Neuroimmunology of Autism Spectrum Disorder
David Marc, B.Sc.a, & Kelly Olson Ph.Da.
a NeuroScience, Inc., 373 280th St., Osceola, WI 54020, United States
Autism is a developmental disorder characterized by immunological and neurological abnormalities. The role of cytokines in the pathophysiology
of autism has been researched suggesting a relationship with altered blood-brain barrier permeability and subsequent neuroinflammation.
Cytokine recruitment to the CNS may result in altered neurotransmitter signaling and the behavioral manifestation of autism symptoms. Other
immune mediated events such as changes in the number and activity of natural killer cells, macrophages, immunoglobulins, and glutathione may
contribute to altered neuronal signaling and neurotransmitter imbalances. The purpose of this overview is to examine the relationship between
immune system and nervous system dysfunction to determine biomarkers for autism spectrum disorder. We will explore the utility of serum
cytokines and urinary neurotransmitter analyses as biomarkers for autism.
in the central nervous system (CNS) thereby establishing
Autism is a pervasive developmental disorder
communication between peripheral immune cells and CNS
characterized by impaired development of social interaction and
neurons (Dunn, 2006). The purpose of this overview is to identify
communication, and a markedly restricted repertoire of activities
neurological and immunological abnormalities that exist in
and interests (American Psychiatric Association, 1994). The exact
individuals with autism. Further, it will become critically apparent
etiology of autism remains largely unknown, however, literature
that neuroimmune biomarker testing for autism can identify these
has emerged to suggest genetic, neurological, immunological, and
abnormalities and ensure therapeutic effectiveness.
environmental contributions. Immunological and environmental
CYTOKINES AND NEUROTRANSMISSION
factors, such as diet, infection, and xenobiotics play critical roles in
Cytokines released by immune cells, particularly
the development of autism. (Ivarsson, Bjerre, Vegfors, and
interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α),
Ahlfors, 1990; Wakefield et al., 1998; Edelson and Cantor, 2002;
communicate with the CNS to affect neural activity and modify
Fatemi et al., 2002; Kibersti and Roberts, 2002). Abnormalities in
behaviors, hormone release, and “normal” autonomic function
enzymatic function (Fatemi et al., 2002a), autoantibodies to brain
(Dunn, 2006). Cytokines can enter the brain by various
proteins (Vojdani et al., 2002), and maternal infections during
mechanisms including active transport or direct entry through a
pregnancy (Shi et al., 2003) have been indicated in the autism
compromised blood-brain barrier. Active transport mechanisms
population. Additionally, pathological alterations in genes
that involve a saturable system have been documented for IL-1 and
involved in the patterning of the central nervous system,
TNF-α (Dunn, 1992; Gutierrez, Banks, and Kastin, 1993;
biochemical pathways, development of dendrites and synapses,
Gutierrez, Banks, Kastin, 1994). Additionally, Maier and
and genes associated with the immune system have been observed
colleagues (1998) found that cytokines may directly enter the
in this population (Burber and Warren, 1998; Palmen, Engeland,
Hof, and Schmitz, 2004; Polleux and Lauder, 2004; Cohen et al.,
predominantly the area postrema, where the blood brain barrier is
2005; Crawley, 2007; Glessner et al., 2009; Wang et al., 2009).
less protective (Pavlov et al., 2003). Other circumventricular
Interestingly, an emerging body of evidence is growing
regions of potential cytokine entry include the pineal gland,
concerning the link between abnormal immune function and
subfornical organ, organum vasculosum of the lamina terminalis,
neurological dysfunction with autism spectrum disorders. At
choroid plexus, median eminence, subcommissural organ, and
critical times of infantile development, immune dysregulation may
posterior pituitary (Ganong, 2000).
result in the release of immunomodulatory molecules, such as
Upon entry into the CNS, cytokines promote regulatory
chemokines and cytokines, leading to altered neuronal
signals in the brain, through augmentation of hypothalamic-
development and neural function (Cohly & Panja, 2005).
pituitary-adrenal (HPA) axis activity and vagal efferents, which
Chemokines and cytokines are proteins that manage
can modify peripheral immune status. Enhanced HPA axis release
immune cell trafficking and cellular arrangement of immune
of epinephrine and cortisol can decrease the release of pro-
organs and determine appropriate immune responses (Borish &
inflammatory cytokines from macrophages in the periphery
Steinke, 2003). Cytokines can be transported to and/or synthesized
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(Pavlov et al., 2003). In addition, enhanced vagal efferent activity
(IgG4) production in children with autism. Elevated IgG
can trigger the release of acetylcholine from peripheral
antibodies have been identified against brain-specific proteins in
parasympathetic nerve endings, decreasing the release of pro-
the hypothalamus and thalamus of autistic children, again
inflammatory cytokines (Pavlov et al., 2003). It is therefore
suggesting autoimmunity (Cabanlit et al., 2007).
