Immune factors produced by PBMCs upon stimulation with lactobacillus delbrueckii ssp. bulgaricus OLL1073R-1-derived exopolysaccharides inhibit HCoV-229E and SARS-CoV-2 replication.

The COVID-19 pandemic entered a controllable phase due to vaccine and drug development. The development of defense strategies of enhancing host innate immunity remains imperative to counter the emergence of infectious SARS-CoV-2 variants. In this study, we examined the inhibitory effects of Lactobacillus delbrueckii ssp. bulgaricus OLL1073R-1 (OLL1073R-1)-derived exopolysaccharides (R-1 EPS), a natural immunomodulator, against coronaviral infection. The inhibitory effect of R-1 EPS on HCoV-229E was evaluated first and subsequently extended to SARS-CoV-2. Cell culture supernatant was obtained by stimulating human peripheral blood mononuclear cells with R-1 EPS (R-1 sup), and the inhibitory effect of R-1 sup on coronaviruses was evaluated using the human lung cell line, MRC5. High levels of proinflammatory cytokines were detected in R-1 sup, and replication of both HCoV-229E and SARS-CoV-2 was significantly suppressed in MRC5 cells pretreated with R-1 sup. Transcriptomic analysis revealed that type I IFN signaling was upregulated in R-1 sup-treated MRC5 cells. Our results suggest that immune factors, including proinflammatory cytokines, in R-1 sup, enhance type I IFN response, resulting in an inhibitory effect on both HCoV-229E and SARA-CoV-2. Although this study is an in vitro investigation and requires further in vivo validation, the findings provide a scientific basis for the application of OLL1073R-1 and R-1 EPS as preventive strategies against HCoV-229E and SARS-CoV-2.