Dengue Virus Infection-Enhancing Activity in Serum Samples with Neutralizing Activity as Determined by Using FcγR-Expressing Cells

Background: Progress in dengue vaccine development has been hampered by limited understanding of protective immunity against dengue virus infection. Conventional neutralizing antibody titration assays that use Fc gamma R-negative cells do not consider possible infection-enhancement activity. We reasoned that as Fc gamma R-expressing cells are the major target cells of dengue virus, neutralizing antibody titration assays using Fc gamma R-expressing cells that determine the sum of neutralizing and infection-enhancing activity, may better reflect the biological properties of antibodies in vivo. Methods and Findings: We evaluated serum samples from 80 residents of a dengue endemic country, Malaysia, for neutralizing activity, and infection-enhancing activity at 1:10 serum dilution by using Fc gamma R-negative BHK cells and Fc gamma R-expressing BHK cells. The serum samples consisted of a panel of patients with acute DENV infection (31%, 25/80) and a panel of donors without acute DENV infection (69%, 55/80). A high proportion of the tested serum samples (75%, 60/80) demonstrated DENV neutralizing activity (PRNT50 >= 10) and infection-enhancing activity. Eleven of 18 serum samples from patients with acute secondary DENV infection demonstrated neutralizing activity to the infecting serotype determined by using Fc gamma R-negative BHK cells (PRNT50 >= 10), but not when determined by using Fc gamma R-expressing cells. Conclusion: Human serum samples with low neutralizing activity determined by using Fc gamma R-negative cells showed DENV infection-enhancing activity using Fc gamma R-expressing cells, whereas those with high neutralizing activity determined by using Fc gamma R-negative cells demonstrate low or no infection-enhancing activity using Fc gamma R-expressing cells. The results suggest an inverse relationship between neutralizing antibody titer and infection-enhancing activity, and that neutralizing activity determined by using Fc gamma R-expressing cells, and not the activity determined by using Fc gamma R-negative cells, may better reflect protection to DENV infection in vivo.