Background: During pregnancy bacterial pathogens ascend from the vagina to cause infection and inflammation resulting in preterm birth and neonatal sepsis. Streptococcus agalactiae (Group B Streptococcus, GBS) is the leading infectious cause of morbidity and mortality among newborns in the United States. Placental macrophages (PMs) are fetally derived cells present in gestational tissues. We sought to identify roles of these cells during GBS infection.
Methods: PMs were isolated from gestational tissue of term, non-laboring pregnancies. PMs or THP-1 cells were incubated with GBS cells at an MOI 20:1 for 1 hour and then processed for scanning electron (SEM) or confocal microscopy. Cells producing extracellular traps were quantified by blinded review of SEM images at 750X magnification.
Results: Placental macrophages produce extracellular traps (ETs) in response to GBS infection as evidenced by the characteristic appearance via scanning electron microscopy and extracellular DNA staining of these structures by confocal microscopy. Extracellular traps are immune structures composed of DNA and histones and studded with antimicrobial peptides. Quantification of PMs producing ETs demonstrated differences when PMs were infected with GB37, an invasive strain that resists phagocytosis, compared to GB590, a colonizing strain, with cells infected with the colonizing GB590 strain producing significantly more ETs (Figure 1).
Next we infected THP-1 cells, an immortalized macrophage-like line, to determine if macrophage ET formation was a function specific to PMs, or could be produced by less differentiated macrophages. THP-1 cells also produced extracellular traps in response to GB590 infection, and ET formation in these cells was inhibited by pretreatment with cytochalasin D, but not nocodazole, implicating actin polymerization as an essential process for ET formation. Additionally, contact with bacteria was required to induce ET formation, as bacterial culture supernatant failed to induce ETs (Figure 2).
Conclusion: Taken together, these results demonstrate that macrophages produce extracellular traps as a defense response to GBS. Formation of extracellular traps in THP-1 cells requires bacterial cell contact and active actin polymerization.
D. Aronoff, None
J. Gaddy, None