Biomarkers of brain damage in HIV infection

HIV budding from an infected cell. Credit: NIAID

The global HIV/AIDS epidemic is still hitting hard. During 2008, there were 33.4 million people living with HIV and an estimated 2.7 million people newly affected, according to the World Health Organization. These high figures inevitably led to a large number of death from AIDS, an estimated 2 million in that year.

There have been improvements in HIV treatment that control the infection and lengthen the life expectancy of infected people, but 95% of the infections and deaths occurred in developing countries where nutrition and health care are challenging.

There is still some confusion between HIV and AIDS. The initial infection is brought about by the human immunodeficiency virus (HIV) which attacks the white blood cells called CD4+ cells. These are involved in helping other types of immune cell to seek and destroy germs within the body. As HIV infection progresses, more CD4+ cells are killed and the ability to fight infection is weakened.

The acquired immunodeficiency syndrome (AIDS) represents the later stages of HIV infection when a person has precariously low levels of CD4+ cells and suffers from other infections such as pneumonia, tuberculosis, fungal infections and cancer.

A further late-stage problem is dementia, brought about by cognitive deterioration of the brain. Many autopsied brains from affected individuals have revealed the presence of HIV-encephalitis. However, it is not always easy to determine when someone infected by HIV is suffering from brain injury because the condition is clinically silent. It may show up in brain imaging scans but these are expensive to operate and not amenable to screening.

Some biomarkers of neurological damage have been proposed but there are no well-validated procedures as yet. This deficiency in HIV research has prompted researchers in the USA to undertake a proteomics study in search of biomarkers of HIV-induced brain injury, which might be able to indicate the degree of progression and help develop ways to preserve cognitive function.

Ann Ragin and co-researchers from Northwestern University, Johns Hopkins University, the Harvard-MIT Division of Health Science and Technology, North Shore University Health System (Evanston, IL) and the Children's Memorial Hospital, Chicago, cooperated in the study. Their subjects were eight men and two women aged 38-62 who were in advanced-stage HIV infection.

The neurological status of each individual was determined by several standard tests, including the Karnofsky performance scale for assigning functional status and the Memorial Sloan Kettering procedure for cognitive impairment. In addition, the viral loads, absolute CD4 cell counts, body mass index and haemoglobin levels were measured.

The participants were subjected to two principal tests to assess changes that had taken place over a three year period. The levels of 22 potential biomarkers, measured by a proteomics technique, were correlated with the progression of brain injury, determined by quantitative magnetic resonance imaging in vivo.

The biomarkers were determined in plasma using Luminex-based technology in which fluorescent, colour-coded microspheres were coated with antibodies to trap the specific protein targets. The beads were then passed through a laser beam to measure the fluorescence intensities.

The MRI studies used automated brain segmentation algorithms to derive volume fractions of grey and white matter and cerebrospinal fluid. Diffusion tensor imaging (DTI), which has been shown to be sensitive to changes in the brain induced by HIV, was also employed to measure the diffusivity of water molecules.

The data from the tests at year zero revealed a correlation between the level of the monocyte chemoattractant protein-1 [MCP-1, also known as chemokine (C-C motif) ligand 2] and the loss of white matter integrity due to axion disruption.

Three years later, the correlation of MCP-1 to brain injury was more extensive. It was linked to whole-brain DTI measurements, as well as brain volumetric measurements, including reduced grey matter and increased CSF volume. It also correlated with changes in brain parenchyma volume. These neurological modifications were consistent with irreversible neuronal loss and atrophy, both common occurrences in HIV-dementia.

The inferred involvement between MCP-1 and brain damage and resultant cognitive impairment backs up several reported studies. HIV is imported into the brain on activated monocytes in the blood and MCP-1 enhances their progress. Dementia has been linked with the levels of activated monocytes entering the brain. MCP-1 may also be involved with viral replication within the brain.

The combination of multiplexed proteomics techniques with automated image analysis is ideal for high-throughput analysis, requiring little human intervention. It holds great promise for identifying further factors associated with neurological regression in HIV infection, which can be clinically silent, and could ultimately help in the preservation of brain and cognitive function.