Previous studies with anti-prM antibodies have shown that while the antibodies are generally non-neutralizing [9, 25] , they are capable of rendering non-infectious immature DENV (imDENV) particles infectious through an FccR-mediated process [9, 12] . Indeed, D29 Fab-IgG failed to neutralize any of the 4 serotypes of DENV at the highest concentration tested (400 mg/ml) in a PRNT assay (Table S1 ). Next, the ability of D29 Fab-IgG to enhance infection of DENV2 and imDENV2 was determined by carrying out ADE assays with FccR-bearing K562 cells. The specific infectivity of the viruses was established by qPCR and plaque assay to be 8.5610 5 :1 for imDENV2, which was significantly higher than that of the standard-grown DENV2 at 240:1 (Fig. 7A) .