Transplantation immunology of the brain as a privileged site for neural grafting.

The course of intracerebral transplant rejection differs from rejection of grafts placed elsewhere in the body. There are many factors which may modulate immune responses in the central nervous system (CNS). Low expression of major histocompatibility gene complex (MHC) products on nervous tissue and the existence of the blood - brain barrier (BBB) seem to be the central components of this immune protection. Lymphatic drainage of the brain is limited, yet antigens introduced into the brain are drained to the lymph nodes. Some investigators highlight a lack of dendritic cells in the CNS, however, microglia, astrocytes and probably endothelial cells may act as antigen presenting cells in certain circumstances. Brain residual perivascular macrophages found in the Virchow-Robin spaces may be also involved in the process of graft recognition and rejection. Some neural cells (e.g. astrocytes) produce local immunosuppressive factors which may also contribute to prolonged neural graft survival. All these factors are not able to protect neural allo- and xenografts from rejection response. The rejection of neural intracerebral allo- and xenografts occurs suggesting the brain immune privilege is not absolute.