Comparison with literature and interpretation
All previous studies on FLG mutations and immune cell numbers have been performed within mice models or smaller numbers of AD patients.10-12,23 This is the first study that provides insight in the role of FLG mutations on immune cell numbers in school-aged children of a general population. The setting of this study within a population-based pediatric cohort study is unique to study the association of FLG on immune cell numbers in a general population.
In line with a previous study in mice, we observed higher Th22 cell numbers in children with FLG mutations than in to children with wild type FLG alleles. 23 Th22 cells are characterized by the production of IL-22 and contributes to skin integrity.24,25 Moreover, IL-22 is known for its role in the defense of different pathogens in the skin by the production of antimicrobial proteins.24,25 In AD, acute skin lesions are associated with an upregulated Th22 immune response.26,27 We speculate that the observed higher number of Th22 cells is an immunological defense mechanism to the damaged skin barrier. Nevertheless, future studies should confirm this finding. When studying Th22 numbers only in AD children, no differences were observed between both groups. This is could be explained by the fact that children with AD have higher Th22 independent of theirFLG genotype. 26 In addition, the expression of the protein filaggrin is decreased in AD which can cause Th22 alterations through skin barrier dysfunction independent of FLGmutation status.1,5 Unfortunately, literature on the effect of FLG mutations on the numbers of Th22 in AD is lacking.
In contrast with small previous case studies in AD, we did not observe differences in Th2, Th17 and Treg cell numbers between children with and without FLG mutations both in the total study population and in the subgroup of patients with AD. 10-12,23 The associations of Th2, Th17 and Treg cell numbers with filaggrin in previous studies were explained by increased TSLP activity due to impaired skin barrier and impaired thymus functioning.11,28 In the thymus, an altered filaggrin expression affects maturation of T cells and induces dendritic cells to stimulate the differentiation of naive Treg cells into functionally different Treg cells. 10,29 The discrepancies between previous studies and our current study could be explained by differences in investigated populations and species. The mice studies studiedFLG null mutations, in which no expression of filaggrin is present.10 This is in contrast with our population, encompassing mainly haploinsufficient children which leads to a 50% reduction in filaggrin expression. 1 In addition,FLG knockdown skin equivalents in previous studies could represent a different immunological setting than is present in human skin. 23 Furthermore, it is likely that previous results on immune cell numbers in AD populations are affected by disease severity. Namely, immune cells in active AD skin can induce downregulation of filaggrin protein expression in the skin independent of FLG mutations, subsequently affecting immune cell composition. Although we do not have information on disease severity in our AD population, this study included a relatively healthy cohort in which we expect relatively mild AD. A more severe AD phenotype has been linked to homozygous and compound heterozygous FLG mutation phenotype.1 In addition, activated immune cells in more active AD skin can in itself downregulate filaggrin protein expression in the skin independent of FLG mutations and thereby affect skin barrier function and subsequently immune cell alteration.1,5In accordance, a previous study in AD patients observed that circulating Treg cell numbers were associated with AD activity and not withFLG genotype.11 In addition, we previously showed that children with AD had higher Th17 and Treg memory cell numbers.16 Therefore, alteration in immune cell numbers is probably not only dependent on FLG mutation genotype, but also on AD severity and epigenetic and environmental factors that affect these two, as our data suggest.
In addition, we did not observe differences in memory B cell numbers between children with and without FLG mutations, nor any associations between memory B cell numbers and AD in our previous study.16 No previous studies have investigated B cell subsets in relation to FLG mutations. Possibly, T cells are more affected by FLG mutations than B cells because of the effects of filaggrin in the thymus.10