FAQs: The meaning of neuroinflammatory findings in autism
  1. What type of immune reactions are present in the brain of autistic patients?
    In our study, we have demonstrated a marked increase in neuroglial responses, characterized by activation of microglia and astroglia, in the brains of autistic patients. These increased neuroglial responses are likely part of neuroinflammatory reactions associated with the central nervous system's (CNS) innate immune system. In innate immune reactions of the CNS, microglial activation is the main cellular response to CNS dysfunction. This is in contrast to adaptive immune responses, in which lymphocyte and/or antibody mediated reactions are the dominant responses. In our sample of autistic patients, microglial and astroglial activation was present in the absence of lymphocyte infiltration or immunoglobulin deposition in the CNS. It also was associated with increased production of pro-inflammatory and anti-inflammatory cytokines such as MCP-1 and TGF-1 by neuroglia.
  2. Is neuroinflammation always present in the brain of autistic patients?
    NOT necessarily. Since autism is a disorder that is highly variable in the ways it presents, and may be associated with multiple causes, it is possible that our sample of cases does not represent the entire autistic spectrum. Also, some of our patients had other associated neurological disorders frequently found in autism, such as epilepsy and mental retardation. However, the presence of microscopic and immunological findings showing neuroimmune reactions in all of our autistic patients and the cytokine findings in the cerebrospinal fluid (CSF) support a potential role for neuroglia and neuroinflammation in the CNS effects in a number of individuals with autism.
  3. Why are neuroglial activation and neuroinflammation relevant to the neurobiology of autism?
    The presence of increased neuroglial responses is relevant to the neurobiological mechanisms involved in autism, as both microglia and astroglia are essential for neuronal activity and synaptic [neural transmission] function, neuronal-neuroglial interactions, as well as for cerebral cortex modeling, organization and remodeling during brain development. Furthermore, microglial and astroglial activation seems to play a major role in the neuroimmune mechanisms of disease in the CNS. These cells are part of the first-line response of the innate immune system of the CNS. They contribute to the modulation of immune responses by producing both pro-inflammatory and anti-inflammatory cytokines as well as growth and differentiation factors.
  4. Are microglial and astroglial reactions always bad for the brain?
    NO. The microglia and astroglia in the CNS may have a two-sided role in the inflammatory responses of the brain: they can act both as direct effectors of injury and on the other hand as protectors of the brain. In some situations, microglia and astroglia may produce neurotoxic reactions that damage other cells such as neurons and oligodendrocytes in the brain. However, there is strong evidence from experimental models that in some situations, both microglia and astroglia also contribute to the repair and restoration of neuronal connections and produce growth factors to maintain normal CNS function.
  5. Are microglial and astroglial activation the result of residual damage or ongoing damage to the brain?
    It remains unclear how and when microglia and astroglia become activated in the brain of autistic patients. Neuroglial responses in autism may be part of primary (intrinsic) reactions that result from disturbances in neuroglial function or neuronal-neuroglial interactions during brain development. They may also be secondary (extrinsic), resulting from unknown factors that disturb prenatal or postnatal CNS development (e.g. infections, toxins, etc). Both astrocytes and microglia are critical for brain development. MHC class II (HLA-DR antigen)-positive microglia colonize the developing CNS during the second trimester of pregnancy. It is possible that the presence of activated microglia in the brain in autism may reflect abnormal persistence of fetal patterns of development. This may be in response to genetic or environmental (e.g. intrauterine, maternal) factors. Our study of brains of autistic patients showed that regardless of age, history of epilepsy, developmental regression or mental retardation, microglial and astroglial reactions were consistently present. Our findings support the view that chronic and sustained neuroglial inflammatory responses are part of an ongoing and active process rather than a residual change. These changes may be involved in mechanisms associated with abnormal function of neurons and synapses in the brain in autism.

