Promises and challenges for the development of Listeria monocytogenes-based immunotherapies

Combination immunotherapy with two attenuated Listeria strains carrying shuffled HPV-16 E6E7 protein causes tumor regression in a mouse tumor model

Cervical cancer continues to impose a heavy burden worldwide, and human papilloma virus (HPV) infection, especially persistent infection with type 16 (HPV-16), is known to be the primary etiological factor. Therapeutic vaccines are urgently needed because prophylactic vaccines are ineffective at clearing pre-existing HPV infection. Here, two recombinant Listeria strains (LMΔ-E6E7 & LIΔ-E6E7) with deletions of the actA and plcB genes, expressing the shuffled HPV-16 E6E7 protein were constructed. The strains were delivered into the spleen and liver by intravenous inoculation, induced antigen-specific cellular immunity and were eliminated completely from the internal organs several days later. Intravenously treating with single strain for three times, or with both strains alternately for three times significantly reduced the tumor size and prolonged the survival time of model mice. Combination immunotherapy with two strains seemed more effective than immunotherapy with single strain in that it enhanced the survival of the mice, and the LMΔ-E6E7-prime-LIΔ-E6E7-boost strategy showed significant stronger efficacy than single treatment with the LIΔ-E6E7 strain. The antitumor effect of this treatment might due to its ability to increase the proportion of CD8⁺ T cells and reduce the proportion of T regulatory cells (Tregs) in the intratumoral milieu. This is the first report regarding Listeria ivanovii-based therapeutic vaccine candidate against cervical cancer. Most importantly we are the first to confirm that combination therapy with two different recombinant Listeria strains has a more satisfactory antitumor effect than administration of a single strain. Thus, we propose a novel prime-boost treatment strategy.

Cyclooxygenase-1 and -2 Play Contrasting Roles in Listeria-Stimulated Immunity

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX) activity and are commonly used for pain relief and fever reduction. NSAIDs are used following childhood vaccinations and cancer immunotherapies; however, how NSAIDs influence the development of immunity following these therapies is unknown. We hypothesized that NSAIDs would modulate the development of an immune response to Listeria monocytogenes-based immunotherapy. Treatment of mice with the nonspecific COX inhibitor indomethacin impaired the generation of cell-mediated immunity. This phenotype was due to inhibition of the inducible COX-2 enzyme, as treatment with the COX-2-selective inhibitor celecoxib similarly inhibited the development of immunity. In contrast, loss of COX-1 activity improved immunity to L. monocytogenes Impairments in immunity were independent of bacterial burden, dendritic cell costimulation, or innate immune cell infiltrate. Instead, we observed that PGE₂ production following L. monocytogenes is critical for the formation of an Ag-specific CD8⁺ T cell response. Use of the alternative analgesic acetaminophen did not impair immunity. Taken together, our results suggest that COX-2 is necessary for optimal CD8⁺ T cell responses to L. monocytogenes, whereas COX-1 is detrimental. Use of pharmacotherapies that spare COX-2 activity and the production of PGE₂ like acetaminophen will be critical for the generation of optimal antitumor responses using L. monocytogenes.

Activation of the Listeria monocytogenes Virulence Program by a Reducing Environment

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

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

Overcoming the resistance of pancreatic cancer to immune checkpoint inhibitors

Immunotherapy has become a new modality of cancer treatment, but has had a limited success in treating PDAC. A combination approach to immunotherapy, using both immune checkpoint inhibitors and immune activating agonists, is needed, as PDAC does not respond to single-agent checkpoint inhibitors. Studies have also supported using vaccine-based therapies to prime the tumor microenvironment of PDAC with effector T-cells. Other therapeutic strategies including epigenetic agents, stroma modulators, radiotherapy, and T-cell transfer therapies may also prime the tumor microenvironment to overcome resistance to immune checkpoint inhibitors.

Mechanistic insights into ADXS11-001 human papillomavirus-associated cancer immunotherapy

Immune responses to the facultative intracellular bacterium Listeria monocytogenes (Lm) are robust and well characterized. Utilized for decades as a model of host-disease immunology, Lm is well suited for use as an immunotherapeutic bacterial vector for the delivery of foreign antigen. Genetic modification of Lm has been undertaken to create an attenuated organism that is deficient in its master transcriptional regulator, protein-related factor A, and incorporates a truncated, nonhemolytic version of the listeriolysin O (LLO) molecule to ensure its adjuvant properties while also preventing escape of the live organism from the phagolysosome. Delivery of a vaccine construct (Lm-LLO-E7; axalimogene filolisbac [AXAL] or ADXS11-001) in which the modified LLO molecule is fused with the E7 oncoprotein of human papillomavirus type 16 (HPV-16) consistently stimulates strong innate and E7 antigen-specific adaptive immune responses, resulting in reduction of tumor burden in animal cancer models. In the clinical setting, AXAL has shown early promise in phase I/II trials of women with cervical cancer, and several more trials are currently underway to assess the efficacy and safety of this antitumor vaccine in patients with HPV-positive head and neck and anal cancers.

