|
 How
can autoimmunity cause illness? Over the years, immunologists have debated
this issue regarding many different diseases. The consensus today is that
several criteria must be fulfilled to demonstrate an autoimmune etiology.
For those disorders in which a humoral etiology is proposed, autoantibodies
should be found in the sera of affected individuals. In addition, removal
of these autoantibodies should lead to an improvement of symptoms. Finally,
transfer of the autoantibodies to an experimental animal should reproduce
some of the clinical symptoms of the disorder. In this column, we review
the evidence that myasthenia gravis and Rasmussen’s encephalitis
are autoimmune disorders. Similar strategies will need to be used by investigators
attempting to demonstrate that illnesses such as Tourette’s syndrome
and obsessive-
compulsive disorder are caused by an autoimmune mechanism.
Myasthenia
gravis is a relatively rare neurological illness that affects approximately
1 of every 200,000 individuals. A key clinical finding is muscle weakness,
especially after repeated activity. Although the course of the illness
is highly variable, the disease typically affects eye and eyelid muscles
leading to double vision and ptosis. In more severe cases, muscles controlling
swallowing, speaking, and breathing become affected.
The
typical treatment is to administer inhibitors of acetylcholinesterase.
Acetylcholine is a neurotransmitter normally found at the neuromuscular
junction. To initiate a muscle contraction, the nerve terminal releases
the neurotransmitter acetylcholine into the synaptic cleft. The neurotransmitter
rapidly diffuses across and binds to acetylcholine receptors on the surface
of the muscle cell. The binding of neurotransmitter to its receptor leads
to a rapid change in ion flow across the membrane, which in turn propagates
a muscle contraction.
Acetylcholine
must be rapidly removed from the synaptic cleft to allow for multiple
signals to arrive at the muscle and to allow for repeated contractions
in sustained muscle activity. The rapid removal is accomplished in part
by the action of acetylcholinesterase that degrades the neurotransmitter.
The finding that drugs that inhibit this enzyme improved the muscle weakness
in patients with myasthenia gravis suggested that the illness was caused
by a disturbance in some component of the acetylcholine signaling pathway.
Little
progress was made, however, until the chance observations of two investigators
at the Salk Institute in 1973. Jim Patrick and Jon Lindstrom were interested
in determining the location of the nicotinic acetylcholine receptor within
the CNS. In order to do this, they wanted to generate antibodies against
the receptor that could be used in immunocytochemical localization studies.
Generating antibodies is a time-honored procedure by which investigators
produce a probe that will bind to and help visualize a specific protein
under investigation.
Typically,
rabbits are used to produce such antibodies. When a protein or a portion
of the protein is injected into a rabbit, the animal will mount a humoral
response against the foreign antigen. Large amounts of antibodies are
thereby produced that are capable of recognizing different portions of
the injected protein. Repeated boosts with the antigen lead to the production
of large amounts of antibodies in the sera of these animals. The antibodies
can then be separated from other components of the sera and used in immunocytochemical
studies within the CNS.
Unexpectedly,
the rabbits that were immunized with the acetylcholine receptor developed
severe muscle weakness. Patrick and Lindstrom saw a similarity between
the animals’ behavior and the weakness seen in patients with myasthenia
gravis. When they treated the animals with an acetylcholinesterase inhibitor,
the rabbits got better. For the first time, the specific hypothesis that
the acetylcholine receptor was the target of autoantibodies could be tested.
Relatively
quickly, researchers established that the sera of patients with myasthenia
gravis contained antibodies and that these antibodies recognized a subunit
of the acetylcholine receptor complex of proteins (Fig.
1). This was an important first step. It is currently believed that
the immunogenic portion of the acetylcholine receptor lies on one of the
five subunits that assemble to form the receptor. The a
subunit is not only the site for binding of acetylcholine, but it also
contains the amino acid sequence that elicits the antibody reaction. When
the autoantibody binds to the receptor, it is believed that the receptor
is internalized by the muscle cell and degraded. In some rare cases, the
binding of autoantibody to the a subunit blocks
access of acetylcholine to its binding site. The binding of autoantibodies,
however, initiates a series of events that promotes complement binding
and focal lysis of the postsynaptic membrane. The net result is that there
are fewer functional acetylcholine receptors at the neuromuscular junction.Muscle
cells are innervated by axons with terminal arbors that form synapses
at structures called neuromuscular junctions. The neuromuscular
junction is composed of a nerve terminal that releases acetylcholine neurotransmitter
and the postsynaptic component on the muscle cell where the acetylcholine
receptors are found. Autoantibodies produced in myasthenia gravis recognize
amino acid sequences on the a subunit of the
acetylcholine receptor. Binding of the antibody to the receptor is thought
to lead to its internalization, leading to a loss of functional receptors
at the neuromuscular junction, as well as the eventual attack of the muscle
cell by lymphocytes.}]
If
true, this decline in receptors would explain why treatment with inhibitors
of acetylcholinesterase is effective. The increase in the relative amount
of neurotransmitter at the neuromuscular junction compensates for the
loss of functional receptors. Removal of the antibodies from sera of affected
individuals leads to improvement of clinical symptoms. In fact, plasmapheresis
has now become a standard treatment for myasthenia gravis.