    Although our studies did not show any difference in neuroglial activation among patients with autism with respect to history of developmental regression or mental retardation, further studies that include larger series of cases may help to clarify whether these factors contribute to the ongoing neuroinflammatory responses.
  6. Is the neuroinflammation observed in autism similar to encephalitis or meningitis ?
    NO. In meningitis and encephalitis, the most prominent immune reaction is one of adaptive immunity, the main feature being infiltration of the CNS by inflammatory cells such as T lymphocytes and B lymphocytes along with the production of antibodies. There is also activation of astroglia and microglia in meningitis and encephalitis, but the dominant immune reaction is due to adaptive immunity. In contrast, in autism, there is NO evidence of lymphocyte infiltration or antibody mediated reactions; the most prominent immune response is characterized by activation of microglia and astrocytes, features that characterize innate immune responses within the CNS. These observations suggest that the adaptive immune system does not play a significant pathogenic role in this disorder, at least not during its chronic phase, and that the main immune mechanism involves predominantly innate immune reactions. Since our study focused on autopsy tissues, we cannot exclude the possibility that specific immune reactions, mediated by T-cell and/or antibody responses, occurred at the onset of disease, during prenatal or postnatal stages of development.
  7. Is there any brain region particularly affected by neuroinflammation?
    Yes. Our study showed the cerebellum exhibited the most prominent neuroglial responses. The marked neuroglial activity in the cerebellum is consistent with previous observations that the cerebellum is a major focus of pathological abnormalities in microscopic and neuroimaging studies of patients with autism. Based on our observations, selective processes of neuronal degeneration and neuroglial activation appear to occur predominantly in the Purkinje cell layer (PCL) and granular [needs to be defined] cell layer (GCL) areas of the cerebellum in autistic subjects. These findings that are consistent with an active and ongoing postnatal process of neurodegeneration and neuroinflammation. Our observations suggest that the pathological changes observed in the cerebellum in autistic patients do not occur exclusively during prenatal development, but appear to involve an ongoing chronic neuroinflammatory process that involves both microglia and astroglia. Furthermore, this process continues beyond early neurodevelopment, and is even present at very late stages in the life of patients with autism. These findings also support the hypothesis that selective vulnerability of Purkinje cells plays a role in the pathological process of autism
  8. Is there any other evidence to support the presence of neuroinflammation in the brain of autistic patients?
    Yes. Our study has also demonstrated the presence of unique profiles of cytokine expression in the brain and CSF of subjects with autism. Two pro-inflammatory chemokines, MCP-1 and TARC, and an anti-inflammatory and modulatory cytokine, TGF-1, were consistently elevated in the brain regions studied. MCP-1, a chemokine involved in innate immune reactions and an important mediator for monocyte and T-cell activation, and for trafficking into areas of tissue injury, appeared to be one of the most relevant proteins found in cytokine protein array studies. It was significantly elevated in both brain tissues and CSF. The presence of MCP-1 is of particular interest, since it facilitates the infiltration and accumulation of monocytes and macrophages in inflammatory CNS disease. Our immunocytochemical studies of the cerebral cortex and cerebellum showed that MCP-1 is produced by activated and reactive astrocytes, showing that these cells play an effector role in the disease process in autism. The increased expression of MCP-1 has relevance to the pathogenesis of autism as we believe its elevation in the brain is linked to microglial activation and perhaps also to the recruitment of additional macrophages and microglia to areas of neurodegeneration, such as those we observed in the cerebellum.
  9. . Is the elevation of MCP-1 unique to autism?