Dendritic cell-based immunotherapy

Immunotherapy using dendritic cell (DC)-based vaccination is an approved approach for harnessing the potential of a patient's own immune system to eliminate tumor cells in metastatic hormone-refractory cancer. Overall, although many DC vaccines have been tested in the clinic and proven to be immunogenic, and in some cases associated with clinical outcome, there remains no consensus on how to manufacture DC vaccines. In this review we will discuss what has been learned thus far about human DC biology from clinical studies, and how current approaches to apply DC vaccines in the clinic could be improved to enhance anti-tumor immunity.

Dendritic Cell Vaccines

Exploitation of the patient's own immune system to induce antitumor immune responses using dendritic cell (DC) immunotherapy has been established in early clinical trials as a safe and promising therapeutic approach for cancer. However, their limited success in larger clinical trials highlights the need to optimize DC vaccine preparations. This chapter describes the methodologies utilized for the preparation of the DC vaccine most commonly used in clinical trials. Optional variations at different stages in DC vaccine preparation, based on the nature of antigen, delivery of antigen, maturation stimuli, and mode of administration for DC vaccines, are also presented for consideration as these are often dependent on the disease setting, desired immune response, and/or resources available.

Adjuvant HER2/neu peptide cancer vaccines in breast cancer

Active cancer immunotherapy remains an exciting and rapidly advancing field in oncology. Peptide cancer vaccines are an attractive therapeutic option as they are safe and easily produced and administered. Peptide cancer vaccines may be most effective in patients with a lower disease burden, when cancer tolerance is minimized. Our experience with three peptide cancer vaccines, E75, GP2 and AE37, in clinically disease-free breast cancer patients provides encouraging results that this method may be effective. Furthermore, the combined results of the initial trials suggest that the vaccine administered may need to be tailored to the specific subtype of cancer and tumor antigen expression level to achieve maximum effectiveness. The results also suggest that combining peptide vaccines with other immunotherapy may lead to a synergistic effect.

Therapeutic vaccines as a promising treatment modality against prostate cancer: rationale and recent advances

Cancer immunotherapy was deemed the medical breakthrough of 2013, in part because it can induce a rapid, durable, self-propagating and adaptable immune response. Specifically in prostate cancer, immunotherapy has emerged as a viable and attractive treatment strategy. To date, therapeutic cancer vaccines and immune checkpoint inhibitors are the two classes of immunotherapy that have demonstrated improvements in overall survival in patients with advanced tumors. The 2010 Food and Drug Administration approval of sipuleucel-T for asymptomatic or minimally symptomatic metastatic prostate cancer set the stage for ongoing phase III trials with the cancer vaccine PSA-TRICOM and the immune checkpoint inhibitor ipilimumab. A class effect of these approved immune-based therapies is a benefit in overall survival without short-term changes in disease progression, apparently due to modulation of tumor growth rate kinetics, in which the activated immune system exerts constant immunologic pressure that slows net tumor growth. A growing body of evidence suggests that the ideal population for clinical trials of cancer vaccines as monotherapy is patients with lower tumor volume and less aggressive disease. Combination strategies include immunotherapy with standard therapies or with other immunotherapies. Here we review emerging data on immunotherapy for patients with prostate cancer.

A Listeria vaccine and depletion of T-regulatory cells activate immunity against early stage pancreatic intraepithelial neoplasms and prolong survival of mice

BACKGROUND & AIMS

Premalignant lesions and early stage tumors contain immunosuppressive microenvironments that create barriers for cancer vaccines. Kras(G12D/+);Trp53(R172H/+);Pdx-1-Cre (KPC) mice, which express an activated form of Kras in pancreatic tissues, develop pancreatic intraepithelial neoplasms (PanIN) that progress to pancreatic ductal adenocarcinoma (PDA). We used these mice to study immune suppression in PDA.

