Current and Future Applications of Viral-Vectored Recombinant Vaccines in Poultry

Intro by Dr. Stephen Collett: Vectored vaccines have from their conception been promoted as the ultimate answer to the problems encountered when using immunization to reduce the impact of disease challenge on flock health and productivity. This interesting and thorough but succinct mini-review shows that the quest has unfortunately proven more complex than initially anticipated. Only recently have scientists been able to provide the field veterinarian and producer with a glimpse of what will likely become a standard and invaluable part of our disease management programs.

 

The poultry industry has witnessed a boom in the development and use of viral-vectored recombinant vaccines in recent years. These vaccines have been used with some success for the control of a number of poultry diseases. Recombinant vaccine development is a rapidly evolving field, as scientists learn more about pathogen virulence factors, host-pathogen interactions, host immune response mechanisms, and how best to harness the immune response to stimulate a protective immunity. In this article we spotlight the current commercially available and experimental viral-vectored vaccines, and discuss the advantages and disadvantages of these vaccines in the light of their published safety and efficacy. This article is not intended to be an exhaustive review of vectored vaccines; the objective is instead to provide a broad overview of our current knowledge of these vaccines.

 

Viral-vectored recombinant vaccines comprise viruses serving as vectors, into which foreign viral or bacterial genes coding for immunogenic antigens are integrated for expression by the vector. The immune response generated after infection of the host with these vectors targets both the vector and the expressed foreign antigen(s), and is thus bi- or multivalent. With a view to augmenting the immune response, some vectored vaccines also incorporate genes for the expression of immunostimulatory molecules (e.g. interferon-γ (IFN-γ) and interleukin-18 (IL-18)). Herpesvirus of turkey (HVT), Mareks Disease virus (MDV), Fowlpox virus (FPV), Newcastle Disease virus (NDV) and Adenovirus (AdV) have all been used as vectors into which selected genes of infectious laryngotracheitis virus (ILTV), NDV, infectious bronchitis virus (IBV), avian influenza virus (AIV), infectious bursal disease virus (IBDV) and Mycoplasma gallisepticum (MG) have been inserted to generate recombinant viral-vectored vaccines.

 

The efficacy of vectored vaccines is dependent on a number of variables; including the viral vector used (latency, replication, immunogenicity), the gene(s) inserted and promoter(s) selected (immunogenicity, expression), the route of administration (in ovo, subcutaneous, eye-drop, etc.), the characteristics of the pathogen targeted (intracellular vs. extracellular, systemic vs. local infection) and the nature of the immune response stimulated (humoral vs. cell-mediated immunity). The relative efficacy of these vaccines is often difficult to deduce, because few studies have evaluated vectored vaccines alongside other vectored and live vaccines, and different bird types, challenge routes, dosages, strains and criteria for protection are common. Many studies have, for instance, evaluated the efficacy of vectored vaccines only in SPF chickens, which does not facilitate evaluation of the effect of maternally-derived antibody (MDA). In addition, several vaccines targeting respiratory pathogens (e.g. NDV) have been evaluated using only intramuscular challenge models, which may not accurately represent their efficacy at the level of the respiratory tract. There is currently no standardized protocol facilitating comparison of the efficacy of viral vectored vaccines for specific poultry pathogens. In the table shown below we present a list of current commercially available and experimental viral-vectored poultry vaccines.

 

A technological challenge for vectored vaccine development is the construction of a recombinant virus whose replication competency is not compromised by insertion of the foreign genetic material, since this would compromise the generation of protective immunity. The use of viral vectors with large DNA genomes (e.g. FPV and HVT) allows the insertion, by homologous recombination, of larger gene inserts or multiple genes; however, the careful selection of insertion sites is vital. Negative-sense RNA virus vectors like NDV have a more limited capacity for the insertion of genetic material, and require the use of reverse genetics technology for their development. The selection of a suitable promoter for optimal expression of the foreign gene(s) inserts is also an important determinant of vectored vaccine efficacy.

