Generation and immunological evaluation of SARS-CoV-2 membrane protein virus-like particles

The emergence of the SARS-CoV-2 pandemic led to the spread of highly transmissible variants, such as the Delta variant, which originated in India, underscoring the urgent need to develop new antivirals, therapeutics, and vaccines. In our previous study, we showed that Membrane-Envelope Virus-like Particles exhibit antigenicity and neutralization activity. Hence, our present study was conducted to evaluate whether the M protein alone can form VLPs that elicit an immune response.

Using computational methods, we identified key interacting residues in M-protein that contribute to VLP formation and interact with other structural proteins, including Spike (S), Nucleocapsid (N), and Envelope (E). The SARS-CoV-2-M protein was expressed in Sf-21 insect cells, and the resulting VLPs were purified, analyzed for shape and size, and characterized using DLS, FESEM, and TEM. The purified VLPs were injected into BALB/c mice to evaluate their immune response compared with uninfected controls.

The biophysical analysis confirms that the particles are round and have a size of ~ 180-200 nm. The serum levels of IgG, IgM, and IgA were found to be higher in immunized mice than in uninfected mice. Further qRT-PCR analysis demonstrated the levels of IFN-γ, IL-2, and IL-12, indicating a TH1-biased immune response against the M protein.

Our study demonstrates that the highly conserved M protein can self-assemble into VLPs and elicit humoral and cellular immune response. Furthermore, our study indicates that while M-protein VLPs elicit significant antibodies and cytokine responses, they do not induce detectable neutralizing activity when given alone.