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Previous studies have shown that two CCAAT/enhancer binding protein (C/EBP) binding sites (sites I and II) within the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) are critically important for efficient virus replication within cells of the monocyte lineage, a primary cell type infected by HIV-1. Sequence variation at C/EBP sites I and II has been shown to alter the affinity of C/EBP factors to these sites. Specifically, sequence variation within C/EBP binding site II has been shown to alter binding of purified C/EBP beta protein and basal activity of the HIV-1 LTR. We have previously demonstrated that the C/EBP site II consensus cladeB (ConB) variant was highly conserved in brain- and peripheral blood-derived LTRs of individuals with advanced HIV-1 disease. Given these important observations, the regional distribution of LTRs containing the C/EBP site II ConB variant derived from brain tissues of patients with and without HIV-1-associated dementia (HIVD) was examined. A statistically significant difference was found in the distribution of LTRs containing the C/EBP site II ConB variant in brain regions derived from patients with and without HIVD. In addition, we have previously shown that LTRs containing C/EBP site II 4C and 6G variants (designated according to the position at which nucleotide change occurred relative to ConB, followed by the actual nucleotide found at the variant position) were only found in brain tissue of patients with HIVD. As an extension of these observations, the regional distribution of LTRs containing C/EBP site II 4C or 6G variants derived from the brains of patients with HIVD was examined and a statistically significant difference was observed. We have shown that LTRs containing a low-affinity C/EBP site II 4C variant accumulated in the cerebellum. LTRs containing the 4C site variant in conjunction with the consensus cladeB (ConB) site I exhibited the lowest basal LTR activity of any of the LTRs examined. These results suggest that LTRs containing the C/EBP site II 4C configuration may promote the establishment of a latent provirus in the cerebellum, a region of the HIVD brain that exhibits little viral gene expression. Furthermore, LTRs containing a high affinity C/EBP site II 6G variant accumulated in the mid-frontal gyrus, a site of highly productive replication. In addition, LTRs containing the C/EBP site II 6G variant with the ConB at site I exhibited the highest basal LTR activity. In conclusion, distinct LTR populations with specific C/EBP site II configurations were found in different neuroanatomical regions of the brain, potentially due to differences in the molecular architecture of the LTR, viral entry pathways, and/or brain microenvironments.

This article reviews the changing epidemiology of HIV-associated dementia, current concepts of the different patterns of dementia under the influence of highly active antiretroviral therapy, and reviews therapeutic aspects.

The appearance and progression of human immunodeficiency virus type 1 (HIV-1)-associated pathogenesis in the immune and central nervous systems is dependent on the ability of the virus to replicate in these compartments, which is, in turn, controlled by numerous factors, including viral binding and entry, receptor and coreceptor usage, and regulation of viral expression by the long terminal repeat (LTR). The LTR promotes viral expression in conjunction with viral and cellular regulatory proteins, including members of the CCAAT/enhancer binding protein (C/EBP) family, which modulate LTR activity through at least two cis-acting binding sites. Previous studies have shown that these sites are necessary for HIV-1 replication in cells of the monocyte/macrophage lineage, but dispensable in T lymphocytes. To establish potential links between this important family of transcription factors and HIV-1-associated pathogenesis, C/EBP site I and II sequence variation in peripheral blood mononuclear cell (PBMC)-derived LTRs from HIV-1-infected patients with varying degrees of disease severity was examined. A high prevalence of C/EBP site variants 3T (site I) and consensus B (site II) within PBMC-derived HIV-1 LTRs was shown to correlate with late stage disease in HIV-1-infected patients. These results suggest that the increased prevalence in the PBMCs of HIV-1 LTRs containing the 3T C/EBP site I variant and the consensus B site II variant may serve as a molecular marker for disease progression within the immune system. The relative low or high binding affinity of C/EBP beta to sites I and II in electrophoretic mobility shift (EMS) analyses correlated with low or high LTR activity, respectively, in transient expression analyses during both early and late disease stages. The 3T C/EBP site I was the only variant examined that was not found in LTRs derived from PBMCs of patients at early stages of HIV-1 disease, but was found at increasing frequencies in patients with late stage disease. Furthermore, the 3T C/EBP site I was not found in brain-derived LTRs of patients without HIV-1-associated dementia (HIVD), but was found in increasing numbers in brain-derived LTRs from patients diagnosed with HIVD. The C/EBP site I 3T variant appears to be exclusive to patients progressing to increasingly severe HIV-1-associated immunologic and neurologic disease.

