Stimulation of Peripheral Blood Mononuclear Cells with Lactococcus lactis Strain Plasma Elicits Antiviral Effects Against H1N1 and SARS-CoV-2.

Viruses, like influenza and Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), remain major causes of upper respiratory tract infections worldwide, with symptoms ranging from asymptomatic to lethal outcomes. While antivirals and vaccines have helped ameliorate disease morbidity and mortality, these infections still pose significant challenges. Probiotics, including Lactococcus lactis strain plasma (LC-Plasma), have recently shown antiviral effects by activating plasmacytoid dendritic cells (pDCs), though their detailed mechanism remains unclear. In this study, we stimulated peripheral blood mononuclear cells (PBMCs) collected from healthy participants with LC-Plasma and conducted immunological analyses to investigate the immunomodulatory mechanisms of LC-Plasma. The supernatant derived from LC-Plasma-stimulated PBMCs (LCP Sup) exhibited dose-dependent inhibition of replication in Influenza A virus subtype H1N1 (H1N1) and SARS-CoV-2. LCP Sup significantly reduced the SARS-CoV-2 viral load in Huh-7 cells. However, in the H1N1 antiviral assay using A549 cells, LCP Sup was required at a higher concentration against H1N1 in A549 cells compared with SARS-CoV-2 in Huh-7 cells. Treatment with LCP Sup significantly upregulated interferon-stimulated genes (ISG) expression, particularly MxA, in A549 cells. While MxA showed the most notable increase, other ISGs also exhibited elevated expression levels compared with the negative control. Other cytokines, chemokines, and growth factors were also induced by LC-Plasma and CpG-DNA stimulation, and the effects of LC-Plasma were much higher than those of CpG-DNA. These results provide in vitro evidence of the antiviral mechanisms of LC-Plasma via upregulation of interferon-α (IFN-α) and related ISGs for host defense against respiratory viruses.