Centrocytes rapidly adopt a memory B cell phenotype on co-culture with autologous germinal centre T cell-enriched preparations

B cells, after mutating their Ig B-region genes in germinal centres (GC), undergo apoptosis, unless they receive antigen-dependent selection signals. The signals appear to be delivered by GC T cells, require CD40 ligand expression and may induce differentiation to memory cells. Cultured GC B cells are prevented from entering apoptosis by ligating their surface CD40, but the resulting phenotype is not associated with B cells found in vivo. Conversely, GC B cells rapidly adopt a memory B cell phenotype on culture with autologous memory CD4(+) T cells that have been induced to express CD40 ligand transiently. This effect is prevented by blocking CD40 ligand. Naive CD4(+) T cells, induced to express CD40 ligand, do not prevent GC B cells undergoing apoptosis.