Toll-like receptors (TLR) represent an ancient front-line defence system that enables the host organism to sense the presence of microbial components within minutes. As inducers of inflammation, TLR act as important triggers of distinct entities such as sepsis or autoimmune disease exacerbation. We report here that vitamin D3 [1a,25-dihydroxycholecalciferol, 1,25(OH) 2 D3] suppresses the expression of TLR2 and TLR4 protein and mRNA in human monocytes in a time-and dose-dependent fashion. Despite 1,25(OH) 2 D3-induced up-regulation of CD14, challenge of human monocytes with either LPS or lipoteichoic acid resulted in impaired TNF-a and procoagulatory tissue factor (CD142) production, emphasizing the critical role of TLR in the induction of inflammation. Moreover, reduced TLR levels in 1,25(OH) 2 D3-treated phagocytes were accompanied by impaired NF-jB/RelA translocation to the nucleus and by reduced p38 and p42/44 (extracellular signal-regulated kinase 1/2) phosphorylation upon TLRligand engagement. Both TLR down-regulation and CD14 up-regulation were substantially inhibited by the vitamin D receptor (VDR) antagonist ZK 159222, indicating that the immunomodulatory effect of 1,25(OH) 2 D3 on innate immunity receptors requires VDR transcription factor activation. Our data provide strong evidence that 1,25(OH) 2 D3 primes monocytes to respond less effectively to bacterial cell wall components in a VDR-dependent mechanism, most likely due to decreased levels of TLR2 and TLR4.
The impaired infection control related to the functional immaturity of the neonatal immune system is an important cause of infection in preterm newborns. We previously reported that constitutive Toll-like receptor (TLR) 4 expression and cytokine secretion on lipopolysaccharide (LPS) stimulation increases with gestational age. Here, we analyzed constitutive monocyte TLR2 expression and evaluated the expression profiles of the proximal downstream adapter molecule myeloid differentiation factor 88 (MyD88). We further investigated activation of protein kinases p38 and extracellular regulated kinsase (ERK) 1/2 in CD14 monocytes after ex vivo stimulation with bacterial TLR ligands (LPS and lipoteichoic acid [LTA]). The functional outcome of the stimulation was determined by cytokine secretion. Monocytes from 31 preterm newborns (<30 weeks of gestation, n=16; 30-37 weeks of gestation, n=15), 10 term newborns, and 12 adults were investigated. In contrast to TLR4 expression, TLR2 levels did not differ between age groups. However, MyD88 levels were significantly lower in preterm newborns. Activation of p38 and ERK1/2 was impaired in all newborn age groups after stimulation with TLR-specific ligands. Accordingly, after LTA stimulation, the levels of interleukin (IL)-1 beta , IL-6, and IL-8 cytokine production were substantially lower (P<.001) in preterm newborns than in adults. The reduced functional response to bacterial cell wall components appears to be part of the functional immaturity of the neonatal immune system and might predispose premature newborns to bacterial infection.
Premature newborns are highly susceptible to severe bacterial infections. This is partially due to their immature innate immune system, characterized by decreased neutrophil and monocyte activity as well as by reduced concentrations of complement factors. However, additional mechanisms might be important for innate immunity and are still the subject of considerable debate. The importance of pattern recognition domains such as Toll-like receptors (TLR) has been fully acknowledged within the last few years. Therefore, we investigated age-related monocyte TLR4 expression and lipopolysaccharide-induced cytokine secretion from very low birth weight infants (VLBWI) and from newborns after wk 30 of gestation in comparison to healthy adults. In VLBWI, expression of TLR4 surface protein, detected by flow cytometry, and TLR4-specific mRNA, quantified by real time-PCR, were significantly reduced in comparison to mature infants and to adults. Reduced TLR4 expression was paralleled by significantly diminished ex vivo LPS stimulated IL-1, IL-6, and tumor necrosis factor-␣ secretion into whole blood. We conclude that, in VLBWI, the minimized expression of TLR4 contributes to the susceptibility of VLBWI to infections with Gram-negative bacteria due to the lack of cytokines to boost initial immune response. (Pediatr Res 58: 121-124, 2005) Abbreviations CB, cord blood LPS, lipopolysaccharide PAMP, pathogen-associated molecular pattern PB, peripheral blood PBMC, peripheral blood mononuclear cells TLR, Toll-like receptor TNF, tumor necrosis factor VLBWI, very low birth weight infants Bacterial infections are a major cause of neonatal morbidity (1,2), and premature newborns are highly susceptible to infections (3). This increased susceptibility appears to be related to immaturity of the immune system. Maturation of the immune system includes specific humoral and cellular responses as well as nonspecific innate immune mechanisms, such as factors of the complement system and phagocyte activity (3-5).Monocytes and macrophages play a central role in both specific and nonspecific immunity against bacterial, viral, and fungal infection. In VLBWI, a substantial decrease of fully functional phagocytes has recently been reported (6). The Toll protein has been identified as a receptor controlling dorsoventral polarization during embryonic development of Drosophila larvae and serves a defensive function (7). Structures homologous to Toll receptors were found in mammalian cells (8,9) and termed TLR. Within the last few years, 10 human TLR have been described and characterized as important sensors of the innate immune system (9,10). These TLR are phylogenetically conserved and are involved in recognition of microbial PAMP. TLR4 functions as the transmembrane component of the LPS receptor. In addition to LPS, the major glycolipid component of the outer membrane of Gram-negative bacteria, TLR4 recognizes saturated and unsaturated fatty acids such as lipoteichoic acids from Gram-positive bacteria, and heat-shock proteins (8 -10). Binding of these ...
