In vitro enhancement of respiratory syncytial virus infection of U937 cells by human sera.

Human sera containing respiratory syncytial (RS) virus-specific antibodies enhance RS virus infection of the U937 macrophage cell line. There was an increase in the number of cells expressing virus antigen when U937 cells were infected with RS virus in the presence of human serum compared to cells infected in the absence of human serum. Human sera enhanced virus yield, as measured by the cell-released infectious virus, by an average of 50-fold compared to virus infection in the absence of human serum. The comparison of the enhancing activities of paired acute and convalescent human sera showed that the titre of enhancing antibody increased in parallel with the titre of RS virus-specific antibody measured by complement fixation and virus neutralization. An RS virus-specific neutralizing monoclonal antibody directed to the virus F protein enhanced virus infection of U937 cells. A non-neutralizing monoclonal antibody directed to the virus nucleoprotein did not enhance virus infection. The possible role of enhancing antibodies in vivo is discussed.