Ten years ago, few people over 50 were being treated for HIV; today they make up one third of the HIV- positive population. Dr. Alan Winston, with the Department of Medicine at Imperial College of London, noted that this is due both to more effective therapy and to more people being infected after age 50. Dr. Winston leads the POPPY study in the UK, a large cohort which compares people over 50 living with HIV to a control group of HIV-negative people with similar lifestyle characteristics.
Definitions of disease and aging
The function of all organs, including the brain, wanes with age; once decline occurs beyond a certain threshold we consider it disease. An insult to the body (infection, drug toxicity, etc.) can increase the rate of decline; this is called accelerated aging. Premature aging occurs when an insult causes a decline in organ function, but the decline does not continue at a faster rate. Real world circumstances are complex, and there may be multiple insults that have separate or cumulative effects. This makes longitudinal cohorts useful tools for studying these conditions.
Pathogenesis of accelerated aging in HIV
Several potential pathogenic mechanisms may occur in people with HIV:
- Antiretroviral therapy (ART) factors – An extended period of inadequate ART treatment (experienced by many early in the epidemic) can accelerate aging; this has been relatively well studied. However, treatments may also accelerate disease, and this may be true even with newer, less toxic drugs. For example, increased insomnia was noted in the dolutegravir trials, a newer, frontline drug. This could be a flag for central nervous system toxicities, so clinicians must remain vigilant as treatments change. Some toxicities only emerge with long-term treatment.
- Comorbidities – Many studies have shown higher rates of non-infectious comorbidities in people living with HIV compared to lifestyle-matched controls. Comorbidities may affect brain function. Depression has a profound adverse effect on cognitive function – and depression rates are very high, up to 50%, in people with HIV.
- Inflammatory responses in the brain – The increase in comorbidities may be partly due to inflammation, which may also directly impact the brain – as brain imaging has revealed.
- Lifestyle factors – Lifestyle is an important factor in accelerated aging; for example, smoking has many negative effects. Careful matching of controls in studies such as POPPY is vital to tease out these effects – even in POPPY, non-medicinal drug use is much higher among people living with HIV than among controls.
The definitions of cognitive decline used are very important in this discussion. Different definitions/tests identify different people as having cognitive impairment.
Dr. Winston cited an Italian study, presented at CROI in 2015, that showed a decreasing prevalence of HIV-associated neurocognitive disorder. An ever smaller proportion of the population experienced the inadequate treatment of the early epidemic, which may reduce cognitive decline. However, there is a steadily increasing duration of infection in populations living with HIV, which may increase cognitive concerns.
HAND peripheral and CSF biomarkers: Pitfalls and promises
Dr. Bruce J Brew, of St. Vicent’s Hospital in Australia, began by comparing the prevalence and severity of HIV-associated neurocognitive disorders (HAND) before and after the advent of effective antiretroviral therapies (ART). Severity has markedly declined, but neurocognitive impairments persist, even with modern treatments and suppressed viral load – as does chronic low-grade inflammation in the brain.
Dr. Brew presented six principles that need to be considered when selecting/analysing biomarkers:
- HAND is not a single condition – Milder forms, especially in people who are virally suppressed, may be associated with viral proteins and latent infection, while more severe disease is likely driven by the whole virus.
- HAND is driven by systemic inflammation as well as CNS inflammation – Less severe forms of HAND (particularly among people with higher CD4 counts) are likely driven primarily by systemic inflammation. To use a cancer analogy, when inflammation metastasizes to the brain it is more likely to be associated with severe cognitive disorders.
- Stable or inactive disease – These simmering pot conditions exist, but what do they mean for long-term outcomes?
- Reparative and remodelling evidence – There is some evidence that repair mechanisms exist in people who have been on ART for a while. These mechanisms change the biomarkers, so it is critical to know when a person began ART when assessing biomarkers.
- Confounds – There are many potentially confounding factors in populations of HIV-positive people, such as substance use, hepatitis C, and conditions of aging.
- Selectivity – All people with HIV are not equally vulnerable to cognitive impairment, and not everyone will get it, particularly in the age of ART – need to target vulnerable patients.
To identify useful biomarkers, it will be necessary to take a combination approach considering neural, viral, and immunologic markers, as well as metabolic and vascular markers. Solutions will rely on large, well-characterized datasets underpinned by neuropathology data.
