Antigen encoded by vaccine vectors derived from human adenovirus serotype 5 is preferentially presented to CD8+ T lymphocytes by the CD8α+ dendritic cell subset

Nanoparticle-Based Adjuvants and Delivery Systems for Modern Vaccines

Ever since the development of the first vaccine, vaccination has had the great impact on global health, leading to the decrease in the burden of numerous infectious diseases. However, there is a constant need to improve existing vaccines and develop new vaccination strategies and vaccine platforms that induce a broader immune response compared to traditional vaccines. Modern vaccines tend to rely on certain nanotechnology platforms but are still expected to be readily available and easy for large-scale manufacturing and to induce a durable immune response. In this review, we present an overview of the most promising nanoadjuvants and nanoparticulate delivery systems and discuss their benefits from tehchnological and immunological standpoints as well as their objective drawbacks and possible side effects. The presented nano alums, silica and clay nanoparticles, nanoemulsions, adenoviral-vectored systems, adeno-associated viral vectors, vesicular stomatitis viral vectors, lentiviral vectors, virus-like particles (including bacteriophage-based ones) and virosomes indicate that vaccine developers can now choose different adjuvants and/or delivery systems as per the requirement, specific to combatting different infectious diseases.

Evaluation of the immunogenicity of prime-boost vaccination with the replication-deficient viral vectored COVID-19 vaccine candidate ChAdOx1 nCoV-19

Clinical development of the COVID-19 vaccine candidate ChAdOx1 nCoV-19, a replication-deficient simian adenoviral vector expressing the full-length SARS-CoV-2 spike (S) protein was initiated in April 2020 following non-human primate studies using a single immunisation. Here, we compared the immunogenicity of one or two doses of ChAdOx1 nCoV-19 in both mice and pigs. Whilst a single dose induced antigen-specific antibody and T cells responses, a booster immunisation enhanced antibody responses, particularly in pigs, with a significant increase in SARS-CoV-2 neutralising titres.

The porcine trophoblast cell line PTr2 is susceptible to porcine reproductive and respiratory syndrome virus-2 infection

INTRODUCTION

Porcine reproductive and respiratory syndrome virus-2 (PRRSV-2) breaches the maternal-fetal interface (MFI) to infect porcine fetuses, yet the exact mechanism(s) of transmission is not understood. The objective of this study was to determine the susceptibility of porcine trophoblast cell line (PTr2) to PRRSV-2 infection to understand the potential role of the trophoblast in viral transmission to fetuses in vivo.

METHODS

PTr2 cells were exposed in vitro to PRRSV-2 and then subjected to immunofluorescence analysis (IF), flow cytometry (FCM), real-time quantitative PCR (RT-qPCR), transmission electron microscopy (TEM) and immunogold electron microscopy (IEM) to assess viral infection. The effects of PRRSV-2 on PTr2 cell cycle progression and apoptosis, as well as the ability of PTr2 cells to produce infectious viral particles were also examined.

RESULTS

PRRSV-2 was readily detected in PTr2 cells by IF, FCM, RT-qPCR, TEM and IEM techniques. RT-qPCR and FCM results of a time course of infection of PTr2 cells indicated PRRSV-2 load decreased over time after initial infection up to 72 h. PRRSV-2 infection altered PTr2 cell cycle with a selective increase of cells within the G2/M phase and also induced apoptosis. TEM and IEM demonstrated PRRSV-2 within and on the surface of PTr2 cells and PRRSV-2 virions released from PTr2 cells infected naïve MARC-145 cells inducing cytopathic effects.

DISCUSSION

Trophoblast cells are susceptible to PRRSV-2 infection and release live virions capable of inducing cytopathic effects in naïve cells. This suggests a possible mechanism by which PRRSV-2 can breach the MFI resulting in fetal infection and death.