    No. Our observations resemble findings in other neurological disorders in which elevation of MCP-1 is associated with the pathogenesis of neuroinflammation and neuronal injury. These diseases include HIV dementia, ALS, stroke and multiple sclerosis. It remains unclear whether MCP-1 plays multiple roles in the CNS or whether its presence is only associated with inflammatory conditions. It has been speculated that MCP-1 may be involved in neuronal survival and brain protection mechanisms in addition to monocyte activation and trafficking or even in non-lymphocytic-mediated neuronal injury. Expression of MCP-1 in the CNS appears to be developmentally regulated, and previous studies have shown its expression in the cerebellum during prenatal development, a finding that may suggest an association with maturation of Purkinje cells. Like MHC-class II expression in microglia during CNS modeling, MCP-1 elevation in the brain of autistic patients may reflect persistent fetal patterns of brain development.
  10. What is the role of other cytokines found in the brain of autistic patients?
    Our study found that other cytokines with pro-inflammatory and anti-inflammatory effect were also increased in the brain of patients with autism. An example of anti-inflammatory cytokines is TGF-1, a key anti-inflammatory cytokine involved in tissue remodeling following injury. It can suppress specific immune responses by inhibiting T-cell proliferation and maturation and downregulates MHC class II expression. In our immunocytochemical studies, TGF- 1 was localized mostly within reactive astrocytes and neurons in the cerebellum. Purkinje cells that exhibited microscopic features of degeneration showed marked reactivity for TGF-1. These findings suggest that the elevation of this cytokine in autism may reflect an attempt to modulate neuroinflammation or remodel and repair injured tissue.
  11. What is the significance of the cerebrospinal fluid findings in autistic patients?
    Cerebrospinal fluid (CSF) studies also confirmed a prominent inflammatory cytokine profile in patients with autism. The presence of a marked increase of MCP-1 in CSF supports the hypothesis that pro-inflammatory pathways are activated in the brain of autistic patients. This increase in MCP-1 may be associated with the mechanisms of macrophage/microglia activation observed in the brain tissue studies. The elevation of MCP-1 in the CSF resembles observations in other conditions in which microglia/macrophage activation is important, such as in HIV dementia and multiple sclerosis, diseases in which neuroinflammation plays a prominent role. In addition to the marked elevation in MCP-1, the presence of elevated levels of IFN?, IL-8, IP-10, angiogenin and LIF strongly supports the view that active neuroinflammatory reactions and a network of multiple cytokines are likely involved in immune-mediated mechanisms in the CNS of autistic patients. These cytokines play important roles in immune mediated processes and their presence in the CSF in autistic patients may reflect an ongoing stage of inflammatory reactions associated with neuroglial activation and/or neuronal injury. There was a greater increase in these cytokines in the CNS compared to the brain tissue. The reasons for the difference is unknown.. It could be that the brain cytokines are produced from neuroglial and neuronal sources as demonstrated by our immunocytochemical assessment. The cytokines in CSF could result from other sources of production, such as the meningeal lining around the brain or choroid plexus (where CSF is produced) The persistent elevation of cytokines in CSF also might reflect a neurodevelopmental arrest, as some of the cytokines are normally elevated during phases of neurodevelopment. Since the CSF is easily accessible for clinical studies, CSF cytokine profiling may be useful in the future to diagnose, characterize and follow the clinical course of autistic disorders.
  12. If there is neuroinflammation in the brain of some autistic patients, is treatment with anti-inflammatory or immunomodulatory medications indicated?
    At present, THERE IS NO indication for using anti-inflammatory medications in patients with autism. Immunomodulatory or anti-inflammatory medications such as steroids (e.g. prednisone or methylprednisolone), immunosupressants (e.g. Azathioprine, methotrexate, cyclophosphamide) or modulators of immune reactions (e.g. intravenous immunoglobulins, IVIG) WOULD NOT HAVE a significant effect on neuroglial activation because these drugs work mostly on adaptive immunity by reducing the production of immunoglobulins, decreasing the production of T cells and limiting the infiltration of inflammatory cells into areas of tissue injury. Our study demonstrated NO EVIDENCE at all for these types of immune reactions. There are ongoing experimental studies to examine the effect of drugs that limit the activation of microglia and astrocytes, but their use in humans must await further evidence of their efficacy and safety.