METHODS

We immunized KPC and Kras(G12D/+);Pdx-1-Cre mice with attenuated intracellular Listeria monocytogenes (which induces CD4(+) and CD8(+) T-cell immunity) engineered to express Kras(G12D) (LM-Kras). The vaccine was given alone or in sequence with an anti-CD25 antibody (PC61) and cyclophosphamide to deplete T-regulatory (Treg) cells. Survival times were measured; pancreatic and spleen tissues were collected and analyzed by histologic, flow cytometry, and immunohistochemical analyses.

RESULTS

Interferon γ-mediated, CD8(+) T-cell responses were observed in KPC and Kras(G12D/+);Pdx-1-Cre mice given LM-Kras, but not in unvaccinated mice. Administration of LM-Kras to KPC mice 4-6 weeks old (with early stage PanINs), depleted of Treg cells, significantly prolonged survival and reduced PanIN progression (median survival, 265 days), compared with unvaccinated mice (median survival, 150 days; P = .002), mice given only LM-Kras (median survival, 150 days; P = .050), and unvaccinated mice depleted of Treg cells (median survival, 170 days; P = .048). In 8- to 12-week-old mice (with late-stage PanINs), LM-Kras, alone or in combination with Treg cell depletion, did not increase survival time or slow PanIN progression. The combination of LM-Kras and Treg cell depletion reduced numbers of Foxp3(+)CD4(+) T cells in pancreatic lymph nodes, increased numbers of CD4(+) T cells that secrete interleukin 17 and interferon γ, and caused CD11b(+)Gr1(+) cells in the pancreas to acquire an immunostimulatory phenotype.

CONCLUSIONS

Immunization of KPC mice with Listeria monocytogenes engineered to express Kras(G12D), along with depletion of Treg cells, reduces progression of early stage, but not late-stage, PanINs. This approach increases infiltration of the lesion with inflammatory cells. It might be possible to design immunotherapies against premalignant pancreatic lesions to slow or prevent progression to PDA.

Live-attenuated bacteria as a cancer vaccine vector

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

A Listeria monocytogenes-based vaccine that secretes sand fly salivary protein LJM11 confers long-term protection against vector-transmitted Leishmania major

Cutaneous leishmaniasis is a sand fly-transmitted disease characterized by skin ulcers that carry significant scarring and social stigmatization. Over the past years, there has been cumulative evidence that immunity to specific sand fly salivary proteins confers a significant level of protection against leishmaniasis. In this study, we used an attenuated strain of Listeria monocytogenes as a vaccine expression system for LJM11, a sand fly salivary protein identified as a good vaccine candidate. We observed that mice were best protected against an intradermal needle challenge with Leishmania major and sand fly saliva when vaccinated intravenously. However, this protection was short-lived. Importantly, groups of vaccinated mice were protected long term when challenged with infected sand flies. Protection correlated with smaller lesion size, fewer scars, and better parasite control between 2 and 6 weeks postchallenge compared to the control group of mice vaccinated with the parent L. monocytogenes strain not expressing LJM11. Moreover, protection correlated with high numbers of CD4(+), gamma interferon-positive (IFN-γ(+)), tumor necrosis factor alpha-positive/negative (TNF-α(+/-)), interleukin-10-negative (IL-10(-)) cells and low numbers of CD4(+) IFN-γ(+/-) TNF-α(-) IL-10(+) T cells at 2 weeks postchallenge. Overall, our data indicate that delivery of LJM11 by Listeria is a promising vaccination strategy against cutaneous leishmaniasis inducing long-term protection against ulcer formation following a natural challenge with infected sand flies.

Vaccine therapy for pancreatic cancer

Pancreatic cancer is a lethal disease and currently available therapies have significant limitations. Pancreatic cancer is thus an ideal setting for the development of novel treatment modalities such as immunotherapy. However, relevant obstacles must be overcome for immunotherapeutic regimens against pancreatic cancer to be successful. Vaccine therapy relies on the administration of biological preparations that include an antigen that (at least ideally) is specifically expressed by malignant cells, boosting the natural ability of the immune system to react against neoplastic cells. There are a number of ways to deliver anticancer vaccines. Potent vaccines stimulate antigen presentation by dendritic cells, hence driving the expansion of antigen-specific effector and memory T cells. Unlike vaccines given as a prophylaxis against infectious diseases, anticancer vaccines require the concurrent administration of agents that interfere with the natural predisposition of tumors to drive immunosuppression. The safety and efficacy of vaccines against pancreatic cancer are nowadays being tested in early phase clinical trials.

