ T-cell epitope-based heterologous prime-boost vaccination potentiates anti-tumor immunity and PD-1/PD-L1 immunotherapy

BACKGROUND

Antitumor therapeutic vaccines are generally based on antigenic epitopes presented by major histocompatibility complex (MHC-I) molecules to induce tumor-specific CD8⁺ T cells. Paradoxically, continuous T cell receptor (TCR) stimulation from tumor-derived CD8⁺ T-cell epitopes can drive the functional exhaustion of tumor-specific CD8⁺ T cells. Tumor-specific type-I helper CD4⁺ T (T_H1) cells play an important role in the population maintenance and cytotoxic function of exhausted tumor-specific CD8⁺ T cells in the tumor microenvironment. Nonetheless, whether the vaccination strategy targeting MHC-II-restricted CD4⁺ T-cell epitopes to induce tumor-specific T_H1 responses can confer effective antitumor immunity to restrain tumor growth is not well studied. Here, we developed a heterologous prime-boost vaccination strategy to effectively induce tumor-specific T_H1 cells and evaluated its antitumor efficacy and its capacity to potentiate PD-1/PD-L1 immunotherapy.

METHODS

Listeria monocytogenes vector and influenza A virus (PR8 strain) vector stably expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein-specific I-A^b-restricted CD4⁺ T cell epitope (GP_61-80) or ovalbumin-specific CD4⁺ T cell epitope (OVA_323-339) were constructed and evaluated their efficacy against mouse models of melanoma and colorectal adenocarcinoma expressing lymphocytic choriomeningitis virus glycoprotein and ovalbumin. The impact of CD4⁺ T cell epitope-based heterologous prime-boost vaccination was detected by flow-cytometer, single-cell RNA sequencing and single-cell TCR sequencing.

RESULTS

CD4⁺ T cell epitope-based heterologous prime-boost vaccination efficiently suppressed both mouse melanoma and colorectal adenocarcinoma. This vaccination primarily induced tumor-specific T_H1 response, which in turn enhanced the expansion, effector function and clonal breadth of tumor-specific CD8⁺ T cells. Furthermore, this vaccination strategy synergized PD-L1 blockade mediated tumor suppression. Notably, prime-boost vaccination extended the duration of PD-L1 blockade induced antitumor effects by preventing the re-exhaustion of tumor-specific CD8⁺ T cells.

CONCLUSION

CD4⁺ T cell epitope-based heterologous prime-boost vaccination elicited potent both tumor-specific T_H1 and CTL response, leading to the efficient tumor control. This strategy can also potentiate PD-1/PD-L1 immune checkpoint blockade (ICB) against cancer.

Bacterial inhibition of Fas-mediated killing promotes neuroinvasion and persistence

Infections of the central nervous system are among the most serious infections^1,2, but the mechanisms by which pathogens access the brain remain poorly understood. The model microorganism Listeria monocytogenes (Lm) is a major foodborne pathogen that causes neurolisteriosis, one of the deadliest infections of the central nervous system^3,4. Although immunosuppression is a well-established host risk factor for neurolisteriosis^3,5, little is known about the bacterial factors that underlie the neuroinvasion of Lm. Here we develop a clinically relevant experimental model of neurolisteriosis, using hypervirulent neuroinvasive strains⁶ inoculated in a humanized mouse model of infection⁷, and we show that the bacterial surface protein InlB protects infected monocytes from Fas-mediated cell death by CD8⁺ T cells in a manner that depends on c-Met, PI3 kinase and FLIP. This blockade of specific anti-Lm cellular immune killing lengthens the lifespan of infected monocytes, and thereby favours the transfer of Lm from infected monocytes to the brain. The intracellular niche that is created by InlB-mediated cell-autonomous immune resistance also promotes Lm faecal shedding, which accounts for the selection of InlB as a core virulence gene of Lm. We have uncovered a specific mechanism by which a bacterial pathogen confers an increased lifespan to the cells it infects by rendering them resistant to cell-mediated immunity. This promotes the persistence of Lm within the host, its dissemination to the central nervous system and its transmission.

Bacteria and bacterial derivatives as delivery carriers for immunotherapy

There is growing interest in the role of microorganisms in human health and disease, with evidence showing that new types of biotherapy using engineered bacterial therapeutics, including bacterial derivatives, can address specific mechanisms of disease. The complex interactions between microorganisms and metabolic/immunologic pathways underlie many diseases with unmet medical needs, suggesting that targeting these interactions may improve patient treatment. Using tools from synthetic biology and chemical engineering, non-pathogenic bacteria or bacterial products can be programmed and designed to sense and respond to environmental signals to deliver therapeutic effectors. This review describes current progress in biotherapy using live bacteria and their derivatives to achieve therapeutic benefits against various diseases.

Interrogating in vivo T-cell metabolism in mice using stable isotope labeling metabolomics and rapid cell sorting

T cells are integral players in the adaptive immune system that readily adapt their metabolism to meet their energetic and biosynthetic needs. A major hurdle to understand physiologic T-cell metabolism has been the differences between in vitro cell culture conditions and the complex in vivo milieu. To address this, we have developed a protocol that merges traditional immunology infection models with whole-body metabolite infusion and mass-spectrometry-based metabolomic profiling to assess T-cell metabolism in vivo. In this protocol, pathogen-infected mice are infused via the tail vein with an isotopically labeled metabolite (2-6 h), followed by rapid magnetic bead isolation to purify T-cell populations (<1 h) and then stable isotope labeling analysis conducted by mass spectrometry (~1-2 d). This procedure enables researchers to evaluate metabolic substrate utilization into central carbon metabolic pathways (i.e., glycolysis and the tricarboxylic acid cycle) by specific T-cell subpopulations in the context of physiological immune responses in vivo.

Why is Listeria monocytogenes such a potent inducer of CD8+ T-cells?

Listeria monocytogenes is a rapidly growing, Gram-positive, facultative intracellular pathogen that has been used for over 5 decades as a model to study basic aspects of infection and immunity. In a murine intravenous infection model, immunisation with a sublethal infection of L. monocytogenes initially leads to rapid intracellular multiplication followed by clearance of the bacteria and ultimately culminates in the development of long-lived cell-mediated immunity (CMI) mediated by antigen-specific CD8+ cytotoxic T-cells. Importantly, effective immunisation requires live, replicating bacteria. In this review, we summarise the cell and immunobiology of L. monocytogenes infection and discuss aspects of its pathogenesis that we suspect lead to robust CMI. We suggest five specific features of L. monocytogenes infection that positively impact the development of CMI: (a) the bacteria have a predilection for professional antigen-presenting cells; (b) the bacteria escape from phagosomes, grow, and secrete antigens into the host cell cytosol; (c) bacterial-secreted proteins enter the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation; (d) the bacteria do not induce rapid host cell death; and (e) cytosolic bacteria induce a cytokine response that favours CMI. Collectively, these features make L. monocytogenes an attractive vaccine vector for both infectious disease applications and cancer immunotherapy.

