Role of Id proteins in B lymphocyte activation: new insights from knockout mouse studies.

Id (inhibitor of differentiation) proteins play important roles in cell differentiation, cell cycle control, and apoptosis. They act as negative regulators of basic helix-loop-helix-type transcription factors, which positively regulate differentiation of various cell types. Id proteins work to block B lymphocyte (B cell) maturation at an early differentiation step, as demonstrated by gain-of-function studies. In recent years a series of gene-targeted mice lacking different Ids have been generated. Analyses of these gene-targeted mice provide information useful for understanding the physiological roles of Ids in B cell biology. Id3 is required for proper B cell functions and acts by controlling the cell cycle. Upon B cell activation, Id2 acts as a negative regulator to prevent potentially harmful effects brought about by excessive immunological reactions; one of its special roles is to maintain low serum concentrations of immunoglobulin E (IgE). The Id2 protein does this by antagonizing E2A and Pax5 activities, both of which are required for proper B cell activation. This review presents several new insights into B cell differentiation and activation programs and the physiological role of Id proteins in B cell activation.