DISCUSSION
Minimally invasive biomarkers, such as serum proteins, are widely sought
after for their value in defining disease states and outcomes. In
adolescent and adult AD patients, a high total serum IgE level was a
predictive factor for poor disease prognosis [19]. Interestingly, in
these pediatric patients, many inflammatory serum biomarkers
demonstrated stronger correlation to disease severity better than IgE
alone [20]. Thus, the focus of our study was to highlight key
biomarkers in early AD that correlate with disease and potentially
predict the persistence or resolution of disease.
Multiple analytes correlated strongly with pediatric AD severity and,
most significantly, known disease markers, IL-13, MCP-4, and TARC,
correlated with SCORAD across all time points from infancy to 5 years of
age, recapitulating the importance of type 2 immune response early in AD
pathogenesis. Correlations with chemokines and monocyte chemoattractant
proteins underscores the role of eosinophils and monocytes in early AD,
while TARC and IL-13 are both mediators of Th2 inflammation and
implicated in pediatric and adult AD, emphasizing that these analytes
could represent critical nodes for better understanding AD heterogeneity
and disease progression [7, 21, 22]. Although standard of care
treatments lead to a decreased concentration of these biomarkers, the
observations and changes observed in our study demonstrate the strong
relationship these markers have with the disease pathology in pediatric
patients. Importantly, this study included ultrasensitive measurements
of numerous cytokines for which quantitative assessment has only
recently been possible, including IL-13, which is implicated as a driver
of AD based on clinical responses to lebrikizumab and dupilumab, further
implicating IL-13 in disease severity even in infancy [5, 23, 24].
Analytes described as drivers of adult AD, such as IL-22 and IL-17A
[7, 18], did not possess strong correlations to disease severity
within these pediatric AD samples. This is likely due to the slow
increase in serum concentration over time, limiting the role of IL-22 in
pediatric AD as seen in previous reports [7]. Though levels of
IL-17A were not correlated to SCORAD and present in very low levels, as
seen in previous pediatric studies [25], Th17-related fibroblast
markers, CCL19 and CCL20, were observed at various timepoints throughout
our study and CCL20 positively correlated with Th2 circulating cell
populations at Y1 pointing toward an intriguing immunological overlap of
inflammatory fibroblasts and T cells. Pathway analysis of proteins
upregulated in Y5 samples reveals the emergence of innate pathways over
time and multiple gene ontology results indicate that dendritic cells
(DCs) may increase with age. Increased conventional dendritic cells were
previously described as being protective for wheezing in these pediatric
eczema patients [10], and stimulation of these conventional DCs
isolated from atopic infants produced more interferon and IL-10 than
those derived from non-atopic infants at initial visit and the one year
follow-up visit [26]. Mounting evidence demonstrates this appearance
of symptoms known as the atopic march tracks with early disruption of
immune development and can have lasting effect into adulthood [27].
This link between increased CCL19 and DCs further indicates a role in
atopic march from AD to asthma for CCR7, the receptor for CCL19, which
can coordinate T cell, B cell, and DC responses across pathologies
[28].
Matrix remodeling proteins (MMP-10) and chemoattractant molecules
(MCP-3, MCP-2, CXCL5) provide further insights into the complexity of
disease mechanisms significantly associated with the progression of the
pediatric form of AD. The connections of these markers to disease
severity contextualize a shift toward Th1 immunity and myeloid
involvement developing throughout the course of the pediatric AD patient
journey concomitant with dominant Th2 responses throughout. The
identification of CXCL5 and IL-5 as important disease severity
predictive biomarkers for pediatric AD highlights the innate allergic
response early in disease progression. The high levels of sCD40L that we
observed in pediatric patients aligned with prior published observations
that this protein is elevated in the blood of pediatric patients
[6]. Additionally, sCD40L concentrations correlated throughout the
study with CD4+Th2 cells which might link to the role of CD4+CD40L+
cells in the priming of other immune cell types including CD14+
monocytes and CD8+ T cells [29]. This work aids our
contextualization of a shift toward Th1 immunity and myeloid involvement
developing throughout the course of the pediatric AD patient journey
concomitant with Th2 responses presenting initially and persisting
throughout the course of disease.
The previously reported circulating protein profiles of pediatric AD
have provided robust assessment of correlation analysis at a single
timepoint [6]. Our study builds upon these important observations
via the monitoring of circulating biomarkers over a development period
of four years adding value to the concept that objective biomarkers
could be used to objectively assess disease severity, as has been
demonstrated in adult AD [30]. We observed subsets of inflammatory
proteins that correlated with each other in a manner consistent with
early disease skewing toward type 2 immune responses. Importantly,
forward selection modeling identified 18 analytes, such as IL-18, IL-13,
and MMP-10, in infant serum that could be used to predict AD severity
four years later; therefore, these analytes might represent critical
nodes to better understand AD heterogeneity and disease progression. The
measurement of markers identified in Figure 6 should provide guidance
for therapeutic intervention strategies at early stages of life for
pediatric patients suffering from AD. This model will likely have
predictive value for AD persistence past infancy and will be useful in
further defining the pathogenesis of atopic disease allowing us to begin
to see ever branching nodes delineating the complex underlying immune
regulation. This, in turn, may guide therapeutic intervention and the
prognostic value of such serum protein measurement, outlining the need
for early treatment options in pediatric patients.
Our data highlights additional questions for the field regarding the
progression of AD as children age through adolescence into adulthood. We
did not observe elevation of IL-4 in the serum of these patients, which
could be attributed to the technical limitations of assays available at
time of testing, considering that IL-13 was measured on an
ultra-sensitive platform. We also note that though MCP-4 and TARC were
moderately correlated with disease severity, they did not factor into
the predictive modeling of disease progression possibly due to these
proteins being more broadly involved in atopic mechanisms beyond AD
[31]. Future studies with focused outcomes on other atopy measures
can provide predictive insight into mechanisms of wheezing or lung
function. Correlations to SCORAD allow us to minimize potential error
that is introduced in a longitudinal study, especially in pediatrics as
maturation and growth are heavily involved in immune system development.
This cohort and data included in this manuscript enhance our knowledge
regarding the systemic presentation of AD in children and potential
biomarker profiles which aid in predication of disease persistence or
resolution.