Later work using the transgenic system described by Nemazee and Burki showed that negative selection of immature B cells does not always result in their immediate deletion (Figure 11-5c). Instead, maturation of the self-reactive cell is arrested while the B cell "edits" the light-chain gene of its receptor. In this case, the H-2d/k transgenics produced a few mature B cells that expressed mIgM containing the ^ chain encoded in the transgene, but different, endogenous light chains. One explanation for these results is that when some immature B cells bind a self-antigen, maturation is arrested; the cells up-regulate RAG-1 and RAG-2 expression and begin further rearrangement of their endogenous light-chain DNA. Some of these cells succeed in replacing the k light chain of the self-antigen reactive antibody with a X chain encoded by endogenous X-chain gene segments. As a result, these cells will begin to express an "edited" mIgM with a different light chain and a specificity that is not self-reactive. These cells escape negative selection and leave the bone marrow.
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