TABLE 1.

Characteristics of the two CNS immune compartments: (a) the brain ventricular immune system, and (ii) parenchymal immune systems

(a) The brain ventricular immune system

Anatomy: -comprises the choroid plexus, ventricles, CSF and meninges -characterized by the presence of all vascular and lymphatic channels, and immune cells, found throughout other organs of body, i.e., skin, lungs, etc.

Physiology: -elicits innate and adaptive immune responses to challenge with infectious particulate antigens (i.e., BCG), soluble antigens (i.e., ovalbumin), and viral-vector-mediated gene transfer (i.e., Adv, AAV, lentivirus).

(b) The brain parenchymal immune system

Anatomy: -comprises the CNS parenchyma proper -lacks classical lymphatics, but outflow from CSF and brain parenchyma to cervical lymphatics has been described -presence of blood–brain barrier -lacks professional afferent APCs (i.e. DCs) able to prime naive lymphocytes by migrating from the brain to secondary lymphoid tissue in the absence of inflammation -myeloid-derived, but not lymphoid-derived, DCs are present in the brain only during infectious, autoimmune- or DTH-induced brain inflammation, or following expression of the DC growth factor Flt3L -presence of monocyte-derived cells (e.g. macrophages and microglial cells) -presence of all complement-activation pathways -presence of proteosomes

Physiology: -elicits only innate immune responses to challenge with infectious particulate infectious antigens and viral-vector-mediated gene transfer -CTL responses are not primed following challenge with infectious particulate antigens or viral vectors -neutralizing antibody responses are not primed following challenge with infectious particulate antigens -slow recruitment of neutrophils