The evolution of dengue viruses has resulted in four antigenically similar yet distinct serotypes. Infection with one serotype likely elicits lifelong immunity to that serotype, but generally not against the other three. Secondary or sequential infections are common, as multiple viral serotypes frequently cocirculate. Dengue infection, although frequently mild, can lead to dengue hemorrhagic fever (DHF) which can be life threatening. DHF is more common in secondary dengue infections, implying a role for the adaptive immune response in the disease. There is currently much effort toward the design and implementation of a dengue vaccine but these efforts are made more difficult by the challenge of inducing durable neutralizing immunity to all four viruses. Domain 3 of the dengue virus envelope protein (ED3) has been suggested as one such candidate because it contains neutralizing epitopes and it was originally thought that relatively few cross-reactive antibodies are directed to this domain. In this study, we performed a detailed analysis of the anti-ED3 response in a cohort of patients suffering either primary or secondary dengue infections. The results show dramatic evidence of original antigenic sin in secondary infections both in terms of binding and enhancement activity. This has important implications for dengue vaccine design because heterologous boosting is likely to maintain the immunological footprint of the first vaccination. On the basis of these findings, we propose a simple in vitro enzyme-linked immunosorbent assay (ELISA) to diagnose the original dengue infection in secondary dengue cases.Dengue virus is an insect-borne flavivirus transmitted to humans by the bite of an infected mosquito, usually Aedes aegypti (20). There are four circulating serotypes of dengue (dengue serotype 1 [Den1] to Den4) that show up to 70% sequence homology across their genomes (4, 17), and it is common for multiple viral serotypes to cocirculate in countries where dengue is endemic. Most dengue infections are either asymptomatic or lead to uncomplicated dengue fever (DF). However, in 1 to 5% of cases, symptoms can be more severe with the development of plasma leakage and hemorrhage. Such dengue hemorrhagic fever (DHF) can lead to circulatory collapse, resulting in a mortality rate of around 20% if left untreated.The more frequent occurrence of DHF in secondary dengue infections in children and adults suggests a role for the acquired immune system in disease pathogenesis, and there has been considerable research into both the B-and T-cell responses. Antibody-dependent enhancement (ADE) of infection, proposed by Halstead in 1977 (24, 25), is one hypothesis for this increase in severity in secondary infections (23,36). During a primary infection, antibodies that cross-react with the remaining 3 serotypes are induced. After a few months, when heterologous protection is no longer observed (54), it is hypothesized that these cross-reactive antibodies decline to subneutralizing levels, meaning that a heterologous infecting serot...

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