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B cell development requires the ordered rearrangement of Ig genes encoding H and L chain proteins that assemble into BCRs or Abs capable of recognizing specific Ags. Igκ rearrangement is promoted by chromatin accessibility and by relative abundance of RAG1/2 proteins. Expression of the E26 transformation-specific transcription factor Spi-C is activated in response to dsDNA double-stranded breaks in small pre-B cells to negatively regulate pre-BCR signaling and Igκ rearrangement. However, it is not clear if Spi-C regulates Igκ rearrangement through transcription or by controlling RAG expression. In this study, we investigated the mechanism of Spi-C negative regulation of Igκ L chain rearrangement. Using an inducible expression system in a pre-B cell line, we found that Spi-C negatively regulated Igκ rearrangement, Igκ transcript levels, and Rag1 transcript levels. We found that Igκ and Rag1 transcript levels were increased in small pre-B cells from Spic−/− mice. In contrast, Igκ and Rag1 transcript levels were activated by PU.1 and were decreased in small pre-B cells from PU.1-deficient mice. Using chromatin immunoprecipitation analysis, we identified an interaction site for PU.1 and Spi-C located in the Rag1 promoter region. These results suggest that Spi-C and PU.1 counterregulate Igκ transcription and Rag1 transcription to effect Igκ recombination in small pre-B cells.

B cell acute lymphoblastic leukemia (B-ALL) is frequently associated with mutations or chromosomal translocations of genes encoding transcription factors. Conditional deletion of genes encoding the E26-transformation-specific transcription factors, PU.1 and Spi-B, in B cells (ΔPB mice) leads to B-ALL in mice at 100% incidence rate and with a median survival of 21 wk. We hypothesized that PU.1 and Spi-B may redundantly activate transcription of genes encoding tumor suppressors in the B cell lineage. Characterization of aging ΔPB mice showed that leukemia cells expressing IL-7R were found in enlarged thymuses. IL-7R-expressing B-ALL cells grew in culture in response to IL-7 and could be maintained as cell lines. Cultured ΔPB cells expressed reduced levels of B cell linker protein (BLNK), a known tumor suppressor gene, compared with controls. The Blnk promoter contained a predicted PU.1 and/or Spi-B binding site that was required for promoter activity and occupied by PU.1 and/or Spi-B as determined by chromatin immunoprecipitation. Restoration of BLNK expression in cultured ΔPB cells opposed IL-7-dependent proliferation and induced early apoptosis. We conclude that the tumor suppressor BLNK is a target of transcriptional activation by PU.1 and Spi-B in the B cell lineage. Copyright © 2012 by The American Association of Immunologists, Inc.

Splenic B-2 cells can be divided into two major subsets: follicular (FO) and marginal zone (MZ) B cells. FO and MZ B cells are generated from immature transitional B cells. Few transcription factors have been identified that regulate FO B cell differentiation. The highly related proteins PU.1, Spi-B, and Spi-C are transcription factors of the E26-transformation-specific family and are important for B cell differentiation and function. To determine whether these proteins play a role in the differentiation of FO B cells, we performed a detailed analysis of splenic B cells in mice with inactivating mutations in the genes encoding PU.1 (Sfpi1) or Spi-B (Spib). Sfpi1+/− Spib−/− (PUB) mice had a 9-fold reduction in the frequency of CD23+ FO B cells compared with that of wild-type mice. In contrast, PUB mice had a 2-fold increase in the frequency of MZ B cells that was confirmed by immunofluorescence staining. Expression of Spi-C in Eμ-Spi-C transgenic PUB mice partially rescued frequencies of CD23+ B cells. Gene expression analysis, in vitro reporter assays, and chromatin immunoprecipitation experiments showed that transcription of the Fcer2a gene encoding CD23 is activated by PU.1, Spi-B, and Spi-C. These results demonstrate that FO B cell differentiation is regulated by the E26-transformation-specific transcription factors PU.1, Spi-B, and Spi-C.

