Midway through India’s monsoon season, the dengue virus has predictably emerged once again, cocking a snook at both civic agencies unable to control the mosquito that spreads the virus and scientists’ efforts at a treatment or vaccine. Amidst the seemingly losing battle against the virus that is endemic in more than 100 countries across the tropics and sub-tropics, recent research has kindled hopes over ‘designer antibodies’ engineered to elicit the kind of immune response needed to beat the virus.
The World Health Organisation (WHO) estimates 50-100 million infections with the dengue virus (DENV) that exists in four forms, called ‘serotypes’ (DENV 1, 2, 3, 4), each with characteristic protein pieces called antigens that generate specific antibodies. Antibodies against one serotype confer immunity against it, but far from cross-protecting against the other three, worsen any subsequent infection by them. Some scientists are trying to tackle the virus by developing antibodies against the dominant serotype, usually DENV2. Others are aiming at a cocktail of antibodies that work against all four.
New research indicates scientists need to think beyond the natural antibodies generated during an infection, and consider ‘designer antibodies’ tailored to generate the specific immune response needed to beat the virus. “So far, scientists have been focussing mostly on live, weakened viruses for a vaccine. But it is time to think of designer antibodies as an option,” Navin Khanna, a virologist at the International Centre for Genetic Engineering and Biotechnology (ICGEB), New Delhi, told The Wire.
In July two international teams of scientists, drawn from Singapore and the US, reported results of engineered anti-dengue antibodies in animal studies. A team from the Massachusetts Institute of Technology (MIT), Duke University, National University of Singapore, Nanyang Technological University (NTU), and a biotech company, Visterra, reported in the journal Cell that their genetically-engineered antibody fights all four circulating types of the dengue virus in mice.
The team picked a specific part of an antigen to which an antibody binds – called an epitope – present in envelope protein that forms the outer sheath for the dengue virus. This piece, though functionally relevant, was not involved in evoking an immune reaction. The team next genetically engineered a new antibody, Ab513, that could bind with the epitope.
The engineered antibody “demonstrated in mice an ability to resolve symptoms of severe dengue infection” and neutralised all four types of the dengue virus, according to their report.
The team next assessed whether the antibody showed potential for treatment by testing it against clinical features of human dengue infections in mice. These included the abnormally low level of blood platelets characteristic of dengue infection and mild bleeding, usually from nose, gums, or easy bruising. The antibody reduced low blood platelet counts, contained the bleeding, and reduced the numbers of viral particles to nearly undetectable levels.
“The results demonstrate that Ab513 may reduce the public health burden from dengue,” they reported.
“The paper is an interesting piece of work,” says Easwaran Sreekumar, a scientist at the Rajiv Gandhi Centre for Biotechnology (RGCB), Thiruvananthapuram, who is working on chikungunya and dengue viruses. “The team has developed a new therapeutic antibody, which can be administered in patients suffering from dengue, may be in the early days, so that this antibody neutralises the virus and also alleviates the harmful effect of the disease such as platelet reduction and development of shock syndrome.”
“One advantage of the newly developed antibody is that it can work against all the four serotypes of dengue virus, and you do not need to do a prior determination of the virus serotype that has affected the patient before administering the antibody,” he added.
The research indicates that designer antibodies could hold the clue to developing any treatment or vaccine, says Khanna. It also “establishes the unique importance of the epitope when designing a next-generation dengue vaccine candidate.”
Use of such antibodies not generated by the body’s immune system is called passive immunotherapy. And the snag with it, says Khanna, is that it requires huge amounts of antibodies – two grams per kg body weight – that would be expensive to make. “Deployment of passive immunotherapy at such high concentrations is extremely challenging for dengue-infected developing countries.”
Meanwhile, Science also reported in July the results of animal studies by another team of US and Singapore scientists on a new antibody against an epitope of the second dengue type, which could be a potential candidate for a vaccine or drug.
Scientists from the National University of Singapore and Vanderbilt University, US, developed antibodies against specific antigens present on the envelope of the second dengue type, DENV2, using frozen samples and simulation studies to observe antibody-antigen reactions almost down to the atomic level. The new antibody locked against several proteins present in the virus envelope, and prevented the virus from infecting in studies of mice.
