We use atomic force microscopy (AFM) to investigate the nanomechanical properties of virus-like particles (VLPs) assembled from Acinetobacter phage protein AP205 (forming a scaffold) conjugated with B cell epitopes for surface glycoprotein of SARS-CoV-2 virus. The scaffold is found to have a stiffness and diameter equal to about 100 pN/nm and 20 nm respectively from direct force measurements in PeakForce Tapping mode with peak forces below 400 pN (Fig. 1a-c). Small angle X-ray scattering reveals an undeformed diameter of 26.2 nm and protein shell thickness of 3.0 nm which together with the stiffness give an estimation for the elastic modulus equal to 65 MPa in agreement with other virus capsids [1]. Addition of B cell epitope amino acids (up to about 25% in weight as compared to the molecular weight of AP205) to the surface of scaffold is found to increase the diameter by about 5 nm but this leads to only a slight increase in the stiffness. When the epitope is repeated one more time on the surface of the scaffold, the diameter is found to increase by an additional 5 nm, and in this case, a rise in the stiffness by about two times is observed. Furthermore, some particles were found to be dumbbell-like showing that the assembly from peptide-conjugated AP205 may deviate from a spherical particle if the peptide is long. Preliminary in vivo investigations were performed in mice and the results showed variations among these particles in antibody response. We aim to find the relations between physical properties including size, shape, stiffness and modulus and immunogenicity leading to optimization of the former properties in order to increase the immune response induced by these VLPs.

Figure 1: Schematic of atomic force microscopy investigations of virus like particles (a) leading to the evaluation of height profile from imaging (b) and stiffness  from direct measurements of force-indentation (F - δ) curves (c) at single particle level.

[1] Andreas Fery, Richard Weinkamer, Polymer, 2007, 48, 7221–7235.