Enhancement of Human Immunodeficiency Virus (HIV) Replication in Human Monocytes by Low Titres of Anti-HIV Antibodies in Vitro

Low concentrations of serum obtained from a patient with acquired immunodeficiency syndrome (AIDS) enhanced the replication of human immunodeficiency virus type 1 (HIV-1) in a particular subclone of the CD-4-positive monocytoid cell line U937 clone 2. Cells of this subclone have a high expression of Fc receptors and a considerable degree of Fc-mediated phagocytic activity. IgG purified from the serum was also able to enhance the replication. These results indicate that low concentrations of human anti-HIV antibody may enhance HIV replication on human monocyte-macrophages. Furthermore, two mouse IgG1 monoclonal antibodies against gp120, the envelope glycoprotein of HIV-1, also induced enhancement at low concentrations. The binding of radiolabelled gp120 to the cells was increased at the same low concentrations. Antibodies against envelope glycoproteins may cause enhancement of HIV infection. Both normal and enhanced replication of HIV were completely inhibited by the masking of the binding site of CD4 molecules with F(ab')2 fragments of anti-CD4 antibody. Moreover, CD4-positive, Fc gamma RI-negative K562 cells and mouse macrophages failed to show any infection in the presence of antibody. These results suggest that CD4 molecules on the cell surface are necessary to cause enhancement of infection of HIV on monocyte-macrophages.