Live attenuated oral Salmonella platform for effective T- and B-cell targeting of PD-L1

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Background: Significant progresses have been recently achieved in cancer vaccines, yet novel immunization solutions to deliver efficiently tumor-associated antigens to professional antigen-presenting cells, and to overcome the peripheral tolerance and the immunosuppressive tumor microenvironment that prevent the eradication of cancer in most of patients, are urgently needed. VAXIMM is developing a unique and versatile oral T-cell vaccination platform based on the FDA-approved live-attenuated Salmonella Typhi strain Ty21a vaccine Vivotif, capable of delivering tumor-associated antigens encoded in DNA expression construct to the gut-associated lymphoid tissue, breaking immune tolerance and inducing effective anti-tumor immunity. Methods: This study summarizes the immunogenicity and antileukemia efficacy of VXM10 vaccines based on the live-attenuated Salmonella Typhimurium strain SL7207, transformed with a eukaryotic expression plasmid encoding different variants of the murine PD-L1 protein. Results: The antileukemia activity of VXM10 was evaluated in the FBL-3 disseminated model of leukemia, in which the tumor cells strongly express PD-L1. Multiple oral administrations of VXM10 vaccines produced a strong anti-tumor effect, with 100% of surviving animals 80 days after challenge with FBL3 leukemia in the highest dose groups. Moreover, 100% of long-term surviving mice resisted re-challenge with FBL-3 cells, demonstrating that vaccination with VXM10 generated a potent memory T cell response against the leukemia. Importantly, full leukemia control was achieved in both prophylactic and therapeutic settings. Upon immunization with VXM10 vaccines, T cell response was raised against PD-L1 epitopes after in vitro restimulation of the splenocytes, and anti-PD-L1 antibodies were detected in the serum. The precise mechanism of action of VXM10 vaccines is currently being investigated. Conclusions: This study provides further evidence that VAXIMM’s oral T-cell vaccination platform can be employed to stimulate anti-tumor immunity against antigens of the immune checkpoint regulatory protein PD-L1. These data paved the way for advancing the development of VXM10 into clinical development.

A phase 1 trial extension to assess immunologic efficacy and safety of prime-boost vaccination with VXM01, an oral T cell vaccine against VEGFR2, in patients with advanced pancreatic cancer

VXM01 is a first-in-kind orally applied tumor vaccine based on live attenuated Salmonella typhi carrying an expression plasmid encoding VEGFR2, an antigen expressed on tumor vasculature and a stable and accessible target for anti-angiogenic intervention. A recent randomized, placebo-controlled, phase I dose-escalation trial in advanced pancreatic cancer patients demonstrated safety, immunogenicity and transient, T-cell response-related anti-angiogenic activity of four priming vaccinations applied within one week. We here evaluated whether monthly boost vaccinations are safe and can sustain increased frequencies of vaccine-specific T cells. Patients with advanced pancreatic cancer were randomly assigned at a ratio of 2:1 to priming with VXM01 followed by up to six monthly boost vaccinations, or placebo treatment. Vaccinations were applied orally at two alternative doses of either 10⁶ colony-forming units (CFU) or 10⁷ CFU, and concomitant treatment with standard-of-care gemcitabine during the priming phase, and any treatment thereafter, was allowed in the study. Immunomonitoring involved interferon-gamma (IFNγ) ELIspot analysis with long overlapping peptides spanning the entire VEGFR2 sequence. A total of 26 patients were treated. Treatment-related adverse events preferentially associated with VXM01 were decreases in lymphocyte numbers in the blood, increased frequencies of neutrophils and diarrhea. Eight out of 16 patients who received at least one boosting vaccination responded with pronounced, i.e. at least 3-fold, increase in VEGFR2-specific T cell response over baseline levels. In the VXM01 vaccination group, VEGFR2-specific T cells peaked preferentially during the boosting phase with an average 4-fold increase over baseline levels. In conclusion, prime/boost vaccination with VXM01 was safe and immunogenic and increased vaccine specific T cell responses compared with placebo treatment.

Anti-angiogenic activity of VXM01, an oral T-cell vaccine against VEGF receptor 2, in patients with advanced pancreatic cancer: A randomized, placebo-controlled, phase 1 trial

