Dysfunction of plasmacytoid dendritic cells in preterm neonates – a cause of severe infections?
Objective: Bacterial and viral infections are associated with high rates of morbidity and mortality in neonates, especially in preterm newborns. The mechanisms of altered immunological functions in neonates leading to severe courses of infections are yet a question of debate. Plasmacytoid dendritic cells (pDCs), although low in numbers in peripheral blood (PB), are strong producers of interferon alpha (IFN-a) after TLR9 stimulation. Patients and Methods: In order to analyze pDCs' impact on neonatal immune responses, we analyzed the frequency of pDCs in cord blood (CB) from term (n=20) and preterm neonates (n=20) in comparison to DC numbers in PB from healthy adults (n=20) using specific surface staining and flow cytometry. Moreover, the functional capacity of pDCs after stimulation with ODN2216, a synthetic TLR9 agonist was analyzed in CB from term (n=20) and preterm (n=20) neonates in comparison to adult PB (n=20). pDCs were characterized by BDCA-2 and CD123. Upon stimulation with ODN2216 IFN-a production was quantified by intracellular IFN-a staining using FACS analysis. Additionally, the morphology of pDCs was visualized by electron microscopy. Results: We found comparable amounts of absolute pDC numbers and expression of the surface marker BDCA-2 and TLR9. Upon stimulation with ODN2216 preterm pDCs, displayed significantly lower IFN-a production and the percentage of pDCs capable of producing IFN-a was reduced compared to pDCs from term newborns and adults. Transmission electron microscopical investigations demonstrated a weaker formation of dendritical cell projections in 8 hours cultivated preterm versus term PDCs. Conclusion: While equal in absolute numbers and TLR9 expression, naive pDCs in preterm neonates show a slightly different phenotype. After stimulation with a synthetic TLR9 agonist they produce significantly less IFN-a and show an altered morphology compared to term neonates and adults. These in vitro observations of reduced responsiveness and impaired functional capacity of pDCs might help to understand the susceptibility of preterm neonates for severe course of bacterial and viral diseases.