Increased antibody secretion by B cells co-cultured with oestrogen-stimulated CD4+ T cells. (a, b) CD4+ T cells from systemic lupus erythematosus (SLE) patients were treated with 1000 nm 17β-oestradiol for 48 hr. CD4+ T cells and culture supernatants were then separated by centrifugation. B cells isolated from the peripheral blood of healthy controls were cultured for 96 hr with oestrogen-pretreated T cells (a) and the culture supernatant (b), respectively, in the absence or presence of interleukin-21 (IL-21) blocking antibody. The concentrations of secreted IgG, IgG1 and IgG2a by B cells were measured by ELISA. Data are expressed as means ± SD of three independent experiments using cells from three patients and three healthy controls. (*P < 0·05, **P < 0·01, versus untreated cells, #P < 0·05, ##P < 0·01, versus 1000 nm 17β-oestradiol-treated cells).