evident that the immune system and nervous system communicate
Although limited studies on autism and autoimmunity
to maintain homeostasis, yet under excessive immune challenges
exist, it has been hypothesized that the excess transport and
alterations in neuronal signaling can develop.
synthesis of proinflammatory chemokines, cytokines, and
Studies have shown that peripheral activation of
immunoglobulins from the periphery to the CNS contribute to the
cytokines can lead to CNS release of various neurotransmitters.
development of autoimmune responses (Cohly & Panja, 2005).
Specifically, IL-1 administration may promote CNS release of
Autoimmunity may lead to dysregulated neuronal signaling
norepineprhine, serotonin, dopamine, glutamate, and gamma-
causing behavioral manifestation of autism symptoms. Therefore,
amino-butyric-acid (GABA) (Dunn, 1992; Zalcman et al., 1994;
assessment of immune and nervous system function may provide
Casamenti et al., 1999; Luk et al., 1999; Huang and O’Banion,
biochemical targets to treat patients with these behavioral
1998). With enhanced turnover of these neurotransmitters,
significant neurological and behavioral alterations transpire.
NERVOUS SYSTEM BIOMARKERS AND AUTISM
Research has shown how immune challenges can alter
Biomarkers are substances used as indicators of a
neurotransmission leading to behavioral changes and psychiatric
biologic state. Research has revealed the clinical utility of urinary
disorders (Kronfol & Remick, 2000). For example, elevated
neurotransmitters as practical biomarkers to associate with
levels of interleukin-6 (IL-6) have been associated with depressive
neurotransmission (Kusaga et al., 2002; Hughes et al., 2004).
symptoms (Bob et al., 2009).
Urinary neurotransmitter analysis is an innovative, minimally
In Autism, alteration in immune system function may
invasive method to assess peripheral neurotransmitter levels, and
contribute to impaired neurological signaling. A possible
has a breadth of data to support its usefulness in clinical practice.
mechanism contributing to neuronal dysfunction in the autistic
In the 1950’s, research uncovered correlations between urinary
brain is the transport of noxious substances across the blood-brain
catecholamine levels and psychiatric symptoms, such as depression
barrier into the CNS leading to autoimmunity. Studies have shown
and anxiety (Bergsman, 1959; Carlsson et al., 1959). Recent
how cytokines, chemokines, immunoglobulins, and natural killer
research has examined the utility of urinary neurotransmitter
cells promote the recruitment of noxious chemicals in the brains of
analysis to categorize subsets of depression and anxiety, and to
autistic individuals, as well as contribute to autoimmunity
determine pharmaceutical intervention(s) (Hughes et al., 2004;
(Ashwood et al., 2006). Proinflammatory chemokines, such as
Otte et al., 2005). Notwithstanding, urinary neurotransmitter
monocyte chemotactic protein-1 (MCP-1) and thymus activation-
analysis can further be used to assess Attention-Deficit-
regulated chemokine (TARC), along with cytokines, such as TNF-
Hyperactivity Disorder (ADHD). Subjects with ADHD tend to
α, were consistently elevated in the brains of individuals with
have decreased urinary monoamine neurotransmitter levels
autism (Cohly & Panja, 2005). The transport or synthesis of
(specifically, beta-phenylethylamine (PEA)) that can impair mood
cytokines in the brain may contribute to neuroinflammation and
and attention (Kusaga et al., 2002). What’s more, decreased beta-
possible neurotransmitter imbalances (Cohly & Panja, 2005).
PEA levels may contribute to symptoms of inattentiveness (Berry,
Furthermore, Ashwood and colleagues (2008) found that reduced
levels of the modulatory cytokine, transforming growth factor-β1
Overall, urinary neurotransmitter assessment can be a
(TGF-β1), in autistic children contributed to the dysregulation of
useful tool in any clinical practice, especially those managing
adaptive behaviors and predisposal for autoimmune responses.
psychiatric disorders. Urinary neurotransmitter analysis can
Autoimmunity can be detrimental to normal neuronal signaling and
identify neurotransmitter abnormalities that may contribute to
result in significant behavioral abnormalities (Ashwood et al.,
behavioral changes, and thereby allow more appropriate treatment
2006). Vojdani and colleagues (2008) reported decreased natural
selection (Kahane, 2009).
