Tuberculosis, caused by infection with Myobacterium tuberculosis (M.tb), is the world’s leading cause of death brought on by a single pathogen. More than 10 million new cases of TB are diagnosed each year, and almost two million people die from the disease. Globally, 1.7 billion people are estimated to be infected with M.tb.
A new blood test has been found to more accurately predict the development of tuberculosis up to two years before its onset in people living with someone with active TB, according to research published online in the http://www.atsjournals.org/journal/ajrccm (AJRCCM), an American Thoracic Society journal. Those in close contact with a TB patient are at risk for developing the disease, which can be prevented by treatment. However, if all contacts were to receive prevention, up to 85% would be treated unnecessarily. A blood test that predicts the development of TB is not currently available, but could help focus the preventative treatment on those with the highest risk.

In “Four-Gene Pan-African Blood Signature Predicts Progression to Tuberculosis,” researchers from a multinational research consortium report that they developed and validated a blood test that measures the expression levels of four genes selected by a sophisticated computational algorithm that can more accurately predict the development of TB in high-risk individuals in Sub-Saharan Africa than currently available tests.

The most significant finding was that prediction of progression to active tuberculosis in household contacts of active TB cases up to two years before the onset of the disease is possible through measurements of a specific combination of a four-gene signature in the blood. This signature, known as ‘RISK4,’ is a combination of four genes associated with inflammatory responses and was found to apply to cohorts from South Africa, Gambia and Ethiopia. Prediction tests that have been published before were also tested on this cohort but were not successful on all cohorts from across Africa. The work potentially paves the way for targeted preventative treatment in specific high-risk populations: household contacts of active TB cases.

The work resulted from a Grand Challenges in Global Health initiative that was launched in 2003 by the Bill & Melinda Gates Foundation (BMGF), in partnership with the National Institutes of Health, to harness the power of science and technology to dramatically improve health in the world’s poorest countries. The initiative supported groundbreaking research projects to discover and develop scientific breakthroughs for preventing, treating, and curing diseases that kill millions of people each year in developing countries. The work reported here was part of Grand Challenge 6-74, ‘Biomarkers of protective immunity and surrogate markers of TB disease in Africa’, a 14-year collaboration of researchers from Europe, Africa and the United States. Co-funding was provided by the NIH, European Union, the Wellcome Trust and the South African Medical Research Council. The initial grant, led by Prof Stefan Kaufmann from the Max Planck Institute for Infection Biology, in Berlin, Germany entailed the identification of 4,466 HIV-negative, healthy study participants from the households of 1,098 index cases (people with active TB, who allowed the researchers to enroll members of their household who did not have TB in its active stage). Blood samples were taken from all study participants, and stored. A study-specific biorepository was established that will allow additional analyses in future. Two follow-on grants, focusing on the analyses of the progressor and non-progressor samples, were awarded by the BMGF: one was also led by Prof Kaufmann while the other was initially led by Prof Willem Hanekom from the South African TB Vaccine Initiative (SATVI) at the University of Cape Town and subsequently co-led by Prof Thomas Scriba (SATVI) and Prof Gerhard Walzl from Stellenbosch University. The analysis of the gene expression work was led by the Centre for Infectious Diseases Research (US).

At the end of the initial study period, when it was apparent who had progressed to TB and who had not, the blood samples of 79 TB progressors and 328 controls who remained healthy during the two years of follow up, were analyzed.

A number of companies and laboratories have the technology to develop blood tests that measure the expression of these four genes and apply the acquired algorithms in devices that allow rapid and cost-effective testing. The researchers aim to develop tests that will be used at primary health clinics, similar to the tests that are currently used to diagnose TB.

While there are currently tests on the market that may predict progression to TB, the test developed by the GC6-74 team gives positive results for a significantly smaller percentage of high-risk household contacts than the current tests, which only identify infection rather than predict progression. This translates to fewer people being treated unnecessarily in order to prevent TB. Preventative treatment is several weeks to months long and has potential side effects, such that one wants to limit the number people who have to undergo such treatment to those most likely to be at risk of developing active TB.

An example of preventative treatment is a 12-week regimen of the antibiotics isoniazid, which has side effects in some patients that may include liver damage and numbness in the hands and feet, and rifapentine, which may also cause liver problems and other serious side effects.

Overall, the study highlights the value of building multi-national consortia, to develop tests that will work in different locations, and strengthening clinical and laboratory research infrastructures where TB burden is high. Other leading international contributors to the work include the Medical Research Council Unit in The Gambia, the London School of Hygiene and Tropical Medicine (UK), Case Reserve Western University, Leiden University Medical Centre (Netherlands), the Armauer Hansen Research Institute (Ethiopia), Makerere University (Uganda), the Ethiopian National Research Institute (Ethiopia), the Karonga Prevention Study (Malawi), Statens Serum Institute (Denmark), AERAS Global TB Vaccine Foundation (US), Stanford University (US) and Sanquin Research (Netherlands).

This study is an important step towards targeted prevention, and now the impact of this test on prevention of TB will need to be tested in multicenter clinical trials. In addition, the validity of the prediction in high-risk individuals in Asia, South America and other high-priority areas needs to be assessed.