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Abstract
Huge productivity loss due to infectious diseases in chickens is a major problem and, hence, robust development of the poultry industry requires control of poultry health. Immunization using vaccines is routine practice; however, to combat infectious diseases, conventional vaccines as well as new-generation recombinant vaccines alone, due to relatively weak immunogenicity, may not be effective enough to provide optimum immunity. With this in mind, there is a need to incorporate better and more suitable adjuvants in the vaccines to elicit the elevated immune response in the host. Over last few decades, with the increase in the knowledge of innate immune functioning, efforts have been made to enhance vaccine potency using novel adjuvants like Toll-like receptor based adjuvant systems. In this review, we will discuss the potential use of toll-like receptor ligands as an adjuvant in vaccines against the infectious diseases of chickens.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
Key issues
For the successful development of the poultry industry, efficacious vaccines are needed to fight against the infectious diseases of chicken.
Conventional vaccines, including live and killed vaccines, suffer from many disadvantages. New-generation recombinant and often inactivated vaccines require assistance in the form of adjuvants.
Toll-like receptor (TLR) ligands have emerged as potential adjuvants for the use in vaccines against the infectious diseases. TLR adjuvants can induce both humoral and cell-mediated immune responses required to combat with deadly infectious agents.
TLR ligands boost up the host’s immune response against the infectious diseases by upregulating the various cytokines, interferons and other molecules or processes involved in the development of an immune response.
Many chicken infectious diseases are zoonotic in nature and pose a threat to public health apart from affecting chickens and, hence, the commercial poultry industry.
TLR ligands such as flagellin, CpG-ODN, poly I:C and loxoribine have successfully been used as adjuvants with the inactivated influenza vaccines as well as with other viral vaccines.
DNA vaccination with TLR ligand as an adjuvant has also been tried against the IBD in chicken and against avian influenza in mice and ferrets. Recombinant fusion protein containing protective antigen fused to flagellin has also shown success in mice and non-human primates.
TLR ligands have shown promising results as adjuvants against bacterial infectious diseases such as Salmonella, Campylobacter and Escherichia coli.
In ovo delivery of these ligands has shown great results, thus reducing the stress and discomfort to birds and inducing early immunity against the infectious diseases when the birds are very vulnerable to diseases.
TLR ligands have shown tremendous potential as an adjuvant against parasitic infections as well. CpG-ODN and flagellin fusion protein with recombinant parasite antigens have induced augmented immunity against the infections.