Spatiotemporal immunofluorescent evaluation of porcine reproductive and respiratory syndrome virus transmission across the maternal-fetal interface

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes severe reproductive failure characterized by high fetal morbidity and mortality leading to substantial economic losses to the swine industry. Evaluation of spatiotemporal transmission of PRRSV at the maternal-fetal interface (MFI) is critical for understanding fetal infection. Localization of PRRSV-2 strain NVSL 97-7895 at different regions of the MFI in 20 pregnant gilts at 2, 5, 8, 12 and 14 days post-inoculation (dpi) were analyzed by immunofluorescence (IF). Samples of MFI were collected from 15 inoculated and 5 control gilts and transplacental PRRSV transmission assessed in randomly selected fetuses from each litter. Localization of NVSL 97-7895 antigen immunoreactivity in the MFI was focused in three major areas: endometrial connective tissues (ENDO), the feto-maternal junction (FMJ) and fetal placenta (PLC). NVSL 97-7895 was detected at the FMJ by 2 dpi. At 2, 5 and 8 dpi, NVSL 97-7895 was localized within the ENDO and FMJ, whereas at 12 and 14 dpi, it was mainly localized in the PLC. Using a novel IF strategy for counting and size sorting NVSL 97-7895 viral antigen in situ, results of this study indicate that non-cell-associated mechanisms are involved in PRRSV transmission across the MFI.

Classification of fetal resilience to porcine reproductive and respiratory syndrome (PRRS) based on temporal viral load in late gestation maternal tissues and fetuses

Although porcine reproductive and respiratory syndrome virus (PRRSV) readily crosses the maternal fetal interface (MFI) in third trimester, fetal resilience varies within litters. The aim of this study was to characterize PRRSV-2 concentration in MFI and fetuses at five time points after experimental inoculation of late gestation gilts and use this information to classify potentially resistant, resilient and susceptible fetuses. The secondary objective was to verify the relationship between PRRS viral load and intrauterine growth retardation (IUGR). Three PRRSV-inoculated pregnant gilts and 1 sham-inoculated control were euthanized at five time points in days post infection (DPI; 2, 5, 8, 12, 14). The preservation status of each fetus was determined and MFI samples adjacent to the umbilical stump of each fetus, as well as serum, thymus, umbilical cord and amniotic fluid were collected. Viral load was quantified using probe-based reverse-transcriptase quantitative PCR (RT-qPCR) targeting PRRSV NVSL 97-7895 ORF7. Our result show the MFI was largely PRRSV infected by 2 DPI and virus was first detected in fetal sera and umbilical cord by 5 DPI, and in fetal thymus and amniotic fluid by 8 DPI. This indicates that PRRSV-2 quickly crossed the placenta and traveled toward the fetus via umbilical circulation within one week of the dam's inoculation. Fetal compromise was first observed on 8 DPI and increased progressively through to 14 DPI. However, several factors were associated with fetal resilience. The random forest model identified that 'viral load in fetal thymus' and duration of infection ('DPI') as the most important factors predicting fetal resilience and resistance. Moreover, IUGR fetuses had lower viral load and were less frequently compromised or dead compared to non-IUGR and average cohorts. Understanding the mechanisms of fetal resilience to PRRSV will improve selection strategies for replacement gilts.

Capsid Engineering of Adenovirus Vectors: Overcoming Early Vector-Host Interactions for Therapy

Adenovirus-based vectors comprise the most frequently used vector type in clinical studies to date. Both intense lab research and insights from the clinical trials reveal the importance of a comprehensive understanding of vector-host interactions. Especially for systemic intravenous adenovirus vector delivery, it is paramount to develop safe and efficacious vectors. Very early vector-host interactions that take place in blood long before the first cell is being transduced are phenomena triggered by the surface, shape, and size of the adenovirus vector particles. Not surprisingly, a multitude of different technologies ranging from genetics to chemistry has been developed to alter the adenovirus vector surface. In this review, we discuss the most important technologies and evaluate them for their suitability to overcome hurdles imposed by early vector-host interactions.

