Celiac disease is an autoimmune disease that has become increasingly prevalent in
recent years. It is an immune reaction
that occurs with gluten, which is a protein found in wheat, barley, and malt. This reaction causes inflammation, which
damages the small intestine. You may be
thinking that wheat, malt, and barley are in everything! So, what do people who suffer from celiac
eat? As someone who suffers from celiac,
I can tell you the transition to a gluten free diet wasn’t easy, but luckily
there are a lot of good, gluten free alternatives that have been
developed. Even though celiac can be
helped with dietary accommodation, it is still important to try to understand
what is causes the autoimmune response in order to potentially develop a
treatment for celiac disease someday.
Unfortunately, it is not well understood how the autoreactive B cells
become activated in celiac, but it is known that celiac disease is associated
with autoantibodies produced that are specific for the transglutaminase-2 (TG2)
enzyme. In the paper Transglutaminase 2-Specific Autoantibodies
in Celiac Disease Target Clustered, N-Terminal Epitopes Not Displayed on the
Surface of Cells, the authors Iversen et al. investigated the mechanism
that controls the formation of the TG2 autoantibodies.
Transglutaminase-2
is an enzyme that is involved in the deamination of
glutamine residues, during them into glutamic acid. This process increases their affinity for the
HLA molecules associated with the disease, thus increasing the reactivity of
gluten. The anti-TG2 autoantibodies are
an important marker of celiac disease, but it is unknown how these
autoantibodies are contributing the disease.
In the lab’s previous research, they have identified various monoclonal
(target 1 particular antigen) TG2-reactive antibodies (mAbs), and they have
found that the antibodies target distinct (but close or overlapping)
regions. They define these regions as
epitopes 1-4 throughout the paper.
In order to
determine the specificity of the mAbs, they stained the small intestinal tissue
sections with immunofluorescent dye in either wild-type or mice deficient in
TG2. There was only fluorescence (indicating mAb
binding) in the tissues containing TG2, which suggests that the mAbs are highly
specific to TG2. They also attempted to
determine whether the mAbs would react with other members of the
transglutaminase family, TG3 and TG6, which have also been associated with
celiac disease. They did not find any
reactivity, which indicated that autoantibodies against other transglutaminases
in celiac disease are created in dependently than the anti-TG2 autoantibodies.
TG2 has
both an open and closed conformation. It
was thought that the epitopes on TG2 targeted by the autoantibodies were
conformational, it would be expected that the mAbs would differ in binding
strength to the two conformations. Using
an ELISA assay, the authors used the natural conformational regulators
of TG2, Ca2+ (open conformation) and GTP (closed conformation) to
measure the binding strength of the mAbs in each of the conformations. The binding affinity of the mAbs was
increased by Ca2+, but decreased by GTP. This suggests that in celiac disease the open
conformation of TG2 is the one that is targeted by the autoantibodies.
In a recent study done by Simon-Vecsei et al. , they identified an epitope targeted by celiac disease
serum autoantibodies. The authors wanted
to know if their mAbs would react with the newly discovered epitope. They constructed a triple mutation in the
epitope and measured the binding strength with the mAbs by ELISA. These mutations resulted in loss in
reactivity for the mAbs in the epitope 2 group and the epitope 3 group. This suggests that while this epitope is one
of the major targets of autoantibodies in celiac disease, it is not the only
one since the mAbs in the epitope 1 and 4 groups do not interact with it.
TG2 is
found both in the cytosol and on the cell surface. The authors wanted to determine how the mAbs
interact with TG2 in the different locations.
When TG2 is expressed on the cell surface, the mAbs did not react with
it. This indicates that the epitopes are
hidden when TG2 is bound to the surface.
When they tested TG2 inside cells, they found that all the mAbs did
react with TG2. They also tested the
cytosolic TG2 for binding with polyclonal sera (antibodies from the blood) of a
celiac patient and found that antibodies also recognize TG2.
Main paper: Rasmus Iversen, Roberto Di Niro, Jorunn Stamnaes, Knut E. A. Lundin, Patrick C. Wilson and Ludvig M. Sollid. Transglutaminase 2−Specific Autoantibodies in Celiac Disease Target Clustered, N-Terminal Epitopes Not Displayed on the Surface of Cells. J Immunol 2013; 190:5981-5991.
Images:
http://www.scientificamerican.com/media/inline/celiac-disease-insights_1.jpg
http://celiac.org/wp-content/uploads/2009/11/celiac.gif
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