Concomitant delivery of a CTL-restricted peptide antigen and CpG ODN by PLGA microparticles induces cellular immune response

Poly(lactide-co-glycolide) (PLGA) microparticles (MP) possess immunological adjuvant properties. Yet, exploitation of their full potential has just begun. The purpose of this study was to explore opportunities arising from surface modifications, and attachment and entrapment of combinations of antigen and a Toll-like receptor (TLR) ligand. The cytotoxic T lymphocyte (CTL)-restricted OVA ovalbumin peptide SIINFEKL was microencapsulated into bare, chitosan-coated, and protamine-coated PLGA MP using a microextrusion-assisted solvent extraction process. A TLR-ligand (CpG ODN) was either covalently coupled or physically adsorbed onto the MP surface. The peptide encapsulation efficiency decreased from 71% for uncoated particles to 62% and 45% upon coating with chitosan and protamine, respectively. CpG adsorption efficiency decreased from 93% for protamine-coated particles to 19% and 8% for chitosan and bare particles. Release of the adsorbed CpG was slow and incomplete (23% within 7 days) with the protamine coating, intermediate (>90% within 3 days) with the chitosan coating, and immediate (100% within 3 h) without coating. Interestingly, only the uncoated PLGA MP with adsorbed CpG mediated a prominent CTL response in mice at 6 days after immunization, as determined from IFN-gamma release from antigen-specific CD8(+) cells; failure of the other MP formulations was ascribed to the low release of antigen and CpG within the first week after immunization. The study illustrates novel opportunities for PLGA MP vaccines by combining antigens and immunostimulatory ligands.