The B cell to plasma cell transition represents a dramatic molecular switch, from a transcriptional program controlling the B cell phenotype (sensing antigen, cytokines and Th cells), to the plasma cell phenotype dedicated to antibody production, secretion and long-term survival.
Bcl6, a transcriptional repressor of the bric-a-brac, tramtrack, broad complex/Pox virus zinc finger (BTB/POZ) family, is critically important during B and Tfh cell differentiation, enabling the formation and operation of germinal centres (GC), and inhibiting premature B cell differentiation. It is also an oncogene frequently implicated in B cell lymphomas. Bcl6 protein stability is post-translationally regulated in response to B cell receptor signaling, through phosphorylation and subsequent proteasome-mediated degradation. This degradation is postulated to reflect a strong BCR signal through a high affinity antigen receptor, and to signal exit from the GC and initiation of differentiation.
We have examined Bcl6 protein stability in B cells under a number of activating regimens in vitro. We confirmed that a BCR signal causes rapid Bcl6 degradation that is proteasome-mediated. We also found that the follicular Th cell cytokines IL4 and IL21 are able to inhibit this degradation, allowing Bcl6 proteins levels to remain high for several days. The IL4 effect required Stat6, and both transcription and translation. We propose that this is a means through which Tfh cells support B cells during the GC reaction. In vivo, we found that GC B cell numbers declined more rapidly in Stat6-/- mice than controls.
Irf4, a transcription factor of the Interferon response family, is also required for GC formation and is essential for terminal differentiation to antibody secreting cells in vivo and in vitro. We describe a hypomorphic allele of Irf4, derived via enu mutagenesis, that selectively enables T cell independent differentiation in vitro, and restores the plasma cell compartment of the spleen, but not the bone marrow, in vivo. The mechanisms of action are being explored, and will be discussed.