Firewalls Prevent Systemic Dissemination of Vectors Derived from Human Adenovirus Type 5 and Suppress Production of Transgene-Encoded Antigen in a Murine Model of Oral Vaccination

To define the bottlenecks that restrict antigen expression after oral administration of viral-vectored vaccines, we tracked vectors derived from the human adenovirus type 5 at whole body, tissue, and cellular scales throughout the digestive tract in a murine model of oral delivery. After intragastric administration of vectors encoding firefly luciferase or a model antigen, detectable levels of transgene-encoded protein or mRNA were confined to the intestine, and restricted to delimited anatomical zones. Expression of luciferase in the form of multiple small bioluminescent foci in the distal ileum, cecum, and proximal colon suggested multiple crossing points. Many foci were unassociated with visible Peyer's patches, implying that transduced cells lay in proximity to villous rather than follicle-associated epithelium, as supported by detection of transgene-encoded antigen in villous epithelial cells. Transgene-encoded mRNA but not protein was readily detected in Peyer's patches, suggesting that post-transcriptional regulation of viral gene expression might limit expression of transgene-encoded antigen in this tissue. To characterize the pathways by which the vector crossed the intestinal epithelium and encountered sentinel cells, a fluorescent-labeled vector was administered to mice by the intragastric route or inoculated into ligated intestinal loops comprising a Peyer's patch. The vector adhered selectively to microfold cells in the follicle-associated epithelium, and, after translocation to the subepithelial dome region, was captured by phagocytes that expressed CD11c and lysozyme. In conclusion, although a large number of crossing events took place throughout the intestine within and without Peyer's patches, multiple firewalls prevented systemic dissemination of vector and suppressed production of transgene-encoded protein in Peyer's patches.

Vaccination with recombinant adenovirus expressing peste des petits ruminants virus-F or -H proteins elicits T cell responses to epitopes that arises during PPRV infection

Peste des petits ruminants virus (PPRV) causes an economically important disease that limits productivity in small domestic ruminants and often affects the livestock of the poorest populations in developing countries. Animals that survive PPRV develop strong cellular and humoral responses, which are probably necessary for protection. Vaccination should thus aim at mimicking these natural responses. Immunization strategies against this morbillivirus using recombinant adenoviruses expressing PPRV-F or -H proteins can protect PPRV-challenged animals and permit differentiation of infected from vaccinated animals. Little is known of the T cell repertoire these recombinant vaccines induce. In the present work, we identified several CD4⁺ and CD8⁺ T cell epitopes in sheep infected with PPRV. We also show that recombinant adenovirus vaccination induced T cell responses to the same epitopes, and led to memory T cell differentiation. T cells primed by these recombinant adenovirus vaccines expanded after PPRV challenge and probably contributed to protection. These data validate the use of recombinant adenovirus expressing PPRV genes as DIVA strategies to control this highly contagious disease.

Recall T cell responses to bluetongue virus produce a narrowing of the T cell repertoire

In most viral infections, recall T cell responses are critical for protection. The magnitude of these secondary responses can also affect the CD8 and CD4 epitope repertoire diversity. Bluetongue virus (BTV) infection in sheep elicits a T cell response that contributes to viremia control and could be relevant for cross-protection between BTV serotypes. Here, we characterized CD4⁺ and CD8⁺ T cell responses during primary and recall responses. During primary immune responses, both CD4⁺ and CD8⁺ T cell populations expanded by 14 days post-infection (dpi). CD4⁺ T cell populations showed a lower peak of expansion and prolonged contraction phase compared to CD8⁺ T cell populations. Recall responses to BTV challenge led to BTV-specific expansion and activation of CD8⁺ but not of CD4⁺ T cells. The evolution of the BTV-specific TCR repertoire was also characterized in response to VP7 peptide stimulation. Striking differences in repertoire development were noted over the time-course of infection. During primary responses, a broader repertoire was induced for MHC-I and MHC-II epitopes. However, during memory responses, a narrowed repertoire was activated towards a dominant motif in VP7 comprising amino acids 139–291. Monocytes were also examined, and expanded during acute infection resolution. In addition, pro-inflammatory cytokine levels increased after BTV inoculation and persisted throughout the experiment, indicative of a prolonged inflammatory state during BTV infections. These findings could have implications for vaccine design as the narrowing memory T cell repertoire induced after BTV re-infection could lead to the development of protective immunodominant TCR repertoires that differs between individual sheep.

