The Science

The Emerging Picture of Persistent Measles Vaccine Virus in Autism
Andrew Wakefield FRCP FRCPath
The possible association between MMR vaccine, persistent measles virus infection, intestinal inflammation and autism was first suggested by a team of medical researchers from the Royal Free Medical School, London, in 1998 (1). The same research team, in collaboration with researchers from Trinity College, Dublin and Paediatric Gastroenterologists from the Royal Free Hospital, have since confirmed, in a series of further scientific studies, the validity of the original study’s findings.

This research and additional work by independent physicians and scientists in the US, has now provided the following pieces of evidence:
►Many children with regressive autism and intestinal symptoms have a novel and characteristic inflammatory disease of their intestine (1-4), a finding that has been confirmed by independent studies in the US (5-7).
►This disease is not found in developmentally normal control children (2-4).
►This disease is consistent with a viral cause (8).
►This disease appears to be associated with raised blood ammonia, intestinal dysbiosis (overgrowth of harmful bacteria) and cognitive improvement in response to orally-administered, luminal-acting antibiotics, as with hepatic encephalopathy and D-lactic acidosis in patients with short-bowel syndrome. As such, the intestinal disease in children with autism may be the source of toxic damage to the developing brain (9-11).
►As one possible source of toxic damage to the brain, the disease is associated with opioid peptiduria from bovine casein (?-caseomorphine 1-7) and gluten (gluteomorphine) substrate with cognitive response to dietary exclusion in a blinded controlled study (12).
►Measles virus (MV) genomic RNA has been identified in the diseased intestine in the majority of children with regressive autism studied, precisely where it would be expected if it were the cause of the intestinal pathology, that is, the reactive lymphoid follicles (13).
►MV genomic RNA has been found in only a small minority of developmentally normal children (13).
►The strain of MV in the diseased intestine of autistic children, as determined by allelic discrimination, is consistent with being from the MMR vaccine (14).
►Affected children appear to have an abnormal antibody response to MV (I5, 16) that is associated with the presence of autoantibodies to myelin basic protein.
►These findings are consistent with many parental reports that their normally developing child regressed into autism following exposure to MMR vaccine.

Epidemiological studies
In population-based studies an association between MMR vaccination and autism has not been found (17-22). Ecological studies have shown a rise in autism following the introduction of MMR but until recently, all epidemiological studies that have examined a causal relationship specifically between MMR and autism, were inadequate. Madsen et al (22) recently summarised the status of current epidemiological evidence, stating, “The evidence [for a negative association between MMR and autism] is weak and based on case-series, cross-sectional and ecologic studies. No studies have had statistical power to detect an association and none had a population-based cohort design”. They went on to report a population based cohort study in which, once again, no association was found. The principal limitation of their study was that they failed to take account of the proposed relationship between MMR and autism (see below), and failed to disaggregate the relevant autism subset – one which they accurately described in the introduction to their paper – from the overall autism population. More importantly, however, these studies failed to test the proposed hypothesis (New England Journal of Medicine; Wakefield AJ. Measles, mumps, and rubella vaccination and autism. N Engl J Med. 2003;348:951-954)

Defining the hypothesis
Our studies have been concerned with examining the aaetiology and pathogenesis of autism in a subset of children who regress after a period of normal development and suffer an immune-mediated gastrointestinal pathology. Because of referral bias to a GI clinic, we are not in position to say what proportion of the autism population this subset represents. Moreover, it is only possible to describe the parameters of this disease now that we have investigated in excess of 200 similarly affected children and have made comparisons with appropriate control groups. We have not, at any stage, put forward a hypothesis that has been examined at the population level. One of the key questions in seeking to understand this phenomenon better is “what factors might make a particular subset of children susceptible to an adverse reaction to MMR, when for the majority it appears safe?” Within the relevant subset of children we have observed a high prevalence, within the first year of life, of atopy (especially food allergy) including cow’s milk intolerance, antibiotic use, ear infection, multiple concurrent vaccine exposure and a strong family history of atopy and autoimmune disease, as reported by others (23).

In 1999 we wrote, “At the level of the immune response, the newborn tends towards a TH2 response to pathogens and gradually shifts towards a TH1 response with age. If this transition does not take place appropriately, the infant is likely to be at greater risk of mounting aberrant immune responses in later life, as seen in patients with allergies. Given that, under normal circumstances the age of this transition will be different for different children, it seems inevitable that a ubiquitous viral exposure of all 15-month-old children could induce an immune response that is consistent with the individual dynamics of this TH2-TH1 transition.” (24)

