Adjuvant combination enhances immune response of equine influenza virus vaccine in horses.

Background: Despite routine vaccination, equine influenza outbreaks occur due to disparities of virus strains between vaccine and field, and immunity gaps between vaccinations.

Objectives: To evaluate whether a combination of adjuvants can enhance immune responses to equine influenza virus (EIV) vaccines.

Study design: In vivo experimental study.

Methods: Eight mixed-breed horses were immunised intramuscularly with a recombinant canarypox-based EIV vaccine, either alone or in combination with monophosphoryl lipid A (MPL) and polyinosinic-polycytidylic acid (poly I:C). A single vaccine dose was administered, and serum samples were collected before immunisation and at 2, 8, and 24 weeks post-immunisation to assess the humoral immune response. Peripheral blood mononuclear cells (PBMCs) were isolated to evaluate immunomodulatory cytokine expression and characterise recall cellular immune responses. Additionally, bone marrow and lymph node cells were analysed to assess memory cell responses.

Results: The combination of MPL and poly I:C significantly enhanced serum IgG antibody concentrations and haemagglutination inhibition (HI) titres against H3N8 A/equine/Kyonggi/SA01/2011 (Florida sub-lineage clade 1) and maintained these for up to 24 weeks post-immunisation. This combination led to increased cytokine mRNA expression and interferon-gamma (IFN-γ) responses in PBMCs. Enhanced memory B cell responses were also observed in the bone marrow and lymph nodes at 8 weeks post-immunisation. MPL and poly I:C induced higher frequencies of CD4⁺ and CD8⁺ T cells following in vitro restimulation with H3N8 EIV.

Main limitations: A viral challenge experiment was not included.

Conclusions: The use of MPL and poly I:C as adjuvants in EIV vaccines significantly improved immune responses by enhancing humoral immunity, memory cell responses, and antigen-specific cellular immunity.