Feline herpesvirus-1 (FHV-1) is an important primary viral pathogen of cats worldwide. The clinical signs in cats include upper respiratory and ocular disease. As with all alphaherpesviruses, induction of lifelong latency with periodic reactivation is also an important part of the epidemiology of this disease. Commercial vaccines for FHV-1 only reduce clinical signs but don't prevent virus shedding and the subsequent establishment of latency. This presents an obstacle for controlling disease especially in animal shelters where infected cats, even when vaccinated, will keep spreading the virus to most of the unvaccinated population. Such circumstances lead to the recurrence of the disease and significant loss of neonatal and unvaccinated cats. Thus, there is a strong demand to develop a next-generation vaccine, which provides more efficient and effective protection, both against clinical signs and infection itself. It is hypothesized that the insufficient effectiveness of current FHV-1 vaccines is due to the fact that current vaccines contain the complete set of virulence genes and can not be safely administered mucosally, where induction of immunity is critical for protection. In fact, the genome sequences of vaccine strains are almost identical to those of clinical isolates. Moreover, several virulence factors of alphaherpesviruses, including glycoprotein C (gC), glycoprotein E (gE), and Us3-encoded serine/threonine protein kinase (PK), also have immune modulatory function that prevent induction of strong host immunity. Studies involving other alphaherpesviruses, such as pseudorabies virus, bovine herpesvirus-1, and equine herpesvirus-1, have demonstrated that experimental immunization with mutants in which virulence-associated or immune modulatory genes were deleted could induce immune responses that protect the host from wild-type virus infection. This data led us to explore the use of deletion mutants of FHV-1 to achieve the same goal. Four FHV-1 deletion mutants, including a glycoprotein C gene-deleted mutant (gC-), a glycoprotein E gene-deleted mutant (gE-), a Us3-encoded serine/threonine protein kinase gene-deleted mutant (PK-), as well as a mutant with deletions of both the gE and thymidine kinase (TK) genes (gE-TK-), were constructed previously in our lab by bacterial artificial chromosome (BAC) mutagenesis. The research described in this thesis is composed of 1) an in vitro study with these mutants in a primary feline respiratory cell culture (FREC) model, 2) an ex vivo study performed using a feline tracheal tissue explant culture system, and 3) an in vivo study. Together, these studies provide comprehensive data regarding safety, induction of innate and adaptive immune responses and efficacy of these deletion mutants and will guide moving forward with the most promising candidate for vaccine development.
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