Oncolytic bacteria: past, present and future

More than a century ago, independent groups raised the possibility of using bacteria to selectively infect tumours. Such treatment induces an immune reaction that can cause tumour rejection and protect the patient against further recurrences. One of the first holistic approximations to use bacteria in cancer treatment was performed by William Coley, considered the father of immune-therapy, at the end of XIX century. Since then, many groups have used different bacteria to test their antitumour activity in animal models and patients. The basis for this reactivity implies that innate immune responses activated upon bacteria recognition, also react against the tumour. Different publications have addressed several aspects of oncolytic bacteria. In the present review, we will focus on revisiting the historical aspects using bacteria as oncolytic agents and how they led to the current clinical trials. In addition, we address the molecules present in oncolytic bacteria that induce specific toxic effects against the tumors as well as the activation of host immune responses in order to trigger antitumour immunity. Finally, we discuss future perspectives that could be considered in the different fields implicated in the implementation of this kind of therapy in order to improve the current use of bacteria as oncolytic agents.

Persistent Zika Virus Clinical Susceptibility despite Reduced Viral Burden in Mice with Expanded Virus-Specific CD8+ T Cells Primed by Recombinant Listeria monocytogenes

Vaccines against Zika virus (ZIKV) infection that target CD8⁺ T cells are of considerable interest because Abs may enhance infection susceptibility. However, whether CD8⁺ T cells are protective or promote susceptibility to clinical infection symptoms remains uncertain. To more precisely investigate ZIKV-specific CD8⁺ T cells in isolation, we engineered a Listeria monocytogenes-based vector to express a single MHC class I-restricted immune dominant peptide, E294-302, from ZIKV envelope protein. We show accumulation of activated ZIKV-specific CD8⁺ T cells primed by recombinant L. monocytogenes is associated with reductions in circulating virus levels after ZIKV challenge in type I IFN receptor-deficient mice and wildtype mice administered neutralizing Abs against type I IFN receptor. Interestingly, susceptibility to ZIKV clinical infection including weight loss and mortality each persists and is neither significantly improved nor worsened compared with isogenic L. monocytogenes-primed control mice. These data demonstrating persistent ZIKV clinical susceptibility despite reduced viral burden in mice with expanded virus-specific CD8⁺ T cells highlights the need for targeting other adaptive immune components in developing vaccines against ZIKV infection.

The histone methyltransferase EZH2 primes the early differentiation of follicular helper T cells during acute viral infection

Epigenetic modifications to histones dictate the differentiation of naïve CD4+ T cells into different subsets of effector T helper (TH) cells. The histone methyltransferase enhancer of zeste homolog 2 (EZH2) has been implicated in the mechanism regulating the differentiation of TH1, TH2 and regulatory T (Treg) cells. However, whether and how EZH2 regulates follicular helper T (TFH) cell differentiation remain unknown. Using a mouse model of acute lymphocytic choriomeningitis virus (LCMV) infection, we observed abundant EZH2 expression and associated H3K27me3 modifications preferentially in the early committed virus-specific TFH cells compared to those in TH1 cells. Ablation of EZH2 in LCMV-specific CD4+ T cells leads to a selective impairment of early TFH cell fate commitment, but not late TFH differentiation or memory TFH maintenance. Mechanistically, EZH2 specifically stabilizes the chromatin accessibility of a cluster of genes that are important for TFH fate commitment, particularly B cell lymphoma 6 (Bcl6), and thus directs TFH cell commitment. Therefore, we identified the chromatin-modifying enzyme EZH2 as a novel regulator of early TFH differentiation during acute viral infection.

Alpha-defensin 5 differentially modulates adenovirus vaccine vectors from different serotypes in vivo

Adenoviral vectors have shown significant promise as vaccine delivery vectors due to their ability to elicit both innate and adaptive immune responses. α-defensins are effector molecules of the innate immune response and have been shown to modulate natural infection with adenoviruses, but the majority of α-defensin-adenovirus interactions studied to date have only been analyzed in vitro. In this study, we evaluated the role of α-defensin 5 (HD5) in modulating adenovirus vaccine immunogenicity using various serotype adenovirus vectors in mice. We screened a panel of human adenoviruses including Ad5 (species C), Ad26 (species D), Ad35 (species B), Ad48 (species D) and a chimeric Ad5HVR48 for HD5 sensitivity. HD5 inhibited transgene expression from Ad5 and Ad35 but augmented transgene expression from Ad26, Ad48, and Ad5HVR48. HD5 similarly suppressed antigen-specific IgG and CD8+ T cell responses elicited by Ad5 vectors in mice, but augmented IgG and CD8+ T cell responses and innate cytokine responses elicited by Ad26 vectors in mice. Moreover, HD5 suppressed the protective efficacy of Ad5 vectors but enhanced the protective efficacy of Ad26 vectors expressing SIINFEKL against a surrogate Listeria-OVA challenge in mice. These data demonstrate that HD5 differentially modulates adenovirus vaccine delivery vectors in a species-specific manner in vivo.

A pipeline for identification and validation of tumor-specific antigens in a mouse model of metastatic breast cancer

Cancer immunotherapy continues to make headway as a treatment for advanced stage tumors, revealing an urgent need to understand the fundamentals of anti-tumor immune responses. Noteworthy is a scarcity of data pertaining to the breadth and specificity of tumor-specific T cell responses in metastatic breast cancer. Autochthonous transgenic models of breast cancer display spontaneous metastasis in the FVB/NJ mouse strain, yet a lack of knowledge regarding tumor-bound MHC/peptide immune epitopes in this mouse model limits the characterization of tumor-specific T cell responses, and the mechanisms that regulate T cell responses in the metastatic setting. We recently generated the NetH2pan prediction tool for murine class I MHC ligands by building an FVB/NJ H-2q ligand database and combining it with public information from six other murine MHC alleles. Here, we deployed NetH2pan in combination with an advanced proteomics workflow to identify immunogenic T cell epitopes in the MMTV-PyMT transgenic model for metastatic breast cancer. Five unique MHC I/PyMT epitopes were identified. These tumor-specific epitopes were confirmed to be presented by the class I MHC of primary MMTV-PyMT tumors and their T cell immunogenicity was validated. Vaccination using a DNA construct encoding a truncated PyMT protein generated CD8 + T cell responses to these MHC class I/peptide complexes and prevented tumor development. In sum, we have established an MHC-ligand discovery pipeline in FVB/NJ mice, identified and tracked H-2D^q/PyMT neoantigen-specific T cells, and developed a vaccine that prevents tumor development in this metastatic model of breast cancer.

Clinical Response of Live-Attenuated, Listeria monocytogenes Expressing Mesothelin (CRS-207) with Chemotherapy in Patients with Malignant Pleural Mesothelioma

PURPOSE

Malignant pleural mesothelioma (MPM) is an aggressive cancer associated with poor prognosis. CRS-207 is a live-attenuated Listeria monocytogenes engineered to express mesothelin, a tumor-associated antigen highly expressed in MPM. CRS-207 induces antitumor immune responses and increases susceptibility of neoplastic cells to immune-mediated killing.

PATIENTS AND METHODS

Patients with unresectable MPM, ECOG 0 or 1, and adequate organ and pulmonary function were enrolled in this multicenter, open-label phase Ib study. They received two priming infusions of 1 × 109 CFU CRS-207, followed by pemetrexed/cisplatin chemotherapy, and CRS-207 booster infusions. Primary objectives were safety and induction of immune response. Secondary/exploratory objectives included tumor response, progression-free survival (PFS), overall survival (OS), immune subset analysis, and gene-expression profiling of tumor.

