Dendritic Cells in Inflammatory Responses in the CNS

Abstract

There are a number of bone marrow derived mononuclear phagocytes associated with the central nervous system. These include: microglia in the brain parenchyma, perivascular macrophages and macrophages in the meninges and in the choroid plexus¹. MHC class Il is not expressed on microglia in the normal rodent brain, but it can be rapidly upregulated during an inflammatory response. Other macrophages associated with the CNS express MHC class II constitutively. Although the expression of MHC antigens is a prerequisite for antigen presentation, it is not by itself sufficient to stimulate T cell responses. In vitro studies by Fontana and his colleagues showed that both astrocytes and microglia purified from the brains of new born animals, cultured and stimulated with INF-γ can present an antigen to already primed T cells². In contrast, recent in vitro studies have shown that MHC class II+ microglia rapidly isolated from the adult rat brain are poor accessory cells, even when presenting an antigen to primed T cells, but other macrophages associated with the CNS can efficiently stimulate already primed T cells³. Using bone marrow chimeras Hickey and Kimura showed that perivascular macrophages are important accessory cells for the induction of immune responses in the CNS following peripheral activation of T cells⁴. However, it is still debatable whether any of the MHC class II+ cells in the CNS can induce a primary immune response.