In vitro virulence marker: growth of dengue-2 virus in human leukocyte suspension cultures.

We wished to find a simple, biologically relevant method to evaluate the virulence of dengue viruses for human beings. Since cells of mononuclear phagocyte lineage may be important sites of dengue infection in primates, we evaluated the permissiveness of these cells to dengue virus as a correlate of virus virulence. Two wild-type, large-plaque, monkey-virulent dengue-2 virus strains and two small-plaque, monkey-avirulent dengue-2 virus strains were evaluated for their ability to replicate in human peripheral blood leukocyte cultures supplemented with enhancing antibody. One of the small-plaque strains was demonstrated to have reduced virulence for man. Wild-type dengue-2 viruses replicated readily in peripheral blood leukocyte suspension cultures, whereas small-plaque dengue-2 strains did not. Differences between our data and results obtained by other workers employing adherent peripheral blood leukocytes are discussed. Antibody-enhanced growth of dengue virus in suspension cultures of human peripheral blood leukocytes gives promise of being a simple in vitro system for characterizing dengue virus virulence.