RESULTS

Of 35 evaluable patients, 89% (31/35) had disease control with one complete response (3%), 19 partial responses (54%), and 10 stable disease (29%). The estimated median duration of response was 5.0 months (95% CI, 3.9-11.5). The median PFS and OS were 7.5 (95% CI, 7.0-9.9) and 14.7 (95% CI, 11.2-21.9) months, respectively. Tumor size reduction was observed post-CRS-207 infusion prior to chemotherapy in 11 of 35 (31%) patients. No unexpected treatment-related serious adverse events or deaths were observed. IHC analysis of pre- and post-CRS-207 treatment tumor biopsies revealed possible reinvigoration and proliferation of T cells, increased infiltration of dendritic and natural killer cells, increased CD8:Treg ratio, and a shift from immunosuppressive M2-like to proinflammatory M1-like macrophages following CRS-207 administration.

CONCLUSIONS

Combination of CRS-207 and chemotherapy induced significant changes in the local tumor microenvironment and objective tumor responses in a majority of treated patients.

Oncolytic Bacteria and their potential role in bacterium-mediated tumour therapy: a conceptual analysis

As the human microbiota has been confirmed to be of great significance in maintaining health, the dominant bacteria in them have been applied as probiotics to treat various diseases. After the detection of bacteria in tumours, which had previously been considered a sterile region, these bacteria have been isolated and genetically modified for use in tumour therapy. In this review, we sum up the main types of bacteria used in tumour therapy and reveal the mechanisms of both wild type and engineered bacteria in eliminating tumour cells, providing potential possibilities for newly detected, genetically modified, tumour-associated bacteria in anti-tumour therapy.

A Potent and Effective Suicidal Listeria Vaccine Platform

Live-attenuated Listeria monocytogenes has shown encouraging potential as an immunotherapy platform in preclinical and clinical settings. However, additional safety measures will enable application across malignant and infectious diseases. Here, we describe a new vaccine platform, termed Lm-RIID (L. monocytogenes recombinase-induced intracellular death), that induces the deletion of genes required for bacterial viability yet maintains potent T cell responses to encoded antigens.

Live-attenuated Listeria monocytogenes has shown encouraging potential as an immunotherapy platform in preclinical and clinical settings. However, additional safety measures will enable application across malignant and infectious diseases. Here, we describe a new vaccine platform, termed Lm-RIID (L. monocytogenes recombinase-induced intracellular death), that induces the deletion of genes required for bacterial viability yet maintains potent T cell responses to encoded antigens. Lm-RIID grows normally in broth but commits suicide inside host cells by inducing Cre recombinase and deleting essential genes flanked by loxP sites, resulting in a self-limiting infection even in immunocompromised mice. Lm-RIID vaccination of mice induces potent CD8⁺ T cells and protects against virulent challenges, similar to live L. monocytogenes vaccines. When combined with α-PD-1, Lm-RIID is as effective as live-attenuated L. monocytogenes in a therapeutic tumor model. This impressive efficacy, together with the increased clearance rate, makes Lm-RIID ideal for prophylactic immunization against diseases that require T cells for protection.

Combined HDAC and BET Inhibition Enhances Melanoma Vaccine Immunogenicity and Efficacy

The combined inhibition of histone deacetylases (HDAC) and the proteins of the bromodomain and extraterminal (BET) family have recently shown therapeutic efficacy against melanoma, pancreatic ductal adenocarcinoma, testicular, and lymphoma cancers in murine studies. However, in such studies, the role of the immune system in therapeutically controlling these cancers has not been explored. We sought to investigate the effect of the HDAC inhibitor romidepsin (RMD) and the BET inhibitor IBET151, both singly and in combination, on vaccine-elicited immune responses. C57BL/6 mice were immunized with differing vaccine systems (adenoviral, protein) in prime-boost regimens under treatment with RMD, IBET151, or RMD+IBET151. The combined administration of RMD+IBET151 during vaccination resulted in a significant increase in the frequency and number of Ag-specific CD8⁺ T cells. RMD+IBET151 treatment significantly increased the frequency of vaccine-elicited IFN-γ⁺ splenic CD8⁺ T cells and conferred superior therapeutic and prophylactic protection against B16-OVA melanoma. RNA sequencing analyses revealed strong transcriptional similarity between RMD+IBET151 and untreated Ag-specific CD8⁺ T cells except in apoptosis and IL-6 signaling-related genes that were differentially expressed. Serum IL-6 was significantly increased in vivo following RMD+IBET151 treatment, with recombinant IL-6 administration replicating the effect of RMD+IBET151 treatment on vaccine-elicited CD8⁺ T cell responses. IL-6 sufficiency for protection was not assessed. Combined HDAC and BET inhibition resulted in greater vaccine-elicited CD8⁺ T cell responses and enhanced therapeutic and prophylactic protection against B16-OVA melanoma. Increased IL-6 production and the differential expression of pro- and anti-apoptotic genes following RMD+IBET151 treatment are likely contributors to the enhanced cancer vaccine responses.

Transplantation preferentially induces a KLRG-1lo CD127hi differentiation program in antigen-specific CD8+ T cells

Models of infection have shaped our understanding of programmed memory T cell differentiation, yet whether these models apply to memory programming in the context of transplantation has yet to be defined. Previous work has identified differences in the response of antigen-specific CD8⁺ T cells to cognate antigen based on the environment in which the antigen is presented. Thus, we hypothesized that programming of antigen specific CD8⁺ T cells responding to graft and pathogen may be dissimilar. Here we find that antigen-specific CD8⁺ T cells primed by a skin graft contract faster than those primed by gammaherpesvirus (gHV), yet are able to expand more rapidly upon rechallenge. Moreover, graft-primed antigen-specific CD8⁺ T cells exhibited higher frequencies of cells secreting IL-2 and demonstrate lower expression of KLRG-1, which are qualities suggestive of increased recall potential. Additionally, the expression of CD127 at a memory time point suggests graft-elicited CD8⁺ antigen specific T cells are maintained in a less terminally-differentiated state compared to gHV-elicited CD8⁺ antigen specific T cells, despite fewer cells being present at that time point. Taken together, our findings suggest that the surface marker expression and functional profiles of T cells depends on the priming conditions and may be used to predict immunologic risk following transplantation after traditional allosensitization or heterologous immune priming.

The Kinase Complex mTOR Complex 2 Promotes the Follicular Migration and Functional Maturation of Differentiated Follicular Helper CD4+ T Cells During Viral Infection

Follicular helper CD4⁺ T (T_FH) cells are critical for optimal B-cell-mediated humoral immunity by initiating, fueling, and sustaining germinal center reactions. The differentiation of T_FH cells relies on multiple intrinsic and extrinsic factors; however, the details by which these factors are integrated to coordinate T_FH differentiation are largely unknown. In this study, using a mouse model of acute lymphocytic choriomeningitis virus (LCMV) viral infection, we demonstrate that mTOR complex 2 (mTORC2) kinase integrates TCR signaling and ICOS-mediated co-stimulation to promote late differentiation and functional maturation of virus-specific T_FH cells. Specifically, mTORC2 functions to maintain T_FH lineage specifications, including phenotypes, migratory characteristics, and functional properties. Thus, our results highlight the importance of mTORC2 in guarding T_FH phenotypic and functional maturation.

