Dendritic cells in the initiation of immune responses against central nervous system-derived antigens

Antigen presentation is essential for the activation and maintenance of antigen-specific T cell responses in the nervous tissue. Generally, it is becoming well accepted that the antigen presenting cell (APC) type responsible for the initiation of the primary immune response through the exclusive ability to activate naïve T cells is the dendritic cell (DC); however, the role of these cells in central nervous system (CNS) immunity is unclear at this time. The diverse phenotypes and origins of DCs make the characterization of their function in the CNS even more difficult. It is believed that DCs can influence the immune response in several ways: these cells are not only capable of initiating the immune response but they are also a major determinant of peripheral tolerance. DCs are characterized by the constitutive ability to express MHC class II molecules as well as high-level upregulation of these molecules in response to inflammatory stimuli. A pan DC marker that has proved to be useful in identifying them is CD11c (the alpha-chain of CR4); other markers include CD205 and MHC class II. DCs also actively participate in the humoral immune response. In this review, we would like to discuss how DCs appear in the CNS and their roles in initiation, maintenance and tolerance in the immune reactions in the CNS.