CNS and inflammation

CNS immune privilege (or not?) The central nervous system (CNS) immune privilege is an experimentally defined phenomenon. Tissues that are rapidly rejected by the immune system when grafted in sites, such as the skin, show prolonged survival when grafted into the CNS. Initially, CNS immune privilege was construed as CNS isolation from the immune system by the blood-brain barrier (BBB), the lack of draining lymphatics, and the apparent immunoincompetence of microglia, the resident CNS macrophage. Moreover, except for astrocytes, there is no constitutive expression of MHC molecules in the cells of the CNS. The most convincing evidence of all is that tissue transplanted from one individual into the brain of another individual survives for extended periods of time. The 'immune privilege' of the CNS is indispensable for damage limitation during inflammation in a sensitive organ with poor regenerative capacity. It is a longstanding notion which, over time, has acquired several misconceptions and a lack of precision in its definition. There is no doubt that the brain differs significantly from other tissues in its responses to pathogenic challenges. Infection or inflammation elicits rather different responses in the brain to those in other tissues. This is most evident in leukocyte recruitment, which is rapid in many systemic organs, but modest and delayed in the brain. In spite of these notable differences, the brain does exhibit key features of inflammation (Table 1).