Analysis of adenovirus-induced immunity to infection with Listeria monocytogenes: Fading protection coincides with declining CD8 T cell numbers and phenotypic changes

Defining correlates of T cell mediated protection is important in order to accelerate the development of efficient T cell based vaccines conferring long-term immunity. Extensive studies have provided important insight regarding the characteristics and functional properties of the effector and memory CD8 T cells induced by viral vector based vaccines. However, long-term protection has been difficult to achieve with T cell inducing vaccines, and the determinants underlying this loss in protection over time are still not fully defined. In this study we analyzed different parameters of the CD8 T cell response as a function of time after vaccination with a human serotype 5 adenovector expressing the glycoprotein (GP) of LCMV tethered to the MHC class II-associated invariant chain. Using this vector we have previously found that CD8 T cells mediate protection from challenge with GP-expressing Listeria monocytogenes at 60 days post vaccination, but only little protection after further 60 days, and we now confirm this observation. A comparison of vaccine-primed CD8 T cells early and late after vaccination revealed a minor decline in the overall numbers of antigen specific memory CD8 T cells during this interval. More importantly, we also observed phenotypic changes over time with a distinct decline in the frequency and number of KLRG1⁺ CD8 T cells, and, notably, adoptive transfer studies confirmed that memory CD8 T cells expressing KLRG1 are central to protection from systemic L. monocytogenes infection. Together these findings imply that multiple factors including changes in memory T cell numbers and phenotypic composition over time influence the longevity of CD8 T-cell mediated protection.

Hollow microneedle-mediated intradermal delivery of model vaccine antigen-loaded PLGA nanoparticles elicits protective T cell-mediated immunity to an intracellular bacterium

The skin is an attractive organ for immunization due to the presence of a large number of epidermal and dermal antigen-presenting cells. Hollow microneedles allow for precise and non-invasive intradermal delivery of vaccines. In this study, ovalbumin (OVA)-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles with and without TLR3 agonist poly(I:C) were prepared and administered intradermally by hollow microneedles. The capacity of the PLGA nanoparticles to induce a cytotoxic T cell response, contributing to protection against intracellular pathogens, was examined. We show that a single injection of OVA-loaded PLGA nanoparticles, compared to soluble OVA, primed both adoptively transferred antigen-specific naïve transgenic CD8⁺ and CD4⁺ T cells with markedly high efficiency. Applying a triple immunization protocol, PLGA nanoparticles primed also endogenous OVA-specific CD8⁺ T cells. Immune response, following immunization with in particular anionic PLGA nanoparticles co-encapsulated with OVA and poly(I:C), provided protection against a recombinant strain of the intracellular bacterium Listeria monocytogenes, secreting OVA. Taken together, we show that PLGA nanoparticle formulation is an excellent delivery system for protein antigen into the skin and that protective cellular immune responses can be induced using hollow microneedles for intradermal immunizations.

Suppression of autoimmune demyelinating disease by preferential stimulation of CNS-specific CD8 T cells using Listeria-encoded neuroantigen

CD8 T-cells predominate in CNS lesions of MS patients and display oligoclonal expansion. However, the role of myelin-specific CD8 T-cells in disease remains unclear, with studies showing protective and pathogenic roles in EAE. We demonstrated a disease-suppressive function for CNS-specific CD8 T-cells in a model where the antigen is exogenously administered in vivo and used for in vitro activation. To probe the nature of the CD8 response elicited by endogenously presented myelin antigens in vivo, we developed a novel approach utilizing infection with Listeria monocytogenes (LM) encoding proteolipid protein peptide (PLP) amino acids 178-191 (LM-PLP). LM-PLP infection preferentially induced PLP-specific CD8 T-cell responses. Despite the induction of PLP-specific CD8 T-cells, LM-PLP infection did not result in disease. In fact, LM-PLP infection resulted in significant amelioration of PLP178-191-induced EAE. Disease suppression was not observed in mice deficient in CD8 T-cells, IFN-γ or perforin. DTH responses and CNS infiltration were reduced in protected mice, and their CD4 T-cells had reduced capacity to induce tissue inflammation. Importantly, infection with LM-PLP ameliorated established disease. Our studies indicate that CD8 T-cells induced by endogenous presentation of PLP178-191 attenuate CNS autoimmunity in models of EAE, implicating the potential of this approach as a novel immunotherapeutic strategy.

Efficient control of chronic LCMV infection by a CD4 T cell epitope-based heterologous prime-boost vaccination in a murine model

CD4⁺ T cells are essential for sustaining CD8⁺ T cell responses during a chronic infection. The adoptive transfer of virus-specific CD4⁺ T cells has been shown to efficiently rescue exhausted CD8⁺ T cells. However, the question of whether endogenous virus-specific CD4⁺ T cell responses can be enhanced by certain vaccination strategies and subsequently reinvigorate exhausted CD8⁺ T cells remains unexplored. In this study, we developed a CD4⁺ T cell epitope-based heterologous prime-boost immunization strategy and examined the efficacy of this strategy using a mouse model of chronic lymphocytic choriomeningitis virus (LCMV) infection. We primed chronically LCMV-infected mice with a Listeria monocytogenes vector that expressed the LCMV glycoprotein-specific I-A^b-restricted CD4⁺ T cell epitope GP61-80 (LM-GP61) and subsequently boosted the primed mice with an influenza virus A (PR8 strain) vector that expressed the same CD4⁺ T cell epitope (IAV-GP61). This heterologous prime-boost vaccination strategy elicited strong anti-viral CD4⁺ T cell responses, which further improved both the quantity and quality of the virus-specific CD8⁺ T cells and led to better control of the viral loads. The combination of this strategy and the blockade of the programmed cell death-1 (PD-1) inhibitory pathway further enhanced the anti-viral CD8⁺ T cell responses and viral clearance. Thus, a heterologous prime-boost immunization that selectively induces virus-specific CD4⁺ T cell responses in conjunction with blockade of the inhibitory pathway may represent a promising therapeutic approach to treating patients with chronic viral infections.

Experimental Infection with Listeria monocytogenes as a Model for Studying Host Interferon-γ Responses

L. monocytogenes is a gram-positive bacterium that is a cause of food borne disease in humans. Experimental infection of mice with this pathogen has been highly informative on the role of innate and adaptive immune cells and specific cytokines in host immunity against intracellular pathogens. Production of IFN-γ by innate cells during sublethal infection with L. monocytogenes is important for activating macrophages and early control of the pathogen^1-3. In addition, IFN-γ production by adaptive memory lymphocytes is important for priming the activation of innate cells upon reinfection⁴. The L. monocytogenes infection model thus serves as a great tool for investigating whether new therapies that are designed to increase IFN-γ production have an impact on IFN-γ responses in vivo and have productive biological effects such as increasing bacterial clearance or improving mouse survival from infection. Described here is a basic protocol for how to conduct intraperitoneal infections of C57BL/6J mice with the EGD strain of L. monocytogenes and to measure IFN-γ production by NK cells, NKT cells, and adaptive lymphocytes by flow cytometry. In addition, procedures are described to: (1) grow and prepare the bacteria for inoculation, (2) measure bacterial load in the spleen and liver, and (3) measure animal survival to endpoints. Representative data are also provided to illustrate how this infection model can be used to test the effect of specific agents on IFN-γ responses to L. monocytogenes and survival of mice from this infection.