Bacterial and viral infections cause high rates of morbidity and mortality in premature newborns. In the setting of viral infection, pDCs play a key role as strong producers of IFN-α upon TLR9 activation. We analyzed pDC frequency, phenotype, morphology, and function in CB of preterm and term newborns in comparison with adults. Whereas all age groups show similar pDC numbers, BDCA-2, CD123, and TLR9 levels, the expression of BDCA-4 and capacity to produce IFN-α upon TLR9 challenge were decreased significantly in preterm neonates. Furthermore, we show by means of electron microscopy that pDCs from preterm newborns exhibit a distinct, "immature" morphology. Taken together, these findings suggest decreased functionality of pDCs in the premature newborn. The reduced capacity to produce IFN-α is likely to render such infants more susceptible to viral infections.
Background: Concise regulation of the Toll signaling pathway is mandatory in neonatal innate immunity. The microRNA-146 family (miR-146a/b) was recently reported to be a regulator of Toll-like receptor 4 (TLR4) through a negative feedback loop mechanism. Acting as a potent regulator, miRNA helps to protect the organism from developing overwhelming proinflammatory immune responses leading to septic shock or chronic inflammatory diseases. Objective: We investigated for the first time whether miRNA-146a/b plays a regulatory role in human monocytes derived from infant cord or adult blood, and whether differences in miRNA-146 expression exist. Methods: Expression profiles of miR-146a/b and TLR4 were studied by real-time PCR upon stimulation with lipopolysaccharide. Results: Both members of the miRNA-146 family showed a time-dependent upregulation. For miR-146a, a statistically higher significant increase was found after 24 h of stimulation in monocytes from cord blood compared to those derived from adults. In contrast, no differences were found for miR-146b and TLR4, respectively. Conclusion: We conclude that differences between the negative regulatory role for miR-146a obviously exist in neonatal and adult TLR4 signaling, and suggest that more intense research in the involvement of miRNA in immune regulation will facilitate the understanding of the development and function of the innate immune system of neonates.
Premature infants are highly susceptible to severe bacterial infections. The impaired infection control related to the functional immaturity of the neonatal innate immune system is an important cause of infection. Different monocyte subpopulations have been described and functionally characterized. However, data from preterm infants are scarce. We analyzed constitutive monocyte TLR2, TLR4, CD163, and HLA-DR expression in preterm cord blood. We further investigated activation of the signaling proteins ERK1/2 and NF-κB in monocyte subpopulations after ex vivo stimulation with the bacterial TLR agonists LPS and lipoteichoic acid. The functional outcome of the stimulation was determined by the intracellular production of TNF. Furthermore, the phagocytic activity was measured via flow cytometry. TLR4 and HLA-DR showed a gestational age-dependent increase. However, activation of ERK1/2 and NF-κB was impaired in neonatal monocyte subpopulations after stimulation with TLR agonists. Accordingly, intracellular TNF was diminished in preterm monocytes, especially in nonclassic monocytes. Premature monocytes showed high phagocytic activity, with significantly lower acidification of the phagosome. The reduced functional response of nonclassic monocytes of preterm neonates appears to be part of the diminished early immune response to bacterial cell wall components and is likely to contribute to their susceptibility to bacterial infection.
SummaryBioactive components of human milk, such as human lactoferrin (hLF), play an essential role in gut microbiome homeostasis and protection against neonatal inflammatory diseases. Neonatal intestinal macrophages display a proinflammatory profile that might contribute to inflammatory mucosal injury. Therefore, the aim of the study was to investigate the immunomodulatory effects of hLF on differentiation and activation of monocyte‐derived macrophages (moMϕ). Monocytes isolated from umbilical cord blood of term neonates and peripheral blood of healthy adults were differentiated in the absence or presence of hLF, and differentiation, apoptosis and phagocytosis were evaluated. Cytokine production, Toll‐like receptor (TLR) signalling and activation marker expression were investigated upon activation with lipopolysaccharide (LPS) and lipoteichoic acid (LTA) challenge. We demonstrate that hLF‐differentiated moMϕ exhibit decreased TLR‐4 expression, TLR signalling, proinflammatory cytokine secretion and intracellular tumour necrosis factor (TNF)‐α production. Investigation of differentiation markers, morphology and induction of apoptosis showed no alteration in lactoferrin‐differentiated moMϕ. Taken together, hLF promote anergic/anti‐inflammatory effects by TLR expression and pathway interference, resulting in a diminished proinflammatory moMϕ phenotype. The anergic/anti‐inflammatory properties of hLF might contribute to the prevention of harmful TLR‐mediated inflammatory disorders in the developing gut of premature infants.
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