VEGFR-2 is expressed on tumor vasculature and a target for anti-angiogenic intervention. VXM01 is a first in kind orally applied tumor vaccine based on live, attenuated Salmonella bacteria carrying an expression plasmid, encoding VEGFR-2. We here studied the safety, tolerability, T effector (Teff), T regulatory (Treg) and humoral responses to VEGFR2 and anti-angiogenic effects in advanced pancreatic cancer patients in a randomized, dose escalation phase I clinical trial. Results of the first 3 mo observation period are reported. Locally advanced or metastatic, pancreatic cancer patients were enrolled. In five escalating dose groups, 30 patients received VXM01 and 15 placebo on days 1, 3, 5, and 7. Treatment was well tolerated at all dose levels. No dose-limiting toxicities were observed. Salmonella excretion and salmonella-specific humoral immune responses occurred in the two highest dose groups. VEGFR2 specific Teff, but not Treg responses were overall increased in vaccinated patients. We furthermore observed a significant reduction of tumor perfusion after 38 d in vaccinated patients together with increased levels of serum biomarkers indicative of anti-angiogenic activity, VEGF-A, and collagen IV. Vaccine specific Teff responses significantly correlated with reductions of tumor perfusion and high levels of preexisting VEGFR2-specific Teff while those showing no antiangiogenic activity had low levels of preexisting VEGFR2 specific Teff, showed a transient early increase of VEGFR2-specific Treg and reduced levels of VEGFR2-specific Teff at later time points - pointing to the possibility that early anti-angiogenic activity might be based at least in part on specific reactivation of preexisting memory T cells.

VXM01, an oral T-cell vaccine targeting the tumor vasculature: Results from a randomized, controlled, first-in-man study in pancreatic cancer patients

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Background: VXM01 is an orally available, bacterially transmitted DNA vaccine targeting VEGFR-2. Pre-clinically, VXM01 showed anti-tumor activity in multiple tumor types. This first-in-human study was designed to evaluate the safety and tolerability of VXM01. Secondary endpoints included VEGFR-2 specific T-cell responses, tumor perfusion changes, and related biomarkers. Methods: A randomized, double-blinded, placebo-controlled, dose-escalation study was conducted in 45 patients with advanced pancreatic cancer. VXM01 or placebo was given on days 1, 3, 5, and 7. Doses were escalated from 10(6) CFU to 10(10) CFU over 5 dose groups, each including 6 VXM01 and 3 placebo patients. VEGFR-2 specific T-cell activity was monitored by ELISpot and T(reg) specificity assays before, during and after the vaccination course. Tumor perfusion was assessed by DCE-MRI on days 0 and 38. Biomarkers included CA19-9, VEGF-A and collagen IV. Results: Patients were enrolled from 12/2011 to 10/2012. Most commonly observed AEs were leukopenia, abdominal pain, and diarrhea, which were all equally distributed between treatment and placebo group. While a mild elevation in average blood pressure was observed in the VXM01 group over the placebo group, the hypertension adverse event rate did not differ between both groups. No DLTs were observed. VEGFR-2 specific effector T-cell response was increased in 57% of evaluable VXM01 treated patients, during and after the vaccination course. In 25% of the VXM01 group, the T-cell response score post-vaccination was higher than maximum placebo levels. In contrast, VEGFR-2 specific T(reg) responses were overall reduced in vaccinated patients. DCE-MRI data indicated a >33% drop in K(trans)/tumor perfusion in 35% of evaluable VXM01 treated patients vs. 10% in the placebo group. Mean changes were -4% (VXM01) and +15% (placebo). Reduced tumor perfusion correlated with VEGFR-2 specific T-cell responses and biomarker responses. Conclusions: VXM01 appeared safe and was well tolerated without DLTs across 5 tested dose levels. The data suggest further that VXM01 induces and enhances a VEGFR-2 specific T-cell response and impacts tumor perfusion. Clinical trial information: ISRCTN68809279.

Nuclear localization of the duck hepatitis B virus capsid protein: detection and functional implications of distinct subnuclear bodies in a compartment associated with RNA synthesis and maturation

We have reinvestigated the subcellular distribution of the duck hepatitis B virus (DHBV) core protein in infected duck hepatocytes and in transfected cells. By using indirect immunofluorescence, the protein was found to be localized not only in the cytoplasm, as described previously, but also within the cell nucleus, being concentrated in distinct, brightly staining nuclear core bodies (NCBs). In colocalization studies using confocal microscopy, the NCBs were found exclusively in the periphery of nuclear subdomains characterized as splicing factor compartments and distal to other subnuclear domains. Also relevant for their functional significance is that the NCBs formed during the establishment of virus infection, i.e., at very low overall concentrations of newly synthesized core protein, and persisted throughout all stages of infection. Moreover, a subset of NCBs colocalized with foci of pregenomic DHBV RNA present at concentrations detectable by fluorescence in situ hybridization. Taken together, these findings indicate that a minor fraction of the DHBV core protein molecules escapes the major cytoplasmic assembly pathway to accumulate in specific subnuclear domains, and they furthermore suggest that these NCBs serve a role in the synthesis and/or maturation of the DHBV RNA pregenome.