Dendritic cell immunotherapy in ovarian cancer

Ovarian cancer is one of the most frequent gynecological malignancies. However, as there is no effective screening method to detect early disease, it is usually only diagnosed when already widespread in the abdomen. The majority of patients diagnosed with advanced-stage disease will relapse and require additional therapy. In the search for additional effective treatments for the management of recurrent disease, researchers have focused on the potential usefulness of immunotherapeutic modulation by administering autologous immune cells, such as dendritic cells (DCs), to stimulate antitumor host responses. With the ultimate goal of improved survival, this review addresses mechanisms in ovarian cancer that may limit the expansion of antitumor immunity, discusses the parameters to be considered for optimal DC immunotherapy, outlines evaluation methodology used to monitor the success of treatment regimens and reviews reported DC immunotherapy trials in ovarian cancer.

Prophylactic and therapeutic efficacy of an attenuated Listeria monocytogenes-based vaccine delivering HPV16 E7 in a mouse model

Listeria monocytogenes (L. monocytogenes) has been developed as a cancer vaccine vector due to its ability to elicit strong innate and adaptive immune responses. For clinical application, it is necessary to exploit a Listeria platform strain that is safe and that also retains its immunogenicity to develop vaccine candidates against cancer. In this study, a highly attenuated strain with a deletion of actA/plcB was employed as a vector to deliver the human papillomavirus type 16 (HPV16) E7 antigen, which was stably inserted into the chromosome of L. monocytogenes. The prophylactic and therapeutic efficacy of the recombinant L. monocytogenes strain expressing E7 (LM1-2-E7) were evaluated in C57BL/6 mice. In prophylactic tumor challenge assays, immunization with the recombinant strain LM1-2-E7 was able to protect against tumor formation in 87.5% of the mice, even after a second challenge, suggesting that this prophylactic immunization can provide long-lasting immunity. In the therapeutic setting, immunization with LM1-2-E7 led to tumor regression in 50% of the mice and suppressed tumor growth in the remaining mice. The results showed that the recombinant strain was cleared by the immune system within 5 days after immunization and induced a Th1 immune response against E7 peptide and E7-specific cytotoxic T-lymphocyte (CTL) killing activity without severe inflammatory responses in the spleen and liver. Markedly, recombinant Listeria strain resulted in preferential accumulation within tumor tissues and induced higher numbers of CD8+ T cells that infiltrated into the tumor, which were associated with retardation of tumor growth. Collectively, these data indicate that LM1-2-E7 is a possible vaccine candidate against cervical cancer.

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.

Killed but metabolically active vaccines

Beginning in the 20th century and continuing into the new millennia, vaccines against numerous diseases have had an unquestioned principal role of both enhancing the quality of life and increasing life expectancy (Rappuoli R, Mandl CW, Black S, De Gregorio E: Vaccines for the twenty-first century society. Nat Rev Immunol 2011, 11:865-872). Despite this success and the development of sophisticated new vaccine technologies, there remain multiple infectious diseases including tuberculosis, malaria and AIDS that await an effective prophylactic vaccine. In addition, there have been recent clinical successes among individuals with cancer using vaccine treatment strategies-so-called therapeutic vaccines-that stimulate tumor specific immunity and increase survival (Kantoff PW, Higano CS, Shore ND, Berger ER, Small EJ, Penson DF, Redfern CH, Ferrari AC, Dreicer R, Sims RB, et al.: Sipuleucel-T immunotherapy for castration-resistant prostate cancer. New Engl J Med 2010, 363:411-422). Here we summarize a new class of vaccines termed Killed But Metabolically Active (KBMA). KBMA vaccines are whole pathogenic or attenuated organisms killed through photochemical inactivation and cannot cause disease, yet retain sufficient metabolic activity to initiate a potent immune response. KBMA vaccines have two broad applications. First, recombinant KBMA vaccines encoding selected antigens relevant to infectious disease or cancer can be used to elicit a desired immune response. In the second application, KBMA vaccines can be derived from attenuated forms of a targeted pathogen, allowing for the presentation of the entire antigenic repertoire to the immune system, of particular importance when the correlates of protection are unknown.

Phagosomes induced by cytokines function as anti-Listeria vaccines: novel role for functional compartmentalization of STAT-1 protein and cathepsin-D

Phagosomes are critical compartments for innate immunity. However, their role in the protection against murine listeriosis has not been examined. We describe here that listericidal phago-receptosomes are induced by the function of IFN-γ or IL-6 as centralized compartments for innate and adaptive immunity because they are able to confer protection against murine listeriosis. These phago-receptosomes elicited LLO(91-99)/CD8(+)- and LLO(189-201)/CD4(+)-specific immune responses and recruited mature dendritic cells to the vaccination sites controlled by T cells. Moreover, they present exceptional features as efficient vaccine vectors. First, they compartmentalize a novel listericidal STAT-1-mediated signaling pathway that confines multiple innate immune components to the same environment. Second, they show features of MHC class II antigen-loading competent compartments for cathepsin-D-mediated LLO processing. Third, murine cathepsin-D deficiencies fail to develop protective immunity after vaccination with listericidal phago-receptosomes induced by IFN-γ or IL-6. Therefore, it appears that the connection of STAT-1 and cathepsin-D in a single compartment is relevant for protection against listeriosis.

