Effects of Exogenous SARS-CoV-2 S1 Protein and mRNA Vaccines on Mixed Neuronal-Glial Cell Cultures.

Background and

Objectives

SARS-CoV-2 produces potentially pathogenic molecules, such as single-stranded RNA and spike proteins, which can potentially activate microglial cells. In this study, we aimed to investigate whether SARS-CoV-2 spike protein S1 and mRNA vaccines can cause neurotoxicity directly or through microglial involvement. Materials and

Methods

Primary cerebellar granule cell cultures isolated from Wistar rats and organotypic hippocampal slice cultures from transgenic C57BL/6J mice were used in the experiments. Imaging and quantitative analysis of cell viability, proliferation, and phagocytic activity were performed using light and fluorescence microscopy.

Results

The exogenous SARS-CoV-2 S1 protein at 50 µg/mL concentration induced neuronal cell death in neuronal-glial co-cultures and stimulated microglial proliferation during the first 3 days of exposure without an effect on inflammatory cytokine secretion. Single application of Tozinameran/Riltozinameran and Original/Omicron BA. 4-5 vaccines did not affect neuronal viability and total neuronal number in cell co-cultures after 7 days of exposure. In contrast, three repeated treatments with mRNA vaccines at 6 ng/mL caused microglial proliferation without affecting microglial phagocytosis and TNF-α release. In organotypic brain slice cultures, only Tozinameran/Riltozinameran stimulated microglial cell proliferation in female brain slices, while male brain slices remained unaffected by both vaccines, indicating sex-dependent effects.

Conclusions

The findings suggest that mRNA vaccines do not exert neurotoxic effects in primary neuronal-glial co-cultures, but induce microglial proliferation, particularly in female brains in the absence of inflammatory cytokine release. SARS-CoV-2 S1 protein at high concentrations directly induces neuronal death.