Shortened Intervals during Heterologous Boosting Preserve Memory CD8 T Cell Function but Compromise Longevity

Developing vaccine strategies to generate high numbers of Ag-specific CD8 T cells may be necessary for protection against recalcitrant pathogens. Heterologous prime-boost-boost immunization has been shown to result in large quantities of functional memory CD8 T cells with protective capacities and long-term stability. Completing the serial immunization steps for heterologous prime-boost-boost can be lengthy, leaving the host vulnerable for an extensive period of time during the vaccination process. We show in this study that shortening the intervals between boosting events to 2 wk results in high numbers of functional and protective Ag-specific CD8 T cells. This protection is comparable to that achieved with long-term boosting intervals. Short-boosted Ag-specific CD8 T cells display a canonical memory T cell signature associated with long-lived memory and have identical proliferative potential to long-boosted T cells Both populations robustly respond to antigenic re-exposure. Despite this, short-boosted Ag-specific CD8 T cells continue to contract gradually over time, which correlates to metabolic differences between short- and long-boosted CD8 T cells at early memory time points. Our studies indicate that shortening the interval between boosts can yield abundant, functional Ag-specific CD8 T cells that are poised for immediate protection; however, this is at the expense of forming stable long-term memory.

Splenectomy Alters Distribution and Turnover but not Numbers or Protective Capacity of de novo Generated Memory CD8 T-Cells

The spleen is a highly compartmentalized lymphoid organ that allows for efficient antigen presentation and activation of immune responses. Additionally, the spleen itself functions to remove senescent red blood cells, filter bacteria, and sequester platelets. Splenectomy, commonly performed after blunt force trauma or splenomegaly, has been shown to increase risk of certain bacterial and parasitic infections years after removal of the spleen. Although previous studies report defects in memory B-cells and IgM titers in splenectomized patients, the effect of splenectomy on CD8 T-cell responses and memory CD8 T-cell function remains ill defined. Using TCR-transgenic P14 cells, we demonstrate that homeostatic proliferation and representation of pathogen-specific memory CD8 T-cells in the blood are enhanced in splenectomized compared to sham surgery mice. Surprisingly, despite the enhanced turnover, splx mice displayed no changes in total memory CD8 T-cell numbers nor impaired protection against lethal dose challenge with Listeria monocytogenes. Thus, our data suggest that memory CD8 T-cell maintenance and function remain intact in the absence of the spleen.

The p60 and NamA autolysins from Listeria monocytogenes contribute to host colonization and induction of protective memory

Inducing long-term protective memory CD8(+) T-cells is a desirable goal for vaccines against intracellular pathogens. However, the mechanisms of differentiation of CD8(+) T-cells into long-lived memory cells capable of mediating protection of immunized hosts remain incompletely understood. We have developed an experimental system using mice immunized with wild type (WT) or mutants of the intracellular bacterium Listeria monocytogenes (Lm) that either do or do not develop protective memory CD8(+) T-cells. We previously reported that mice immunized with Lm lacking functional SecA2, an auxiliary secretion system of gram-positive bacteria, did not differentiate functional memory CD8(+) T-cells that protected against a challenge infection with WT Lm. Herein we hypothesized that the p60 and NamA autolysins of Lm, which are major substrates of the SecA2 pathway, account for this phenotype. We generated Lm genetically deficient for genes encoding for the p60 and NamA proteins, ΔiapΔmurA Lm, and further characterized this mutant. Δp60ΔNamA Lm exhibited a strong filamentous phenotype, inefficiently colonized host tissues, and grew mostly outside cells. When Δp60ΔNamA Lm was made single unit, cell invasion was restored to WT levels during vaccination, yet induced memory T-cells still did not protect immunized hosts against recall infection. Recruitment of blood phagocytes and antigen-presenting cell activation was close to that of mice immunized with ΔActA Lm, which develop protective memory. However, key inflammatory factors involved in optimal T-cell programming such as IL-12 and type I IFN (IFN-I) were lacking, suggesting that cytokine signals may largely account for the observed phenotype. Thus, altogether, these results establish that p60 and NamA secreted by Lm promote primary host cell invasion, the inflammatory response and the differentiation of functional memory CD8(+) T-cells, by preventing Lm filamentation during growth and subsequent triggering of innate sensing mechanisms.

Expression of porcine respiratory and reproductive syndrome virus membrane-associated proteins in Listeria ivanovii via a genome site-specific integration and expression system

Listeria ivanovii (LI) possesses a facultative intracellular life cycle in macrophages and non-professional phagocytes thus making it a potential novel vaccine vector. We have developed a genome site-specific integration system as a universal tool for constructing the live LI-based vaccines. Using this system, the porcine reproductive and respiratory syndrome virus (PRRSV) orf5 and orf6 genes, coding for PRRSV membrane-associated protein GP5 and M, respectively, were integrated into the LI genome. Genome PCR and sequencing results showed that the orf5 or orf6 gene had been integrated into the LI genome downstream of orfXYZ or mpl gene. Western blot analysis of the non-secreted and secreted bacterial protein samples showed the expression and secretion of GP5 or M protein from the recombinant LI strains. The secretion of the heterologous proteins is a key step for bacterial vaccine vectors to deliver the antigens to the immune system to stimulate antigen-specific immune responses. Therefore, this integration system may be a potential tool for constructing novel vaccines using live LI as vector.

Longitudinal requirement for CD4+ T cell help for adenovirus vector-elicited CD8+ T cell responses

Despite the widespread use of replication-incompetent recombinant adenovirus (Ad) vectors as candidate vaccine platforms, the mechanism by which these vectors elicit CD8(+) T cell responses remains poorly understood. Our data demonstrate that induction and maintenance of CD8(+) T cell responses by Ad vector immunization is longitudinally dependent on CD4(+) T cell help for a prolonged period. Depletion of CD4(+) T cells in wild type mice within the first 8 d following Ad immunization resulted in dramatically reduced induction of Ag-specific CD8(+) T cells, decreased T-bet and eomesodermin expression, impaired KLRG1(+) effector differentiation, and atypical expression of the memory markers CD127, CD27, and CD62L. Moreover, these CD8(+) T cells failed to protect against a lethal recombinant Listeria monocytogenes challenge. Depletion of CD4(+) T cells between weeks 1 and 4 following immunization resulted in increased contraction of memory CD8(+) T cells. These data demonstrate a prolonged temporal requirement for CD4(+) T cell help for vaccine-elicited CD8(+) T cell responses in mice. These findings have important implications in the design of vaccines aimed at eliciting CD8(+) T cell responses and may provide insight into the impaired immunogenicity of vaccines in the context of AIDS and other CD4(+) T cell immune deficiencies.