Innate immune pathways triggered by Listeria monocytogenes and their role in the induction of cell-mediated immunity

Acquired cell-mediated immunity to Listeria monocytogenes is induced by infection with live, replicating bacteria that grow in the host cell cytosol, whereas killed bacteria, or those trapped in a phagosome, fail to induce protective immunity. In this chapter, we focus on how L. monocytogenes is sensed by the innate immune system, with the presumption that innate immunity affects the development of acquired immunity. Infection by L. monocytogenes induces three innate immune pathways: an MyD88-dependent pathway emanating from a phagosome leading to expression of inflammatory cytokines; a STING/IRF3-dependent pathway emanating from the cytosol leading to the expression of IFN-β and coregulated genes; and very low levels of a Caspase-1-dependent, AIM2-dependent inflammasome pathway resulting in proteolytic activation and secretion of IL-1β and IL-18 and pyroptotic cell death. Using a combination of genetics and biochemistry, we identified the listerial ligand that activates the STING/IRF3 pathway as secreted cyclic diadenosine monophosphate, a newly discovered conserved bacterial signaling molecule. We also identified L. monocytogenes mutants that caused robust inflammasome activation due to bacteriolysis in the cytosol, release of DNA, and activation of the AIM2 inflammasome. A strain was constructed that ectopically expressed and secreted a fusion protein containing Legionella pneumophila flagellin that robustly activated the Nlrc4-dependent inflammasome and was highly attenuated in mice, also in an Nlrc4-dependent manner. Surprisingly, this strain was a poor inducer of adaptive immunity, suggesting that inflammasome activation is not necessary to induce cell-mediated immunity and may even be detrimental under some conditions. To the best of our knowledge, no single innate immune pathway is necessary to mount a robust acquired immune response to L. monocytogenes infection.

Tumor-associated antigen expressing Listeria monocytogenes induces effective primary and memory T-cell responses against hepatic colorectal cancer metastases

Purpose

Despite advances in therapy for the treatment of metastatic colorectal cancer, many patients die of hepatic disease. Current immunotherapeutic strategies are likely limited by inhibitory signals from the tumor. To successfully eliminate tumor deposits within an organ, an appropriate immunologic milieu to amplify antitumor responses must be developed.

Methods

We used a murine model utilizing the CT26 colon cancer cell line to analyze primary and memory tumor-specific T-cell responses induced by an attenuated actin A and internalin B deleted immunodominant tumor-associated antigen expressing strain of Listeria monocytogenes for the treatment of metastatic colorectal cancer.

Results

Treatment of mice bearing established hepatic metastases with this L. monocytogenes strain led to the generation of a strong initial tumor-specific cytotoxic CD8⁺ T-cell response that successfully treated 90% of animals. Tumor antigen-specific central and effector memory T cells were also generated and protected against tumor rechallenge. These cell populations, when measured before and after tumor rechallenge, showed a marked expansion of antigen-specific effector CD8⁺ effector memory T cells. This strain of L. monocytogenes was able to down-modulate the expression of the immune checkpoint molecule, PD-1, within the tumor microenvironment but had variable effects on CTLA-4 expression.

Conclusions

This L. monocytogenes strain generated a highly effective antitumor T-cell response, providing a basis for the development of this vaccine platform in patients with liver metastases.

Electronic supplementary material

The online version of this article (doi:10.1245/s10434-011-2037-0) contains supplementary material, which is available to authorized users.

