The overall goal is to develop an animal model to examine the role of the blood brain barrier (BBB) in HIV-induced neuropathogenesis. The BBB is a network of capillary endothelial cells and perivascular astrocytes located at the blood-CNS interface that restricts the transendothelial passage of blood-borne substances into the brain. Several groups have reported abnormal BBB permeability in AIDS patients, suggesting that barrier perturbation plays a role in neurologic disease. Normally, BBB integrity is maintained by specialized features of brain capillary endothelial (BCE) cells, such as the presence of intercellular tight junctions and restricted transcellular transport mechanisms. BCE cells are susceptible to HIV infection in vivo and in vitro. Infection of BCE cells may alter BBB physiology and influence the progression of CNS disease. Firstly, infection of BCE cells may allow HIV passage across the BBB. Virus entry may be either a direct consequences of BCE cell infection or due to extravasation of infected leucocytes in response to HIV-induced endothelial adhesion molecules. Secondly, infection of BCE cells may increase vascular permeability by disrupting tight junctions or altering transcellular transport mechanisms. We have shown that different strains of HIV display a preferential tropism for human BCE (HBCE) cells. Evolution of endothelial cell tropic HIV strains in the periphery many therefore predict initial invasion of the CNS and influence the progression of neurologic disease. Investigating these important parameters is not however feasible in the human system due to the impossibility using defined inocula or of controlled sampling of CNS tissue. The simian system is a good candidate for an animal model to study HIV infection of the brain, because inoculation of rhesus macaques with SIV results in a neuropathology similar to that described in AIDS patients. We have recently demonstrated that a neuroinvasive strain of SIV productively infects simian BEC (SBEC) cells, but not the parental strain from which is evolved. In this project we will use the simian model to identify viral determinants of SBCE cell tropism, to follow the evolution of SBCE cell tropism in vivo, and to define the role of SBCE cell tropic V for development of BBB malfunction. In summary, these experiments should allow definition of viruses with an increased potential to cause CNS disease and a greater understanding of the mechanisms of BBB malfunction.