Infections and Inflammation in the Brain and Spinal Cord: A Dangerous Liaison

Abstract

Unicellular microorganisms developed on Earth approximately 3–4 billion years ago, and since the evolution of modern man (Homo sapiens) about 200,000 years ago, there been a close interaction between them which has not always been beneficial for the host. Diseases resulting from microbial infections have for long been a bane of human society and with the discovery of viruses and prions, the array of infectious agents has further widened. An infectious agent may target either specific or multiple cell types, organs, or organ systems. As a response to the infections, the body ‘fights back’ with its own set of defenders, i.e., the immune system. In this chapter, we focus on the various types of infections that can affect our central nervous system (CNS), arguably the most complicated organization of matter that we have the knowledge about, and the immune responses against them. The CNS had been long considered to be ‘immune-privileged’ due to its apparent separation from the rest of the body by specialized barriers. However, these barriers have been found to be dynamic in nature, regulating the flow of material across them. Also, the cells in the brain are themselves equipped with various mechanisms to detect the presence of the infectious agents and respond accordingly to contain or neutralize the threat posed by them. The response mechanism often results in a condition termed as inflammation, which in itself is a complex process involving multiple mediators. Inflammation is often referred as ‘double-edged’ sword as, if un-controlled, it results in severe damage to the host itself. In a non-regenerating organ system such as the CNS this has detrimental ramifications that are commonly termed as neurodegeneration. Thus, in this chapter we have strived to provide the reader not only on the different types of pathogens infecting human CNS but also the immune response associated with them. More specifically, we have tried to provide information about how these pathogens are detected/recognized by cells of the CNS, how the cells respond following the detection, and how is the response regulated (if there is any regulation at all). Even though we have separated the response against bacteria, viruses, fungi, parasites, and prions in different sections, the readers will no doubt notice a certain degree of overlap in the mechanism of response of these different types of pathogens indicating the plasticity of the immune system. However, there is some uniqueness associated with each pathogen infection which makes the immune systems task even more difficult. In this chapter, we have strived to incorporate multitude of information in a concise manner; however, we do stress that this is by no means all-encompassing. Hence, the readers are encouraged to follow-up any particular point of interest from the cited publications.