Abstract
Dengue viruses are distributed widely in the tropical and subtropical areas of the world and cause dengue fever and its severer form, dengue hemorrhagic fever. While neutralizing antibodies are considered to play a major role in protection from these diseases, antibody-dependent enhancement (ADE) of infection is an important mechanism involved in disease severity, in addition to the involvement of T lymphocytes. Here, we analyzed relationships between neutralizing and enhancing activities at a clonal level using models of dengue type 2 virus (DENV2) and dengue type 4 virus (DENV4). Totals of 33 monoclonal antibodies (MAbs) against DENV2 and 43 against DENV4 were generated, all directed to the envelope protein. In these MAbs, enhancing activities were shown at subneutralizing doses under normal ADE assay conditions where test samples were heat inactivated. However, the inclusion of commercial rabbit complement or fresh sera from healthy humans in the ADE assay system abolished the enhancing activities of all these MAbs. The reductive effect of fresh sera on enhancing activities was significantly reduced by their heat inactivation or the use of C1q- or C3-depleted sera. In some fresh sera, enhancing activities were shown within a range of 20 to 80% of normal complement levels in a dose-dependent fashion. These results indicate that a single antibody species possesses two distinct activities (neutralizing/enhancing), which are controlled by the level of complement, suggesting the involvement of complement in dengue disease severity. Fresh human sera also tended to reduce enhancing activities more effectively in homologous than heterologous combinations of viruses (DENV2/DENV4) and MAbs (against DENV2/DENV4).