Covalent decoration of adenovirus vector capsids with the carbohydrate epitope αGal does not improve vector immunogenicity, but allows to study the in vivo fate of adenovirus immunocomplexes

Adenovirus-based vectors are promising tools for genetic vaccination. However, several obstacles have to be overcome prior to a routine clinical application of adenovirus-based vectors as efficacious vectored vaccines. The linear trisaccharide epitope αGal (alpha-Gal) with the carbohydrate sequence galactose-α-1,3-galactosyl-β-1,4-N-acetylglucosamine has been described as a potent adjuvant for recombinant or attenuated vaccines. Humans and α-1,3-galactosyltransferase knockout mice do not express this epitope. Upon exposure of α-1,3-galactosyltransferase-deficient organisms to αGal in the environment, large amounts of circulating anti-Gal antibodies are produced consistently. Immunocomplexes formed between recombinant αGal-decorated vaccines and anti-Gal antibodies exhibit superior immunogenicity. We studied the effects of the trisaccharide epitope on CD8 T cell responses that are directed specifically to vector-encoded transgenic antigens. For that, covalently αGal-decorated adenovirus vectors were delivered to anti-Gal α-1,3-galactosyltransferase knockout mice. We generated replication-defective, E1-deleted adenovirus type 5 vectors that were decorated with αGal at the hexon hypervariable regions 1 or 5, at fiber knob, or at penton base. Surprisingly, none of the adenovirus immunocomplexes being formed from αGal-decorated adenovirus vectors and anti-Gal immunoglobulins improved the frequencies of CD8 T cell responses against the transgenic antigen ovalbumin. Humoral immunity directed to the adenovirus vector was neither increased. However, our data indicated that decoration of Ad vectors with the αGal epitope is a powerful tool to analyze the fate of adenovirus immunocomplexes in vivo.

Methods and clinical development of adenovirus-vectored vaccines against mucosal pathogens

Adenoviruses represent the most widely used viral-vectored platform for vaccine design, showing a great potential in the fight against intracellular infectious diseases to which either there is a lack of effective vaccines or the traditional vaccination strategy is suboptimal. The extensive understanding of the molecular biology of adenoviruses has made the new technologies and reagents available to efficient generation of adenoviral-vectored vaccines for both preclinical and clinical evaluation. The novel adenoviral vectors including nonhuman adenoviral vectors have emerged to be the further improved vectors for vaccine design. In this review, we discuss the latest adenoviral technologies and their utilization in vaccine development. We particularly focus on the application of adenoviral-vectored vaccines in mucosal immunization strategies against mucosal pathogens including Mycobacterium tuberculosis, flu virus, and human immunodeficiency virus.

Evaluation of a Fiber-Modified Adenovirus Vector Vaccine against Foot-and-Mouth Disease in Cattle

Novel vaccination approaches against foot-and-mouth disease (FMD) include the use of replication-defective human adenovirus type 5 (Ad5) vectors that contain the capsid-encoding regions of FMD virus (FMDV). Ad5 containing serotype A24 capsid sequences (Ad5.A24) has proved to be effective as a vaccine against FMD in livestock species. However, Ad5-vectored FMDV serotype O1 Campos vaccine (Ad5.O1C.2B) provides only partial protection of cattle against homologous challenge. It has been reported that a fiber-modified Ad5 vector expressing Arg-Gly-Asp (RGD) enhances transduction of antigen-presenting cells (APC) in mice. In the current study, we assessed the efficacy of a fiber-modified Ad5 (Adt.O1C.2B.RGD) in cattle. Expression of FMDV capsid proteins was superior in cultured cells infected with the RGD-modified vector. Furthermore, transgene expression of Adt.O1C.2B.RGD was enhanced in cell lines that constitutively express integrin αvβ6, a known receptor for FMDV. In contrast, capsid expression in cattle-derived enriched APC populations was not enhanced by infection with this vector. Our data showed that vaccination with the two vectors yielded similar levels of protection against FMD in cattle. Although none of the vaccinated animals had detectable viremia, FMDV RNA was detected in serum samples from animals with clinical signs. Interestingly, CD4(+) and CD8(+) gamma interferon (IFN-γ)(+) cell responses were detected at significantly higher levels in animals vaccinated with Adt.O1C.2B.RGD than in animals vaccinated with Ad5.O1C.2B. Our results suggest that inclusion of an RGD motif in the fiber of Ad5-vectored FMD vaccine improves transgene delivery and cell-mediated immunity but does not significantly enhance vaccine performance in cattle.

