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 Useful
animal models for the investigation of autoimmune disorders that involve
the brain are essential, especially in children in whom access to the
CNS is limited. The paucity of such models can hinder an understanding
of causation, pathogenic mechanisms, and treatments. This is particularly
true when one is investigating postulated antibody-mediated disorders
in which evidence supporting pathogenesis is derived from in vitro studies.
The interpretation of such studies is confounded by the frequent nonpathogenic
antineuronal antibody binding associated with chronic disorders, such
as autism, type 1 diabetes mellitus, as well as normal aging. Differentiating
pathogenic from nonpathogenic antibodies cannot be done without a biological
assay such as an animal model. This need is seen in the ongoing discussion
of the pathogenesis of autoantibodies in childhood Tourette syndrome (TS),
obsessive-compulsive disorder (OCD), pediatric autoimmune neuropsychiatric
disorders associated with streptococcus (PANDAS), and Sydenham chorea
(SC).
An
animal model should reflect the type of immune response occurring or postulated
to occur in the brain, utilize brain regions involved in the disorder,
and maintain the brain’s unique immune environment. When studying
putative immune brain disorders, it is useful to initially group disorders
by the presence or absence of acute inflammation. This distinction is
important because it determines whether the normal immunosuppressive environment
of the brain parenchyma is maintained. Acute inflammation is associated
with the disruption of the blood-brain barrier (BBB), a major anatomical
and physiological barrier for peripheral immune responses. Loss of this
barrier allows the peripheral immune cascade to operate within the brain
parenchyma without its normal down-regulating influences. This inflammatory
disruption of the BBB in humans is most often associated with vasculitis
or perivasculitis of the cerebral vessels.
In
addition to categorizing autoimmune-mediated brain disorders by the presence
or absence of acute inflammation, it is useful to separate them further,
into one of the four established hypersensitivity responses. In general,
autoimmune responses in the nervous system are either type 2 (antibody-mediated,
often g-immunoglobulins [IgG]) or type 4 (cell-mediated)
hypersensitivity. The immune-mediated mechanism postulated in TS, SC,
OCD, and PANDAS has been conceptualized by most investigators as a noninflammatory
antibody-mediated response, a type 2 response. This is a reasonable assumption
because the evidence supporting acute inflammation in these disorders
is meager. For this reason, further discussion of animal models will be
limited to models in which antibodies are the predominant immune effectors.
This is not to indicate that other humoral components, such as cytokines,
do not have a pathogenic role. However, current evidence suggests that
if they have a role, it is complementary.
Several
animal protocols are available for studying noninflammatory type 2 hypersensitivity
while maintaining the BBB and the brain’s immunosuppressive environment.
Peripheral immunization with neuroantigens or neural tissue is one approach.
Studies of an autoimmune mechanism in Parkinson disease were conducted
by immunizing guinea pigs with bovine substantia nigra. Nigral injury
was detected in the guinea pigs, although it was not expressed clinically.
Similarly, antibodies generated by immunizing rabbits with glutamate 3
receptors (GluR3) bound to cortical GluR3 and resulted in seizures (see
this column, XXX). Subsequent identification of anti-GluR3 antibodies
in childhood Rasmussen’s encephalitis suggests that an immunization
protocol is a useful animal model for further investigations of this disorder.
The
immunization model is most effectively studied when the neuroantigen involved
in a disorder is known. This knowledge is not yet available for many disorders
such as TS, PANDAS, or SC. Results need cautious interpretation because
immunological processes in this model are only partially understood. How
do serum antibodies gain access to brain regions with an intact BBB? Does
this protocol affect only cortical surface antigens? Is perivasculitis
a prerequisite? How dependent is the neural effect on the peripheral immunization
protocol, i.e., use of Freund’s adjuvant and booster immunizations?
Brain dysfunction has not been reported in rabbits immunized with other
glutamate receptors. Why? Immunizing different strains of mice with GluR3
results in a range of postimmunization serum titers, brain lesions, and
the absence of seizures. What is the influence of genetic background on
the process?
Infusion
of antibodies into CSF (subarachnoid or intraventricular) is a second
experimental design that is used for studying putative antibody-medicated
neural disorders. This is, in essence, a modification of the traditional
immunological technique of adoptive transfer. The approach is an effort
to determine whether antibodies are pathogenic by passively transferring
them from an affected individual to naïve animals, thereby inducing
the disorder in the animals. Induction of ataxia in mice after subarachnoid
infusion of anti-GluR1 antibodies isolated from patients with paraneoplastic
syndromes is an example of this approach.
The
simplicity of this protocol is appealing, but the slow movement of IgG,
a globular macromolecule, through the dense tortuous cellular architecture
and metabolic active brain environment can confound results. Infused antibodies
are rapidly diluted and are removed with CSF turnover. If deep brain structures
are targeted, the size of the brain will influence the duration of time
that a particular concentration of antibodies is maintained in the CSF.
Diffusion will facilitate movement over short distances, but movement
over longer distances utilizes bulk flow, which can have a heterogeneous
distribution. Longer infusions also must control for the possibility that
the infusion rate and volume per se may alter the brain environment, leading
to abnormal behavior. The specificity of the response to the IgG will
also diminish as the duration increases because its movement throughout
the brain is not confined. This can be minimized by infusing monoclonal
antibodies, an option that is not available for most disorders.
An
alternative to CSF infusion is direct infusion of antibodies into specific
brain regions. Although CSF infusion is less invasive, direct infusion
into the brain offers specificity. Using this approach, we have demonstrated
that some TS-IgG cross-reacts with neural antigens and causes behavioral
changes in rats after infusion into lateral striata. Direct infusion into
a brain region allows an investigator to select regions implicated in
a disorder. The lateral striatum was selected for TS-IgG infusion because
stereotypic behaviors, analogous to those in TS, had been induced by pharmacological
manipulation of this region. Behavioral changes were also induced after
the subthalamic nucleus, which is postulated to have a role in SC, was
infused with SC sera. During placement of a cannula the BBB is breached,
resulting in a temporarily alteration of the brain’s environment.
Therefore, it is critical to allow adequate time for the BBB to reestablish
itself before infusion. As with CSF infusion, the influence of the infusion
rate, duration, concentration, and volume on the brain microenvironment
must be minimized.
Direct
infusion into brain regions has also been used to study mechanisms by
which antibodies can gain entry to a brain with an intact BBB. In this
approach, an antigen is infused into the brain and animals are subsequently
immunized with the same antigen. This produces high titers of serum antibodies
specific for a unique, albeit foreign, brain antigen. The infusion of
specific antigens allows investigators to predetermine the location (at
the cannula tip) at which neuroimmunological activity should occur (Figure
1). This greatly limits the search for an effect and facilitates the
separation of antigen-specific antibodies from constituent antibodies.
Using this protocol, we found that antigen-specific B lymphocytes traffic
from the peripheral circulation to the striatum and, after encountering
their cognate antigen, transform into antibody-secreting plasma cells.
Clinical
and experimental interest in antibody-mediated brain dysfunction under
noninflammatory conditions is growing. In TS, OCD, PANDAS, and SC, clinical
and immunohistochemical studies have uncovered an association with serum
antibodies. Emerging animal models are now available for investigation
of causal relationships. However, until we understand more about these
models and neuroimmunological mechanisms operating in the unique brain
environment, results from animal studies require a cautious extrapolation
to the human situation.
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