Maternal immunity partially protects newborn mice against a Chlamydia trachomatis intranasal challenge

To determine the role of maternal immunity in protecting newborn mice against a Chlamydia trachomatis infection, female BALB/c mice were immunized intranasally (i.n.) with 10⁴ inclusion forming units (IFU) of the C. trachomatis mouse pneumonitis biovar (MoPn). As a control, another group of female mice was sham-immunized i.n. with HeLa cell extracts. Immunized animals mounted strong immune responses as evidenced by high Chlamydia-specific antibody titers in serum and milk. Newborn mice born from immunized and sham-immunized dams were challenged i.n. with 10³IFU of MoPn at two post-natal days (PND). Following inoculation, newborn mice were euthanized at 7- and 18-PND and the lungs, spleen and intestine were cultured for Chlamydia. Overall, no significant differences were observed between the mice born from and fed by immunized dams and mice born from and fed by sham-immunized dams. Of the mice born from immunized dams, 75 and 25% had positive lung cultures at 7- and 18-PND, respectively. Of the mice born from sham-immunized dams, 82 and 50% had positive lung cultures for those same days. When the number of IFU recovered from the lungs and spleens was compared between the two groups no significant differences were observed. However, when the number of IFU recovered from the small intestine was compared, significant differences were observed between the two groups of newborn mice (2×10⁵ versus 32×10⁶ at 7-PND and 9.2×10⁶ versus 85×10⁶ at 18-PND). In conclusion, maternal immunity plays a limited role in protecting newborn mice against a Chlamydia infection.