Signal transducer and activator of transcription 3 (STAT3) is a key transcriptional mediator for many cytokines and is essential for normal embryonic development. We have generated a unique strain of mice with tissue-specific disruption of STAT3 in bone marrow cells during hematopoiesis. This specific STAT3 deletion causes death of these mice within 4 -6 weeks after birth with Crohn's disease-like pathogenesis in both the small and large intestine, including segmental inflammatory cell infiltration, ulceration, bowel wall thickening, and granuloma formation.

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Pax5-deficient progenitor B (pro-B) cells are thought to be severely defective for recombination of all immunoglobulin heavy chain (IgH) V gene segments, but the mechanism by which Pax5 regulates this process has not been defined. To address this issue, we have examined the assembly of the IgH locus in Pax5-deficient pro-B cells and find, unexpectedly, that 3 IgH V gene segments, which lie closest to the D-J-Cµ region, recombine efficiently, but progressively more distal V gene segments recombine progressively less efficiently. Histone acetylation and germ-line transcription correlate strongly with an open or an accessible chromatin structure thought to be permissive for V(D)J recombination, and defects in recombination are typically accompanied by deficits in these processes. We were therefore surprised to observe that distal V H gene segments in Pax5−/− pro-B cells exhibit no defect in these measures of accessibility. The finding of transcribed, histone acetylated gene segments that fail to recombine suggests that a Pax5-dependent regulatory mechanism is required in addition to standard constraints of accessibility to control V H gene recombination. During lymphocyte development, V(D)J recombination assembles the variable region of antigen receptor loci from individual V, D, and J gene segments. This reaction is initiated by binding of the RAG1 and RAG2 proteins to conserved recombination signal sequences (RSSs) that flank each of these gene segments. After binding to two RSS elements, the RAG1/RAG2 complex introduces DNA double-strand breaks between the RSSs and their flanking gene segments. Importantly, the two RSSs must first be brought into close proximity and assembled into a synaptic complex before cleavage can occur (Fugmann et al. 2000;Gellert 2002), ensuring that the recombining partners are juxtaposed before potentially dangerous DNA breaks are created. After DNA cleavage, the RAG proteins and DNA repair factors process and join the ends to complete the recombination reaction.Temporal and developmental specificity of V(D)J recombination are achieved in part by regulating the accessibility of RSSs to the recombination machinery (Yancopoulos and Alt 1985;Hesslein and Schatz 2001). Although the underlying structural basis of accessibility has not been determined, V(D)J recombination of the antigen receptor loci is tightly correlated with locus changes such as nuclease sensitivity, germ-line (sterile) transcription, DNA demethylation, and histone acetylation (Krangel 2001). Cis-acting transcriptional elements found in the antigen receptor loci have been implicated in the regulation of gene rearrangement, but the mechanisms by which these elements control V(D)J recombination are unknown.Bone marrow B-cell development has been divided into distinct stages based on cell size and surface markers by Hardy (Hardy et al. 1991) and Rolink and Melchers (Rolink et al. 1994). The earliest B lineage cells (Hardy fractions A and B or Rolink and Melcher pro/pre-B-I cells) express c-kit and B220 and actively undergo D...

Immunoglobulin heavy chain rearrangement (V H -to-DJ H ) occurs only in B cells, suggesting it is inhibited in other lineages. Here we found that in the mouse V H locus, methylation of lysine 9 on histone H3 (H3-K9), a mark of inactive chromatin, was present in non-B lineage cells but was absent in B cells. As others have shown that H3-K9 methylation can inhibit V(D)J recombination on engineered substrates, our data support the idea that H3-K9 methylation inhibits endogenous V H -to-DJ H recombination. We also show that Pax5, a transcription factor required for B cell commitment, is necessary and sufficient for the removal of H3-K9 methylation in the V H locus and provide evidence that one function of Pax5 is to remove this inhibitory modification by a mechanism of histone exchange, thus allowing B cell-specific V H -to-DJ H recombination.During hematopoiesis, transcription factors initiate and maintain lineage-specific commitment 1 . In B and T lymphocytes, the unique variable (V), diversity (D) and joining (J) (V(D)J) recombination of immunoglobulin and T cell receptor (TCR) gene segments provides critical developmental checkpoints as well as diverse, clonotypic antigen recognition capability 2 . All immunoglobulin and TCR loci use a common recombinase machinery, including proteins encoded by the recombination activating genes Rag1 and Rag2 that recognize and cut at common DNA recognition elements called recombination signal sequences (RSSs). However, V(D)J recombination proceeds with strict T lineage-B lineage specificity and developmentally determined order. The mechanisms responsible for these aspects of V(D)J regulation remain poorly understood.