Increased susceptibility to infection and a tendency toward more severe outcomes than in healthy adults both illustrate the prematurity of neonatal innate immunity mediated via TLRs. However, the details of TLR-mediated neonatal innate immunity are not fully understood. Here, we investigated the differences in TLR-mediated immune responses between the human neonate and adult, focusing on the cytokine profiles of monocytes, dendritic cells (DCs), and monocyte-derived DCs (MoDCs) in cord and adult blood. Purified monocytes, DCs, and MoDCs were stimulated with LPS (TLR4 ligand), Pam3CSK4 (TLR1/2 ligand), flagellin (TLR5 ligand) or zymosan (TLR2 ligand). IL-8, IL-6, and TNF concentrations were analysed in culture supernatants. Compared with the effects in adult blood, LPS-, Pam3CSK4-, and flagellin-stimulated cytokine production in cord blood was weak in monocytes, comparable in DCs, and elevated in MoDCs. In contrast, zymosan stimulation gave comparable cytokine profiles in the monocytes, DCs, and MoDCs of cord and adult blood. The immaturity of TLR-mediated innate immunity in neonates thus depends on monocytes rather than DCs. Zymosan, a cell wall extract from Saccharomyces cervisiae, is known to show vaccine adjuvant activity in adult animal, but the adjuvant activity was unclear in neonatal animal. Our results indicate that zymosan-mediated effective TLR2 signalling in neonates may be useful for developing a neonatal vaccine adjuvant.
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