c-Myb coordinates survival and expression of key signaling components required for the pre-BCR checkpoint (HEM1P.220)

The c-Myb transcription factor is abundantly expressed during B cell development and is down regulated as the cells mature. We targeted Myb in mice with loxP sites (Mybf/f). By crossing these mice to other strains that direct Cre expression to different stages of B cell development (Rag1-cre, Mb1-cre, CD19-cre) we are able to assess subsequent B-lineage maturation. In Mybf/f Mb1-cre mice, c-Myb is required for B cell differentiation beyond the pre-pro-B cell stage and survival of CD19+ pro-B cells. c-Myb controls survival of pro-B cells by suppression of the proapoptotic factors Bmf and Bim. We find that IL-7 signaling represses the expression of Bmf and Bim. c-Myb controls the expression of CD127 but forced expression of CD127 is not sufficient to repress Bmf and Bim or rescue survival of c-Myb deficient pro-B cells. Examination of negative mediators of IL-7 signaling revealed increased SOCS3 mRNA in c-Myb deficient pro-B cells and forced expression of SOCS3 suppressed proliferation and survival of CD19+ pro-B cells in the presence of IL-7. Surprisingly, Mybf/f CD19-cre Bcl2-Tg mice contain severely reduced large and small pre-B cell compartments. Examination of mRNA expression in these cells revealed that c-Myb controls expression of key regulators of the pre-BCR checkpoint, including CD127, lambda-5, cyclin D3 and Cxcr4. c-Myb coordinates survival in pro-B cells with the expression of key components of the signaling pathways that drive the pro-B to pre-B cell transition.

Rpl22 deficiency predisposes to leukemic transformation by blunting DNA damage responses (HEM2P.263)

Mutations that impair DNA repair mechanisms lead to accumulation of DNA damage within hematopoietic stem cells (HSC), resulting in their functional exhaustion or development into leukemic stem cells (LSC). Surprising, we have found that one such mutation in the ribosomal protein Rpl22 is able to predispose to leukemic transformation and this appears to involve impaired responses to DNA damage. Indeed, Rpl22-defiicency renders lineage negative Sca-1+, c-kit+ (LSK) hematopoietic progenitors resistant to killing by the DNA-damaging agent 5-fluorouricil (5FU), which is used to treat a variety of malignancies. Interestingly, while 5FU treatment increases γH2AX phosphorylation in Rpl22-deficeint LSK, indicating that cellular DNA has been damaged, these cells fail to induce transcriptionally active p53 because of a defect in their DNA-damage response signaling. This leads to an accumulation of LSK harboring damaged DNA, which we hypothesize predisposes them to transformation. Consistent with this viewpoint, we have found that Rpl22-/- LSK do exhibit an enhanced predisposition to transformation by the oncogenic function of MLL-AF9 in mouse models. This also appears to be true in human cancer patients, since AML patients that express less Rpl22 exhibit reduced survival. Taken together, these data support our hypothesis that Rpl22-deficiency impairs the activation of p53 in response to DNA damage and this predisposes cells to transformation and more aggressive leukemias.

Regulation of γδ T cell effector fate specification by ribosomal protein paralogs (HEM3P.285)

γδ T cells are key mediators of inflammation that play a crucial role in preserving epithelial barriers and eradicating cutaneous tumors. The molecular mechanisms that control γδ T cell development and function, however, remain poorly understood. We have previously reported that the ribosomal protein paralogs Rpl22 and Rpl22l1 have antagonistic roles during hematopoietic stem cell emergence. Consequently, we sought to determine if Rpl22 and Rpl22l1 might play roles in more differentiated hematopoietic lineages. We have now demonstrated that γδ T cell development is not arrested in the absence of Rpl22 or Rpl22l1, suggesting these proteins are dispensable γδ T cell development; however, Rpl22 and Rpl22l1 do appear to regulate the specification of γδ effector fate during development in the thymus and have opposing roles in this process. Specifically, Rpl22-deficiency enhances the generation of IL-17-producing γδ T cells. In contrast, Rpl22l1-deficiency leads to an expansion of IFN-γ-producing γδ T cells and, in particular, the Vγ1.1+ innate-like γδ T cell subset. Rpl22 and Rpl22l1 are RNA-binding proteins that exert their control over developmental processes extraribosomally (i.e., away from the ribosome) by binding and regulating the expression of cellular RNA targets. Our efforts to identify the key cellular RNAs through which Rpl22 and Rpl22l1 influence γδ T cell effector fate will be presented.