 

Understanding the pathogenesis of the target pathogen, including its tissue tropism, and the nature of protective immunity against that pathogen is crucial to determining the type of vector that would be most suitable for use in a vectored vaccine. The method and age of application of vectored vaccines is determined by the type of viral vector selected (e.g. HVT – in ovo / subcutaneous at 1 day of age). All commercially available vectored vaccines for poultry utilize the HVT (i.e. recombinant HVT-ILT (rHVTILT), rHVT-ND and rHVT-IBD) or FPV vectors (i.e. rFPV-ILT, rFPV-ND, rFPV-AI and rFPV-MG), and are registered for in ovo and/or parenteral (subcutaneous and/or wing-web) application.

 

Currently available vectored vaccines (which are applied by the in ovo or parenteral routes) tend to be more effective against systemically-invasive pathogens (e.g. IBDV, HPAIV), compared with pathogens which replicate primarily in the respiratory tract and do not typically invade systemically (e.g. MG, IBV, ILTV). A mucosally-applied respiratory virus such as lentogenic NDV, which replicates in the upper respiratory tract and stimulates local immune responses might be a suitable alternate vector for the expression of antigens of such non-invasive respiratory pathogens. In addition, vectored vaccines targeting viral pathogens with only one (or few) immunodominant antigens (e.g. IBDV) are likely to stimulate a more effective immune response than those targeting bacterial pathogens with complex and variable surface antigen profiles (e.g. MG), or viruses with complex host-pathogen interactions (e.g. ILTV). In general, currently available viral-vectored vaccines appear to provide good protection against AIV and IBDV challenge, moderate protection against NDV, ILTV and IBV challenge and limited protection against MG challenge. The ability of vectored vaccines to protect against viral shedding is often limited. This has been observed for vectored ILTV vaccines; vaccinated broilers show significant reductions in clinical signs, but not in virus shedding after ILTV challenge (Johnson et al., 2010; Vagnozzi et al., 2012).

 

HVT is the most commonly used vaccine vector, and is characterized by its ability to establish persistent infection in the host; this has facilitated the development of long-lasting immunity with HVT recombinant vaccines, e.g. rHVT-ND (Esaki et al., 2013) and rHVT-IBD (Tsukamoto et al., 2002). FPV vectored vaccines are also capable of stimulating long-lived immune responses, e.g. rFPV-AI (Swayne et al., 1997; Bublot et al., 2006). Several studies with rFPV and rHVT vectored vaccines have, however, indicated that the development of protective immunity may be delayed compared with that induced by live vaccines (Palya et al., 2012; Vagnozzi, et al., 2012; Esaki, et al., 2013).

 

The effect of circulating antibodies on the efficacy of viral-vectored vaccines is an important consideration; particularly for vaccines that are applied in the presence of MDA (passive immunity), or in flocks previously vaccinated with or exposed to the pathogen(s) (active immunity). Circulating vector- or insert -specific antibodies may interfere with the generation of immunity to either the vector, or the insert, or both (Morgan et al., 1993; Taylor et al., 1996; Sakaguchi et al., 1998; Sonoda et al., 2000; Sarfati- Mizrahi et al., 2010; Mesonero et al., 2011; Schroer et al., 2011). This highlights the importance of assessing vectored vaccine efficacy in chickens with antibodies against both the vector and the insert.

 

The efficacy of rFPV vaccines is significantly compromised by previous exposure to FPV (activeimmunity); this has been observed for rFPV-ND (Iritani et al., 1991) and rFPV-AI (Swayne et al., 2000a; Bublot, et al., 2006) vaccines. In general, FPV- and HVT-vectored vaccines are not significantly affected by thepresence of vector-targeted MDA (passive immunity), as immunity against the vector is primarily cellmediated. HVT-vectored vaccine efficacy is probably compromised to a lesser extent by the presence of MDA, due to the longer infection periods characteristic of HVT.

 

An inhibitory effect of MDA against the inserted antigen has been reported for both rFPV-ND (Taylor, et al., 1996) and rHVT-ND (Morgan, et al., 1993) vaccines; with somewhat reduced protection against NDV seen in chickens with ND MDA (Taylor, et al., 1996). The field performance of rHVT-IBD vaccines in the presence of high levels of insert -specific MDA suggests that MDA does not compromise the efficacy of this vaccine. This is not the case for live IBD vaccines, which are neutralized by MDA when applied at an early age (Goutebroze et al., 2003).