The TH1-TH2 paradigm an oversimplification of T-lymphocyte functionality, but it serves as a template for current purposes. We had hypothesised that susceptibility to an adverse reaction to a live viral vaccine might be determined by the immune status of the child at the time of MMR administration. This hypothesis is not difficult to justify, since live viral vaccines are contraindicated in those with certain immunodeficiencies. Therefore, according to the hypothesis, a precursor to an adverse reaction to MMR vaccine would be a genetic and/or acquired aberrant immune programming towards a T helper cell type 2 (TH2) bias (24). A TH2 immune bias, that has been described in affected children (3,25), would increase the likelihood of an inadequate antiviral immune response in the face of a live viral vaccine and may facilitate viral persistence, as described for measles virus in affected children. MMR vaccine itself, further promotes TH2 skewing of the immune system (25) and might exacerbate mucosal immune dysregulation. In affected children, the reported excess TH2 cytokine profile in peripheral blood mononuclear cells and intestinal lymphocyte populations, in association with eosinophilic infiltration of the intestinal mucosa is consistent with such a proposition.

Therefore, the key to defining the “child at risk” is an examination of the co-factors that may interfere with, or modify the appropriate TH2-TH1 transition prior to, or concomitant with, MMR exposure. One of the key factors may be organic mercury.

Mercury, Neurodevelopmental Disorders and Response to Infection
The hypothesis outlined above, was put forward prior to our knowledge that certain infant vaccines contained mercury. Since it was proposed, several lines of evidence have indicated a role for the preservative thimerosol (ethyl mercury) in childhood developmental disorders. Thimerosol is present at potentially toxic levels in many infant vaccines, from which it is now being removed. Both organic and inorganic mercury – the effects of which are qualitatively similar – have profound, toxic effects upon the immune system Methyl mercury and to a lesser extent mercuric chloride inhibit most human and animal lymphocyte functions including proliferation, expression of cell activation markers and cytokine production (27), and have been linked to allergic sensitisation and autoimmune disease.

The immunosuppressive and immunomodulatory effects associated with mercury exposure are accompanied by increased susceptibility to challenge with infectious agents. Murine susceptibility to Leishmania major infection is genetically restricted, and is dependent upon a TH2 skewed response to infection. Resistant animals, that exhibit a TH1 response to L.major, are rendered susceptible by prior exposure to mercury. In resistant animals, subtoxic doses of mercuric chloride induce an autoimmune syndrome characterised by the expansion of TH2 cells, IL-4 production by splenocytes and IgG1 and IgE production (28). Prior mercury exposure in previously resistant mice induced a non-healing phenotype with increased footpad swelling and increased parasite burden. The authors conclude that these findings have implications for mercury contamination in areas of endemic leishmaniasis.

Methyl mercury enhances the immune damage and chronicity of coxsackie B3 myocarditis in mice, compared with mice infected without prior mercury exposure (29), Similarly, mercuric chloride exposure significantly impaired macrophage mediated resistance to generalised infection with herpes simple type-2 in a murine model (30).

Thimerosol may induce TH2 immune skewing in humans. Thimerosol is a potent allergic sensitiser. The clinical expression of a TH2-skewed immune response in humans would be suggested by an excess risk of allergic diseases such as asthma, eczema, hay fever, rhinitis etc in thimerosol recipients compared with controls. In support of this, Hurwitz and Morgenstern identified a significant excess risk for allergy among 13,328 recipients of DPT vaccine compared with unvaccinated controls in the recent NHANES III study (31), consistent with the findings of previous population-based studies (32-33).

Epidemiology may not be able to address the proposed hypothesis. The rapidly expanded infant vaccination schedule in the US, the differential regimen of infant vaccine exposure between countries, the heterogeneity of autistic spectrum disorders, the apparently idiosyncratic nature of thimerosol-associated adverse effects upon the immune system and the absence of prospectively collected data sets make this a likely impossible task. Relevant and necessary adjustments would reduce the statistical power to an untenable extent.

Summary
The data are consistent with the hypothesis that, as a consequence of prior immunomodulatory exposures, including to mercury containing vaccines, a proportion of the paediatric population are at excess risk of expressing an inappropriately biased immune response. Such a response may lead to an inadequate antiviral response in the face of a challenge with a live virus vaccine, with subsequent measles virus persistence and chronic immunopathology. Chronic viral immunopathology in the GI tract might cause failure of detoxification combined with the egress of neurotoxins to the systemic circulation and the CNS, where they may impact upon the rapidly developing brain (34-40). An analogous encephalopathic response occurs in hepatic encephalopathy, short bowel syndrome, and infantile intussussception. Alternatively, cerebral autoimmunity and/or direct viral invasion by, for example measles virus, might account for the encephalopathy in affected children.

Alternatively, although not exclusively, cerebral autoimmunity and/or direct viral invasion by, for example measles virus, or direct mercury neurotoxicity might account for or contribute to the observed encephalopathy in affected children. In light of the detection of vaccine strain measles virus in bowel biopsies, blood and CSF by one laboratory, there is an urgent need to either confirm or refute these observations in an independent data set, using an independent laboratory.

References
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