In vitro triggering of somatic mutation in human naive B cells.

During T cell-dependent immune response, germinal center B cells accumulate somatic mutations in their Ig V(D)J genes and give rise to affinity-selected B cells. We tested several culture conditions for triggering somatic mutation in human tonsillar naive slgD+CD23+ cells after cross-linking their membrane Igs. CD40 activation, in the presence of exogenous cytokines (IL-2, IL-4, and IL-10), induced proliferation and isotype switch without somatic mutation. In contrast, after coculture with anti-CD3-activated cloned T cells, somatic mutation accumulated in a fraction of naive B cells. Mutations included shared as well as independent events in clonally related sequences, allowing reconstitution of genealogic trees generated in vitro. Naive tonsillar B cells sorted for slgD expression can be induced to mutate their Ig V(H) gene upon coculture with activated T cells, thereby providing a model to study somatic hypermutation in vitro.