Endothelial cell-mediated uptake of a hepatitis B virus: a new concept of liver targeting of hepatotropic microorganisms

The liver is a target for many infectious agents, most notably hepatitis viruses. However, several receptor molecules identified so far for hepatitis viruses were found to be ubiquitously expressed and can thus not account for efficient liver targeting. Using a model hepatitis B virus, the duck hepatitis B virus (DHBV), we have obtained data indicating that scavenging liver sinusoidal endothelial cells (LSEC), rather than hepatocytes themselves, play the key role in the initial uptake of viral pathogens into the liver. Experiments with fluorescent viral particles and coated gold particles in test animals, as well as in primary liver cell culture, demonstrated a preferential uptake of the viral substrates into LSEC. Intracellularly, fluorescent virus particles internalized by LSEC colocalized with the DHBV receptor, carboxypeptidase D, suggesting receptor-mediated rescue from lysosomal degradation. To comply with the high efficiency by which hepatitis B viruses infect hepatocytes in vivo, we propose that viruses initially scavenged by LSEC are thereafter released to infect adjacent hepatocytes, the only cells capable of replicating these viruses. Such a model of primary uptake into LSEC may illustrate a general mechanism by which blood-borne hepatotropic agents are targeted to the hepatocytes in the liver.

Signals for bidirectional nucleocytoplasmic transport in the duck hepatitis B virus capsid protein

Hepadnavirus genome replication involves cytoplasmic and nuclear stages, requiring balanced targeting of cytoplasmic nucleocapsids to the nuclear compartment. In this study, we analyze the signals determining capsid compartmentalization in the duck hepatitis B virus (DHBV) animal model, as this system also allows us to study hepadnavirus infection of cultured primary hepatocytes. Using fusions to the green fluorescent protein as a functional assay, we have identified a nuclear localization signal (NLS) that mediates nuclear pore association of the DHBV nucleocapsid and nuclear import of DHBV core protein (DHBc)-derived polypeptides. The DHBc NLS mapped is unique. It bears homology to repetitive NLS elements previously identified near the carboxy terminus of the capsid protein of hepatitis B virus, the human prototype of the hepadnavirus family, but it maps to a more internal position. In further contrast to the hepatitis B virus core protein NLS, the DHBc NLS is not positioned near phosphorylation target sites that are generally assumed to modulate nucleocytoplasmic transport. In functional assays with a knockout mutant, the DHBc NLS was found to be essential for nuclear pore association of the nucleocapsid. The NLS was found to be also essential for virus production from the full-length DHBV genome in transfected cells and from hepatocytes infected with transcomplemented mutant virus. Finally, the DHBc additionally displayed activity indicative of a nuclear export signal, presumably counterbalancing NLS function in the productive state of the infected cell and thereby preventing nucleoplasmic accumulation of nucleocapsids.

Envelope protein-mediated down-regulation of hepatitis B virus receptor in infected hepatocytes

Entry of duck hepatitis B virus (DHBV) is initiated by specific interaction of its large envelope protein (L) with a cellular entry receptor, recently identified as carboxypeptidase D (CPD; historically gp180). In this report, we present evidence demonstrating that this receptor is down-regulated as a result of DHBV infection: (i) receptor levels determined by Western blot were much reduced in DHBV-infected duck livers and undetectable by immunostaining in infected cultured hepatocytes; (ii) results from metabolic labeling experiments indicate enhanced receptor protein turnover; (iii) the kinetics of receptor loss from newly infected cells correlated with the accumulation of newly synthesized viral protein; (iv) expression of DHBV L protein, transduced from a recombinant adenovirus, was sufficient to eliminate gp180/CPD from the Golgi compartment, its normal predominant location; (v) gp180/CPD remained absent from the Golgi compartment in infected hepatocytes, even after overexpression from a recombinant adenovirus, while residual amounts subsequently became detectable in a perinuclear compartment, containing DHBV L protein; (vi) expression of DHBV L protein in a HepG2 cell line, stably expressing gp180/CPD, leads to incomplete receptor maturation and induces its degradation. Taken together, these data are consistent with a model in which the virus receptor interacts early in the biosynthetic pathway with the viral L protein, leading to its retention in a pre-Golgi compartment and to subsequent degradation, thus preventing receptor interference with the export of DHBV via the secretory pathway which it shares with its receptor. Accordingly, and analogously with receptor down-regulation in retroviral systems, DHBV receptor down-modulation may account for the much-reduced efficiency of DHBV superinfection of preinfected hepatocytes.

