Molecular Ig gene analysis reveals that monocytoid B cell lymphoma is a malignancy of mature B cells carrying somatically mutated V region genes and suggests that rearrangement of the kappa-deleting element (resulting in deletion of the Ig kappa enhancers) abolishes somatic hypermutation in the human

Five cases of monocytoid B cell lymphoma (MBCL) were analyzed for somatic mutations in the rearranged V region genes. Somatic mutations were found in four of the five cases. whereas one unusual CD5(+) lymphoma harbored unmutated V region genes Since somatic mutations are introduced into V region genes of antigen-activated B cells in the course of T cell-dependent immune responses, these results suggest a derivation of the tumor B cells in MBCL from antigen-experienced mature B cells. An analysis of the kappa-deleting element in two of the cases in which mutated V-H but unmutated and nonfunctional V-kappa gene rearrangements were found suggests that somatic hypermutation does not take place in human rearranged V-kappa region genes when the C-kappa gene and the kappa enhancers have been deleted in cis by rearrangement of the kappa-deleting element.