Infancy and early childhood are a time of increased susceptibility to infectious disease with demonstrated immaturity of function in both the innate and adaptive arms of the immune system compared with adults. Toll-like receptors (TLRs 1–10) are transmembrane proteins that play an important role in directing the adaptive immune response and providing an essential bridge between the two arms of immunity. Developmental immaturity of TLR function may be implicated in age-related susceptibility to infection. We measured the basal expression of TLR2 and TLR4 on peripheral blood monocytes (PBMC) in healthy children aged between 1 month and 16 years undergoing minor surgical procedures. After written consent was obtained, 1 mL of blood was drawn at anaesthetic induction. PBMC were isolated and cell surface labeling was performed using techniques previously described in our laboratory. Stained cells were analysed using a fluorescence-activated cell sorter flow cytometer. TLR2 and TLR4 surface expression on monocytes was expressed as a mean fluorescence intensity (MFI) ratio, that is, the ratio of the MFI of the marker to the MFI of the isotype control. Kruskal–Wallis and two-tailed Mann–Whitney tests were employed for non-parametric comparison because of the relatively small sample size, with statistical significance P < 0.05 assigned. Eighty children (77% male) prospectively recruited were grouped into: <12 months (n = 25), 1 to 5 years (n = 32) and over 5 years (n = 23). TLR2 and TLR4 expression did not vary significantly across the age groups, suggesting that basal TLR2 and TLR4 expression on monocytes is not ontogenically regulated (Fig. 1). There was no significant gender difference in TLR2 expression; however basal TLR4 expression was higher in boys (1.11 vs. 0.99, P = 0.03).
This study provides data on the baseline expression of TLR2 and TLR4 in the largest cohort of children from birth to 18 years. The lack of a significant change across the age groups suggests that reduced function rather than reduced basal expression may be an explanation for the increased infection burden in younger children. Several recent papers have addressed ontogeny of cytokine responses in response to TLR stimulation in limited age groups and have found a qualitative and quantitative difference in cytokine production for different TLRs depending on the ligand and the age. Although the mechanisms underlying these observations are uncertain, they suggest the concept of age-specific responses rather than a global maturation process.
Gender differences described in TLR function in adults are thought to be important in the favourable outcome from infectious insults in females versus males. The increased TLR4 expression in males seen in our study is consistent but requires confirmation. We feel such studies will inform strategies to prevent and treat severe infections in young children.