Dr. Brew discussed the wide range of biomarkers currently being discussed to study HAND, which are listed below. To use these markers effectively it is essential to think carefully about what your study is trying to learn. Biomarkers might be used to diagnose HAND focusing on presence of the disease, its severity, or the activity of the virus in the brain. Biomarkers might also be used predictively to anticipate HAND development or response to treatment and to consider questions about whether HAND is primarily driven by virology or immune responses. Last, one might want to consider the role of latent infection in HAND.
Potential peripheral markers
- multiple markers are available but none are specific to HAND
- most robust markers related to monocyte function
- micro-RNA seems promising, but the seminal work by Asahchop et al (2016) needs to be expanded.
In the SIV model, monocyte expansion correlated with the severity of encephalitis, and the best measure of this is soluble CD163 (Burdo et al, 2010). It also increased in cognitive impairment in people with HIV (Burdo et al, 2013) – and seems to be a robust, if not specific, marker.
Potential CSF markers
- CSF NFL correlates with HAND severity, responds to ART, and predicts HAND and encephalitis – however it may not be so useful for milder disease or when an individual is virally suppressed.
- Tau is a less expensive, less effective neural marker.
- CSF S100b and CSF neopterin may have potential but limited use in those on ART as are measures of blood brain barrier impairments.
- Soluble CD163 appears to be a useful immunological marker in CSF as it is in the periphery.
- Quinolinic acid/tryptophan ratios effectively predict neurological disease in SIV and correlate with severity even with ART treatment, but there is no easy assay.
- Other CSF immune markers include CCL2 and YKL-40.
- In terms of viral markers, the single copy assay is promising, but it is not clear whether it is an effective predictor of risk. HIV DNA measures are not practical.
- CSF progranulin is lower on HAND even with suppressive ART, and is the only trophic factor where significant work has been done.
- Metabolic biomarkers such as sphingolipids and ceramide are elevated in HAND. With progression of the disease there is a shift from single to multiple lipid species.
- There is only one potential marker of latent viral infection, Bcl11b, which could help identify patients with a low burden of viral DNA in the brain.
How useful is MRI?
Dr. Lucette Cysique of Neuroscience Research Australia studies ways of monitoring and understanding neurocognitive change in people living with HIV. In her presentation, she summarized a systematic review of the current literature on three approaches to brain imaging:
- Morphometric and volumetric MRI
- Magnetic resonance spectroscopy
- Diffusion tensor imaging
The review looked at naturalistic studies of the brain in the era of highly effective antiretroviral therapy (ART). With ART the impact of HIV on brain structures is much subtler, and measurement is complicated by many factors at the population level.
A. Morphometric and volumetric magnetic resonance imaging (MVMRI)
This review included 25 studies conducted between 2007 and 2016. MVMRI provides a measure of atrophy in different regions of the brain. Many of the MVMRI studies were not appropriately controlled, and a multitude of approaches were used for analysis including automated segmentation (40%) as well as statistical approaches to assess the entire brain. The latter approach generally identified more differences. Most, but not all, studies found correlations between atrophy and cognitive impairment. Low CD4 nadir, longer duration of HIV infection, and longer exposure to ART were the most robust risks associated with atrophy. HIV and aging effects were inconsistent.
Dr. Cysique concluded that standardization of the analytic protocol is needed. Give the variation at a population level in MVMRI, it is likely that longitudinal testing (comparing a brain to its own previous measures) will be the most useful approach.
B. Magnetic resonance spectroscopy (MRS)
The review included 18 studies conducted between 2001 and 2015. MRS looks at brain metabolites that have functional significance in the brain. Again, problems with matching of controls were noted. MRS predicted neurocognitive impairment correctly 72% of the time, with fairly robust profiles of metabolites noted in the frontal white matter in people on ART with HIV-associated injury.
Dr. Cysique concluded that, if good international standardization could be developed, a complex array of metabolites across the brain has the potential to provide useful clinical information.
C. Diffusion tensor imaging
The review included 27 studies conducted between 2006 and 2016. The main measures involved were fractional anisotropy, an indirect measure of axonal integrity (a higher score indicates brain health), and mean diffusivity, an inverse measure of atrophy (a higher score indicates damage). Proof of concept studies involving untreated people living with HIV who had advanced cognitive impairment found changes in DTI, however studies of people treated with effective ART were highly inconsistent.
Dr. Cysique concluded that many methodological issues remain for this technology and that the measurable effects are probably small in people treated with ART.
In summary, Dr. Cysique advocated for a large collaborative international study to define standards for clinical implementation of these tools. She noted that the brain changes associated with HIV infection are a complex system that would benefit from the use of network analysis and other sophisticated methods, rather than traditional approaches comparing findings to a control.