Immunogenicity of targeted lentivectors

To increase the safety and possibly efficacy of HIV-1 derived lentivectors (LVs) as an anti-cancer vaccine, we recently developed the Nanobody (Nb) display technology to target LVs to antigen presenting cells (APCs). In this study, we extend these data with exclusive targeting of LVs to conventional dendritic cells (DCs), which are believed to be the main cross-presenting APCs for the induction of a TH1-conducted antitumor immune response. The immunogenicity of these DC-subtype targeted LVs was compared to that of broad tropism, general APC-targeted and non-infectious LVs. Intranodal immunization with ovalbumin encoding LVs induced proliferation of antigen specific CD4+ T cells, irrespective of the LVs' targeting ability. However, the cytokine secretion profile of the restimulated CD4+ T cells demonstrated that general APC targeting induced a similar TH1-profile as the broad tropism LVs while transduction of conventional DCs alone induced a similar and less potent TH1 profile as the non-infectious LVs. This observation contradicts the hypothesis that conventional DCs are the most important APCs and suggests that the activation of other APCs is also meaningful. Despite these differences, all targeted LVs were able to stimulate cytotoxic T lymphocytes, be it to a lesser extent than broad tropism LVs. Furthermore this induction was shown to be dependent on type I interferon for the targeted and non-infectious LVs, but not for broad tropism LVs. Finally we demonstrated that the APC-targeted LVs were as potent in therapy as broad tropism LVs and as such deliver on their promise as safer and efficacious LV-based vaccines.

An adenoviral vector expressing lipoprotein A, a major antigen of Mycoplasma mycoides subspecies mycoides, elicits robust immune responses in mice

Contagious bovine pleuropneumonia (CBPP), caused by Mycoplasma mycoides subsp. mycoides small colony type (MmmSC), is a devastating respiratory disease of cattle. In sub-Saharan Africa, where CBPP is enzootic, live attenuated vaccines are deployed but afford only short-lived protection. In cattle, recovery from experimental MmmSC infection has been associated with the presence of CD4(+) T lymphocytes that secrete interferon gamma in response to MmmSC, and in particular to the lipoprotein A (LppA) antigen. In an effort to develop a better vaccine against CBPP, a viral vector (Ad5-LppA) that expressed LppA was generated from human adenovirus type 5. The LppA-specific immune responses elicited by the Ad5-LppA vector were evaluated in mice, and compared to those elicited by recombinant LppA formulated with a potent adjuvant. Notably, a single administration of Ad5-LppA, but not recombinant protein, sufficed to elicit a robust LppA-specific humoral response. After a booster administration, both vector and recombinant protein elicited strong LppA-specific humoral and cell-mediated responses. Ex vivo stimulation of splenocytes induced extensive proliferation of CD4(+) T cells for mice immunized with vector or protein, and secretion of T helper 1-associated and proinflammatory cytokines for mice immunized with Ad5-LppA. Our study - by demonstrating the potential of a viral-vectored prototypic vaccine to elicit prompt and robust immune responses against a major antigen of MmmSC - represents a first step in developing a recombinant vaccine against CBPP.

The influence of delivery vectors on HIV vaccine efficacy

Development of an effective HIV/AIDS vaccine remains a big challenge, largely due to the enormous HIV diversity which propels immune escape. Thus novel vaccine strategies are targeting multiple variants of conserved antibody and T cell epitopic regions which would incur a huge fitness cost to the virus in the event of mutational escape. Besides immunogen design, the delivery modality is critical for vaccine potency and efficacy, and should be carefully selected in order to not only maximize transgene expression, but to also enhance the immuno-stimulatory potential to activate innate and adaptive immune systems. To date, five HIV vaccine candidates have been evaluated for efficacy and protection from acquisition was only achieved in a small proportion of vaccinees in the RV144 study which used a canarypox vector for delivery. Conversely, in the STEP study (HVTN 502) where human adenovirus serotype 5 (Ad5) was used, strong immune responses were induced but vaccination was more associated with increased risk of HIV acquisition than protection in vaccinees with pre-existing Ad5 immunity. The possibility that pre-existing immunity to a highly promising delivery vector may alter the natural course of HIV to increase acquisition risk is quite worrisome and a huge setback for HIV vaccine development. Thus, HIV vaccine development efforts are now geared toward delivery platforms which attain superior immunogenicity while concurrently limiting potential catastrophic effects likely to arise from pre-existing immunity or vector-related immuno-modulation. However, it still remains unclear whether it is poor immunogenicity of HIV antigens or substandard immunological potency of the safer delivery vectors that has limited the success of HIV vaccines. This article discusses some of the promising delivery vectors to be harnessed for improved HIV vaccine efficacy.

