The roles of antibody variable region hypermutation and selection in the development of the memory B-cell compartment.

Somatic hypermutation and selection of immunoglobulin (Ig) variable (V)-region genes, working in concert, appear to be essential for memory B-cell development in mammals. There has been substantial progress on the nature of the cis-acting DNA elements that regulate hypermutation. The data obtained suggest that the mechanisms of Ig gene hypermutation and transcription are intimately intertwined. While it has long been appreciated that stringent phenotypic selection forces are imposed on the somatically mutated Ig V regions generated during a T-cell dependent B-cell response, the mechanisms involved in this selection have remained enigmatic. Our studies have questioned the role of foreign antigen deposited on follicular dendritic cells in affinity-based positive selection of V regions, and have shown that this selection takes place in a "clone-autonomous" fashion. In addition, our data strongly suggest that affinity for antigen alone is not the driving force for selection of B-cell clones into the memory compartment. In contrast, we suggest that a combination of positive selection for increased foreign antigen binding, and negative selection of antibody V regions that are autoreactive at the onset of the response, or have acquired autoreactivity via hypermutation, results in the "specificity maturation" of the memory B-cell response.