Evaluation of the microparticle as an adjuvant or coadjuvant in a tumor vaccine

Tumors are usually poor stimulators of immune responses and actually appear to be able to induce tolerance. Tumor antigens are being increasingly identified that react with and stimulate T cell responses. A major goal of this research in cancer vaccines is to target tumor antigens to appropriate bone marrow derived antigen presenting cells using albumin microparticles encapsulated with a shed mixture of tumor derived antigens. The microparticle can function as an adjuvant by presenting the antigens in a particulate form which are more readily taken up by professional APCs, such as dendritic cells and macrophages. The microparticle also allows the antigens to persist at the subcutaneous injection site, forming a depot, as opposed to being washed away quickly when injected as soluble antigens. In addition, microparticles allow the simultaneous packaging of other molecules that can be advantageous in inducing an immune response at the level of antigen processing and presentation. The main objective of this research project is to develop biodegradable non-toxic microparticles containing tumor antigens alone and in combination with the immunostimulatory molecules B7-1 and ICAM-1. A novel method of attaching immunostimulatory molecules to the surface of albumin microparticles was developed. The immunostimulatory molecule was attached to the microparticle through an interaction of the surface of the microparticle and the GPI-anchor of the immunostimulatory protein. The ability of the microparticle to function as an adjuvant and coadjuvant (in the presence of the immunostimulatory molecules) in a tumor vaccine and induce tumor immunity was evaluated in vitro and in vivo, using a murine melanoma model. Shed tumor antigens were derived from the B16 murine melanoma. These antigens were encapsulated and compared with the extracellular antigens in solution. 80% of syngeneic mice remained tumor free at 60 days post tumor challenge, as opposed to 100% tumor incidence in the group receiving antigens in solution, however all mice had developed tumors by day 80. When the GPI-anchored immunostimulatory molecule was attached through a novel method, the best results were seen with ICAM-1 on the vaccine preparation (60% tumor free at 100 days). In light of the lack of immunogenicity of tumors there is much work done today in the area of vaccine adjuvants. The albumin microparticle allows for the unique attachment of immunostimulatory molecules to its surface. Combined with its particulate mode of delivery to phagocytic APCs, this coinciding immunostimulation is worth investigating further.