The opsonic antibody response of female rats to type III group B streptococcal immunization: a model for maternal immunity.

Group B streptococcus (GBS) remains a major cause of neonatal infection. Maternal immunization-induced GBS antibody may protect neonates from GBS disease. Since the opsonophagocytosis assay correlates well with survival in GBS infected suckling rats, we sought to determine an immunization schedule which would induce type III GBS opsonic antibody in rat dams above a predetermined level of 10 dilution-1 (dil-1). This schedule could then be used for future studies of maternal-fetal immunity. Wistar rat dams (n = 12) were given killed GBS type III using three immunization schedules (primary injection, initial booster at 7, 14 or 22 days and then weekly boosters). Opsonic GBS type III antibody and total immunoglobulin (IgG) were measured. Only the schedule with a 7-day initial booster resulted in GBS type-specific opsonic antibody consistently above 10 dil-1. The IgG (467 +/- 83 mg/100 ml) remained constant while the opsonic antibody increased significantly to 50 and 63 dil-1 (P less than 0.01 compared to day 0) after boosters on day 7 and 14 respectively. Eight pregnant dams, who received a primary immunization and boosters at 7 and 14 days, developed GBS type III opsonic antibody titers (72 dil-1) similar to non-pregnant dams and potentially adequate to protect suckling rats from GBS disease. This model may now be used to study other adjuvants, immunogens, and maternal-fetal immunity using established suckling rat models of GBS disease.