To test the antiviral activity of γ-PGA in vivo, MNV-1 (1 × 107 PFU per mouse) was orally inoculated into BALB/c mice that received γ-PGA (50 mg/kg body weight) prior to infection once daily for five days to induce IFN-β expression. Two days after MNV inoculation, viral load in the ileum with PPs and mesenteric lymph nodes (MLNs) were quantified using RT-qPCR and plaque-forming assay. γ-PGA administration resulted in 47% and 53% reduction of MNV-1 genome titer in the ileum with PPs and MLNs, respectively (Fig. 5a). Plaque-formation assay revealed 36% and 33% reduction of MNV titer in the ileum with PPs and MLNs, respectively (Fig. 5b). These results indicate that peroral administration of 2,000-kDa γ-PGA effectively suppresses MNV propagation in mouse ileum and MLNs, indicating its prophylactic antiviral efficacy in vivo.Figure 5γ-PGA inhibits MNV-1 in mice. BALB/c mice (6 weeks old, n = 7–8 per group) were administered with 2,000-kDa γ-PGA as described in Fig. 4 for 5 days. Twelve hours after the last γ-PGA administration, mice were inoculated with MNV-1 (1 × 107 PFU in 100 µl saline) perorally and given γ-PGA once daily for another two days. Viral genome and infectious virus titers in the ileum with PPs and the MLNs were determined by RT-qPCR (a) and plaque-forming assay (b), respectively. In all panels, data are mean ± s.d. *P < 0.05 by two-tailed unpaired Student’s t-test.