Antibody Responses To T Cellindependent Antigens 265

cell cycle entry of germinal center B cells. They also make germinal center B cells sensitive to apoptosis by increasing levels of proapoptotic factors and reducing the levels of antiapoptotic proteins.

Two key steps in becoming a plasma cell are the loss of expression of a transcription factor, Pax-5, which is required for the development and maintenance of a mature B cell (see Chapter 8), and the repression of Bcl-6. During plasma cell differentiation, a transcriptional activator called IRF4 and a transcriptional repressor called Blimp-1 are induced. IRF4 and Blimp-1 together induce the expression and splicing of XBP-1, a transcription factor that plays a critical role in the unfolded protein response. XBP-1 may protect developing plasma cells from the injurious consequences of unfolded proteins (which are produced as a side effect of the massive increase in protein synthesis), or it may contribute to the maturation of plasma cells and the enhanced synthesis of Ig seen in these cells.

How exactly the decision is made for a given germinal center B cell to choose between becoming either a memory B cell or a long-lived plasma cell is unclear. Indeed, transcription factors that delineate memory B cell development remain to be identified. It appears, however, that some of the progeny of an antigen-stimulated B cell clone express low levels of IRF4, and these become functionally quiescent, self-renewing, long-lived memory cells. Whereas high levels of IRF4 lead to plasma cell differentiation, lower levels of IRF4 are insufficient to drive an activated B cell toward plasma cell differentiation and thus may be permissive for memory B cell generation.

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