Comparison of Antiviral Immune Responses in Healthy Cats Induced by Two Immune Therapeutics

Background: Effective immunotherapeutic agents for use in cats are needed to aid in the management of intractable viral diseases, including feline infectious peritonitis (FIP) infection. The objectives of this study were to compare two different immune stimulants for antiviral activity in cats: (1) TLR 2/6-activating compound polyprenyl immunostimulant; (PI) and (2) liposome Toll-like receptor 3/9 agonist complexes (LTCs) to determine relative abilities to stimulate the induction of type I (IFN-α, IFN-β) and type II (IFN-γ) interferon immune responses in vitro and to study the effects of treatment on immune responses in healthy cats.

Methods: Cytokine and cellular immune responses to PI and LTC were evaluated using peripheral blood mononuclear cells (PBMCs) from healthy cats incubated with LTC and PI at indicated concentrations using reverse transcriptase polymerase chain reaction assays and ELISA assays. The effects of the immune stimulants on inhibiting FIPV replication were assessed using a feline macrophage cell line (fcwf-4). Cytokine and cellular immune responses to PI and LTC were evaluated in blood samples from healthy cats treated with PI and LTC, using reverse transcriptase polymerase chain reaction (RT-PCR) and ELISA assays.

Results: In the in vitro studies, both compounds triggered the upregulated expression of IFN-α, IFN-γ, and IL-1β genes in cat PBMC, whereas treatment with LTC induced significantly greater expression of IFN-α and IFN-γ on Day 1 and IL-1b on Day 3. There was significant protection from FIPV-induced cytopathic effects when fcwf-4 cells were treated with conditioned medium from LTC-activated leukocytes. In the healthy cat study (in vivo), both PI and LTC increased the mRNA signal for IFN-α, IFN-γ, and IL-1β above baseline at multiple time points with statistically greater increases in the LTC group on either Day 1 (IFN-α, IFN-γ) or Day 3 (IL-1β). In addition, RANTES increased over time in cats treated with the LTC.

Conclusions: Both LTC and PI protocols induced immune-enhancing effects, suggesting a possible clinical use for the management of chronic infectious diseases like FIP. Activating the TLR 3 and 9 pathways (LTC) induced superior broad interferon production in vitro than the activation of the TLR 2 and 6 pathways (PI).