An in vitro monoclonal model for human immunoglubulin somatic hypermutation. Signaling through the B cell antigen receptor, CD40 and CD80^• are essential to activate the mutational machinery

Somatic hypermutation is a central mechanism in the affinity maturation of the antibody response. To address the requirements for and the modalities of Ig somatic hypermutation in the human, we have taken advantage of our recently identified monoclonal model of germinal center (GC) differentiation, CL-01 cells. CL-01 cells are sIgM⁺IgD⁺ and undergo GC-like differentiation, and effectively switch the expressed IgM to IgG, IgA, and IgE in response to physiological stimuli. By combining Ig V(D)J gene SSCP analysis and DNA sequencing, we have found that CL-01 cells effectively hypermutate the V_HDJ_H and V_λJ_λ gene segments of the expressed IgM and IgD, as well as IgG, IgA, and IgE, upon incubation with T cells and engagement of the B cell Ag receptor. As suggested by blocking experiments, CD40:CD40L and CD80:CD28 interaction are crucial in the B:T cell contact required for the induction of hypermutation. In induced CL-01 cells, point-mutations ranged from 2.67 x 10^-3 to 29.3 x 10^-3 change/base, were distributed over the entire V(D)J gene segment, and did not show any preference for the CDRs. Transitions were favored over transversions, and G and C were preferentially targeted. Thus, CL-01 cells likely constitute an invaluable model of human somatic hypermutation, and provide a useful tool toward a definition of the Ig mutation machinery.