Functional bacterial opsonic activity of human amniotic fluid

There are some data to suggest that amniotic fluid protects the fetus from invasion by pathogenic bacteria. To examine methods by which amniotic fluid may offer such protection, quantitative antibody, complement activity, and functional opsonic capacity were measured. Immunoglobulins were measured by laser nephelometry and total hemolytic complement was determined by radial diffusion; results suggested activity adequate for bactericidal capacity. The chemiluminescence assay was used to quantitate the functional interaction between polymorphonuclear leukocytes and E. coli, group B streptococci (GBS), or zymosan particles preopsonized with amniotic fluid obtained at different stages of gestation. Results were compared to those for normal serum. Data were analyzed by evaluation of the initial slope, area under the curve, and peak chemiluminescence response. Opsonic activity of amniotic fluid for E. coli and GBS was demonstrated, with E. coli showing greater reactivity (maximum = 15,000 to 25,000 cpm) than GBS (10,000 to 20,000 cpm). Specific, as well as nonspecific, opsonic activity was demonstrated by absorption of amniotic fluid with killed bacteria. Concentration of amniotic fluid did not result in an increase in chemiluminescent activity, which demonstrates that optimal opsonic activity already exists. The classical and alternate pathways of complement were assessed for E. coli and GBS. Preterm amniotic fluid did not differ in response from that of amniotic fluid obtained from term pregnancies. This study demonstrates that amniotic fluid can provide the fetus with protection from bacterial pathogens and delineates mechanisms for such protection.