Characterization of antigens adsorbed to anionic PLG microparticles by XPS and TOF-SIMS

The chemical composition of the surface of anionic PLG microparticles before and after adsorption of vaccine antigens was measured using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The interfacial distributions of components will reflect underlying interactions that govern properties such as adsorption, release, and stability of proteins in microparticle vaccine delivery systems. Poly(lactide-co-glycolide) microparticles were prepared by a w/o/w emulsification method in the presence of the anionic surfactant dioctyl sodium sulfosuccinate (DSS). Ovalbumin, lysozyme, a recombinant HIV envelope glyocoprotein and a Neisseria meningitidis B protein were adsorbed to the PLG microparticles, with XPS and time-of-flight secondary mass used to analyze elemental and molecular distributions of components of the surface of lyophilized products. Protein (antigen) binding to PLG microparticles was measured directly by distinct elemental and molecular spectroscopic signatures consistent with amino acids and excipient species. The surface sensitive composition of proteins also included counter ions that support the importance of electrostatic interactions being crucial in the mechanism of adsorptions. The protein binding capacity was consistent with the available surface area and the interpretation of previous electron and atomic force microscope images strengthened by the quantification possible by XPS and the qualitative identification possible with TOF-SIMS. Protein antigens were detected and quantified on the surface of anionic PLG microparticles with varying degrees of efficiency under different adsorption conditions such as surfactant level, pH, and ionic strength. Observable changes in elemental and molecular composition suggest an efficient electrostatic interaction creating a composite surface layer that mediates antigen binding and release.