killer cell activity in autistic children with low intracellular levels
In autism, urinary neurotransmitter analysis has been
of glutathione, IL-2, and IL-15. Decreased natural killer cell
utilized to examine biochemical abnormalities. As such, urinary
activity has been associated with autoimmunity through alteration
serotonin has been the primary urinary neurotransmitter evaluated
of cytokine production (Johansson et al, 2005). Lastly, Entrom
in autistic individuals. Abnormalities in urinary serotonin have
and colleagues (2009) demonstrated elevated immunoglobulin G4
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been linked to immunological disturbances. A recent study found
is due to decreased numbers of Purkinje cells located in the
consistent elevations in the number of mast cells, along with
cerebellum. Altered Purkinje cell population can eventually lead
elevated levels of urinary serotonin, in autistic patients (Castellani
to disrupted and weakened motor coordination (Palmen, Engeland,
et al., 2009). Food, stress, or viruses can stimulate mast cells in the
Hof, & Schmitz, 2004). Taken together, abnormal brain growth
intestines and brains of young children. Localized and systemic
could be another factor that can contribute to peripheral
immune activation can lead to enhanced cytokine and serotonin
neurotransmitter imbalances and behavioral manifestation of
release from mast cells and disruption in the lining of the intestines
and the blood-brain-barrier causing altered neuronal signaling
What’s more, abnormal neural development and
(Castellani et al., 2009). As mentioned previously, a compromised
function may result from cytokine recruitment to the CNS and
blood-brain barrier permits noxious substances entry into the brain
therefore amino acid and neurotransmitter alterations (Cohly &
and contribute to neuroinflammation. CNS neurotransmitter
Panja, 2005). Changes in amino acid levels may lead to elevated
abnormalities may result from neuroinflammation leading to
or insufficient neurotransmitter activity and thus can interfere with
normal cognitive development (Aldred, et al., 2003). During
As identified in autistic individuals, raised peripheral
infancy and adolescence, maintenance of optimal neuronal
glutamate levels may also result from a compromised blood-brain
signaling is essential to ensure normal development of attentional
barrier (Moreno-Fuenmayor, et al, 1996, Yip, 2007). Elevated
processes, memory, and overall cognitive function, lending
plasma glutamate has been attributed to decreased levels of its rate
credence to the importance of early intervention through laboratory
limiting enzyme glutamic acid decarboxylase (GAD) in autistic
analysis of neurotransmitters and cytokines.
individuals (Shinohe, 2006, Yip, 2007). Specifically, Fatemi and
colleagues (2002a) and Yip and others (2007) reported a reduced
Immune system and nervous system activity must be
number of GAD 65 and 67 proteins in Purkinje cells in autistic
viewed and examined as one system functioning in parallel. It is
cerebella. The decreased GAD may be due to autoantibodies
specific for GAD, which has been detected in various neurological
abnormalities exist in autistic individuals, however, the
disorders (Manto et al., 2007). These autoantibodies attack the
relationship between neural and immune function has just recently
body's own cells, tissues, and/or organs, causing inflammation and
been emphasized. Food, stress, and viruses can activate immune
tissue damage. Because GAD converts glutamate to gamma-
cells in the periphery and result in CNS disruptions. This may lead
immunobutyric acid (GABA), a decrease in this enzyme will cause
to inflammation in the brain and eventually to behavior changes
subsequent increases in glutamate levels (Yip, 2007). Clinically,
(Castellani et al., 2009). Healthcare practitioners should
high glutamate levels can be excitotoxic and may lead to
understand and evaluate the status of the nervous system together
neurodegeneration and cognitive dysfunction (Ha et al., 2009).
with the immune system to best optimize therapeutic
Studies have demonstrated that particular biochemical
intervention(s). Through the development of innovative laboratory
measurements, such as in plasma amino acid levels, are elevated in
tests to analyze neurotransmitters and cytokines, comprehensive
children with autism when compared to controls. Autistic children
information can be obtained to determine neurological and
demonstrated elevated levels of plasma glutamate and aspartic acid
immunological abnormalities. These biochemical measures can
along with taurine, phenylalanine, asparagine, tyrosine, alanine,
serve as biomarkers for clinical symptoms, as well as provide
and lysine (Moreno-Fuenmayor, Borjas, arrieta, Valera, and
significant guidance for therapeutic selection to reestablish
Socorro-Candanoza, 1996; Aldred, Moore, Fitzgerald, and Waring,
physiological homeostasis and to benefit overall health and
2003). These amino acid alterations may be caused by immune
neurotransmitter transport, or metabolic derangement.
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