Rearrangement of the BCL-6 proto-oncogene can be detected in 30 to 40% of diffuse large-cell lymphomas (DLCLs) and in 6 to 14% of follicular lymphomas (FLs) (5
). In DLCLs and FLs, chromosomal rearrangements affecting the BCL-6 gene are located within a region spanning approximately 4 kb of the promoter and the first exon and result in the juxtaposition of the BCL-6 coding domains downstream of heterologous promoters derived from other chromosomes (53
). These alterations lead to the production of chimeric transcripts which encode a wild-type BCL-6 protein, suggesting that the functional consequence of these translocations is the deregulation of BCL-6 expression by promoter substitution (53
). The high frequency of dysregulated BCL-6 expression in these tumors suggests that this oncogene plays an important role in the transformation of human B cells.
The BCL-6 gene encodes a polypeptide containing six carboxy-terminal zinc-finger motifs homologous to members of the Krüppel subfamily of zinc-finger proteins (30
). This domain of BCL-6 has been shown to recognize and bind to specific DNA sequences in vitro (4
). The N-terminal portion of BCL-6 contains a ZiN (for zinc-finger N-terminal)/POZ (POX/zinc-finger) domain which is also present in other zinc-finger proteins, including the mammalian transcriptional regulators PLZF, ZF5, and ZID (3
). Transfection experiments have demonstrated that BCL-6 can act as a transcriptional repressor, and its ability to mediate repression requires the N-terminal POZ domain (9
). These results suggest that BCL-6 modulates transcription not simply through competitive binding, but through a mechanism of active repression. Indeed, the POZ domains of both BCL-6 and PLZF have recently been shown to associate with the SMRT corepressor, and, by extension, the histone deacetylase repression complex (17
BCL-6 is normally expressed in a tissue-specific and developmentally regulated manner. Although many tissues express low levels of BCL-6 mRNA, high levels of the BCL-6 protein have been found only in certain B cells and T cells (6
). Within the B-cell lineage, BCL-6 is expressed at high levels in mature, germinal center B cells, but not in other B cells or plasma cells (6
). BCL-6 expression in T cells is limited to cortical thymocytes and a population of CD4+
cells within the germinal center and perifollicular zones of the lymph nodes (6
). The importance of BCL-6 in normal lymphocyte function has recently been demonstrated in mice in which the gene for BCL-6 has been disrupted by homologous recombination (16
). Although these mice contain normal numbers of B and T cells, they fail to form germinal centers or mount T-cell-dependent antibody responses. In addition, many of these mice develop a systemic inflammatory disease characterized by the infiltration of multiple organ systems by eosinophils and immunoglobulin E (IgE)-bearing B cells; these features are indicative of a Th2 polarized inflammatory response, which could potentially result from the inappropriate influence of the Th2 cytokines (interleukin-4 [IL-4], IL-5, and IL-13) on immune function. The striking phenotype of the knockout animal therefore implicates BCL-6 in the normal regulation of the immune system.
The evidence suggesting a disruption of cytokine regulation in the BCL-6−/−
mice prompted the comparison between the in vitro defined binding site of BCL-6 (B6BS) and the binding sites of STAT proteins, molecules which are important mediators of cytokine signal transduction (reviewed in references 27
, and 46
). In fact, B6BS shows a marked similarity to STAT recognition sequences, and one study has demonstrated the ability of BCL-6 to bind to the Stat6 site of the IL-4-inducible CD23b promoter (16
). Furthermore, transient transfection studies have suggested that BCL-6 may regulate the Stat6-dependent transcription of the CD23b gene (16
). However, the regulation of gene expression by BCL-6 under physiological conditions has not yet been tested, and other physiologic targets of BCL-6 repression are so far unknown.
In order to identify physiological targets for BCL-6, we have analyzed its ability to bind and regulate Stat6-dependent promoters in vitro and in vivo. Our results demonstrate that although BCL-6 can bind to the Stat6 sites present in several IL-4-responsive promoters in vitro, it can regulate only a subset of Stat6-dependent promoters in vivo; this subset includes the germ line
promoter, but not the CD23b promoter. The germ line
promoter regulates the production of the Ig sterile transcripts necessary for the Ig isotype class switch to IgE (reviewed in reference 13
). Consistent with a role for BCL-6 in the regulation of class switching, IgE production is increased in B cells lacking BCL-6 in vitro and in vivo. This dysregulation of IgE production is not observed in B cells lacking Stat6 as well as BCL-6. These results provide evidence for the physiologic regulatory activity of BCL-6 on specific Stat6-dependent IL-4 signaling and identify the regulation of IgE class switching as a target of this activity in vivo.