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Human newborns are susceptible to microbial infection related to incompletely defined aspects of the neonatal immune system. To characterize neonatal innate immunity, we studied production of two early response cytokines in response to Toll-like receptor (TLR)-activating microbial stimuli in vitro: the pro-inflammatory cytokine tumor necrosis factor (TNF)-[alpha] and IL-6, a multifunctional cytokine with antiinflammatory and Th2-polarizing properties. Neonatal cord blood responses to multiple TLR agonists, including poly dI:dC (TLR3), lipopolysaccharide (LPS) (TLR4), flagellin (TLR5), and CpG DNA (TLR9), are characterized by a higher IL-6/TNF-[alpha] ratio than in adult peripheral blood. Robust LPS-induced IL-6 production is due to both neonatal cellular (monocyte-) and humoral (serum-) factors. Remarkably, serum collected from newborns during the first week of life demonstrates higher IL-6/TNF-[alpha] ratios than does cord blood, associated with elevations of the IL-6-inducible acute phase reactants CRP and LPS-binding protein in the first days of life. A high ratio of stimulus-induced IL-6/TNF-[alpha] production is likely to profoundly modulate both innate and adaptive immune responses in the human newborn.

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