Development and Characterization of Novel Self-Encapsulating Poly(lactic-co-glycolic acid) Microspheres for Vaccine Delivery.

Abstract

Microspheres composed of poly(lactic-co-glycolic acid) (PLGA) have been a major area of interest for vaccine delivery due to their ability for (1) controlled release of antigen, which may enable a decrease in the number of doses required for protective immunity, and (2) enhancing the immune response against poorly immunogenic antigens. One disadvantage to this delivery system is that antigens encapsulated within PLGA microspheres by traditional techniques are susceptible to instability due to harsh processing conditions, including shear stress and exposure to organic solvent. Our lab has developed a novel method, termed “self-encapsulation,” for the remote loading of large molecules into pre-made PLGA microspheres. These microspheres contain a protein-trapping agent and interconnecting pore network. Simple mixing of the microspheres in an aqueous solution of the antigen enables the protein to diffuse into the polymer pores and bind to the trapping agent. Subsequent heating of the system above the glass transition temperature (Tg) of the polymer causes the pores to close, sealing the protein inside the microspheres. This project further expands upon the self-encapsulation approach by exploring its application to vaccine delivery. A formulation of self-encapsulating microspheres was developed for internalization by antigen-presenting cells (APCs), key cells for antigen processing and adaptive immunity. Using ovalbumin as a model antigen, formulation characteristics, including self-encapsulation ability and protein release kinetics, were defined. Stability of encapsulated protein was investigated and maintenance of antigenicity during release was confirmed. In vitro studies showed successful internalization of the microspheres by murine APCs. To examine the type and magnitude of the immune response induced by the microspheres, C57BL/6 mice were immunized with the formulation by intranasal and subcutaneous administration. Intranasal delivery of the microspheres resulted in a Th2-skewed response in serum and local mucosa. In comparison, subcutaneous delivery of the formulation resulted in a significant ovalbumin-specific cellular immune response and serum IgG subclass titers associated with a mixed Th1/Th2 response. Additionally, the microspheres showed promising ability for single-time injection, which could improve convenience and patient compliance. Due to the results of our studies, this formulation of self-encapsulating microspheres was concluded to have potential for further vaccine development.