Immune activation marker, cheap and easy to measure, compares well with CD4 count

Measuring levels of a protein called SuPAR in the blood using a cheap and simple ELISA-based test may provide information on disease progression and treatment response that is equivalent to the CD4 cell count – but without the need for expensive machines and trained laboratory staff – according to Danish and South African research presented last week at the Sixteenth International AIDS Conference in Toronto, Canada.

SuPAR (soluble urokinase Plasminogen Activator Receptor) is a protein found at high levels when the immune system is activated. In HIV-positive people SuPAR levels have been found to correlate with:

• the risk of disease progression after controlling for CD4 count and disease stage (Sidenius 2000; Ostrowski 2004; Eugen-Olsen 2006b).
• viral load levels: suPAR in urine correlated well with plasma viral load in a small study of 20 HIV-positive patients and ten controls (Lingar Neilson 2001).
• response to antiretroviral therapy in a small longitudinal adult study (Jensen 2001), while baseline levels predicted treatment efficacy in a study of 29 South African children (Eugen-Olsen 2006a).

Researchers from Copenhagen University Hospital, working with a company called ViroGates, set out to develop a simple, low-cost test that can be used for monitoring purposes in low-income settings.

They identified particular SuPar antibodies for inclusion in an ELISA test (a simple assay that is used as the basis for HIV antibody testing), and assessed the correlation between SuPAR antibody levels and HIV disease progression in 81 stored plasma samples from the pre-HAART era.

Patients were divided into three equal groups according to arbitrary definitions of low, medium and high SuPAR levels chosen by the researchers. Jesper Eugen-Olsen of Copenhagen University Hospital, one of the researchers who has been involved in the investigation of this surrogate marker, explained that a cut-off level for clinical progression still needs to be defined by larger clinical studies.

Patients were divided into three equal groups of 27:

• lowest suPAR (below 3.85 ng/ml),
• medium suPAR (3.8 to 5.8 ng per ml),
• high suPAR (5.9 – 12.16 ng/ml).

After two years of follow-up 13 of 27 patients in the high SuPAR group had died, compared to none in the low SuPAR group and one in the medium SuPAR group. This difference was statistically significant (p<0.001). Interestingly, just over half the deaths (n=7) occurred in patients with SuPAR levels between 6 and 8ng/ml, and five of these had CD4 cell counts below 200, whereas deaths at higher SuPAR levels were more evenly spread across a range of CD4 cell counts below 600 cells/mm³. These findings suggest the need for much larger studies to define more accurately the degree of correlation between SuPAR and the CD4 cell count.

However, as a crude marker, a cut-off value of 5.9ng/ml was strongly predictive of poor survival. A Kaplan-Meier analysis showed that 50% of patients with a baseline value of 5.9ng/ml or above died within 18 months, compared with just under 2% of patients with baseline values below 5.9ng/ml.

A South African study in 29 children beginning antiretroviral therapy at the Red Cross Children’s Hospital in Cape Town, carried out with the Copenhagen University Clinical Research Unit, also found that baseline SuPAR predicted the time to viral suppression by Kaplan-Meier analysis (SuPAR < 5.3 significantly more likely to achieve viral suppression, p=0.003). However three children died and four were lost to follow-up in this study, further reducing the numbers available for analysis.