Coexpression of PU.1 and GATA-1 is required for proper specification of the mast cell lineage; however, in the myeloid and erythroid lineages, PU.1 and GATA-1 are functionally antagonistic. In this study, we report a transcriptional network in which PU.1 positively regulates GATA-1 expression in mast cell development. We isolated a variant mRNA isoform of GATA-1 in murine mast cells that is significantly upregulated during mast cell differentiation. This isoform contains an alternatively spliced first exon (IB) that is distinct from the first exon (IE) incorporated in the major erythroid mRNA transcript. In contrast to erythroid and megakaryocyte cells, in mast cells we show that PU.1 and GATA-2 predominantly occupy potential cis-regulatory elements in the IB exon region in vivo. Using reporter assays, we identify an enhancer flanking the IB exon that is activated by PU.1. Furthermore, we observe that in PU.1 -/- fetal liver cells, low levels of the IE GATA-1 isoform is expressed, but the variant IB isoform is absent. Reintroduction of PU.1 restores variant IB isoform and upregulates total GATA-1 protein expression, which is concurrent with mast cell differentiation. Our results are consistent with a transcriptional hierarchy in which PU.1, possibly in concert with GATA-2, activates GATA-1 expression in mast cells in a pathway distinct from that seen in the erythroid and megakaryocytic lineages. Copyright © 2010 by The American Association of Immunologists, Inc.

The prevailing model of peripheral tolerance necessitates tissue-resident dendritic cells migrating to lymph nodes and cross presenting self-antigen to T cells, resulting in their deletion or anergy. Our lab has identified a novel mechanism of peripheral tolerance involving a subset of lymph node stromal cells called lymphatic endothelial cells (LEC). LEC in the lymph node induce peripheral tolerance by presenting epitopes derived from peripheral tissue antigens (PTAs) in conjunction with expression of PD-L1, leading to CD8 T cell deletion. However, PTAs and PD-L1 are not expressed on LEC in the colon or diaphragm. We performed RNAseq analysis on LEC from lymph node and diaphragm to identify differences in expression of transcription factors that might control expression of PD-L1 and PTA. We found that Spi-B is the most differentially expressed transcription factor in the lymph node compared to the diaphragm, with a fold change of over 500. Spi-B has been implicated in the development and function of medullary thymic epithelial cells, a key cell type in central tolerance. Lymph node LEC harvested from Spi-B deficient mice have a significant shift in markers associated with LEC tolerogenicity. Expression levels of PD-L1, ICAM-1, MAdCAM-1, and LTbR delineate different lymph node LEC subpopulations that vary in their ability to express PTAs and tolerize CD8 T cells. LEC from Spi-B deficient mice have decreased MAdCAM-1 expression and increased LTbR and ICAM-1 expression suggesting a population shift of non-tolerogenic lymph node LEC to tolerogenic lymph node LEC. This data suggests Spi-B may be involved in a negative feedback mechanism to limit the number of tolerogenic LEC during development.

Transcription factors of the E26 transformation-specific family are critically important for regulating B cell development and function. Spi-B and Spi-C are highly related E26 transformation-specific transcription factors that can bind to identical DNA sequences in the genome. Spib-/- mice have reduced follicular and increased marginal zone B cell frequencies compared to WT mice. Spib-/- B cells are defective in BCR signaling, and cannot generate T-dependent antibody responses. Spi-C is expressed in developing B cells, but its function in B cell development and function is not clear. To determine if Spi-B and Spi-C are functionally redundant, we generated mice that are germline knockout for Spib and heterozygous for Spic (Spib-/-Spic+/-). Spib-/-Spic-/- mice were not born, potentially due to embryonic lethality. Interestingly, Spib-/-Spic+/- mice had restored number of B cells in the spleen, including a rescue of follicular B cells, and had restored proliferative responses compared to Spib-/- mice. Transcripts of genes important for BCR signaling were rescued in sorted follicular Spib-/-Spic+/- B cells compared to Spib-/- B cells. These results suggest a novel role for Spi-C in opposing Spi-B in B cell development and function. These findings have important implications for understanding antibody forming responses, such as those seen in vaccinations.