Short Report: Antigenic Relationships between Sylvatic and Endemic Dengue Viruses Nikos Vasilakis, Anna P. Durbin, Amelia P. A. Travassos da Rosa, Jorge L. Munoz-Jordan, Robert B. Tesh, and Scott C. Weaver* Center for Biodefense and Emerging Infectious Diseases, and Department of Pathology, University of Texas Medical Branch, Galveston, Texas; Center for Immunization Research, Department of International Health, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland; Molecular Virology and Surveillance Laboratory, Centers for Disease Control and Prevention Dengue Branch, San Juan, Puerto Rico Abstract. Sylvatic dengue viruses (DENVs) are transmitted between non-human primates and arboreal Aedes spp. mosquitoes in Southeast Asia and west Africa. Recent evidence suggests that the risk for re-emergence of sylvatic DENV into the urban endemic/epidemic cycle may be high, which could limit the potential for eradicating the human transmission cycle with vaccines now under development. We assessed the likelihood of sylvatic DENV re-emergence in the face of immunity to current endemic strains or vaccines by evaluating the neutralization capacity of sera from DENV vaccinees and convalescent patients after primary infection with DENV-2 and DENV-3 serotypes. Our data indicate robust homotypic cross-immunity between human sera and sylvatic DENV strains, but limited heterotypic neutralization. Should a licensed vaccine lead to the eradication of the urban transmission cycle in the future, re- emergence of sylvatic strains into the urban cycle would be limited by homotypic immunity mediated by virus- neutralizing antibodies. Sylvatic dengue viruses (DENVs) are both ecologically and evolutionary distinct from urban DENVs and are transmitted in an enzootic cycle most likely between non-human primates and arboreal Aedes spp. mosquitoes. In Southeast Asia, iso- lations of zoonotic DENV-1, DENV-2, and DENV-4 suggest an association between Macaca and Presbytis spp. monkeys with Aedes niveus mosquitoes as the primary vectors. 1 Syl- vatic DENV-3 has not been isolated but is presumed to exist in Malaysian sylvatic cycle on the basis of seroconversions in sentinel monkeys. 2 Sylvatic DENV cycles have also been documented in west Africa, where DENV-2 circulates among Erythrocebus patas and possibly other monkeys, and sylvatic Aedes mosquitoes, including Ae. taylori, Ae. furcifer, and Ae. luteocephalus, in a sylvatic focus near Kedougou, Senegal. 3–5 Limited spillover transmission of sylvatic DENV to humans has been documented in west Africa, 6,7 but until recently, the available data suggested that these sylvatic viruses were con- fined to forest habitats. However, recent phylogenetic and seroepidemiologic evidence suggests that spillover epidemics do occasionally occur into urban settings. 8,9 Phylogenetic analyses suggest that the recent emergence of endemic, human DENV from sylvatic progenitors in Asia occurred within the past 1,000 years. 10 However, hyperen- demic, urban DENV probably arose in Southeast Asia and eventually globally after the invasion of the highly efficient, anthropophilic vector Ae. aegypti aegypti. This process may also have been facilitated by adaptation of DENV from the arboreal canopy-dwelling, ancestral Aedes spp. vectors (other than Ae. albopictus and Ae. aegypti) to the peridomestic, an- thropophilic mosquitoes Ae. aegypti and Ae. albopictus. 11 However, recent analyses using a larger number and diversity of isolates have not supported this hypothesis (Vasilakis N, unpublished data, and Hanley K, unpublished data). Recent reports have also shown that the forest canopy-dwelling mos- quito Ae. furcifer is highly susceptible to sylvatic DENV in- fection. 12 Aedes furcifer shows a pattern of movement into villages in eastern Senegal, 5 which suggests that this species may act as a bridge vector for exchange of DENV between forest and peridomestic habitats. Furthermore, the ability of Ae. aegypti and Ae. albopictus to transmit sylvatic DENV-2 12 indicates that little or no change in vector infectivity accom- panied endemic emergence from sylvatic progenitors. Finally, the ability of sylvatic DENV-2 to replicate efficiently in two model systems for human infection, monocyte-derived, hu- man dendritic cells (maDCs), and mice engrafted with human hepatoma cells, 13 as well as recent evidence of rapid sylvatic DENV turnover caused by their high nucleotide substitution rates across the genome, 14 suggest the risk for sylvatic DENV-2 re-emergence into an urban cycle may be high. This finding could limit the potential for eradicating the human transmission cycle with the dengue vaccines now under de- velopment. 15–18 We assessed the likelihood of current sylvatic DENV re- emergence in the face of immunity to current urban strains or vaccine candidates by evaluating the neutralizing capacity of sera from DENV vaccinees and convalescent patients against geographically and genetically diverse sylvatic and urban DENV strains (Table 1). This information may allow us to predict the long-term success of candidate vaccines currently under development, and evaluate the degree to which pre- existing antibodies to urban DENV-2 in the human popula- tion could prevent the re-emergence of sylvatic DENV-2. In other words, if endemic DENV cycles were eradicated by vaccination, would cross-immunity against sylvatic strains prevent their re-emergence as long as herd immunity re- mained above a threshold level needed to prevent efficient urban transmission? First, we used a modified plaque reduction neutralization test (PRNT) to evaluate the neutralizing capacity of sera from 26 (18 test and 8 placebo), 27 (19 test and 8 placebo), and 24 (20 test and 4 placebo) DENV vaccinees 16,19,20 collected 42 days post-vaccination with the rDEN130, rDEN2/430, and rDEN430 vaccine candidates, respectively. The vaccine can- didate rDEN430, a live recombinant DENV-4 virus, was derived from the 814669 (Dominica/81) strains and contains a 30-nucleotide deletion in the 3' non-coding genome region. 21 Strain rDEN130 was derived from the Nauru/74 strain and * Address correspondence to Scott C. Weaver, Center for Biodefense and Emerging Infectious Diseases, and Department of Pathology Keiller 3.135, 301 University Boulevard, University of Texas Medical Branch, Galveston, TX 77555-0609. E-mail: sweaver@utmb.edu Am. J. Trop. Med. Hyg., 79(1), 2008, pp. 128–132 Copyright © 2008 by The American Society of Tropical Medicine and Hygiene 128