Chitosan particle-emulsion complex adjuvants: The effect of particle distribution on the immune intensity and response type

Particle-emulsion complex adjuvants as a new trend in the research of vaccine formulation, can improve the immune strength and balance the immune type. However, the location of the particle in the formulation is a key factor that has not been investigated extensively and its type of immunity. In order to investigate the effect of different combining modes of emulsion and particle on the immune response, three types of particle-emulsion complex adjuvant formulations were designed with the combination of chitosan nanoparticles (CNP) and an o/w emulsion with squalene as the oil phase. The complex adjuvants included the CNP-I group (particle inside the emulsion droplet), CNP-S group (particle on the surface of emulsion droplet) and CNP-O group (particle outside the emulsion droplet), respectively. The formulations with different particle locations behaved with different immunoprotective effects and immune-enhancing mechanisms. Compared with CNP-O, CNP-I and CNP-S significantly improve humoral and cellular immunity. CNP-O was more like two independent systems for immune enhancement. As a result, CNP-S triggered a Th1-type immune bias and CNP-I had more of a Th2-type of the immune response. These data highlight the key influence of the subtle difference of particle location in the droplets for immune response.