(545g) Synthesis and Characterization of Chemically Modified Immunostimulatory Polysaccharide Serving Dual Function As Adjuvant and Protein Antigen Delivery Vehicle

Aluminum salt-based (alum) adjuvants are currently the only Food and Drug Administration (FDA)-approved adjuvants for subunit vaccines.  However, alum-based formulations have several drawbacks.  Antigens with certain surface charges and hydrophobicity are required to bind to alum, which limits its applicability, and protein adsorption to alum can lead to undesired protein denaturation.  Alum typically results in the production of antibodies against the antigen and polarization towards a cluster of differentiation 4 (CD4+) Type 2 helper T cell (Th2) response, whereas a more balanced response that also includes a CD8+ and Type 1 helper T cell (Th1) response is desired.  Effective vaccines should create long-term adaptive immunity against the target pathogen through a combined humoral and cell-mediated response that incorporates all of these responses.  This multifaceted response is preferred because many viruses and bacteria exist intracellularly, rendering an antibody (i.e. humoral) response unable to fully prevent or clear an infection.  Furthermore, long-term storage of antigens often requires certain cold-chain conditions, but alum is not structurally stable enough to be easily frozen or lyophilized.  Structural damage may disrupt its adjuvanticity and therefore limits the long-term storage options of alum-containing vaccines.  Other adjuvants that elicit a Th1 response have been considered but are often too cytotoxic to use clinically (e.g. Freund’s complete adjuvant and cholera toxin).  An alternative approach to using alum or toxic Th1 adjuvants is an immunostimulatory polysaccharide.  Here, we have chemically modified a naturally occurring polysaccharide, which has recognized immunostimulatory properties, that fulfills a dual function as both the adjuvant and acid-sensitive delivery vehicle.  The vehicle has been prepared to passively target antigen presenting cells, and in vivo immunization with an encapsulated model antigen has led to the production of antibody levels comparable to an alum-based formulation.  Moreover, the polysaccharide has previously illustrated cryoprotectant properties, which could facilitate extended storage outside the cold-chain.  Our work demonstrates the potential for this modified immunostimulatory polysaccharide to serve a dual function as a subunit vaccine adjuvant and antigen delivery vehicle.