Dengue virus (DENV), is the causal infectious agent of dengue fever, a mosquito-borne virus that has multiple serotypes. Infection causes flu-like symptoms and can occasionally develop into a more severe disease called dengue hemorrhagic fever/ dengue shock syndrome (DHF/DSS). Severe disease is characterized by vascular leakage, hemorrhagic manifestations, and hypotensive shock, all of which can lead to death. Incidence of DHF/DSS is thought to increase in persons who have been infected with more than one DENV serotype. Other factors may include interval between infections, age, viral factors, host-specific factors and antibody characteristics. One theory for why secondary infection enhances disease is antibody-dependent enhancement (ADE). AED occurs when antibodies bind a pathogen but do not-neutralize. This allows the pathogen bound to the antibody free entry into new host cells by interaction of the antibody with the Fc receptor on the new cell. This potentially allows the virus to infect cells lacking the normal viral receptor. This effect, coupled with the fact that DENV has multiple serotypes allowing for multiple infections, are thought to cause DHF/DSS. However, until now, there has yet to be conclusive evidence in humans that links antibody levels with DHF/DSS risk. In work recently published in Science, a cross-institute collaboration involving Dr. Halloran from the Vaccine and Infectious Disease Division at Fred Hutch studied the relationship between preexisting DENV antibodies and disease severity in a large pediatric Nicaraguan cohort.
Understanding ADE is very important in light of the increase of dengue disease and the current lack of a vaccine. By understanding the peak risk titer (amount of antibody) for inducing ADE, researchers would have a known enhancement titer to avoid when creating vaccines. In this study, 6684 children provided blood samples that were tested for DENV antibodies. Titer was tested by inhibition enzyme-linked immunosorbent assay (iELISA). This assay takes serially diluted serum and competes it with a DENV-specific antibody conjugated to peroxidase for binding to antigens from the four DENV serotypes.
Using titer data and disease severity information, a Cox proportional hazards model was used to calculate hazard ratios. Interestingly the hazard of DHF/DSS was similar between naïve and high titer (sera dilution >1:1280) DENV children but showed a 7-fold increase in the middle titer group (1:21 to 1:80). The hazard for children classified as DHF/DSS was 11.4%, almost twice that of children with low DENV titer (<1:21, at 6.6%). The hazard for naïve and high titer was 1.6% and 1.5%, respectively, suggesting that high titer does not provide benefit or determent compared to individuals not previously exposed to DENV.
In this study, the average half-life of DENV antibodies was four years, and by three years an estimated 22% of children had titers in the peak risk range (1:21 to 1:80). This titer was also the peak titer for hospitalizations due to DENV. As expected, the titer associated with correlates of protection (>1:320) is distinct from that of enhancement. Thus, when testing a vaccine, not only protection but also enhancement titers should be regarded. This study directly shows the level at which DENV antibodies are associated with increase disease severity, thus verifying enhancement in humans. Results from this study should be considered in future DENV vaccine development.
Katzelnick LC ,Gresh L, Halloran ME, Mercado JC, Kuan G, Gordon A, Balmaseda A, Harris E. (2017). Antibody-dependent enhancement of severe dengue disease in humans. Science.