Influence of FcγRIIa-Expressing Cells on the Assessment of Neutralizing and Enhancing Serum Antibodies Elicited by a Live-Attenuated Tetravalent Dengue Vaccine

Background.  Recent trials of recombinant, live-attenuated chimeric yellow fever-dengue tetravalent dengue vaccine (CYD-TDV) demonstrated efficacy against symptomatic, virologically confirmed dengue disease with higher point estimates of efficacy toward dengue virus (DENV)3 and DENV4 and moderate levels toward DENV1 and DENV2. It is interesting to note that serotype-specific efficacy did not correlate with absolute neutralizing antibody (nAb) geometric mean titer (GMT) values measured in a Vero-based plaque reduction neutralization test assay. The absence of Fcγ receptors on Vero cells may explain this observation. Methods.  We performed parallel seroneutralization assays in Vero cells and CV-1 cells that express FcγRIIa (CV-1-Fc) to determine the neutralizing and enhancing capacity of serotype-specific DENV Abs present in CYD-TDV clinical trial sera. Results.  Enhancement of DENV infection was observed in CV-1-Fc cells in naturally exposed nonvaccine sera, mostly for DENV3 and DENV4, at high dilutions. The CYD-TDV-vaccinated sera showed similar enhancement patterns. The CV-1-Fc nAb GMT values were 2- to 9-fold lower than Vero for all serotypes in both naturally infected individuals and CYD-TDV-vaccinated subjects with and without previous dengue immunity. The relative (CV-1-Fc/Vero) GMT decrease for anti-DENV1 and anti-DENV2 responses was not greater than for the other serotypes. Conclusions.  In vitro neutralization assays utilizing FcγRIIa-expressing cells provide evidence that serotype-specific Ab enhancement may not be a primary factor in the serotype-specific efficacy differences exhibited in the CYD-TDV trials.