Attenuated Listeria monocytogenes: a powerful and versatile vector for the future of tumor immunotherapy

For over a century, inactivated or attenuated bacteria have been employed in the clinic as immunotherapies to treat cancer, starting with the Coley's vaccines in the 19th century and leading to the currently approved bacillus Calmette-Guérin vaccine for bladder cancer. While effective, the inflammation induced by these therapies is transient and not designed to induce long-lasting tumor-specific cytolytic T lymphocyte (CTL) responses that have proven so adept at eradicating tumors. Therefore, in order to maintain the benefits of bacteria-induced acute inflammation but gain long-lasting anti-tumor immunity, many groups have constructed recombinant bacteria expressing tumor-associated antigens (TAAs) for the purpose of activating tumor-specific CTLs. One bacterium has proven particularly adept at inducing powerful anti-tumor immunity, Listeria monocytogenes (Lm). Lm is a gram-positive bacterium that selectively infects antigen-presenting cells wherein it is able to efficiently deliver tumor antigens to both the MHC Class I and II antigen presentation pathways for activation of tumor-targeting CTL-mediated immunity. Lm is a versatile bacterial vector as evidenced by its ability to induce therapeutic immunity against a wide-array of TAAs and specifically infect and kill tumor cells directly. It is for these reasons, among others, that Lm-based immunotherapies have delivered impressive therapeutic efficacy in preclinical models of cancer for two decades and are now showing promise clinically. In this review, we will provide an overview of the history leading up to the development of current Lm-based immunotherapies, the advantages and mechanisms of Lm as a therapeutic vaccine vector, the preclinical experience with Lm-based immunotherapies targeting a number of malignancies, and the recent findings from clinical trials along with concluding remarks on the future of Lm-based tumor immunotherapies.

Role of tumor suppressor TSC1 in regulating antigen-specific primary and memory CD8 T cell responses to bacterial infection

The serine/threonine kinase mammalian/mechanistic target of rapamycin (mTOR) integrates various environmental cues such as the presence of antigen, inflammation, and nutrients to regulate T cell growth, metabolism, and function. The tuberous sclerosis 1 (TSC1)/TSC2 complex negatively regulates the activity of an mTOR-containing multiprotein complex called mTOR complex 1. Recent studies have revealed an essential cell-intrinsic role for TSC1 in T cell survival, quiescence, and mitochondrial homeostasis. Given the emerging role of mTOR activity in the regulation of the quantity and quality of CD8 T cell responses, in this study, we examine the role of its suppressor, TSC1, in the regulation of antigen-specific primary and memory CD8 T cell responses to bacterial infection. Using an established model system of transgenic CD8 cell adoptive transfer and challenge with Listeria monocytogenes expressing a cognate antigen, we found that TSC1 deficiency impairs antigen-specific CD8 T cell responses, resulting in weak expansion, exaggerated contraction, and poor memory generation. Poor expansion of TSC1-deficient cells was associated with defects in survival and proliferation in vivo, while enhanced contraction was correlated with an increased ratio of short-lived effectors to memory precursors in the effector cell population. This perturbation of effector-memory differentiation was concomitant with decreased expression of eomesodermin among activated TSC1 knockout cells. Upon competitive adoptive transfer with wild-type counterparts and antigen rechallenge, TSC1-deficient memory cells showed moderate defects in expansion but not cytokine production. Taken together, these findings provide direct evidence of a CD8 T cell-intrinsic role for TSC1 in the regulation of antigen-specific primary and memory responses.

Ikaros imposes a barrier to CD8+ T cell differentiation by restricting autocrine IL-2 production

Naive CD4(+) T cells require signals from the TCR and CD28 to produce IL-2, expand, and differentiate. However, these same signals are not sufficient to induce autocrine IL-2 production by naive CD8(+) T cells, which require cytokines provided by other cell types to drive their differentiation. The basis for failed autocrine IL-2 production by activated CD8(+) cells is unclear. We find that Ikaros, a transcriptional repressor that silences IL-2 in anergic CD4(+) T cells, also restricts autocrine IL-2 production by CD8(+) T cells. We find that CD8(+) T cell activation in vitro in the absence of exogenous cytokines and CD4 help leads to marked induction of Ikaros, a known repressor of the Il2 gene. Naive murine CD8 T cells haplo-insufficient for Ikzf1 failed to upregulate Ikaros, produced autocrine IL-2, and differentiated in an IL-2-dependent manner into IFN-γ-producing CTLs in response to TCR/CD28 stimulation alone. Furthermore, Ikzf1 haplo-insufficient CD8(+) T cells were more effective at controlling Listeria infection and B16 melanoma growth in vivo, and they could provide help to neighboring, non-IL-2-producing cells to differentiate into IFN-γ-producing effectors. Therefore, by repressing autocrine IL-2 production, Ikaros ensures that naive CD8(+) T cells remain dependent on licensing by APCs and CD4(+) T cells, and it may therefore act as a cell-intrinsic safeguard against inappropriate CTL differentiation and immunopathology.

Comparison of widely used Listeria monocytogenes strains EGD, 10403S, and EGD-e highlights genomic variations underlying differences in pathogenicity

For nearly 3 decades, listeriologists and immunologists have used mainly three strains of the same serovar (1/2a) to analyze the virulence of the bacterial pathogen Listeria monocytogenes. The genomes of two of these strains, EGD-e and 10403S, were released in 2001 and 2008, respectively. Here we report the genome sequence of the third reference strain, EGD, and extensive genomic and phenotypic comparisons of the three strains. Strikingly, EGD-e is genetically highly distinct from EGD (29,016 single nucleotide polymorphisms [SNPs]) and 10403S (30,296 SNPs), and is more related to serovar 1/2c than 1/2a strains. We also found that while EGD and 10403S strains are genetically very close (317 SNPs), EGD has a point mutation in the transcriptional regulator PrfA (PrfA*), leading to constitutive expression of several major virulence genes. We generated an EGD-e PrfA* mutant and showed that EGD behaves like this strain in vitro, with slower growth in broth and higher invasiveness in human cells than those of EGD-e and 10403S. In contrast, bacterial counts in blood, liver, and spleen during infection in mice revealed that EGD and 10403S are less virulent than EGD-e, which is itself less virulent than EGD-e PrfA*. Thus, constitutive expression of PrfA-regulated virulence genes does not appear to provide a significant advantage to the EGD strain during infection in vivo, highlighting the fact that in vitro invasion assays are not sufficient for evaluating the pathogenic potential of L. monocytogenes strains. Together, our results pave the way for deciphering unexplained differences or discrepancies in experiments using different L. monocytogenes strains.

Over the past 3 decades, Listeria has become a model organism for host-pathogen interactions, leading to critical discoveries in a broad range of fields, including bacterial gene regulation, cell biology, and bacterial pathophysiology. Scientists studying Listeria use primarily three pathogenic strains: EGD, EGD-e, and 10403S. Despite many studies on EGD, it is the only one of the three strains whose genome has not been sequenced. Here we report the sequence of its genome and a series of important genomic and phenotypic differences between the three strains, in particular, a critical mutation in EGD’s PrfA, the main regulator of Listeria virulence. Our results show that the three strains display differences which may play an important role in the virulence differences observed between the strains. Our findings will be of critical relevance to listeriologists and immunologists who have used or may use Listeria as a tool to study the pathophysiology of listeriosis and immune responses.