Cellular receptor traffic is essential for productive duck hepatitis B virus infection

We have investigated the mechanism of duck hepatitis B virus (DHBV) entry into susceptible primary duck hepatocytes (PDHs), using mutants of carboxypeptidase D (gp180), a transmembrane protein shown to act as the primary cellular receptor for avian hepatitis B virus uptake. The variant proteins were abundantly produced from recombinant adenoviruses and tested for the potential to functionally outcompete the endogenous wild-type receptor. Overexpression of wild-type gp180 significantly enhanced the efficiency of DHBV infection in PDHs but did not affect ongoing DHBV replication, an observation further supporting gp180 receptor function. A gp180 mutant deficient for endocytosis abolished DHBV infection, indicating endocytosis to be the route of hepadnaviral entry. With further gp180 variants, carrying mutations in the cytoplasmic domain and characterized by an accelerated turnover, the ability of gp180 to function as a DHBV receptor was found to depend on a wild-type-like sorting phenotype which largely avoids transport toward the endolysosomal compartment. Based on these data, we propose a model in which a distinct intracellular DHBV traffic to the endosome, but not beyond, is a prerequisite for completion of viral entry, i.e., for fusion and capsid release. Furthermore, the deletion of the two enzymatically active carboxypeptidase domains of gp180 did not lead to a loss of receptor function.

Avian hepatitis B virus infection is initiated by the interaction of a distinct pre-S subdomain with the cellular receptor gp180

Functionally relevant hepadnavirus-cell surface interactions were investigated with the duck hepatitis B virus (DHBV) animal model by using an in vitro infection competition assay. Recombinant DHBV pre-S polypeptides, produced in Escherichia coli, were shown to inhibit DHBV infection in a dose-dependent manner, indicating that monomeric pre-S chains were capable of interfering with virus-receptor interaction. Particle-associated pre-S was, however, 30-fold more active, suggesting that cooperative interactions enhance particle binding. An 85-amino-acid pre-S sequence, spanning about half of the DHBV pre-S chain, was characterized by deletion analysis as essential for maximal inhibition. Pre-S polypeptides from heron hepatitis B virus (HHBV) competed DHBV infection equally well despite a 50% difference in amino acid sequence and a much-reduced infectivity of HHBV for duck hepatocytes. These observations are taken to indicate (i) that the functionality of the DHBV pre-S subdomain, which interacts with the cellular receptor, is determined predominantly by a defined three-dimensional structure rather than by primary sequence elements; (ii) that cellular uptake of hepadnaviruses is a multistep process involving more than a single cellular receptor component; and (iii) that gp180, a cellular receptor candidate unable to discriminate between DHBV and HHBV, is a common component of the cellular receptor complex for avian hepadnaviruses.

Carboxypeptidase D (gp180), a Golgi-resident protein, functions in the attachment and entry of avian hepatitis B viruses

Carboxypeptidase D (gp180), one of many candidate receptors proposed for hepatitis B viruses (HBVs), was examined and found to be the actual cellular receptor for avian HBVs. This conclusion was based on the following observations: (i) gp180 was the only host protein that bound with high affinity to the pre-S ectodomain of the large duck hepatitis B virus (DHBV) envelope protein, which is known to be essential for virus infection; (ii) a pre-S subdomain which determines physical binding to gp180 was found to coincide with a domain functionally defined in infection competition experiments as a receptor binding domain; (iii) soluble gp180, lacking the membrane anchor, efficiently inhibited DHBV infection; (iv) efficient interspecies gp180-pre-S interaction was limited to the natural hosts of avian hepadnaviruses; and (v) expression of gp180 in a heterologous hepatoma cell line mediated cellular attachment and subsequent internalization of fluorescently labeled viral particles into vesicular structures. However, gp180 expression did not render transfected heterologous cells permissive for productive infection, suggesting that a species-specific coreceptor is required for fusion to complete viral entry. In contrast to the case for known virus receptors, gp180 was not detected on the hepatocyte cell surface but was found to be concentrated in the Golgi apparatus, from where it functions by cycling to and from the plasma membrane.

Effects of secondary structure on DNA and RNA cleavage by diplatinum(II)

The photochemistry of Pt2(pop)44- with nucleic acids has been studied using radiolabeled oligomers of DNA and RNA and high-resolution electrophoresis (pop is P2O5H22-). Photolysis of Pt2(pop)44- with duplex DNA produces an even cleavage ladder and relatively little enhancement of cleavage upon treatment with piperidine. In contrast, the cleavage pattern is far less regular with single-stranded DNA, and there is a large enhancement in cleavage upon treatment with piperidine. Accordingly, photolysis of Pt2(pop)44- with the DNA hairpin 5'-d[ATCCTATTTATAGGAT] produces a much larger piperidine enhancement at the loop and end nucleotides than in the stem. There is an additional piperidine enhancement that occurs selectively at guanine residues either in RNA or in DNA at low Mg2+ concentrations that is attributed to outer-sphere electron transfer on the basis of the known excited-state redox potentials of Pt2(pop)44- and the expected oxidative chemistry of guanine. The extent of guanine oxidation is higher compared to the extent of sugar oxidation at low Mg2+ concentrations, which can be attributed to a shallower distance dependence for electron transfer compared to that for atom transfer. The effects of Mg2+ and piperidine or aniline treatment were examined on stem-loop structures of DNA and RNA and gave partial images of the expected secondary structures.