A new approach to studying tolerance

Antigen-receptor-transgenic mice are commonly used to study lymphocyte responses because they increase the frequency of antigen-specific cells to easily detectable levels. However, such systems are designed to distort lymphocyte development and generate quasi-monoclonal immune systems. These non-physiological changes mean that results from receptor-transgenic mice require verification in normal, polyclonal immune systems. Nemazee and colleagues have developed a new approach using single-chain antibodies that allows them to do just that, by creating a superantigen that binds a high frequency of polyclonal B-cell receptors (BCRs).

They generated a single-chain antibody consisting of an Fv domain specific for the constant region of mouse Igκ light chain, hinge regions and the Fc portion from rat IgG1, and transmembrane and cytoplasmic regions from H-2K^b. Cells transfected with the chimeric gene expressed a stable cell-surface protein with the desired specificity for mouse Igκ. Transgenic mice were then created that have uniform and ubiquitous expression of this protein under control of the ubiquitin C promoter. This 'macro-self' antigen can bind the BCRs of a high proportion of polyclonal B cells (those that are Igκ⁺) without requiring skewing of the lymphocyte repertoire.

Almost all of the B cells in these transgenic mice expressed Igλ light chains rather than Igκ, indicating that the antigen can induce B-cell tolerance. One potential mechanism for such tolerance is receptor editing involving secondary light-chain rearrangements, but this has not been shown in vivo in non-receptor-transgenic mice. Using various approaches, the authors showed that, in their system, tolerance is the result of developmental arrest followed by receptor editing concomitant with upregulated expression of recombination-activating genes (Rag1 and Rag2). It was not associated with clonal deletion of Igκ⁺ B cells or proliferation of Igλ⁺ cells.

This technique could, in theory, be used for any type of antigen receptor for which there is a specific monoclonal antibody. The authors also suggest that an adoptive-transfer approach, using macro-self-transgenic mice as hosts for the adoptive transfer of normal or mutant bone marrow, could speed up the screening of mutant mice for immune-tolerance phenotypes, which currently requires time-consuming and expensive crossing with receptor-transgenic mice.