Letter to Editor
To the Editor
Egg-allergic children have higher ovomucoid (OVM)-specific IgD (sIgD)
levels compared to the atopic controls.1 Within the
egg-allergic group, children with higher levels of OVM-sIgD have a
decreased risk of anaphylactic reactions. Ovalbumin (OVA)-sIgD increases
in egg-allergic children desensitized by oral immunotherapy (OIT) but
not in children unresponsive to OIT or with sustained unresponsiveness
to OVA challenge.2 The natural development of
tolerance and the acquisition of sustained unresponsiveness by OIT in
egg-allergic children are associated with an increase in OVM-specific
IgG4 (sIgG4) levels and a decrease in OVM-specific IgE (sIgE)
levels.3 To elucidate the potential role of IgD in the
outgrowing of egg allergy, we analyzed levels of egg white (EW)-, OVM-,
and OVA-sIgD and sIgG4 in sera from 57 egg-allergic children (28 avoided
all forms of egg in the diet (complete avoidance of egg: CAE), 18 were
able to ingest at least 1/32 cooked whole egg but not one cooked whole
egg (partial avoidance of egg: PAE), and 11 outgrew egg allergy (OGE))
and 23 healthy non-egg allergic children (non-egg allergy: NEA) (Table
S1). The study was approved by The Research Ethics Committee of
University of Fukui (#20110052), and written informed consent was
obtained from the parent or guardians.
EW-, and OVM-sIgE levels measured using ImmunoCAP (Thermo-Fisher Inc.,
MA) were higher in the CAE group, followed by the PAE, OGE, and NEA
groups (Fig 1, Fig S1). The CAE group exhibited lower serum levels of
EW- and OVA-sIgD compared to the NEA group and the PAE group,
respectively, and had the lowest OVM-sIgD serum levels among all groups,
suggesting that OVM-sIgD levels are associated with outgrowing egg
allergy. We observed the lowest serum levels of EW-, OVA-, and OVM-sIgG4
in the CAE group, followed by the PAE and OGE groups. The ratio of
OVM-sIgD to OVA-sIgD increased as children outgrew egg allergy, whereas
the ratio of OVM-sIgG4 to OVA-sIgG4 did not change. Thus,
the production of OVM-sIgD differs
from OVM-sIgG4 as children naturally outgrow egg allergy.
High-affinity, but not low-affinity, IgE is known to cause
anaphylaxis.4 High-affinity IgE is derived from memory
IgG1+ B cells, whereas low-affinity IgE is derived
from naïve IgM+IgD+ B cells.
Considering class switching pathways, switching direction from IgM to
IgD, and from IgG1 to IgE or IgG4, elevated OVM-sIgD levels might be
associated with low-affinity OVM-sIgE levels as children outgrow egg
allergy, resulting in hypo-responsiveness to OVM.
A recent study found that OVM-sIgE avidity was more effective at
differentiating clinically reactive egg-allergic patients from those
tolerant of heated egg compared to EW-sIgE.5 The ratio
of OVM-sIgE to OVM-sIgD or sIgG4 in the CAE group was significantly
higher compared to the PAE, OGE, and NEA groups (Fig 2). Receiver
operating analysis revealed that the ratio of OVM-sIgE to OVM-sIgD
discriminated non-tolerant from partially-tolerant egg-allergic patients
with the largest area under the curve (AUC = 0.965) compared with levels
of OVM-sIgE or the ratio of OVM-sIgE to OVM-sIgG4. The optimal cutoff
for the ratio of OVM-sIgE to OVM-sIgD had 86.5% sensitivity and 96.4%
specificity to identify high-risk subjects (Table S2).
There are several limitations to this study. First, there was a small
number of patients. Second, all children were only challenged with
heated egg and were instructed to avoid egg of any form if they tested
positive with less than 1/32 cooked whole egg. Finally, there was a lack
of trajectory of sIgD levels during natural tolerance development.
In conclusion, the ratio of OVM-sIgE to OVM-sIgD is a useful marker to
identify high-risk egg-allergic patients capable of ingesting a low-dose
of cooked whole egg who might be a good candidate for low-dose OIT.