Molecular fine-tuning of affinity maturation in germinal centers.

The development of high-affinity antibodies in response to infection is an iterative process in which B cells cycle between proliferation/somatic hypermutation and antigen-driven selection. These processes occur within specific regions of the secondary lymphoid structures known as germinal centers (GCs) and the environmental and signaling cues provided by these regions guide the GC reactions that drive B cell maturation and antibody production, ultimately determining B cell fate. In this issue of the JCI, Nakagawa and colleagues examine the role of miR-155, a microRNA that is required for GC development and the production of high-affinity antibodies. They show that miR-155 is highly expressed in positively selected B cells and promotes survival of these cells by orienting the Myc transcription program toward survival rather than apoptosis through the inhibition of the transcriptional regulator JARID2. These findings illustrate the fine balance between apoptosis and proliferation that is required for the development of high-affinity antibodies.