Role of MyD88 in Diminished Tumor Necrosis Factor Alpha Production by Newborn Mononuclear Cells in Response to Lipopolysaccharide

Abstract

Human newborns are more susceptible than adults to infection by gram-negative bacteria. We hypothesized that this susceptibility may be associated with a decreased response by leukocytes to lipopolysaccharide (LPS). In this study, we compared LPS-induced secretion of tumor necrosis factor alpha (TNF-␣) by mononuclear cells (MNC) from adult peripheral blood and newborn umbilical cord blood in vitro and attempted to determine the mechanisms involved in its regulation. At a high concentration of LPS (10 ng/ml) and in the presence of autologous plasma, MNC from adults and newborns secreted similar amounts of TNF-␣. However, in the absence of plasma, MNC from newborns secreted significantly less TNF-␣ compared to MNC from adults. Moreover, at a low concentration of LPS (0.1 ng/ml) and in the presence of plasma, TNF-␣ secretion was significantly lower for newborn MNC compared to adult MNC. Adults and newborns had similar numbers of CD14 and Toll-like receptor 4 (TLR-4)-positive cells as measured by flow cytometry. However, the intensity of the CD14 marker was greater for adult than for newborn cells. Incubation of cells with LPS led to an increase in CD14 and TLR-4 intensity for adult cells but not for newborn cells. The effect of LPS stimulation of adult or newborn cells was similar for ERK, p38, and IB␣ phosphorylation, as well as IB␣ degradation. Finally, we assessed levels of the TLR-4 adapter protein, the myeloid differentiation antigen 88 (MyD88). We found a direct relation between adult and newborn TNF-␣ secretion and MyD88, which was significantly decreased in newborn monocytes. Since TLR-4 signals intracellularly through the adapter protein, MyD88, we hypothesize that MyD88-dependent factors are responsible for delayed and decreased TNF-␣ secretion in newborn monocytes.

Hyper innate responses in neonates lead to increased morbidity and mortality after infection

“…Whether and how the innate immune response in neonates contributes to their susceptibility to infection or TLR stimulation is still controversial. In human, it has been reported that mononuclear cells from cord blood secrete lower amounts of TNF than those from healthy adults in response to TLR agonists (10)(11)(12)(13), whereas other studies draw opposite conclusions (14)(15)(16). Results from studies in mice also conflict because neonates produce either lower (17,18) or higher (19) levels of TNF compared with adults.…”

Defective antigen-presenting cell function in human neonates

“…Although neonatal monocytes express similar levels of mRNA for different TLRs than adult monocytes [20,19], these data do not exclude possible differences in the protein expression of such receptors. Some data also support a defect in TLR signaling: (1) the up-regulation of TLR4 and CD14 expression upon LPS stimulation is not observed in neonatal monocytes [19,21]; and (2) the protein expression of MyD88, an important adaptor molecule involved in TLR-mediated signaling, is decreased in neonatal monocytes [21], although MyD88 mRNA levels are similar in neonatal and adult monocytes [19]. It should be noted that the expression of several key molecules involved in TLR signaling, such as TIRAP and IRAK-4, has not been carefully examined.…”

Selective Impairment of TLR-Mediated Innate Immunity in Human Newborns: Neonatal Blood Plasma Reduces Monocyte TNF-α Induction by Bacterial Lipopeptides, Lipopolysaccharide, and Imiquimod, but Preserves the Response to R-848

“…Even in the case of LPS, where several studies have explored the relative responses of newborn and adult cells (17), much of the published work was performed before the realization that commercial preparations of LPS are often contaminated with TLR2 ligands (18). A recent study described a correlation between reduced responsiveness of newborn mononuclear cells to LPS and reduced MyD88 expression (19). However, to our knowledge, there are no published reports of the activity of a range of well-defined TLR ligands and expression of their cognate receptors and signaling intermediates in newborn monocytes.…”