Arid3b Is Critical for B Lymphocyte Development.

Arid3a and Arid3b belong to a subfamily of ARID (AT-rich interaction domain) transcription factors. The Arid family is involved in regulating chromatin accessibility, proliferation, and differentiation. Arid3a and Arid3b are closely related and share a unique REKLES domain that mediates their homo- and hetero-multimerization. Arid3a was originally isolated as a B cell transcription factor binding to the AT rich matrix attachment regions (MARS) of the immunoglobulin heavy chain intronic enhancer. Deletion of Arid3a results in a highly penetrant embryonic lethality with severe defects in erythropoiesis and hematopoietic stem cells (HSCs). The few surviving Arid3a-/- (<1%) animals have decreased HSCs and early progenitors in the bone marrow, but all mature lineages are normally represented in the bone marrow and periphery except for B cells. Arid3b-/- animals die around E7.5 precluding examination of hematopoietic development. So it is unclear whether the phenotype of Arid3a loss on hematopoiesis is dependent or independent of Arid3b. In this study we circumvented this limitation by also examining hematopoiesis in mice with a conditional allele of Arid3b. Bone marrow lacking Arid3b shows decreased common lymphoid progenitors (CLPs) and downstream B cell populations while the T cell and myeloid lineages are unchanged, reminiscent of the adult hematopoietic defect in Arid3a mice. Unlike Arid3a-/- mice, HSC populations are unperturbed in Arid3b-/- mice. This study demonstrates that HSC development is independent of Arid3b, whereas B cell development requires both Arid3a and Arid3b transcription factors.

The mirn23a microRNA cluster antagonizes B cell development.

Ablation of microRNA synthesis by deletion of the microRNA-processing enzyme Dicer has demonstrated that microRNAs are necessary for normal hematopoietic differentiation and function. However, it is still unclear which specific microRNAs are required for hematopoiesis and at what developmental stages they are necessary. This is especially true for immune cell development. We previously observed that overexpression of the products of the mirn23a gene (microRNA-23a, -24-2, and 27a) in hematopoietic progenitors increased myelopoiesis with a reciprocal decrease in B lymphopoiesis, both in vivo and in vitro. In this study, we generated a microRNA-23a, -24-2, and 27a germline knockout mouse to determine whether microRNA-23a, -24-2, and 27a expression was essential for immune cell development. Characterization of hematopoiesis in microRNA-23a, -24-2, and 27a-/- mice revealed a significant increase in B lymphocytes in both the bone marrow and the spleen, with a concomitant decrease in myeloid cells (monocytes/granulocytes). Analysis of the bone marrow progenitor populations revealed a significant increase in common lymphoid progenitors and a significant decrease in both bone marrow common myeloid progenitors and granulocyte monocyte progenitors. Gene-expression analysis of primary hematopoietic progenitors and multipotent erythroid myeloid lymphoid cells showed that microRNA-23a, -24-2, and 27a regulates essential B cell gene-expression networks. Overexpression of microRNA-24-2 target Tribbles homolog 3 can recapitulate the microRNA-23a, -24-2, and 27a-/- phenotype in vitro, suggesting that increased B cell development in microRNA-23a, -24-2, and 27a null mice can be partially explained by a Tribbles homolog 3-dependent mechanism. Data from microRNA-23a, -24-2, and 27a-/- mice support a critical role for this microRNA cluster in regulating immune cell populations through repression of B lymphopoiesis.