Listeria monocytogenes: a promising vehicle for neonatal vaccination

Vaccination as a medical intervention has proven capable of greatly reducing the suffering from childhood infectious disease. However, newborns and infants in particular are age groups for whom adequate vaccine-mediated protection is still largely lacking. With the challenges that the neonatal immune system faces and the required highest level of stringency for safety, designing vaccines for early life in general and the newborn in particular poses great difficulty. Nevertheless, recent advances in our understanding of neonatal immunity and its responses to vaccines and adjuvants suggest that neonatal vaccination is a task fully within reach. Among the most promising developments in neonatal vaccination is the use of Listeria monocytogenes (Lm) as a delivery platform. In this review, we will outline key properties of Lm that make it such an ideal neonatal and early life vaccine vehicle, and also discuss potential constraints of Lm as a vaccine delivery platform.

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

Listeria monocytogenes is a facultative intracellular bacterium that enters the cell by phagocytosis after which it colonizes the cytosol of the host cell. It is thus a potent vaccine vector for the presentation of passenger antigens to the major histocompatability complex class II and class I pathways of antigen processing and presentation. This article shall review the progress made in developing this unusual bacterium as a vaccine vector. In mouse models, recombinant Listeria carrying a number of different antigens have been shown to provide protective immunity against infectious organisms and therapeutic immunity directed towards tumor-associated antigens. Listeria has been engineered to express a number of HIV/SIV antigens. Measurements of immune responses using these recombinant strains in the mouse, after oral and parenteral immunization, and in the rhesus macaque after oral immunization indicate that strong cell-mediated immunity can be induced against these antigens. This review also discusses safety issues associated with live bacterial vaccine vectors and problems to be overcome in developing Listeria as a HIV vaccine for human use.

Smart adjuvants

Although vaccine adjuvants have been used for almost a century, alum is the only adjuvant licensed by the US FDA for human vaccine use. Many adjuvants studied to date have generalized inflammatory properties and lack specificity in terms of targeting immune compartments and cell populations. Indeed, such adjuvants have often been crude in formulation, their effects usually restricted to T-helper 2-type immunity and their use limited owing to inherent toxicity. However, recent advances in immunology have resulted in a number of potential adjuvant candidates that are able to modulate the immune response in a more controlled and specific manner. These novel adjuvants are attractive for inclusion in current and future vaccine strategies since they have better-defined mechanisms of action. In this article, we review several compounds that target specific immune components, such as cells, receptors or signaling pathways, and have termed such reagents 'smart adjuvants'.

B7-1/B7-2 blockade overrides the activation of protective CD8 T cells stimulated in the absence of Foxp3+ regulatory T cells

Although T cell activation has been classically described to require distinct, positive stimulation signals that include B7-1 (CD80) and B7-2 (CD86) costimulation, overriding suppression signals that avert immune-mediated host injury are equally important. How these opposing stimulation and suppression signals work together remains incompletely defined. Our recent studies demonstrate that CD8 Teff activation in response to cognate peptide stimulation is actively suppressed by the Foxp3(+) subset of CD4 cells, called Tregs. Here, we show that the elimination of Treg suppression does not bypass the requirement for positive B7-1/B7-2 costimulation. The expansion, IFN-γ cytokine production, cytolytic, and protective features of antigen-specific CD8 T cells stimulated with purified cognate peptide in Treg-ablated mice were each neutralized effectively by CTLA-4-Ig that blocks B7-1/B7-2. In turn, given the efficiency whereby CTLA-4-Ig overrides the effects of Treg ablation, the role of Foxp3(+) cell-intrinsic CTLA-4 in mitigating CD8 Teff activation was also investigated. With the use of mixed chimera mice that contain CTLA-4-deficient Tregs exclusively after the ablation of WT Foxp3(+) cells, a critical role for Treg CTLA-4 in suppressing the expansion, cytokine production, cytotoxicity, and protective features of peptide-stimulated CD8 T cells is revealed. Thus, the activation of protective CD8 T cells requires positive B7-1/B7-2 costimulation even when suppression by Tregs and in particular, Treg-intrinsic CTLA-4 is circumvented.

High efficiency of antiviral CD4(+) killer T cells

The destruction of infected cells by cytotxic T lymphocytes (CTL) is integral to the effective control of viral and bacterial diseases, and CTL function at large has long been regarded as a distinctive property of the CD8(+)T cell subset. In contrast, and despite their first description more than three decades ago, the precise contribution of cytotoxic CD4(+)T cells to the resolution of infectious diseases has remained a matter of debate. In particular, the CTL activity of pathogen-specific CD4(+) "helper" T cells constitutes a single trait among a diverse array of other T cell functionalities, and overall appears considerably weaker than the cytolytic capacity of CD8(+) effector T cells. Here, using an in vivo CTL assay, we report that cytotoxic CD4(+)T cells are readily generated against both viral and bacterial pathogens, and that the efficiency of MHC-II-restricted CD4(+)T cell killing adjusted for effector:target cell ratios, precise specificities and functional avidities is comparable in magnitude to that of CD8(+)T cells. In fact, the only difference between specific CD4(+) and CD8(+)T cells pertains to the slightly delayed killing kinetics of the former demonstrating that potent CTL function is a cardinal property of both antiviral CD8(+) and CD4(+)T cells.

Disparate roles for STAT5 in primary and secondary CTL responses

IL-2 signals during the primary response to infection are essential in shaping CD8(+) T cell fate decisions. How CD8(+) T cells integrate IL-2 signals in the development of functional memory is not well understood. Because IL-2 induces potent activation of the STAT5 transcription factor, we tested the role of STAT5 in CD8(+) memory T cell differentiation and function using a model system in which STAT5 activity is inducibly abrogated upon CD8(+) T cell activation. We report that STAT5 activity is broadly important for the expansion and effector function of all effector CTL subsets. After pathogen clearance, STAT5 was required for the survival of effector phenotype memory CTLs during the contraction phase. However, despite its role in supporting full primary CD8(+) T cell expansion, and unlike IL-2, STAT5 activity is not required for the development of memory CD8(+) T cells capable of robust secondary expansion upon rechallenge. Our findings highlight differential requirements for survival signals between primary and secondary effector CTL, and demonstrate that IL-2-dependent programming of memory CD8(+) T cells capable of secondary expansion and secondary effector differentiation is largely STAT5 independent.

CD8 T cell memory recall is enhanced by novel direct interactions with CD4 T cells enabled by MHC class II transferred from APCs

Protection against many intracellular pathogens is provided by CD8 T cells, which are thought to need CD4 T cell help to develop into effective memory CD8 T cells. Because murine CD8 T cells do not transcribe MHC class II (MHC-II) genes, several models have proposed antigen presenting cells (APCs) as intermediaries required for CD4 T cells to deliver their help to CD8 T cells. Here, we demonstrate the presence of MHC-II molecules on activated murine CD8 T cells in vitro as well as in vivo. These MHC-II molecules are acquired via trogocytosis by CD8 T cells from their activating APCs, particularly CD11c positive dendritic cells (DCs). Transferred MHC-II molecules on activated murine CD8 T cells were functionally competent in stimulating specific indicator CD4 T cells. CD8 T cells that were “helped” in vitro and subsequently allowed to rest in vivo showed enhanced recall responses upon challenge compared to “helpless” CD8 T cells; in contrast, no differences were seen upon immediate challenge. These data indicate that direct CD8∶CD4 T cell interactions may significantly contribute to help for CD8 T cells. Furthermore, this mechanism may enable CD8 T cells to communicate with different subsets of interacting CD4 T cells that could modulate immune responses.

