Previously, we described some structural features of spherical particles (SPs) generated by thermal remodelling of the tobacco mosaic virus. The SPs represent a universal platform that could bind various proteins. Here, we report that entire isometric virions of heterogeneous nature bind non-specifically to the SPs. Formaldehyde (FA) was used for covalent binding of a virus to the SPs surface for stabilizing the SP-virus complexes. Transmission and high resolution scanning electron microscopy showed that the SPs surface was covered with virus particles. The architecture of SP-virion complexes was examined by immunologic methods. Mean diameters of SPs and SP-human enterovirus C and SP-cauliflower mosaic virus (CaMV) compositions were determined by nanoparticle tracking analysis (NTA) in liquid. Significantly, neither free SPs nor individual virions were detected by NTA in either FA-crosslinked or FA-untreated compositions. Entirely, all virions were bound to the SPs surface and the SP sites within the SP-CaMV complexes were inaccessible for anti-SP antibodies. Likewise, the SPs immunogenicity within the FA-treated SPs-CaMV compositions was negligible. Apparently, the SP antigenic sites were hidden and masked by virions within the compositions. Previously, we reported that the SPs exhibited adjuvant activity when foreign proteins/epitopes were mixed with or crosslinked to SPs. We found that immunogenicity of entire CaMV crosslinked to SP was rather low which could be due to the above-mentioned masking of the SPs booster. Contrastingly, immunogenicity of the FA-untreated compositions increased significantly, presumably, due to partial release of virions and unmasking of some SPs-buster sites after animals immunization.
Keywords: immunogenic compositions; isometric viruses; platforms; spherical particles; tobacco mosaic virus.