Impaired Toll-like receptor 2 signalling in monocytes from 5-year-old allergic children

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Abstract
The relative composition of the two major monocytic subsets CD14+CD16− and CD14+CD16+ is altered in some allergic diseases. These two subsets display different patterns of Toll-like receptor levels, which could have implications for activation of innate immunity leading to reduced immunoglobulin E-specific adaptive immune responses. This study aimed to investigate if allergic status at the age of 5 years is linked to differences in monocytic subset composition and their Toll-like receptor levels, and further, to determine if Toll-like receptor regulation and cytokine production upon microbial stimuli is influenced by the allergic phenotype. Peripheral blood mononuclear cells from 5-year-old allergic and non-allergic children were stimulated in vitro with lipopolysaccharide and peptidoglycan. Cells were analysed with flow cytometry for expression of CD14, Toll-like receptors 2 and 4 and p38-mitogen-activated protein kinase (MAPK). The release of cytokines and chemokines [tumour necrosis factor, interleukin (IL)-1β, IL-6, IL-8, IL-10, IL-12p70] into culture supernatants was measured with cytometric bead array. For unstimulated cells there were no differences in frequency of the monocytic subsets or their Toll-like receptor levels between allergic and non-allergic children. However, monocytes from allergic children had a significantly lower up-regulation of Toll-like receptor 2 upon peptidoglycan stimulation. Further, monocytes from allergic children had a higher spontaneous production of IL-6, but there were no differences between the two groups regarding p38-MAPK activity or cytokine and chemokine production upon stimulation. The allergic subjects in this study have a monocytic population that seems to display a hyporesponsive state as implicated by impaired regulation of Toll-like receptor 2 upon peptidoglycan stimulation.