Protection against influenza virus infection of mice fed Bifidobacterium breve YIT4064

Abstract

Mice fed Bifidobacterium breve YIT4064 and immunized orally with influenza virus were more strongly protected against influenza virus infection of the lower respiratory tract than ones immunized with influenza virus only. The number of mice with enhanced anti-influenza virus immunoglobulin G (IgG) in serum upon oral administration of B. breve YIT4064 and oral immunization with influenza virus was significantly greater than that upon oral immunization with influenza virus only. These findings demonstrated that the oral administration of B. breve YIT4064 increased anti-influenza virus IgG antibodies in serum and protected against influenza virus infection. The oral administration of B. breve YIT4064 may enhance antigen-specific IgG against various pathogenic antigens taken orally and induce protection against various virus infections.

FIG. 1

Protocol for feeding mice with B. breve YIT4064: B, start; ▾, oral immunization with PR8; , inoculation with PR8; , collection of sera; -➤, observation.

FIG. 2

Kinetics of survival rate after inoculation with 1 (A) or 10 (B) μl of a PR8 solution containing various doses into each nostril. Mice were inoculated with 1 μl of a PR8 solution into each nostril for infection of the upper respiratory tract (A) and with 10 μl of a PR8 solution into each nostril for infection of the lower respiratory tract (B). Virus dose group symbols: ⧫, 10^1.5 EID₅₀ of PR8 (n = 6); ○, 10^2.5 EID₅₀ of PR8 (n = 6); ●, 10^3.5 EID₅₀ of PR8 (n = 6); ▵, 10^4.5 EID₅₀ of PR8 (n = 6); ▴, 10^5.5 EID₅₀ of PR8 (n = 6).

FIG. 3

Differences in percentages of anti-PR8 IgG production in serum (A) and survival rates (B) based on the period between oral immunization and nasal infection or on the quantity of immunogen. Nine-week-old female BALB/c mice were immunized with a single dose of 10^5.9 EID₅₀ and with two doses of 10^4.9 EID₅₀ of PR8 at various weeks and at 6 and 2 weeks before nasal inoculation (10^3.5 EID₅₀ of PR8), respectively. The anti-PR8 IgG titer in serum was measured just before nasal inoculation of PR8. The survival rates were observed for 14 days after nasal inoculation of PR8.

FIG. 4

Protection against morbidity due to influenza virus infection in mice administered B. breve YIT4064 orally. Mice were not immunized (A) (groups 1 and 2), were immunized with a single dose of 10^5.9 EID₅₀ of PR8 at 6 (B) (groups 3 and 4), 8 (C) (groups 5 and 6), and 10 (D) (groups 7 and 8) weeks, or were immunized with two doses of 10^4.9 EID₅₀ of PR8 at 6 and 2 weeks (E) (groups 9 and 10) before nasal inoculation (10^3.5 EID₅₀ of PR8). Mice were not fed (○) or were fed (●) B. breve YIT4064 throughout these experiments. After nasal inoculation of PR8, the mice were observed for accumulated symptom rate for 14 to 21 days. and , P < 0.02 and P < 0.05, respectively, versus accumulated symptom rate in groups 9 and 7, respectively, by Fisher’s exact test.

FIG. 5

Protection against mortality due to influenza virus infection in mice administered B. breve YIT4064 orally. Oral immunization and nasal inoculation in each group were performed in the same manner as described in the legend to Fig. 4. After nasal injection of PR8, the mice were observed for survival for 14 to 21 days. , P < 0.05 versus survival rate in group 7 or 9, by Fisher’s exact test.

FIG. 6

Relationship between level of anti-PR8 IgG in serum and survival of mice after infection. The anti-PR8 IgG titers in sera of mice from various groups were measured just before nasal inoculation of PR8 (10^3.5 EID₅₀). The anti-PR8 IgG titers in dead and surviving mice are shown individually.