A Prospective Nested Case-Control Study of Dengue in Infants: Rethinking and Refining the Antibody-Dependent Enhancement Dengue Hemorrhagic Fever Model

Analyses of a prospective case-control study of infant dengue by Daniel Libraty and colleagues casts doubt on the antibody-dependent enhancement model for dengue hemorrhagic fever.Background: Dengue hemorrhagic fever (DHF) is the severe and life-threatening syndrome that can develop after infection with any one of the four dengue virus (DENV) serotypes. DHF occurs almost exclusively in individuals with secondary heterologous DENV infections and infants with primary DENV infections born to dengue immune mothers. The widely accepted explanation for the pathogenesis of DHF in these settings, particularly during infancy, is antibody-dependent enhancement (ADE) of DENV infection. Methods and Findings: We conducted a prospective nested case-control study of DENV infections during infancy. Clinical data and blood samples were collected from 4,441 mothers and infants in up to two pre-illness study visits, and surveillance was performed for symptomatic and inapparent DENV infections. Pre-illness plasma samples were used to measure the associations between maternally derived anti-DENV3 antibody-neutralizing and -enhancing capacities at the time of DENV3 infection and development of infant DHF. Conclusions: This prospective nested case-control study of primarily DENV3 infections during infancy has shown that infants exhibit a full range of disease severity after primary DENV infections. The results support an initial in vivo protective role for maternally derived antibody, and suggest that a DENV3 PRNT50 >50 is associated with protection from symptomatic DENV3 illness. We did not find a significant association between DENV3 ADE activity at illness onset and the development of DHF compared with less severe symptomatic illness. The results of this study should encourage rethinking or refinement of the current ADE pathogenesis model for infant DHF and stimulate new directions of research into mechanisms responsible for the development of DHF during infancy. Trial registration: ClinicalTrials.gov NCT00377754 : Please see later in the article for the Editors' Summary Background: Every year, dengue infects 50–100 million people living in tropical and subtropical areas. The four closely related viruses that cause dengue (DENV1–4) are transmitted to people through the bites of female Aedes aegypti mosquitoes, which acquire the viruses by feeding on the blood of an infected person. Many people who become infected with DENV have no symptoms but some develop dengue fever, a severe, flu-like illness that lasts a few days. Other people—about half a million a year—develop a potentially fatal condition called dengue hemorrhagic fever (DHF). In DHF, which can be caused by any of the DENVs, small blood vessels become leaky and friable. This leakiness causes nose and gum bleeds, bruising and, in the worst cases, failure of the circulatory system and death. There is no vaccine to prevent dengue and no specific treatment for dengue fever or DHF. However, with standard medical care—in particular, replacement of lost fluids—most people can survive DHF. Why Was This Study Done?: DHF is increasingly common, but why do only some people develop DHF after infection with DENV? The widely accepted explanation for the development of DHF is “antibody-dependent enhancement” (ADE) of DENV infection. DHF occurs almost exclusively in two settings; (i) children and adults who become infected with a second DENV serotype after an initial “primary” DENV infection with a different serotype, and (ii) infants with primary DENV infections whose mothers have some DENV immunity. The ADE model suggests that in individuals who develop DHF, although there are some antibodies (proteins made by the immune system to fight infections) against DENV in their blood (in secondary heterologous infections, antibodies left over from the primary infection; in infants with primary infections, antibodies acquired from their mothers before birth), these antibodies cannot “neutralize” the virus. Instead, they bind to it and enhance its uptake by certain immune system cells, thus increasing viral infectivity and triggering an immunological cascade that results in DHF. In this prospective, nested case-control study, the researchers directly test the ADE model for infant DHF. In a prospective study, a group of people is selected and followed to see if they develop a disease; in a nested case-control study, each case is compared with people in the group who do not develop the disease. What Did the Researchers Do and Find?: The researchers collected clinical data and blood samples from 4,441 mothers and their babies at up to two pre-illness study visits. They then followed the infants for a year to see which of them developed symptomatic and symptom-free DENV infections. Finally, they used the pre-illness blood samples to estimate the maternally derived anti-DENV antibody-neutralizing and -enhancing capacities in the infants at the time of DENV infection. 60 infants were infected with DENV—mainly DENV3—during the study. All but one infection was a primary infection. The infected infants showed a wide range of disease severity. Infants who had a high DENV3 neutralizing capacity at birth tended to develop symptomatic DENV3 infections later than infants who had a low DENV3 neutralizing capacity at birth. All the infants who developed a symptomatic DENV3 infection had a low estimated DENV3 neutralizing activity at the time of infection, and nearly all had measurable levels of DENV3 ADE activity. Infants who developed DHF did not have significantly higher frequencies or levels of DENV3 ADE activity than DENV3-infected infants with less severe symptoms. What Do These Findings Mean?: These findings indicate that maternally derived anti-DENV3 antibody initially provides protection against dengue infections. That is, babies born to DENV immune mothers are protected against dengue infections by maternally derived antibodies. Over time, the level of these antibodies declines until eventually the infant becomes susceptible to DENV infections. However, the lack of a significant association between the estimated level of DENV3 ADE activity at illness onset and the development of DHF rather than a less severe illness throws doubt onto (but does not completely rule out) the current ADE pathogenesis model for infant DHF, at least for DENV3 infections. The results of this study, the researchers conclude, should encourage rethinking or refinement of the ADE model for infant DHF and should promote further prospective studies into the development of DHF during infancy. Additional Information: Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000171.