The impact of pre-existing memory on differentiation of newly recruited naive CD8 T cells

One goal of immunization is to generate memory CD8 T cells of sufficient quality and quantity to confer protection against infection. It has been shown that memory CD8 T cell differentiation in vivo is controlled, at least in part, by the amount and duration of infection, Ag, and inflammatory cytokines present early after the initiation of the response. In this study, we used models of anti-vectorial immunity to investigate the impact of pre-existing immunity on the development and differentiation of vector-induced primary CD8 T cell responses. We showed that existing CD8 T cell memory influences the magnitude of naive CD8 T cell responses. However, the differentiation of newly recruited (either TCR-transgenic or endogenous) primary CD8 T cells into populations with the phenotype (CD62L(hi), CD27(hi), KLRG-1(low)) and function (tissue distribution, Ag-driven proliferation, cytokine production) of long-term memory was facilitated when they were primed in the presence of vector-specific memory CD8 T cells of the same or unrelated specificity. Therefore, these data suggested that the presence of anti-vectorial immunity impacts the rate of differentiation of vector-induced naive CD8 T cells, a notion with important implications for the design of future vaccination strategies.

The ubiquitin C-terminal hydrolase UCH-L1 promotes bacterial invasion by altering the dynamics of the actin cytoskeleton

Invasion of eukaryotic target cells by pathogenic bacteria requires extensive remodelling of the membrane and actin cytoskeleton. Here we show that the remodelling process is regulated by the ubiquitin C-terminal hydrolase UCH-L1 that promotes the invasion of epithelial cells by Listeria monocytogenes and Salmonella enterica. Knockdown of UCH-L1 reduced the uptake of both bacteria, while expression of the catalytically active enzyme promoted efficient internalization in the UCH-L1-negative HeLa cell line. The entry of L. monocytogenes involves binding to the receptor tyrosine kinase Met, which leads to receptor phosphorylation and ubiquitination. UCH-L1 controls the early membrane-associated events of this triggering cascade since knockdown was associated with altered phosphorylation of the c-cbl docking site on Tyr1003, reduced ubiquitination of the receptor and altered activation of downstream ERK1/2- and AKT-dependent signalling in response to the natural ligand Hepatocyte Growth Factor (HGF). The regulation of cytoskeleton dynamics was further confirmed by the induction of actin stress fibres in HeLa expressing the active enzyme but not the catalytic mutant UCH-L1(C90S) . These findings highlight a previously unrecognized involvement of the ubiquitin cycle in bacterial entry. UCH-L1 is highly expressed in malignant cells that may therefore be particularly susceptible to invasion by bacteria-based drug delivery systems.

The role of natural killer cells in the defense against Listeria monocytogenes lessons from a rat model

Ly49 receptors in rodents, like killer cell immunoglobulin-like receptors in humans, regulate natural killer (NK) cell activity. Although inhibitory Ly49 receptors clearly recognize classical major histocompatibility complex class I (MHC-I) molecules, the role for the activating Ly49 receptors has been less well understood. Here, we discuss recent data from a rat model for listeriosis. Rats depleted of NK cells, or more specifically the Ly49 receptor-bearing cells, showed increased bacterial loads in their spleen. Athymic nude rats with no functional T cells but increased numbers of Ly49-expressing NK cells were more resistant to infection, indicating a central role of NK cells in early immune defense against Listeria in this species. Listeria infection of macrophages or enteric epithelial cells led to upregulation of MHC-I, including nonclassical (Ib) molecules not regularly recognized by T cells. We have shown that activating Ly49 receptors are more efficiently stimulated when binding to upregulated class Ib antigens on infected cells. From this we postulate that activating Ly49 receptors may have a sentinel function in the early immune response against Listeria in detecting diseased cells 'flagged' by increased MHC-Ib expression.

Prior exposure to an attenuated Listeria vaccine does not reduce immunogenicity: pre-clinical assessment of the efficacy of a Listeria vaccine in the induction of immune responses against HIV

Background

We have evaluated an attenuated Listeria monocytogenes (Lm) candidate vaccine vector in nonhuman primates using a delivery regimen relying solely on oral vaccination. We sought to determine the impact of prior Lm vector exposure on the development of new immune responses against HIV antigens.

Findings

Two groups of rhesus macaques one Lm naive, the other having documented prior Lm vector exposures, were evaluated in response to oral inoculations of the same vector expressing recombinant HIV-1 Gag protein. The efficacy of the Lm vector was determined by ELISA to assess the generation of anti-Listerial antibodies; cellular responses were measured by HIV-Gag specific ELISpot assay. Our results show that prior Lm exposures did not diminish the generation of de novo cellular responses against HIV, as compared to Listeria-naïve monkeys. Moreover, empty vector exposures did not elicit potent antibody responses, consistent with the intracellular nature of Lm.

Conclusions

The present study demonstrates in a pre-clinical vaccine model, that prior oral immunization with an empty Lm vector does not diminish immunogenicity to Lm-expressed HIV genes. This work underscores the need for the continued development of attenuated Lm as an orally deliverable vaccine.

