Molecular determinants of plaque size as an indicator of dengue virus attenuation

The development of live viral vaccines relies on empirically derived phenotypic criteria, especially small plaque sizes, to indicate attenuation. However, while some candidate vaccines successfully translated into licensed applications, others have failed safety trials, placing vaccine development on a hit-or-miss trajectory. We examined the determinants of small plaque phenotype in two dengue virus (DENV) vaccine candidates, DENV-3 PGMK30FRhL3, which produced acute febrile illness in vaccine recipients, and DENV-2 PDK53, which has a good clinical safety profile. The reasons behind the failure of PGMK30FRhL3 during phase 1 clinical trial, despite meeting the empirically derived criteria of attenuation, have never been systematically investigated. Using in vitro, in vivo and functional genomics approaches, we examined infections by the vaccine and wild-type DENVs, in order to ascertain the different determinants of plaque size. We show that PGMK30FRhL3 produces small plaques on BHK-21 cells due to its slow in vitro growth rate. In contrast, PDK53 replicates rapidly, but is unable to evade antiviral responses that constrain its spread hence also giving rise to small plaques. Therefore, at least two different molecular mechanisms govern the plaque phenotype; determining which mechanism operates to constrain plaque size may be more informative on the safety of live-attenuated vaccines

Small Cationic DDA:TDB Liposomes as Protein Vaccine Adjuvants Obviate the Need for TLR Agonists in Inducing Cellular and Humoral Responses

Most subunit vaccines require adjuvants in order to induce protective immune responses to the targeted pathogen. However, many of the potent immunogenic adjuvants display unacceptable local or systemic reactogenicity. Liposomes are spherical vesicles consisting of single (unilamellar) or multiple (multilamellar) phospholipid bi-layers. The lipid membranes are interleaved with an aqueous buffer, which can be utilised to deliver hydrophilic vaccine components, such as protein antigens or ligands for immune receptors. Liposomes, in particular cationic DDA:TDB vesicles, have been shown in animal models to induce strong humoral responses to the associated antigen without increased reactogenicity, and are currently being tested in Phase I human clinical trials. We explored several modifications of DDA:TDB liposomes - including size, antigen association and addition of TLR agonists – to assess their immunogenic capacity as vaccine adjuvants, using Ovalbumin (OVA) protein as a model protein vaccine. Following triple homologous immunisation, small unilamellar vesicles (SUVs) with no TLR agonists showed a significantly higher capacity for inducing spleen CD8 IFNγ responses against OVA in comparison with the larger multilamellar vesicles (MLVs). Antigen-specific antibody reponses were also higher with SUVs. Addition of the TLR3 and TLR9 agonists significantly increased the adjuvanting capacity of MLVs and OVA-encapsulating dehydration-rehydration vesicles (DRVs), but not of SUVs. Our findings lend further support to the use of liposomes as protein vaccine adjuvants. Importantly, the ability of DDA:TDB SUVs to induce potent CD8 T cell responses without the need for adding immunostimulators would avoid the potential safety risks associated with the clinical use of TLR agonists in vaccines adjuvanted with liposomes

Vaccine delivery by penetratin: mechanism of antigen presentation by dendritic cells

Cell-penetrating peptides (CPP) or membrane-translocating peptides such as penetratin from Antennapedia homeodomain or TAT from human immunodeficiency virus are useful vectors for the delivery of protein antigens or their cytotoxic (Tc) or helper (Th) T cell epitopes to antigen-presenting cells. Mice immunized with CPP containing immunogens elicit antigen-specific Tc and/or Th responses and could be protected from tumor challenges. In the present paper, we investigate the mechanism of class I and class II antigen presentation of ovalbumin covalently linked to penetratin (AntpOVA) by bone marrow-derived dendritic cells with the use of biochemical inhibitors of various pathways of antigen processing and presentation. Results from our study suggested that uptake of AntpOVA is via a combination of energy-independent (membrane fusion) and energy-dependent pathways (endocytosis). Once internalized by either mechanism, multiple tap-dependent or independent antigen presentation pathways are accessed while not completely dependent on proteasomal processing but involving proteolytic trimming in the ER and Golgi compartments. Our study provides an understanding on the mechanism of antigen presentation mediated by CPP and leads to greater insights into future development of vaccine formulations

Cellular and humoral responses following different liposome + OVA formulations.

<p>The DDA:TDB+OVA formulations indicated along the X-axis were given to C57/BL6 mice as three homologous injections, two weeks apart. Splenocytes were isolated at the final timepoint (14 weeks) and responses to a CD8 OVA epitope (A) and two CD4 epitopes (B, C) were assessed by <i>ex vivo</i> IFNγ ELISpot. D) Total IgG antibody responses to whole OVA protein at the final time-point, as measured by an end-point titre ELISA. *p<0.05, ** p<0.01.</p

Liposome specifications of the DDA:TDB formulations containing OVA and TLR agonists.

<p>Liposome specifications of the DDA:TDB formulations containing OVA and TLR agonists.</p

Cellular and humoral responses to liposome-adjuvanted viral vectored vaccines.

<p>T cell and antibody responses were assessed in Balb/c mice at peak time-points for Adenovirus (Ad) and Modified Vaccinia Ankara (MVA) viral vectored vaccines expressing TIPeGFP, i.e. two weeks after a single injection of Ad and one week after two injections of MVA, combined with cationic DDA:TDB or neutral DSPC/TDB liposomes. Spleen T cell responses to two CD8 (A, B) and one CD4 epitope (C), contained within the insert of the viral constructs, were measured using IFNγ ELISpot. D) Total IgG titre in the serum of mice immunised with the Ad+liposome formulations at the peak time-point. **p<0.01.</p

Size distribution (bars) and Z-potential (points) of DDA:TDB liposomal formulations with OVA±pIC/CpG.

<p>Size distribution (bars) and Z-potential (points) of DDA:TDB liposomal formulations with OVA±pIC/CpG.</p