How does HIV infection lead to the development of central nervous system disease? Central to this question is identification of the relative contributions of (1) the virus, (2) its host cells, and (3) secondary or downstream events to the pathological process. These are re-examined in this brief review. Also, a greater appreciation for the role of systemic events in neuroinflammation is emerging, with likely relevance to HIV-associated dementia. We propose here a model for HIV neuropathogenesis that highlights the role of systemic monocyte activation and subsequent neuroinvasion in initiating the disease.

Recent studies have shown that two CAAT/enhancer binding protein (C/EBP) sites are critically important for efficient human immunodeficiency virus (HIV) type 1 (HIV-1) replication within cells of the monocyte/macrophage lineage, a primary cell type infected by HIV-1 and a potentially important vehicle for transport of virus to the central nervous system (CNS). Given the relevance of HIV-1 LTR sequence variation with respect to HIV-1 replication within monocyte populations and the important role that monocyte tropism likely plays in HIV-1 infection of the brain, C/EBP site sequence variation was examined within peripheral blood- and brain-derived LTR populations. Brain-derived LTRs commonly possessed a C/EBP site I configuration (6G, comprised of a thymidine to guanosine substitution with respect to the clade B consensus sequence at position 6 of C/EBP site I) that leads to enhanced binding of C/EBP proteins over that observed with the HIV-1 clade B consensus sequence at this site. In contrast, the 6GC/EBP site I configuration appeared infrequently within sequenced peripheral blood-derived LTRs. In addition, C/EBP site II was even more highly conserved in brain-derived HIV-1 LTR populations than site I. This was not the case with peripheral blood-derived LTR C/EBP site II sequences. The high degree of C/EBP site II conservation in brain-derived LTRs was likely important in LTR regulation since the clade B consensus sequence conserved at C/EBP site II recruited high amounts of C/EBP family members. Transient transfection analyses indicated that conservation of the strong C/EBP site II in brain-derived LTRs was likely due to important interactions with Tat. Overall, brain-derived HIV-1 LTRs preferentially contained two highly reactive C/EBP binding sites, which may suggest that these sites play important roles in LTR-directed transcription during invasion and maintenance of HIV-1 in the central nervous system.

Towards understanding the pathogenesis of HIV dementia, we molecularly cloned and sequenced human immundeficiency virus type 1 (HIV-1) gp160 genes from uncultured post-mortem tissues collected from a patient with HIV dementia. Sequences from bone marrow, lymph node, lung, and four regions of brain - the deep white matter, head of caudate, choroid plexus and meninges - were compared. Also included were gp160 sequences recovered from blood monocytes collected 5 months prior to death. Phylogenetic analyses showed that the sequences from deep white matter were more closely related to those from bone marrow, than to those from the other tissues, and moreover, were most closely related to sequences from the blood monocytes. These findings suggest trafficking of bone marrow-derived monocytes into the deep white matter during this late stage of infection. Another cluster included sequences from choroid plexus, meninges and lymph node, and interestingly, identical patterns of four or nine stop codons were shared among these tissues. These mutations appear to be the consequence of G-->A hypermutation, and could reflect independent events, or the movement of virions or infected cells, from the choroid plexus into the cerebrospinal fluid and ultimately, into the lymph node. We propose that a critical step towards the development of HIV dementia is an increase in monocyte trafficking into the brain, and that this process is either initiated and/or accelerated during late-stage infection, which could explain why dementia occurs primarily during this time.