| Abstract |
Multiple sclerosis is an immune-mediated demyelinating disease of the central nervous system. It is considered as a multifactorial disease that arises from a complex reaction of genetic, autoimmune and environmental factors. The investigation of the interaction between the environmental factors and the immune system is of paramount importance because the environment is a modifiable risk factor and thus, the identification of the responsible environmental risk factors could enable the prevention of the disease. At the end of the 19th century, multiple sclerosis was proposed to be of infectious origin. Since then, the theory of the microbial origin of the disease has flourished due to the mainly indirect identification of several microbial factors in multiple sclerosis. Although the evidence that proves the causative association between microbes and multiple sclerosis do not lead to safe results, microbes and especially viruses probably play a critical role in the regulation of the immune system of genetically susceptible individuals and thus in the triggering or exacerbation of the disease. The present review is giving a detailed presentation of the existing data of the international literature that analyze the possible role of infections in the pathogenesis of multiple sclerosis. Furthermore, the possible biological mechanisms that may lead to the disease will be presented. The main mechanisms are molecular mimicry, bystander activation and the persistent microbial infection of the central nervous system. Taking into account the possible detrimental effect of bacterial and viral infections in multiple sclerosis, vaccines appear to be extremely important for the prevention of the disease in genetic susceptible individuals and patients with multiple sclerosis. Nevertheless, the safety of this effective tool against infections has been disputed due to its association from time to time with severeadverse events such as multiple sclerosis. In 1998, the french authorities decided to suspend the school-based routine vaccination against hepatitis B virus due to the increasing number of case reports of multiple sclerosis soon after vaccine administration. This decision had a negative impact on vaccine compliance of the French population. For this reason, the conduction of epidemiological studies and the review of the results of the existing data is of critical importance. The purpose of the present review is to summarize the existing data about the possible association between vaccines and the development or exacerbation of multiple sclerosis. In overall, the existing studies do not indicate any change in risk of developing multiple sclerosis after vaccination against hepatitis B virus, human papilloma virus, seasonal influenza, measles and poliomyelitis virus. No change in risk of relapse was found for influenza vaccine. Further research is needed in order to clarify the potential therapeutic use of the bacillus Calmette-Guerin (BCG) vaccine as well as the potential protective role of the vaccines against tetanus and diphtheria. In conclusion, the possible biological mechanisms of vaccines that could cause the disease as well as the role of each component of the vaccines are presented.
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