Objectives:B-cell hyperactivity and hypergammaglobulinaemia with high levels of HIV-1-specific antibody occur during HIV-1 infection. We investigated the role played by antigen-presenting cells (APC) in the ongoing production of antibody.Methods:Adherent monocytes and non-adherent low density dendritic cells were enriched from peripheral blood of patients at different stages of HIV-1 infection (Centers for Disease Control and Prevention categories II, III and IV) and from control subjects at high or low risk of HIV infection. Different concentrations of lymphocytes were cultured in 20 ml hanging drops in the presence or absence of autologous dendritic cells or monocytes. Antibodies to p24 and gp120 were assessed by enzyme-linked immunosorbent assay.Results:Lymphocytes taken from asymptomatic HIV-1-seropositive subjects and depleted of APC produced low or moderate levels of antibody to gp120 or p24in vitro. However, adding back autologous dendritic cells significantly enhanced antibody production, although fewer samples showed responses to p24 than to gp120. Less antibody production was stimulated using cells from patients with persistent generalized lymphadenopathy, or if monocytes rather than dendritic cells were added back. Little or no antibody was produced by cells from patients with AIDS and no antibody was detected in cultures of cells from seronegative individuals with low or high risk for infection.Conclusions:The evidence suggests that ongoing humoral responses to HIV-1 are fuelled by dendritic cells. Thus, the dominance of humoral over cell-mediated responses in HIV-1 infection may depend upon signalling via dendritic cells. Changes in signalling by dendritic cells could be central to the immunologic features of HIV-1 infection.