Adenovirus-based vaccines for fighting infectious diseases and cancer: progress in the field

The field of adenovirology is undergoing rapid change in response to increasing appreciation of the potential advantages of adenoviruses as the basis for new vaccines and as vectors for gene and cancer therapy. Substantial knowledge and understanding of adenoviruses at a molecular level has made their manipulation for use as vaccines and therapeutics relatively straightforward in comparison with other viral vectors. In this review we summarize the structure and life cycle of the adenovirus and focus on the use of adenovirus-based vectors in vaccines against infectious diseases and cancers. Strategies to overcome the problem of preexisting antiadenovirus immunity, which can hamper the immunogenicity of adenovirus-based vaccines, are discussed. When armed with tumor-associated antigens, replication-deficient and oncolytic adenoviruses can efficiently activate an antitumor immune response. We present concepts on how to use adenoviruses as therapeutic cancer vaccines and consider some of the strategies used to further improve antitumor immune responses. Studies that explore the prospect of adenoviruses as vaccines against infectious diseases and cancer are underway, and here we give an overview of the latest developments.

Induction of alloimmune tolerance in heart transplantation through gene silencing of TLR adaptors

Toll-like receptors (TLRs) activate biochemical pathways that evoke activation of innate immunity, which leads to dendritic cell (DC) maturation and initiation of adaptive immune responses that provoke allograft rejection. We aimed to prolong allograft survival by selectively inhibiting expression of the common adaptors of TLR signaling, namely MyD88 and TRIF, using siRNA. In vitro we demonstrated that blocking expression of MyD88 and TRIF led to reduced DC maturation. In vivo treatment of recipients with MyD88 and TRIF siRNA significantly prolonged allograft survival in the BALB/c > C57BL6 cardiac transplant model. Moreover, the combination of MyD88 and TRIF siRNA along with a low dose of rapamycin further extended the allograft survival (88.8 ± 7.1 days). Tissue histopathology demonstrated an overall reduction in lymphocyte interstitium infiltration, vascular obstruction and hemorrhage in mice treated with MyD88 and TRIF siRNA vector plus rapamycin. Furthermore, treatment was associated with an increase in the numbers of CD4(+) CD25(+) FoxP3(+) regulatory T cells and Th2 deviation. To our knowledge, this study is the first demonstration of prolonging the survival of allogeneic heart grafts through gene silencing of TLR signaling adaptors, highlighting the therapeutic potential of siRNA in clinical transplantation.

Off-the-shelf adenoviral-mediated immunotherapy via bicistronic expression of tumor antigen and iMyD88/CD40 adjuvant

Recent modest successes in ex vivo dendritic cell (DC) immunotherapy have motivated continued innovation in the area of DC manipulation and activation. Although ex vivo vaccine approaches continue to be proving grounds for new DC manipulation techniques, the intrinsic limits of ex vivo therapy, including high cost, minimal standardization, cumbersome delivery, and poor accessibility, incentivizes the development of vaccines compatible with in vivo DC targeting. We describe here a method to co-deliver both tumor-specific antigen (TSA) and an iMyD88/CD40 adjuvant (iMC), to DCs that combines toll-like receptor (TLR) and CD40 signaling. In this study, we demonstrate that simple TSA delivery via adenoviral vectors results in strong antitumor immunity. Addition of iMC delivered in a separate vector is insufficient to enhance this effect. However, when delivered simultaneously with TSA in a single bicistronic vector (BV), iMC is able to significantly enhance antigen-specific cytotoxic T-cell (CTL) responses and inhibit established tumor growth. This study demonstrates the spatial-temporal importance of concurrent DC activation and TSA presentation. Further, it demonstrates the feasibility of in vivo molecular enhancement of DCs necessary for effective antitumor immune responses.