B cell development requires the coordinated action of transcription factors and cytokines, in particular interleukin-7 (IL-7). We report that mice lacking the POZ (Poxvirus and zinc finger) domain of the transcription factor Miz-1 (Zbtb17(ΔPOZ/ΔPOZ)) almost entirely lacked follicular B cells, as shown by the fact that their progenitors failed to activate the Jak-Stat5 pathway and to upregulate the antiapoptotic gene Bcl2 upon IL-7 stimulation. We show that Miz-1 exerted a dual role in the interleukin-7 receptor (IL-7R) pathway by directly repressing the Janus kinase (Jak) inhibitor suppressor of cytokine signaling 1 (Socs1) and by activating Bcl2 expression. Zbtb17(ΔPOZ/ΔPOZ) (Miz-1-deficient) B cell progenitors had low expression of early B cell genes as transcription factor 3 (Tcf3) and early B cell factor 1 (Ebf1) and showed a propensity for apoptosis. Only the combined re-expression of Bcl2 and Ebf1 could reconstitute the ability of Miz-1-deficient precursors to develop into CD19(+) B cells.

IntroductionHematopoietic precursors differentiate into mature blood cell lineages through a series of well-coordinated steps. T cells are generated in the thymus, which is continuously replenished with lymphoid progenitors from the bone marrow via the bloodstream. 1 Early lymphoid progenitors (ELPs) enter the thymus and become early T lineage precursors (ETPs), defined as Lin Ϫ/low , CD117 high , and CD25 Ϫ . 2 The capacity of ELPs to migrate to the thymus has been attributed to their expression of CCR9. 3,4 In addition to CCR9 ϩ ELPs, other progenitors, such as CLPs, may home to the thymus and generate T cells. Recently, Ly6D has been used to identify the branch point of CLPs that gives rise to the first stages of B-cell development, B cell-biased lymphoid progenitor (BLP), and all-lymphoid progenitor (ALP), which contribute to the T-cell development. 5 The subsequent development of ETPs starts with CD4 Ϫ CD8 Ϫ double-negative 1 (DN1) cells. DN1s are subdivided into DN1a-e according to the expression of CD24 and CD117, DN1a/b corresponding to the ETP subset. 6 DN1s give rise to DN2a-b cells, which differentiate into DN3s, subdivided into DN3a-b based on their size and CD27 expression. 7 DN3a cells that have productively rearranged the T-cell receptor ␤-gene (TCR-␤) become activated by TCR-dependent signals (␤-selection), differentiate into DN3b, and become DN4 pre-T cells. The newly developed DN4s become CD4 ϩ CD8 ϩ double-positive (DP) cells and undergo positive/ negative selection before reaching the periphery as mature CD4 ϩ or CD8 ϩ T cells. 8 Pro-T-cell differentiation steps depend on the expression of Notch ligands, mainly ␦-like ligand 1 (DL1) and DL4 on thymic stroma, 9 and on cytokines, such as interleukin-7 (IL-7). 10 Notch signaling assures lineage commitment, survival, and development of ETPs into further DN subsets. 11 The IL-7/IL-7R pathway drives proliferation, survival, and progression of pro-T cells, 12 and also induces the rearrangement and transcription of the TCR-␥ locus. 13 The IL-7R signaling activates Janus kinase 1/3 (Jak1/3), which phosphorylate signal transducer and activator of transcription 5 (STAT5). Phosphorylated STAT5 then activates the transcription of IL-7-dependent target genes. 14 A key player in IL-7R cascade is the maintenance of cell survival by promoting a favorable balance of B-cell lymphoma-2 (Bcl-2) family members. 15 The expression of the antiapoptotic protein Bcl-2 is up-regulated after IL-7 stimulation. Some studies have shown that the up-regulation of Bcl-2 can be STAT5-dependent. [16][17][18] Other studies have shown that STAT5-mediated activation of AKT protein regulates the glucose metabolism of the cell and maintains prosurvival and growth functions. 19 Suppressor of cytokine signaling 1 (SOCS1) is known to inhibit phosphorylation of STAT proteins by directly binding to the Jak proteins and therefore inhibiting all further downstream signaling events to ensure a return to steady-state homeostasis after cytokine responses. 20 Miz-1 (Zbtb17) is a transcription f...

The human-specific p35 isoform of the invariant chain (Ii) includes an R-X-R endoplasmic reticulum (ER) retention motif that is inactivated upon HLA-DR binding. Although the masking is assumed to involve the cytoplasmic tails of class II molecules, the mechanism underlying this function remains to be investigated. Moreover, in light of the polymorphic nature of the class II cytosolic tails, little is known about the capacity of various isotypes or alleles to overcome the retention signal of Iip35. To gain further insights into these issues, we first addressed the proposed role of the HLA-DR cytoplasmic tails. As shown by flow cytometry, the presence of Iip35 in transfected HeLa cells prevented surface expression of HLA-DR molecules lacking their cytoplasmic tails (DRalphaTM/betaTM). These truncated class II molecules and Iip35 accumulated in the ER, and co-localized with calnexin, as determined by confocal microscopy. Sensitivity of DRalphaTM/betaTM to endoglycosidase H treatment confirmed that these molecules do not reach the trans-Golgi network when associated with Iip35. Further characterization revealed that the beta chain cytosolic tail is critical for efficient ER egress of class II/Iip35 complexes. Interestingly, our results clearly demonstrate for the first time that DP and DQ isotypes can also overcome the retention motif of Iip35 through a mechanism involving their very distinctive polymorphic beta chain cytoplasmic tails. Altogether, these results further dissect the masking of di-basic retention signals, and emphasize the interplay between class II molecules and Ii for the transport of the complex to the endocytic pathway.