 

Other vectors that are being investigated for use, but are not yet commercially available, are NDV, MDV and AdV. These vectors have been used to deliver ILTV, NDV, IBV, IBDV and AIV antigens (see table below). Experimentally, NDV has shown great promise as a vector for respiratory vaccines targeting ILTV (Kanabagatte Basavarajappa et al., 2014; Zhao et al., 2014), IBV (Toro et al., 2014b) and AIV (Veits et al., 2006; Romer-Oberdorfer et al., 2008; Nayak et al., 2009; Sarfati-Mizrahi, et al., 2010; Schroer, et al., 2011; Lardinois et al., 2012). One important consideration for the use of NDV as a vector is its efficacy in the presence of both vector- and insert-specific MDAs. The efficacy of rNDV-AI vaccines was reduced when administered in the presence of ND and AI-specific MDAs; however, MDA interference was overcome by increasing the vaccine dose (Sarfati-Mizrahi, et al., 2010; Schroer, et al., 2011).

 

Serotype I MDV has been used as an experimental vector for NDV, IBV, AIV and IBDV. MDV is an attractive alternative vector for IBDV (VP2), as an rMDV-IBD vaccine provided good protection against both IBDV and very virulent (vv) MDV (Tsukamoto et al., 1999). The Serotype 5 replication defective AdV may be a safe alternative for the delivery of AIV antigens (Toro et al., 2007; Toro et al., 2008; Mesonero, et al., 2011), although high MDA levels against the insert (H5) interfered with an AdV-vectored antibody response (Mesonero, et al., 2011). The co-expression of immunostimulatory cytokines (IFN-γ, IL-6, IL-18 or granulocyte-macrophage colony-stimulating factor (GM-CSF) with foreign antigens by virus vectors has been reported to improve the efficacy of several experimental vaccines, including rFPVILT (Chen et al., 2011), rFPV-AI (Mingxiao et al., 2006; Qian et al., 2012), rFPV-IB (Y. F. Wang et al., 2009; Chen et al., 2010; Shi et al., 2011) and rAdV-IB (Zeshan et al., 2011).

 

The safety of viral-vectored vaccines is one of their most appealing characteristics. The viral vectors are characterized by a lack of transmission to non-vaccinated birds, are species-specific, replicate poorly (or not at all) in other hosts and do not revert to virulence (Bublot, et al., 2006). This is a major advantage over many live viral or bacterial vaccines, e.g. live ILTV vaccines (especially ILTV CEO vaccine), for which vaccine reversion to virulence and transmission have been reported (Neff et al., 2008; Oldoni et al., 2008; Chacon et al., 2009).

 

Another argument for the use of vectored vaccines is to facilitate the differentiation between infected and vaccinated animals (DIVA). This is particularly important for notifiable diseases like AI, to ensure that vaccinal immunity does not complicate disease detection and control. Since no AIV-specific nucleoprotein or matrix antibodies are induced by vectored AI vaccines expressing HA, vaccinated uninfected birds will be serologically negative on ELISA and AGP tests, facilitating DIVA strategies (Iqbal, 2012).

 

In ovo vaccination technology has facilitated the delivery of recombinant vaccines, which represents a significant saving in cost and time compared with manual parenteral application of inactivated vaccines. Currently registered viral-vectored vaccines do, however, require in ovo or parenteral administration, as opposed to most live viral and some bacterial vaccines, which can be mass applied by spray or in the drinking water. Vectored vaccines are also more costly to manufacture than conventional live virus vaccines, and are consequently more expensive. The use of respiratory viruses as vectors is likely to facilitate mass application of viral-vectored vaccines in the future, in a similar way to conventional attenuated live virus vaccines.

 

In conclusion, knowledge of the composition and construction of vectored vaccines, host-pathogen interactions and the generation of protective immunity is fundamental to understanding the efficacy and harnessing the success of these vaccines. This field of vaccinology is an exciting and rapidly evolving one, with important implications for the current and future control of poultry diseases.

 

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