Sheemei Lok, associate professor in the Program in Emerging Infectious Diseases, Duke-NUS Graduate Medical School, says that their research “shows the site on the dengue virus that can stimulate a type of antibody that can kill the virus regardless of whether the virus can change in [form].”
“Development of a safe and effective vaccine hasn’t been successful so far especially against DENV 2. One possible reason for the poor performance is the ability of the virus to change in shape and form and these are probably not represented in the vaccine,” she says.
In January 2015, Sanofi Pasteur announced that it hoped to roll out the first dengue vaccine in the second half of the year based on encouraging results published in the New England Journal of Medicine, on a phase 3 Sanofi-sponsored clinical trial of a tetravalent dengue remedy, active against all four types, conducted in Asia and Latin America.
The vaccine is genetically-engineered based on a live but weakened virus, ‘CYD-TDV’. The Sanofi-sponsored trial tested its efficacy in children the two continents, who received three vaccine doses, at zero, six and 12 months, against confirmed dengue infection showing the typical symptoms. The Asian trial involving children between the ages of two and 14 years showed an overall vaccine efficacy of 56.5% after three injections, which increased to 80.8% for efficacy against severe dengue. The results of the Latin American trial, published in NEJM in January, showed that the vaccine had an efficacy of 60.8%, and it was effective against all four types.
But several scientists have pointed out that the numbers are unsatisfactory and leave room for cross-reactions between antibodies that can aggravate future infections.
Tricks up its sleeve
It is these cross-reactions that are proving to be a stumbling block in developing a drug or vaccine. In March 2015, a team of scientists from Singapore and the US reported details of how the dengue virus helps form a complex of highly cross-reactive antibodies that suppress more than 95% of initial infection and later induce lethal dengue infections, irrespective of the amount of virus particles in the blood.
“The major hurdles are that there are four serotypes of dengue viruses. Human immune responses to natural DENV infections are serotype cross-reactive, but not cross-protective,” says Khanna.
And, though the results on the therapeutic antibody reported in Cell are encouraging, one of the issues before it reaches the clinics would be that it needs to be administered early to be effective, as is the case with other anti-dengue molecules under tests, points out Sreekumar.
“There could still be problems of sensitisation, and in endemic areas where all the four serotypes are prevalent, we may have to use this antibody repeatedly in the same patients – at least four times – and the efficacy during these repeated administrations have not been evaluated,” he adds.
The virus also has the ability to undergo genetic and molecular switches, resulting in infections of varying intensities, with milder versions slipping under the radar.
A 2013 study by Sreekumar’s team at RGCB team reported in the Virology Journal subtle changes in the dengue virus’s genetic and molecular patterns in India. He says that the changes in severity of the infection depend in the number of genetic changes that help the virus to multiply and survive.
In fact, the various degrees of infection are adding to confusion over diagnosis in India. A study in Chennai by Johns Hopkins Bloomberg School of Public Health, published in July in the journal PLoS Neglected Tropical Diseases, shows that there are several instances of milder, self-limiting forms of dengue in Chennai alone that go undetected and unreported.
The study estimated that “23% of the susceptible population gets infected by dengue each year, corresponding to approximately 228,000 infections. This transmission intensity is significantly higher than that estimated in known hyper-endemic settings in Southeast Asia and the Americas.”
“This transmission intensity is almost three times larger than that in traditionally hyper-endemic districts in Thailand, and suggests an extremely large proportion of asymptomatic/sub-clinical disease, a lack of recognition of the disease and/or under-reporting,” the authors summarised.
One-size-fits-all vector control programs
Meanwhile, efforts to control the dengue virus vector, Aedes aegypti, face limitations as well. Five years ago, a report in a World Health Organisation (WHO) bulletin based on an analysis of six Asian cities cautioned “public health response should extend beyond larvicidal or focal spraying” as a complex interplay of multiple factors is what helps the mosquitoes breed.
It described dengue vector control programmes as “paternalistic” with flawed a one-size-fits-all approach. Close interaction between public health agencies and local communities and their political and religious leaders, as well as municipal authorities is “critical” to managing the mosquitoes, it said. Clearly new strategies to combat dengue are called for, and there is no immediate answer in sight.
T.V. Padma is a freelancing science journalist based in New Delhi.