ABSTRACT
HIV mainly infects cells bearing the CD4 surface molecule, which acts as a specific receptor for the viral envelope protein, gp120. Such cells are found predominantly within the immune system and include T-helper lymphocytes, monocytes and antigen-presenting cells. However, there are also CD4+cells within the central nervous system [1], these being the microglial cells which are of monocyte or macrophage lineage. These cells can be productively infected by HIV in vitro, and in vivo there is evidence of an HIV-induced cytopathic effect since syncytia-like, multi-nucleated cells are seen in the brains of HIV-infected individuals. This CD4-defined tissue tropism explains the major pathological effects of HIV, which are immunodeficiency and neurological disease. However, HIV may also cause damage at sites where CD4 is not expressed. This may be a result of direct infection of CD4-cells, in which low levels of viral replication can occasionally occur [2] (Table 4.1), or due to infiltration of tissue by HIVinfected lymphocytes and macrophages, which release toxic viral proteins and/or pro-inflammatory cytokines.
