Adjuvants for swine vaccines: mechanisms of actions and adjuvant effects
This review deals with mechanisms of actions (MOA) and adjuvant effects of various types of adjuvants for swine vaccines. A number of different types of adjuvants have been tested with swine vaccines, including oil emulsion, particulate antigen (Ag) carrier, cytokines, pathogen-associated molecular patterns and immune ligands, saponins, and bacterial cells and toxins. In addition, there are a number of chemicals and natural products that possess adjuvant activities when tested with swine vaccines, and are grouped as miscellaneous in this review. The MOA of adjuvants can be generally divided into two categories: delivery vehicles and immunostimulants. Adjuvants serving as delivery vehicle can act as a depot, help deliver Ag to the draining lymph nodes, promote Ag uptake by antigen-presenting cells (APCs), and protect Ag from harsh conditions. Adjuvants possessing immunostimulatory activities may help recruit and activate APCs and T cells, enhance APC functions, and direct T cell differentiation and immunoglobulin isotype switching. Success of adjuvant use on improving immunogenicity and protective efficacy of swine vaccines depends on several factors, including type and stability of vaccine Ag, dose and schedule of vaccination, MOA of adjuvant, route, dose and schedule of adjuvant administration, type of immune response required, and safety from adverse reactions. In addition to the above mentioned factors, cost effectiveness is of concern. Further studies of swine vaccine adjuvants may need to focus on characterization of their MOA and search for more potential adjuvant candidates that can induce mucosal immune response.