The over arching hypothesis is that the brain is an important reservoir for retroviruses because of their ability to infect long lived terminally differentiated cells and viral products released from these cells can cause immune activation and neuronal injury Aim 1: To understand the mechanism of viral persistence in brain If there is any hope to eradicate HIV, close attention to the viral reservoirs in the brain is necessary. They brain is a unique site of viral latency since it infects resident macrophages/microglia and astrocytes. These cells have very low turnover rate, and the mechanism of viral entry and persistence is very different than that of lymphocytes which are the major cell type infected by the virus in the lymphoid organs. Our laboratory has focused on studying the mechanism of viral entry in astrocytes. We have found that while free viral particles can enter these cells, cell to cell contact with lymphocytes is the most efficient way to infect astrocytes. The mechanisms by which T cells facilitate viral entry is being explored. We have discovered that the virus enters by using CXCR4 and is aided by formation of tight junctions between the cells. We have also found that upon viral entry, the virus can enter the endolysosomal pathway which acts as a host defense mechanism. Hence strategies than modulate these pathways could have a significant effect on the establishment of a reservoir in the brain. Two manuscripts are currently being prepared that detail these findings.Hoever the turnover rate of the cells in the brain (microglia and astrocytes) is also critical to the eradication of the reservoir, hence we are also undertaking a study in a mouse model of inflammation to determine if the turnover rate of these cells may be altered during the state of inflammation. These studies have recently been initiated.
Aim 2 : To investigate the mechanism of neuronal injury by HIV and endogenous retroviruses Despite the use of antiretroviral agents and excellent control of the virus in the periphery, HIV infected patients continue to develop cognitive impairment. Currently available antiretroviral agents have no effect on the production of early viral proteins once the virus has integrated into the chromosome. One of these proteins, Tat, has been shown to be neurotoxic. Our laboratory was one of the first to demonstrate its toxic potential and we are now investigating the mechanisms by which it causes neurotoxicity. We have found that the protein can cause synaptic injury at very low concentrations without causing neuronal death. We have characterized the proteins and the morphological changes at the level of the dendrites in human neurons and are further investigating the underlying mechanisms. Using a similar approach we are investigating the mechanisms by which the envelop protein of an endogenous retrovirus-K causes neurotoxicity. We have cloned the gene into an expression vector, created a transgenic line that expresses the protein and are characterizing the pathological findings. Several in invitro experiments are also underway to determine the mechanism by which neuronal injury takes place.
Aim 3 : To investigate the mechanism of HIV-induced lymphocyte activation and subsequent neuronal injury Patients treated with antiretroviral drugs may occasionally develop a T cell mediated encephalitis that can be fatal. Our group has characterized this syndrome as the CNS-immune reconstitutions syndrome and we were one of the first to characterize the clinical and pathological features of this entitity. We have also found these similar clinical syndromes can occur in patients with JC virus infection and we have published several papers characterizing the clinical entity. We have reasoned that because of the inability to control the viral replication fully in the CNS reservoirs, the lymphocytes traffic to the brain along an antigenic gradient. We have shown that the Tat protein can activate lymphocytes in vitro and these activated T cells can cause significant neurotoxicity by the release of granzymes. We have found that the Tat protein enters T cells and then activates these cells in a NF-kB dependent manner. These findings have been recently published (Johnson et al., PNAS 2013) We have also shown that the activated T cells can be detrimental to neural progenitor cells and prevent their differentiation. Similarly they are toxic to neurons which is mediated by effects of granzyme on PAR-1 on the neurons however we found that activated T cells can cause proliferation of olgioprecursor cells. These paradoxical effects are likely mediated by different receptors on these cells and suggest that the role of the T cells is very complex. This remains an active area of investigation in our lab. In summary, we have shown that astrocytes in the brain are an important reservoir for HIV and that cell to cell contact with lymphocytes is necessary for viral entry and the lysosomal pathway in these cells regulates the intracellular trafficking of the virus and its ultimate ability to successfully infect these cells. Further, we have shown that the HIV protein Tat and the env protein of endogenous retrovirus-K are neurotoxic and we are now the underlying mechanisms involved in these effects. Finally, we have also discovered that the Tat protein of HIV can stimulate T cells in a T cell receptor independent manner using a unique mechanism. These activated T cells cause neuronal injury via the release of granzyme B that acts on cell receptors causing a cascade of events leading to neuronal dysfunction.
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