Viral and bacterial minigene products are presented by MHC class I molecules with similar efficiencies

MHC class I molecules present short peptides, usually 8-10 amino acids in length, to CD8(+) T cells. These peptides are typically generated from full-length endogenously synthesized proteins degraded by the antigen processing machinery of the target cell. However, exogenous proteins, whether originating from intracellular bacteria or parasites or via phagocytosis during cross-presentation, can also be processed for presentation by MHC class I molecules. It is currently not known whether endogenously synthesized proteins and proteins acquired from exogenous sources follow the same presentation pathway. One clue that the processing pathways followed by endogenous and exogenous proteins may not be identical is the vastly different presentation efficiencies reported for viral versus bacterial antigens. Because class I antigen processing involves multiple steps, we sought to determine where in the processing pathway these differences in efficiency occur. To accomplish this, we expressed identical minimal peptide determinants from viral and bacterial vectors using a minigene expression system and determined the rate of peptide-MHC generation per molecule of minigene product synthesized. We found that peptides expressed from either the viral or bacterial vector were presented with virtually identical efficiencies. These results suggest that differences in the processing pathways followed by endogenous versus exogenous proteins most likely occur at a point prior to where free peptide is liberated from full-length protein.

Directing dendritic cell immunotherapy towards successful cancer treatment

The use of dendritic cells (DCs) for tumor immunotherapy represents a powerful approach for harnessing the patient's own immune system to eliminate tumor cells. However, suboptimal conditions for generating potent immunostimulatory DCs, as well as the induction of tolerance and suppression mediated by the tumors and its microenvironment have contributed to limited success. Combining DC vaccines with new approaches that enhance immunogenicity and overcome the regulatory mechanisms underlying peripheral tolerance may be the key to achieving effective and durable anti-tumor immune responses that translate to better clinical outcomes.

Strategies for cancer vaccine development

Treating cancer with vaccines has been a challenging field of investigation since the 1950s. Over the years, the lack of effective active immunotherapies has led to the development of numerous novel strategies. However, the use of therapeutic cancer vaccines may be on the verge of becoming an effective modality. Recent phase II/III clinical trials have achieved hopeful results in terms of overall survival. Yet despite these encouraging successes, in general, very little is known about the basic immunological mechanisms involved in vaccine immunotherapy. Gaining a better understanding of the mechanisms that govern the specific immune responses (i.e., cytotoxic T lymphocytes, CD4 T helper cells, T regulatory cells, cells of innate immunity, tumor escape mechanisms) elicited by each of the various vaccine platforms should be a concern of cancer vaccine clinical trials, along with clinical benefits. This review focuses on current strategies employed by recent clinical trials of therapeutic cancer vaccines and analyzes them both clinically and immunologically.

Rhombencephalitis Caused by Listeria monocytogenes in Humans and Ruminants: A Zoonosis on the Rise?

Listeriosis is an emerging zoonotic infection of humans and ruminants worldwide caused by Listeria monocytogenes (LM). In both host species, CNS disease accounts for the high mortality associated with listeriosis and includes rhombencephalitis, whose neuropathology is strikingly similar in humans and ruminants. This review discusses the current knowledge about listeric encephalitis, and involved host and bacterial factors. There is an urgent need to study the molecular mechanisms of neuropathogenesis, which are poorly understood. Such studies will provide a basis for the development of new therapeutic strategies that aim to prevent LM from invading the brain and spread within the CNS.

Listeria monocytogenes - from saprophyte to intracellular pathogen

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

Overcoming cancer immune tolerance and escape

Although HER2/neu-targeted cancer vaccines have shown initial promise in the adjuvant setting, a therapeutic vaccine remains elusive due to the tumor escape mechanisms of established cancer. As described by Seavey et al. in this issue of CCR, a Listeria-delivered vaccine may help overcome immune tolerance, leading to an effective therapeutic vaccine.

A novel human Her-2/neu chimeric molecule expressed by Listeria monocytogenes can elicit potent HLA-A2 restricted CD8-positive T cell responses and impact the growth and spread of Her-2/neu-positive breast tumors

PURPOSE

The aim of this study was to efficiently design a novel vaccine for human Her-2/neu-positive (hHer-2/neu) breast cancer using the live, attenuated bacterial vector Listeria monocytogenes.

EXPERIMENTAL DESIGN

Three recombinant L. monocytogenes-based vaccines were generated that could express and secrete extracellular and intracellular fragments of the hHer-2/neu protein. In addition, we generated a fourth construct fusing selected portions of each individual fragment that contained most of the human leukocyte antigen (HLA) epitopes as a combination vaccine (L. monocytogenes-hHer-2/neu chimera).

