DISCUSSION
In this study, we show that levels of the chemokines CCL18, CXCL10 and CXCL11 in early life and childhood may predict outcomes of allergic disease later in childhood and adolescence. To our knowledge, this is the first study to show that childhood circulating CCL18 levels may affect allergy-related outcomes longitudinally until adolescence.
The main finding was that the dually Th2/Treg regulated chemokine CCL18 predicted development of both asthma and sensitisation, with consistent effects over time. Being constitutively expressed in the lung and lymphoid tissues during homeostatic conditions, CCL18 exhibits both chemotactic and immunoregulatory properties.17 It promotes tolerogenic differentiation of dendritic cells, which in turn may polarise T cells into Tregs, and may polarise memory T cells into FoxP3+ T cells in vitro . In allergic subjects, however, the tolerogenic effects of CCL18 are seemingly abrogated, despite being upregulated in allergic conditions such as atopic dermatitis10,27,28 and asthma.17Described being due to less efficient binding of the protein on immune cells, this possibly could partly explain the loss of tolerance in allergic individuals. Furthermore, CCL18 induces production of collagen both in the skin and lung, implying a role in remodelling of the airways typically seen in asthmatic subjects.17 As alveolar macrophages are the main producers of CCL18 in the lung, where its expression is constitutive, it is tempting to speculate that these levels are augmented in asthmatic individuals owing to dysregulation of these cells. However, as our measurements were performed in plasma samples, and CCL18 may originate from one of many bodily sources, we cannot draw conclusions on tissue specific effects of the observed elevation without performing functional studies. Furthermore, whether heightened CCL18 responses in asthmatic and sensitised children constitute causative mechanisms of allergy induction, or compensatory immune dampening responses, remains to be elucidated. We further examined chemokine expression within allergy clusters previously derived from our cohort.20-23 Indeed, CCL18 levels at age 8 were higher in the multiple early allergic sensitisation cluster. Moreover, children with asthma exacerbations had higher levels of CCL18 at age 8 compared to children without wheeze. Taken together, this suggests that increased CCL18 levels later in childhood may reflect allergic disease severity, although further studies should elaborate on this matter.
Interesting findings also appeared for the Th1-associated chemokines CXCL10 and CXCL11. Elevated levels of CXCL10 in infancy and childhood associated with present and future development of asthma, in line with results from children with wheezing at age 3, who subsequently developed asthma at age 6.16 Additionally, CXCL10 levels are increased in viral-induced asthma29,30, suggesting that viral infections may induce Th1-chemokine responses in asthmatic individuals. On the contrary, low cord blood CXCL10 levels associated with sensitisation in infancy and adolescence. Similarly, decreased CXCL11 levels in early life associated with later development of sensitisation. This corroborates findings from our previous studies, where SPT-positive children had lower levels of CXCL11 at birth and 24 months.10 As sensitisation is a Th2-driven process, and Th1-responses were lessened in sensitised children, diminished neonatal Th1-responses seemingly paves way for development of sensitisation in these children. Additionally, reduced cord blood CXCL11 levels associated with asthma at age 16 and translated into a predicted lower risk with high CXCL11 levels at birth, supporting previous results where children with the highest quartile CXCL11 levels at birth did not become sensitised throughout the first two years of life.10 No long-term effects of CXCL10 and CXCL11 on allergy development could be demonstrated in this study. Possibly, the function of Th1 cells, and their expression of IFN-γ, may become attenuated due to immunoregulatory effects of the highly expressed CCL18 on Tregs, although the findings may also constitute altered patterns of expression in allergic conditions. Collectively, this indicates that although these chemokines are induced by the same cytokine, downstream effects seem to be differentially regulated both in terms of allergy outcome and how levels reflect temporal development of disease.
There are both limitations and strengths to the present study. We evaluated circulating chemokine levels but did not have the opportunity to evaluate functional aspects of the same mediators in different tissues. This would have added mechanistic insights into the findings presented here. Also, the generalisability of these data may be limited, as children in the cohort originate from the Greater Manchester region, with rather homogenous populations. A strength of this study includes the substantial sample size, as few studies have surveyed circulating chemokines at this magnitude. Furthermore, the consistency of the methodologies used compared to previous studies provides another advantage. Moreover, by performing both cross-sectional logistic regression and longitudinal GEE models we have taken into account different temporal perspectives throughout childhood, which is a strength of this study.
In conclusion, we have shown that elevated levels of CCL18 throughout childhood precede the development of asthma and sensitisation, findings that remained solid longitudinally. The Th1-associated chemokines CXCL10 and CXCL11 also predicted development of sensitisation and asthma, with differential regulation at different time points in life. This motivates further investigations of chemokines as biomarkers for allergy development, with putative clinical utility in the prediction of allergic outcomes.