Sera from patients involved in a Peruvian outbreak of dengue virus serotype 1 infection cross-neutralized the American genotype of dengue virus serotype 2 up to 100-fold more efficiently than they did the virulent Asian genotype of dengue virus serotype 2, as determined by a plaque reduction neutralization test (PRNT) with CV-1 fibroblasts modified to express human Fc␥ receptor CD32. The concordant preferential immune enhancement of the Asian genotype of dengue virus serotype 2 in human monocytes suggests that such a modification might strengthen the correlation between the PRNT titer and protection.Dengue fever is typically a self-limited, if incapacitating, illness, but a life-threatening hemorrhagic form (dengue hemorrhagic fever [DHF]) may complicate infection, especially in children. Dengue viruses (DENVs) are single positive-strand RNA viruses that exist as four antigenically distinct serotypes (DENV serotype 1 [DENV-1] to DENV-4) of considerable genotypic diversity that may contribute to pathogenicity (for a review, see reference 10). Durable protection is serotype specific, so that sequential infections with the four DENV serotypes are theoretically possible. The incidence of DHF has been linked to the introduction of new DENV serotypes into a region where dengue is endemic, with the sequence of DENV-1 infection followed by DENV-2 infection being particularly associated with more virulent Southeast Asian dengue epidemics (14). Asian genotype DENV-2 strains have been displacing the relatively more benign American genotype DENV-2 strains in the Americas, notably, accounting for the catastrophic 1981 DENV-2 epidemic in Cuba that followed an uneventful DENV-1 outbreak on the island 4 years earlier (3). The immune enhancement hypothesis offers an explanation for the paradox of increased disease severity in a population immune to DENV: it posits enhanced infection of Fc␥ receptor (Fc␥R)-expressing monocytes (the primary DENV target cell type) by DENV immune complexes (ICs) comprised of antibodies from an earlier heterotypic DENV infection. It was therefore somewhat surprising that DHF was not observed during the 1995 DENV-2 outbreak in Iquitos, Peru, that followed a similarly large but benign primary DENV-1 epidemic (caused by strain IQT6152) 4 years previously (15). The new virus was an American genotype DENV-2 strain (strain IQT2913). Postoutbreak monotypic DENV-1-immune sera exhibited substantial neutralizing activity against the virus by the conventional 50% plaque reduction neutralization test (PRNT) performed with BHK-21 cells (5). Notably, these DENV-1-immune sera were found to exhibit less neutralizing activity against the prototypic virulent Asian genotype DENV-2 strain (strain 16681) (4).Since the monocyte display of Fc␥R may be a pivotal determinant of whether DENV ICs are infectious or are neutralized in vivo, we reasoned that the introduction of Fc␥R expression into a conventional DENV PRNT that employs cells that lack Fc receptors would magnify the previously observed neutralization disparity bet...