RESULTS

Each individual vaccine was able to either fully regress or slow tumor growth in a mouse model for Her-2/neu-positive tumors. All three vaccines could elicit immune responses directed toward human leukocyte antigen-A2 epitopes of hHer-2/neu. The L. monocytogenes-hHer-2/neu chimera was able to mimic responses generated by the three separate vaccines and prevent spontaneous outgrowth of tumors in an autochthonous model for Her-2/neu-positive breast cancer, induce tumor regression in transplantable models, and prevent seeding of experimental lung metastases in a murine model for metastatic breast cancer.

CONCLUSION

This novel L. monocytogenes-hHer-2/neu chimera vaccine proves to be just as effective as the individual vaccines but combines the strength of all three in a single vaccination. These encouraging results support future clinical trials using this chimera vaccine and may be applicable to other cancer types expressing the Her-2/neu molecule such as colorectal and pancreatic cancer.

An anti-vascular endothelial growth factor receptor 2/fetal liver kinase-1 Listeria monocytogenes anti-angiogenesis cancer vaccine for the treatment of primary and metastatic Her-2/neu+ breast tumors in a mouse model

Thirty years after angiogenesis was shown to play an enabling role in cancer, modern medicine is still trying to develop novel compounds and therapeutics to target the tumor vasculature. However, most therapeutics require multiple rounds of administration and can have toxic side effects. In this study, we use anti-angiogenesis immunotherapy to target cells actively involved in forming new blood vessels that support the growth and spread of breast cancer. Targeting a central cell type involved in angiogenesis, endothelial cells, we immunized against host vascular endothelial growth factor receptor 2 to fight the growth of Her-2/neu(+) breast tumors. Using the bacterial vector, Listeria monocytogenes (Lm), we fused polypeptides from the mouse vascular endothelial growth factor receptor 2 molecule (fetal liver kinase-1) to the microbial adjuvant, listeriolysin-O, and used Lm to deliver the Ags and elicit potent antitumor CTL responses. Lm-listeriolysin-O-fetal liver kinase-1 was able to eradicate some established breast tumors, reduce microvascular density in the remaining tumors, protect against tumor rechallenge and experimental metastases, and induce epitope spreading to various regions of the tumor-associated Ag Her-2/neu. Tumor eradication was found to be dependent on epitope spreading to HER-2/neu and was not solely due to the reduction of tumor vasculature. However, vaccine efficacy did not affect normal wound healing nor have toxic side effects on pregnancy. We show that an anti-angiogenesis vaccine can overcome tolerance to the host vasculature driving epitope spreading to an endogenous tumor protein and drive active tumor regression.

Listeria monocytogenes infection in the face of innate immunity

Pathogen recognition and induction of immune responses are important for efficient elimination of infection. However, pathogens such as Listeria monocytogenes employ strategies to evade or modulate these defences, thus creating a more favourable environment that ensures their survival and pathogenesis. New insights into these strategies, particularly those targeting innate immunity, have recently emerged. L. monocytogenes is initially detected at the cell surface or in phagosomes by toll-like receptor 2 and in the cytosol by nuclear oligodimerization domain (NOD)-like receptors (NOD1, NOD2) and NALP3 and Ipaf. It carries out N-deacetylation of peptidoglycan to avoid this detection by toll-like receptor 2 and NOD-like receptors. L. monocytogenes modulates transcription of host immunity genes through modification of histones and chromatin remodelling. Furthermore, L. monocytogenes has recently been shown to avoid autophagy and induce apoptosis in immune effector cells. In this review we discuss some of these strategies, which have provided new insights into the interaction between L. monocytogenes and the immune response at a crucial stage of infection.

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.

Toward scalable parts families for predictable design of biological circuits

Our current ability to engineer biological circuits is hindered by design cycles that are costly in terms of time and money, with constructs failing to operate as desired, or evolving away from the desired function once deployed. Synthetic biologists seek to understand biological design principles and use them to create technologies that increase the efficiency of the genetic engineering design cycle. Central to the approach is the creation of biological parts--encapsulated functions that can be composited together to create new pathways with predictable behaviors. We define five desirable characteristics of biological parts--independence, reliability, tunability, orthogonality and composability, and review studies of small natural and synthetic biological circuits that provide insights into each of these characteristics. We propose that the creation of appropriate sets of families of parts with these properties is a prerequisite for efficient, predictable engineering of new function in cells and will enable a large increase in the sophistication of genetic engineering applications.