No abstract

The availability of monoclonal antibodies has revived interest in immunotherapy. The ability to influence an individual's immune state by administering immunoglobulin of the appropriate specificity may provide a powerful approach to disease control and prevention. Compared with immunoglobulin from other species, human immunoglobulin (Ig) might be best for such therapeutic intervention; it might function better with the recipient's effector cells and should itself be less immunogenic. The success of the mouse hybridoma system suggests that immunoglobulin of virtually any specificity can be obtained from a properly immunized animal. In the human system, however, immunization protocols are restricted by ethical considerations, and it is not yet clear whether human antibody-producing cell lines of the required specificity can be obtained from adventitiously immunized individuals or from in vitro immunized cells. A method which might circumvent these difficulties is to produce antibodies consisting of mouse variable regions joined to human constant regions. Therefore, we have constructed immunoglobulin genes in which the DNA segments encoding mouse variable regions specific for the hapten trinitrophenyl (TNP) are joined to segments encoding human mu and kappa constant regions. These 'chimaeric' genes are expressed as functional TNP-binding chimaeric IgM. We report here some of the properties of this novel IgM.

The production of high-affinity protective antibodies requires somatic hypermutation (SHM) of the antibody variable (V)-region genes. SHM is characterized by a high frequency of point mutations that occur only during the centroblast stage of B-cell differentiation. Activation-induced cytidine deaminase (AID), which is expressed specifically in germinal-centre centroblasts, is required for this process, but its exact role is unknown. Here we show that AID is required for SHM in the centroblast-like Ramos cells, and that expression of AID is sufficient to induce SHM in hybridoma cells, which represent a later stage of B-cell differentiation that does not normally undergo SHM. In one hybridoma, mutations were exclusively in G*C base pairs that were mostly within RGYW or WRCY motifs, suggesting that AID has primary responsibility for mutations at these nucleotides. The activation of SHM in hybridomas indicates that AID does not require other centroblast-specific cofactors to induce SHM, suggesting either that it functions alone or that the factors it requires are expressed at other stages of B-cell differentiation.