As a critical step in optimizing responses to specific pathogens, Ig genes undergo genomic sequence rearrangements in germinal center (GC)4
B cells, a process called Ig gene diversification, which includes somatic hypermutation (SHM) and class switch recombination (CSR). A hallmark for both SHM and CSR is the generation of DNA double-strand breaks (DSB) induced by activation-induced cytidine deaminase (1
). In most somatic cells, the response to DSB is marked by the activation of p53, which in turn induces cell cycle arrest or apoptosis. By activating p21 to induce cell cycle arrest, p53 provides cells with the opportunity to repair DNA DSB before completing the cell cycle (2
). The activation of p53 can also result in cell death, caused by induction of proapoptotic genes (5
), indicating that pathways controlling p53 activity must be exquisitely monitored. GC B cells, however, must be able to tolerate these physiologic DSB without experiencing apoptosis or cell cycle arrest.
Several studies have indicated that BCL6
, a gene critical to the GC reaction, may function in various ways to protect GC B cells from growth arrest and apoptosis. These include direct transcriptional repression of both p53
) and the programmed cell death gene PDCD2
), as well as suppression of p21
transcription by interacting with MIZ1 bound to the p21
). Recently, we reported that expression of BCL6
is regulated in part by the transcription factor IFN regulatory factor 8 (IRF8), also known as IFN consensus sequence binding protein (ICSBP) (9
), suggesting the involvement of IRF8 in GC B cell physiology.
Tyrosine phosphorylation and dephosphorylation have been shown to control the activity of IRF8 (10
), and recent studies have identified contributions of ubiquitylation by TRIM21 and CBL to the function and degradation of the protein (12
). Although IRF8 has been shown to have major effects on the development and function of myeloid and dendritic cells (14
), it is also expressed in B lineage cells. Expression can be detected from the earliest stages of commitment to the lineage (16
), is at highest levels in GC B cells, and is virtually extinguished in plasma cells (9
To further understand the role of IRF8 in GC B cell physiology, we performed an exploratory experiment comparing the gene expression profiles of a GC lymphoma-derived B cell line with small interfering RNA (siRNA)-induced suppression of IRF8 vs unsuppressed cells. Among genes regulated by IRF8, Mdm2
was of particular interest. Mdm2
encodes a regulator of p53, which inhibits p53 function in two distinct ways. It blocks p53
transcriptional activity by binding directly to the transcriptional activation domain of p53
). Additionally, as an E3 ligase, MDM2 ubiquitinates p53, inducing its nuclear export and proteosomal degradation (21
). MDM2 also directly inhibits p21 function by promoting p21 degradation via a ubiquitin-independent pathway (23
). MDM2 is overexpressed in a significant portion of different types of cancers (24
), in which it acts as an oncogene by suppressing p53 activity, thereby permitting the uncontrolled growth of tumor cells.
In this study, we present results that show Mdm2
is a direct transcriptional target of IRF8. We also show that B cells deficient in IRF8 exhibit increased sensitivity to cell death induced by DNA DSB or by IL-21, a cytokine involved in several aspects of B cell survival and differentiation (28
). IRF8 may thus facilitate the expansion of Ag-specific B cells during the course of a GC response. These results also suggest that IRF8 may contribute to lymphomagenesis through its regulation of MDM2 and by collaborating with BCL6 to disable the p53 tumor suppressor pathway.