It
is interesting how history repeats. Twenty years later, in the same neurobiology
department at the Salk Institute, investigators discovered the target
of a second autoimmune disorder. Once again they did so by immunizing
rabbits with a receptor. This time they were interested in localizing
one of the glutamate receptors within the CNS. Surprisingly, some of the
immunized rabbits developed intractable seizures and neuropathological
brain lesions indistinguishable from those seen in humans with Rasmussen’s
encephalitis.
Rasmussen’s
encephalitis is a rare form of epilepsy that is associated with progressive
neurological dysfunction and destruction of a single cerebral hemisphere.
The disorder usually begins during the first decade of life with the appearance
of seizures, hemiparesis, and severe cognitive and language impairments.
The seizures are often unresponsive to standard antiseizure medications,
and surgical removal of the affected hemisphere is the standard treatment.
It was recently discovered that the etiology of this devastating disease
is an autoimmune response to one of the glutamate receptors, GluR3.
The
glutamate neurotransmitter system has captured the attention of so many
investigators for several reasons. Glutamate is the most abundant transmitter
within the CNS. It has been implicated in a wide range of complex neuronal
processes including development, apoptosis, learning, and memory. Its
capacity to participate in these processes is due to its ability to stimulate
a wide variety of intracellular signals, and this is due in part to the
large number of distinct receptors through which glutamate acts.
There
are at least 20 different genes that encode for glutamate receptors. These
receptors can be classified into two broad groups: the metabotropic receptors
and the ionotropic receptors. The metabotropic receptors are members of
the G-coupled family of receptors that are membrane-associated proteins
capable of stimulating a cascade of intracellular pathways when activated.
The ionotropic receptors also bind glutamate directly, but these receptors
act as ion channels. Binding to glutamate rapidly changes ion currents
across the cell membrane. The ionotropic receptors can be further divided
through their distinctive response to pharmacological reagents that activate
them: NMDA (N-methyl-D-aspartate), AMPA (a-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic
acid), and kainic acid.
The
ionotropic glutamate receptors are formed by several subunits that associate
with each other to form a pore through the cell membrane. Each receptor
subtype is formed by combinations of five protein subunits. In this way,
a large number of receptor subtypes can be constructed. It is in fact
the varying subunit compositions that determine exactly which ions are
allowed through the channels. The central pore formed in AMPA/kainate
receptors primarily allows passage of Na+ and K+ ions and in some cases
small amounts of Ca++, while activation of NMDA receptors results in the
influx of large calcium currents.
Once
it was discovered that many different subtypes of glutamate receptors
existed, investigators became interested in determining their localization
patterns. Why were there so many different glutamate receptors, and could
localizing them within the CNS clarify their varying functions? The group
at the Salk was particularly interested in studying the GluR3 receptor,
a subtype of the AMPA family. Rabbits were immunized with the GluR3 protein
in order to make specific antibodies against the receptor. As mentioned
above, several of the immunized rabbits developed intractable seizures
and, on histopathological examination, the brains showed a similar pattern
of perivascular lymphocytic infiltrate and microglial nodules that is
commonly seen in Rasmussen’s encephalitis.
This
initial study suggested that perhaps humans with Rasmussen’s encephalitis
developed their illness as a consequence of an autoimmune response directed
against the GluR3 receptor. Investigators quickly determined that anti-GluR3
antibodies were in fact present in the sera of affected individuals. In
addition, several patients responded dramatically to the removal of the
autoantibodies through plasmapheresis; this finding suggests that the
autoantibodies contributed at least in part to the progressive neuronal
loss and hemispheric atrophy so characteristic of the disease. Attempts
to create an animal model have not been successful to date.
One
question that has puzzled investigators is the fact that tissue destruction
occurs in only one cortical hemisphere. Circulating autoantibody that
crosses the blood-brain barrier should not show unilateral specificity,
especially as the antigen, the GluR3 receptor, is expressed at many sites
in both cortices. A proposed model is that the illness is initiated through
a local traumatic event (such as a blow) to the head that disrupts the
blood-brain barrier in a very limited region. Autoantibody is then able
to leak into a limited area of a single hemisphere.
Exactly
how the autoantibody causes cellular damage is also a matter of considerable
debate. One hypothesis is that the GluR3 autoantibody is excitotoxic.
This would occur if the binding of the antibody to the glutamate receptor
activates the receptor and leads to a massive influx of ions. Activation
of glutamate receptors is a well-known mechanism that precedes neuronal
cell death. Lymphocytic infiltration then occurs and causes local inflammation
and a further disruption of the blood-brain barrier which permits entrance
of additional damaging autoantibodies.
A
competing hypothesis is that the autoantibody that binds to GluR3 receptors
attracts specific components of the complement system. Complement cascades
are activated and lead to neuronal death and lymphocytic infiltrations.
Both hypotheses are being tested and would explain the progressive neuronal
death that occurs in this degenerative seizure disorder.
top of page
|
