Adjuvant Effect of Mesoporous Silica SBA-15 of Different Morphologies on Antidiphtheria Immune Response

Ordered mesoporous silica SBA-15 improves the humoral response as a vaccine adjuvant because of its structural properties. Its morphology is dependent on synthesis conditions and can alter antigen encapsulation and immune response; all tested variants were safe and able to immunize against diphtheria. One of the additional advantages of SBA-15 is that its morphology can be modulated by adjusting the synthesis conditions like temperature, stirring speed, and solvent addition. In this study, SBA-15 was selected as a vaccine adjuvant in immunization against diphtheria by varying four modifications of the synthesis parameters for preparing SBA-15 particles with different morphologies. SEM analyses confirmed that different morphologies were obtained including rope-shaped aggregated rods (S1), filiform rods (S2), hexagons (S3), and nanospheres (S4). All synthesized SBA-15 samples presented an ordered mesoporous structure, with the characteristic reflections of a two-dimensional hexagonal structure and lattice parameter values with small differences (a(hkl) = ∼11 nm at 12), indicating that the silica mesostructure was preserved after incorporation from diphtheria anatoxin (dANA). The SAXS and NAI results indicate that mainly in samples S1 and S4, the dANA is encapsulated in the mesopores as well as in the SBA-15 macropores. Fluorescence analyses revealed the preservation of the aromatic microenvironment of tryptophan, similar to pure protein, except for sample S3, which showed a shift in emission wavelengths to 356 and 372 nm, indicating exposure of tryptophan to the more polar microenvironment. SRCD analyses confirmed the maintenance of dANA's secondary structure in all samples. In the immunogenic assay, the S3dANA sample stood out, presenting a significantly higher primary immune response. However, the immunogenic responses increased and became equal in the secondary response without any variation between different silica morphologies. It is concluded that all SBA-15-based adjuvants with different morphologies are biocompatible and present a good immunogenic response when they are applied as vaccine adjuvants.