If you’ve made it to the end of the summer with no mosquito bites, consider yourself lucky. There are many places in the world where these vexatious insects carry debilitating diseases that can not only adversely affect individuals, but also severely cripple local economies—making it even more difficult to care for the stricken. Affecting up to 400 million people per year, dengue virus (DENV) is one such infectious commuter within these mosquitos, for which there is currently no vaccine or targeted therapies. However, efforts have been made over the past several years to introduce genetically modified mosquitos, in an attempt to control the disease-carrying mosquito population.
Now, researchers from the University of California, Berkeley have identified what they believe is the key protein responsible for the fluid loss and subsequent shock that are the hallmarks of severe—and potentially fatal—DENV infections. Known as nonstructural protein 1 (NS1), the scientists observed that it is the only one of the 10 viral proteins secreted by DENV infected cells to circulate freely throughout the bloodstream.
“This is a missing piece in the puzzle of the pathogenesis of dengue,” explained senior author Eva Harris, Ph.D., professor in the Division of Infectious Diseases and Vaccinology at UC Berkeley. “The role of NS1 itself had been overlooked in severe forms of dengue disease, but we now know that it is an important player. Our findings show that NS1 could be a prime target for drugs and that it should be considered in vaccine development.”
The findings from this study were published recently in Science Translational Medicine through an article entitled “Dengue virus NS1 triggers endothelial permeability and vascular leak that is prevented by NS1 vaccination.”
DENV, or as it’s colloquially referred “breakbone fever”, due to the severe pain that patients experience upon infection, is spread by the Aedes mosquito and has evolved into four different serotypes, ranging in symptom presentation from mild infection to severe disease and sometimes death. The most severe cases are those that develop into dengue hemorrhagic fever and dengue shock syndrome, which is caused by the loss of fluids from blood vessels.
“What is agonizing is that you don't know from the outset of an infection who will die,” said Dr. Harris. “Once the fluid loss begins, it can become fatal in just one to two days.”
Interestingly, individuals at greatest risk for dengue shock are those who have had a prior infection. An initial infection with one of the four serotypes of dengue virus can impart long-term immunity for that specific virus type, but only temporary immunity against the rest. Once the short-term immunity to the other three serotypes wears off, the survivor is at greater risk for more severe disease from a subsequent infection.
The main hypothesis to explain this phenomenon has been that the antibodies from the first infection react with the new serotype in a way that worsens the cellular damage. In this antibody-dependent enhancement, the antibodies inadvertently increase the virus's ability to infect immune cells, leading to more serious symptoms.
“The interaction with the antibodies might be happening, but it never fully explained all cases of dengue hemorrhagic fever,” stated lead author P. Robert Beatty, Ph.D., assistant research scientist at UC Berkeley's School of Public Health. “The toxicity of NS1 makes more sense than just having an over-reactive immune response. NS1 is a more direct pathway toward disease. It is a trigger that causes vessels to become permeable to fluid, causing the vessels to leak plasma.”
To prove their theory, investigators conducted experiments on cultured human lung endothelial cells and in mice, showing that NS1 caused permeability of endothelial cells that line the walls of blood and lymph vessels. Moreover, the team found that mice injected with NS1 alone, without the virus present, developed symptoms of dengue disease that included a cascade of inflammatory cytokines, vascular leakage, and fluid loss. If the researchers added a sublethal dose of DENV, the resulting infection was fatal.
On the other hand, immunization of mice with recombinant NS1 from each of the four serotypes protected mice against vascular leak and the lethal effects DENV infection.
“What's exciting to me is that if we can make antibodies against this toxin and include them in a vaccine, we could potentially prevent a dengue infection from progressing to the more severe symptoms,” noted Dr. Beatty. “The findings open up new intervention strategies where few now exist.”