Methodological considerations
A major strength is that this study investigated the association betweenFLG genotype and a large panel of B and T cells in the general
population for the first time. We had detailed and extensive information
on immune cell numbers from 11‐color flow cytometry and obtained
objective information on genetic ancestry. However, the following
limitations need to be addressed. First, the AD population for the
sensitivity analyses was relatively small which could have limited the
power in the statistical analyses. Nevertheless, in comparison to
previous studies, only including a maximum of 6 AD patients withFLG mutations, this is the largest study on FLG mutations
in both the general population and AD patients. Second, our AD
population was defined by ever having physician diagnosed AD before or
at the age of 10 years. Therefore, it is likely that a subset of the
children has outgrown AD at the age of 10 and this might affect their
immunophenotype. In addition, the use of questionnaires for AD diagnosis
could have introduced recall bias. Third, as mentioned previously, our
study included the four most common FLG mutations in the European
population. To prevent misclassification, we selected children with
genetic European ancestry for the current study. Although the choice for
including the most common FLG mutations in European populations
is in line with previous studies 11,12, other less
frequent FLG mutations could exist in low numbers since up to 113FLG mutations resulting in premature protein termination have
been described. A recent study including patients with AD and Ichthyosis
Vulgaris (IV), showed that screening the entire encoding region ofFLG for mutations led to an improvement of the diagnostic yield.30 Furthermore, biallelic genotypes were only present
in 3 children of the study population and therefore not studied
separately. Since homozygous or compound heterozygous mutations lead to
complete absence of filaggrin expression, these mutations might have
different effects on immune cell numbers compared to heterozygousFLG mutations which lead to 50% reduction in filaggrin
expression.1 Therefore, studies investigating
biallelic mutations compared to heterozygous FLG mutations and
wild type FLG are needed. Furthermore, as this is the first study
in a general cohort addressing the association between FLGmutation and immune cell numbers, future studies are needed for
validation of our results.
In conclusion, within the general population, school-aged children withFLG mutations have higher Th22 cell numbers. Furthermore,
children with and without FLG haploinsufficiency did not differ
in other B and T cell subsets. This suggests that with age, other
factors such as environmental exposure contribute to risk thatFLG mutations poses to the development of allergy.