Maternal immunization with pneumococcal surface protein A protects against pneumococcal infections among derived offspring

Abstract

Pathogen-specific antibody plays an important role in protection against pneumococcal carriage and infections. However, neonates and infants exhibit impaired innate and adaptive immune responses, which result in their high susceptibility to pneumococci. To protect neonates and infants against pneumococcal infection it is important to elicit specific protective immune responses at very young ages. In this study, we investigated the protective immunity against pneumococcal carriage, pneumonia, and sepsis induced by maternal immunization with pneumococcal surface protein A (PspA). Mother mice were intranasally immunized with recombinant PspA (rPspA) and cholera toxin B subunit (CTB) prior to being mated. Anti-PspA specific IgG, predominantly IgG1, was present at a high level in the serum and milk of immunized mothers and in the sera of their pups. The pneumococcal densities in washed nasal tissues and in lung homogenate were significantly reduced in pups delivered from and/or breast-fed by PspA-immunized mothers. Survival after fatal systemic infections with various types of pneumococci was significantly extended in the pups, which had received anti-PspA antibody via the placenta or through their milk. The current findings strongly suggest that maternal immunization with PspA is an attractive strategy against pneumococcal infections during early childhood.

Figure 1. Anti-PspA specific antibodies in sera and milk of mother mice.

Female mice were intranasally immunized twice each week with 1 µg of rPspA and 4 µg CTB for first 2 weeks and with 1 µg rPspA alone for the last week. The levels of anti-PspA specific IgG, IgA and IgM antibodies in sera (A) and breast milk (B) were determined by PspA-specific ELISA on day 0, 7 and 14 after the birth. The values shown are the mean ± S.E. concentrations (ng/ml) taken from PspA-immunized mother (n = 16) and sham-immunized mother (n = 14). The levels of anti-PspA specific antibodies in sera and breast milk from sham-immunized mice were below the limit of detection.

Figure 2. Anti-PspA specific IgG subclasses in sera and milk of mother mice.

Female mice were intranasally immunized twice each week with 1 µg of rPspA and 4 µg CTB for first 2 weeks and with 1 µg rPspA alone for the last week. The levels of anti-PspA specific IgG1, IgG2a, IgG2b and IgG3 antibodies in sera (A) and breast milk (B) were determined by PspA-specific ELISA on day 0, 7 and 14 after the birth. The values shown are the mean ± S.E. concentrations (ng/ml) taken from PspA-immunized mother (n = 16) and sham-immunized mother (n = 14). The mean values of IgG1/IgG2a antibody to PspA in the sera of the individual mother's sera were 3.1, 2.1 and 1.1 for day 0, 7, and 14, respectively. The mean IgG1/IgG2a anti-PspA values for the individual mother's milk samples were 5.7, 6.5, and 1.7 for day 0, 7, and 14, respectively. The levels of anti-PspA specific IgG subclasses in sera and breast milk from sham-immunized mice were below the detections limit. * p<0.05 when compared with mice at day 0 by ANOVA test or Kruskal-Wallis test with Dunn's multiple comparison test. n.d. not determined.

Figure 3. Anti-PspA specific antibodies in sera of offspring.

The levels of anti-PspA specific IgG in sera of offspring were determined by PspA-specific ELISA at days 0, 7, and 14 after the birth. Group A mice were the offspring delivered from PspA-immunized mothers and breast-fed by the same mothers (n = 26). Group B mice were offspring from sham-immunized mothers and breast-fed by PspA-immunized mothers (n = 22). Group C mice were offspring from PspA-immunized mothers and breast-fed by sham-immunized mothers (n = 27). Group D mice were offspring from sham-immunized mother and breast-fed by the same mother (n = 18). The values shown are the mean ± S.E. concentrations (ng/ml). * p<0.05 and ** p<0.01 when compared with offspring in Group D by ANOVA test with Dunn's multiple comparison test.

Figure 4. Anti-PspA specific IgG subclasses in sera of offspring.

The levels of anti-PspA specific IgG subclasses in offspring's sera were determined by PspA-specific ELISA on day 0, 7, and 14 after birth. Group A (n = 26), B (n = 22), C (n = 27), and D (n = 18) mice were the same mice as described in figure 3. The values shown are the mean ± S.E. concentrations (ng/ml). The mean values of IgG1/IgG2a ratio were also shown. * p<0.05 and **p<0.01 are for comparisons with offspring in Group D for PspA-specific IgG subclasses or with offspring on day 0 for the IgG1/IgG2a ratio by ANOVA test with Dunn's multiple comparison test.