Pre-existing immunity against vaccine vectors--friend or foe?

Over the last century, the successful attenuation of multiple bacterial and viral pathogens has led to an effective, robust and safe form of vaccination. Recently, these vaccines have been evaluated as delivery vectors for heterologous antigens, as a means of simultaneous vaccination against two pathogens. The general consensus from published studies is that these vaccine vectors have the potential to be both safe and efficacious. However, some of the commonly employed vectors, for example Salmonella and adenovirus, often have pre-existing immune responses in the host and this has the potential to modify the subsequent immune response to a vectored antigen. This review examines the literature on this topic, and concludes that for bacterial vectors there can in fact, in some cases, be an enhancement in immunogenicity, typically humoral, while for viral vectors pre-existing immunity is a hindrance for subsequent induction of cell-mediated responses.

FoxP3⁺ regulatory CD4 T cells control the generation of functional CD8 memory

During the primary immune response, CD8 memory emerges from an environment of strong immune activation. The FoxP3⁺ regulatory CD4 T-cell subset (Treg) is known as a key suppressive component of the immune system. Here we report that Tregs are required for the generation of functional CD8 memory. In the absence of Tregs during priming, the resulting memory cells proliferate poorly and fail to differentiate into functional cytotoxic secondary effectors following antigen reactivation. We find that the Tregs act early, during the expansion phase of primary CD8 effectors, by fine tuning interleukin-2 exposure of CD8 memory precursors. This crucial new role of Tregs has implications for optimal vaccine development.

The role of CD4⁺ T cells in the generation of memory CD8⁺ T cells is not fully understood. In this study, the exposure of CD8 memory precursors to interleukin-2 during early antigen priming is shown to be controlled by FoxP3⁺ regulatory CD4⁺ T cells, resulting in the production of functional memory cells.

Characterization of CD8+ T cell function and immunodominance generated with an H2O2-inactivated whole-virus vaccine

CD8(+) T cells play an important role in protection against both acute and persistent viral infections, and new vaccines that induce CD8(+) T cell immunity are currently needed. Here, we show that lymphocytic choriomeningitis virus (LCMV)-specific CD8(+) T cells can be generated in response to a nonreplicating H(2)O(2)-inactivated whole-virus vaccine (H(2)O(2)-LCMV). Vaccine-induced CD8(+) T cell responses exhibited an increased ability to produce multiple cytokines at early time points following immunization compared to infection-induced responses. Vaccination with H(2)O(2)-LCMV induced the expansion of a narrow subset of the antigen-specific CD8(+) T cells induced by LCMV strain Arm infection, resulting in a distinct immunodominance hierarchy. Acute LCMV infection stimulated immunodominance patterns that shifted over time or after secondary infection, whereas vaccine-generated immunodominance profiles remained remarkably stable even following subsequent viral infection. Vaccine-induced CD8(+) T cell populations expanded sharply in response to challenge and were then maintained at high levels, with responses to individual epitopes occupying up to 40% of the CD8(+) T cell compartment at 35 days after challenge. H(2)O(2)-LCMV vaccination protected animals against challenge with chronic LCMV clone 13, and protection was mediated by CD8(+) T cells. These results indicate that vaccination with an H(2)O(2)-inactivated whole-virus vaccine induces LCMV-specific CD8(+) T cells with unique functional characteristics and provides a useful model for studying CD8(+) T cells elicited in the absence of active viral infection.

Stability and function of secondary Th1 memory cells are dependent on the nature of the secondary stimulus

Following acute infection in some mouse models, CD4+ memory T cells steadily decline over time. Conversely, in humans, CD4+ memory T cells can be maintained for many years at levels similar to CD8+ T cells. Because we previously observed that the longevity of Th1 memory cell survival corresponded to their functional avidity, we hypothesized that secondary challenge, which enriches for high functional avidity Th1 responders, would result in more stable Th1 memory populations. We found that following a heterologous secondary challenge, Th1 memory cells were maintained at stable levels compared with primary Th1 memory cells, showing little to no decline after day 75 postinfection. The improved stability of secondary Th1 memory T cells corresponded to enhanced homeostatic turnover; enhanced trafficking of effector memory Th1 cells to tissue sites of infection, such as the liver; and acquisition or maintenance of high functional avidity following secondary challenge. Conversely, a weaker homologous rechallenge failed to induce a stable secondary Th1 memory population. Additionally, homologous secondary challenge resulted in a transient loss of functional avidity by Th1 memory cells recruited into the secondary response. Our findings suggest that the longevity of Th1 memory T cells is dependent, at least in part, on the combined effects of primary and secondary Ag-driven differentiation. Furthermore, they demonstrate that the quality of the secondary challenge can have profound effects on the longevity and function of the ensuing secondary Th1 memory population.

Clinical development of Listeria monocytogenes-based immunotherapies

Active immunotherapy targeting dendritic cells (DCs) has shown great promise in preclinical models and in human clinical trials for the treatment of malignant disease. Sipuleucel-T (Provenge, Dendreon, Seattle, WA), which consists of antigen-loaded dendritic cells (DCs), recently became the first targeted therapeutic cancer vaccine to be approved by the US Food and Drug Administration (FDA). However, ex vivo therapies such as Provenge have practical limitations and elicit an immune response with limited scope. By contrast, live-attenuated Listeria monocytogenes (Lm) naturally targets DCs in vivo and stimulates both innate and adaptive cellular immunity. Lm-based vaccines engineered to express cancer antigens have demonstrated striking efficacy in several animal models and have resulted in encouraging anecdotal survival benefit in early human clinical trials. Two different Lm-based vaccine platforms have advanced into phase II clinical trials in cervical and pancreatic cancer. Future Lm-based clinical vaccine candidates are expected to feature polyvalent antigen expression and to be used in combination with other immunotherapies or conventional therapies such as radiotherapy and chemotherapy to augment efficacy.

Protective immunity to Listeria monocytogenes infection mediated by recombinant Listeria innocua harboring the VGC locus

In this study we propose a novel bacterial vaccine strategy where non-pathogenic bacteria are complemented with traits desirable for the induction of protective immunity. To illustrate the proof of principle of this novel vaccination strategy, we use the model organism of intracellular immunity Listeria. We introduced a, low copy number BAC-plasmid harbouring the virulence gene cluster (vgc) of L. monocytogenes (Lm) into the non-pathogenic L. innocua (L.inn) strain and examined for its ability to induce protective cellular immunity. The resulting strain (L.inn::vgc) was attenuated for virulence in vivo and showed a strongly reduced host detrimental inflammatory response compared to Lm. Like Lm, L.inn::vgc induced the production of Type I Interferon's and protection was mediated by Listeria-specific CD8⁺ T cells. Rational vaccine design whereby avirulent strains are equipped with the capabilities to induce protection but lack detrimental inflammatory effects offer great promise towards future studies using non-pathogenic bacteria as vectors for vaccination.