Amniotic Fluid IL-6 and IL-8 Are Superior Predictors of Fetal Lung Injury Compared to Maternal or Fetal Plasma Cytokines or Placental Histopathology in a Nonhuman Primate Model.

BACKGROUND: Intra-amniotic infection or inflammation (IAI) is common in early preterm birth and associated with significant neonatal lung morbidity due to fetal exposure to pro-inflammatory cytokines and infectious organisms. Amniotic fluid interleukin-8 (IL-8), a pro-inflammatory cytokine, was previously correlated with development of neonatal bronchopulmonary dysplasia, but whether amniotic fluid cytokines or placental pathology more accurately predicts neonatal lung pathology and morbidity is unknown. We have used a pregnant nonhuman primate (NHP) model of Group B Streptococcus (GBS) infection to study the pathogenesis of IAI, bacterial invasion of the amniotic cavity and fetus, as well as microbial-host interactions. In this NHP model, we have studied the pathogenesis of several GBS strains with differing potential for virulence, which has resulted in a spectrum of IAI and fetal lung injury that affords the opportunity to study the inflammatory predictors of fetal lung pathology and injury.

OBJECTIVES: To determine whether fetal lung injury is best predicted by placental histopathology or the cytokine response in amniotic fluid or maternal plasma.

STUDY DESIGN: Chronically catheterized pregnant monkeys (Macaca nemestrina, pigtail macaque) at 116-125 days gestation (term=172 days) received a choriodecidual inoculation of either saline (n=5), weakly hemolytic (WH) GBS strain (N=5, low virulence), or hyper-hemolytic (HH) GBS strain (N=5, high virulence). Adverse pregnancy outcomes were defined as either preterm labor, microbial invasion of the amniotic cavity or development of the fetal inflammatory response syndrome (FIRS). Amniotic fluid, maternal and fetal plasma samples were collected after inoculation and pro-inflammatory cytokines (TNF-α, IL-β, IL-6, IL-8) were measured by a multiplex assay. Cesarean section was performed at the time of preterm labor or within one week of inoculation. Fetal necropsy was performed at the time of delivery. Placental pathology was scored by a pediatric pathologist and fetal lung injury was determined by semi-quantitative score from histopathology evaluating inflammatory infiltrate, necrosis, tissue thickening or collapse scored by a veterinary pathologist.

RESULTS: The principal findings in our study are: (1) adverse pregnancy outcomes occurred more frequently in animals receiving HH GBS (80% with preterm labor, 80% with FIRS) compared to WH GBS (40% with preterm labor, 20% with FIRS) and controls (0% preterm labor, 0% FIRS); (2) despite differences in the rate of adverse pregnancy outcomes and FIRS, fetal lung injury scores were similar between animals receiving the WH and HH GBS strains; (3) fetal lung injury score was significantly correlated with peak amniotic fluid cytokines IL-6 and IL-8, but not TNF-α or IL-1β; and (4) fetal lung scores were poorly correlated with maternal and fetal plasma cytokine levels and placental pathology.

CONCLUSIONS: Amniotic fluid IL-6 and IL-8 levels were a superior predictor of fetal lung injury compared to placental histopathology or maternal plasma cytokines. This evidence supports a role for amniocentesis in the prediction of neonatal lung morbidity due to IAI, which cannot be provided by cytokine analysis of the maternal plasma or placental histopathology.