Figure 5. Protection against nasal carriage of pneumococci by maternal immunization with PspA among offspring.

Offspring at 7-day-old were intranasally challenged with 1×10⁵ CFU TIGR4 strain (5 µl/mouse) without anesthesia. Two days after challenge, nasal washes and homogenized washed nasal tissues were collected and the numbers of pneumococci colonies were determined. No evidence of protection was observed in CFUs in nasal washes (not shown). Results are shown for CFU in homogenized washed nasal tissue. Each dot represents the Log₁₀ CFU/mouse. Each horizontal line depicts the median Log₁₀ CFU/mouse. Group A (n = 11), B (n = 10), C (n = 13), and D (n = 15) mice were produced in the same manner as the corresponding groups in figure 3. Group A differed from Group D at p<0.05 by Kruskal-Wallis test with Dunn's multiple comparison test.

Figure 6. Protection against lung infection by maternal immunization with PspA among offspring.

Seven-day-old mice were intranasally challenged with 5×10⁵ CFU TIGR4 strain (10 µl/mouse) with anesthesia. Three days after challenge, lungs were collected and the numbers of pneumococci colonies in the lung homogenate were determined. Each dot shows the Log₁₀ CFU/mouse. Each horizontal line shows the median Log₁₀ CFU/mouse. Group A (n = 18), B (n = 16), C (n = 20), and D (n = 27) mice were produced in the same manner as the corresponding groups in figure 3. p<0.05 and p<0.01 are p-values for differences between the indicated group and the non-immune mice in Group D by Kruskal-Wallis test with Dunn's multiple comparison test.

Figure 7. Protection against fatal systemic pneumococcal infections by maternal immunization with PspA among offspring.

Offspring at 10-days of age were intraperitonally challenged with 1×10⁴ CFU TIGR4 strain (100 µl/mouse) with anesthesia. After challenge, offspring were monitored for 5 days to determine survival. Group A (n = 24), B (n = 28), C (n = 24), and D (n = 38) mice were produced in the same manner as the corresponding groups in figure 3. Group A mice are offspring delivered from PspA-immunized mothers and breast-fed by the same mothers (n = 24). Group B mice are offspring from sham-immunized mothers and breast-fed by PspA-immunized mothers (n = 28). Group C mice are offspring from PspA-immunized mothers and breast-fed by sham-immunized mothers (n = 24). Group D mice are offspring from sham-immunized mothers and breast-fed by the same mothers (n = 38). * p<0.01 when compared with control offspring in Group D by Kaplan-Meier test with Log rank test.

Figure 8. Cross-protection against fatal infections with pneumococcal strains expressing family 1 PspA.

As in the prior studies the mother mice were immunized with a rPspA2 of strain TIGR4. Offspring at 10-days of age were intraperitoneally challenged with D39 (PspA1, serotype 2; 50 CFUs/mouse), BG7322 (PspA1, serotype 6B; 20 CFUs/mouse), EF3030 (PspA1, serotype 19F; 5×10⁶ CFUs/mouse), and L82016 (PspA1 serotype 6B, 5×10⁶ CFUs/mouse) in 100 µl with anesthesia. After challenge, the mice were monitored for 10 days to determine the day of death. Group A mice were offspring delivered from PspA-immunized mothers and breast-fed by the same mothers (n = 6 for D39, n = 10 for BG7322, n = 7 for EF3030, n = 6 for L82016). Group B mice were offspring from sham-immunized mothers and breast-fed by a PspA-immunized mothers (n = 7 for D39, n = 11 for BG7322, n = 4 for EF3030, n = 10 for L82016). Group C were offspring from PspA-immunized mothers and breast-fed by sham-immunized mothers (n = 9 for D39, n = 11 for BG7322, n = 5 for EF3030, n = 11 for L82016). Group D mice were offspring from sham-immunized mothers and breast-fed by the same mothers (n = 9 for D39, n = 5 for BG7322, n = 6 for EF3030, n = 6 for L82016). p<0.05 and p<0.01 for the indicated comparisons with offspring in Group D by Kruskal-Wallis test with Dunn's multiple comparison test are shown.