ERK signals that promote γδ T cell development require ERK interaction with DEF domain-containing targets (HEM3P.276)

Differential induction of ERK signals has been implicated in numerous fate decisions; however, the molecular basis by which gradations in ERK signaling specify alternate fates remains poorly understood. We report here that divergence of the αβ and γδ T cell fates is dependent upon differences in the extent of T cell receptor (TCR) induced activation of ERK signaling. Adoption of the γδ fate is linked to greater amplitude and duration of ERK activation, but ERK activation does not promote γδ development by phosphorylation of substrates like Rsk. Instead, ERK promotes adoption of the γδ fate by physically interacting with DEF domain containing targets through its DEF binding pocket (DBP). The DEF domain-containing targets include immediate early genes (IEG) such as the transcription factor early growth response gene 1 (Egr1). Egr1 protein is normally unstable, but is stabilized by DEF-DBP mediated interaction with ERK. Thus, ERK signals promote γδ development by stabilizing IEG proteins, including transcription factors, thereby enabling them to transactivate targets not possible in the absence of this increase in stability.

Abstract 15: The Helix-Loop-Helix Factor Id3 Links Natural Helper Cells to Proliferation of Atheroprotective B-1a Cells

Natural immunity is emerging as important for atheroprotection. Natural IgM antibodies, such as E06, that recognize oxidative epitopes on LDL or phospholipids prevent atherosclerosis and are produced by the atheroprotective B cell subset, the B-1a B cell. IL5, an important B-1a B cell mitogen, can be produced by the innate lymphoid cell, the natural helper (NH) cell. We previously demonstrated that splenic B cells provide atheroprotection to B cell deficient mice, an effect dependent on the helix-loop-helix transcription factor, Id3. Taken together, these results raise the hypothesis that Id3 may be an important regulator of splenic B-1a B cells and natural immunity in atheroprotection.

Methods and Results

Id3 -/- Apoe -/- mice at 8 weeks old had fewer B-1a cells in the spleen (200672 ±58019, n = 9) compared to Id3 +/+ Apoe -/- mice (511840 ±88811, n = 9) as determined by flow cytometry. Consistent with fewer B-1a B cells, there were lower levels of serum E06 in Id3 -/- Apoe -/- mice compared to Id3 +/+ Apoe -/- mice (1573 ±300 RLU vs. 2807 ±263 RLU respectively, n = 8 for each group) as assessed by ELISA. Id3 -/- Apoe -/- mice had decreased B-1a B cell homeostatic proliferation compared to Id3 +/+ Apoe -/- mice (30.0 ±3.3% vs. 54.4 ± 1.2% respectively, n = 4) as measured by in situ CFSE. However, B-1a B cell proliferation and number and levels of E06 were unchanged in mice with B cell specific deletion of Id3 compared to controls. To determine if Id3 is required for IL33 induced levels of IL5, IL33 or vehicle control was i.p. injected into Id3 +/+ Apoe -/- mice or Id3 -/- Apoe -/- mice every 3 days for 7 days. Id3 -/- Apoe -/- mice had less IL33 induced serum IL5 levels (5.1 ±3.1 pg mL -1 , n = 5) compared to Id3 +/+ Apoe -/- mice (67.4 ±22.9 pg mL -1 , n = 4). Indeed, loss of Id3 significantly attenuated IL33 induced IL5 production in NH cells measured by flow cytometric intracellular cytokine staining ( Id3 -/- Apoe -/- : 31.1 ±5.4%, n = 6 vs. Id3 +/+ Apoe -/- : 50.3 ±4.3%, n = 6). NH cells are also present and can to produce IL5 in response to exogenous IL33 (vehicle: 4.0 ±1.4%, n = 4 vs. IL33: 14.7 ±2.4%, n = 4) in the aortic adventitia/perivascular adipose tissue of Apoe -/- mice.

Conclusion

Results provide the first evidence implicating Id3 as a key regulator of NH cell IL5 production and B-1a B cell proliferation.

Peripheral tolerance induction by PD-L1+ lymphatic endothelial cell is due to deficient costimulation that leads to rapid, high-level expression of PD-1 on CD8 T cells (176.24)

It has been shown that lymph node (LN) stromal cells directly present peripheral tissue antigens (PTA) leading to systemic peripheral tolerance. In particular, lymphatic endothelial cells (LEC) directly present an epitope from tyrosinase to CD8 T cells (TCD8). Here, we determined the roles of costimulatory and inhibitory pathways in the induction of LEC-mediated tolerance. LEC do not express costimulatory ligands but do express ligands for the PD-1, CD80, CD160, BTLA, and LAG-3 inhibitory pathways. Nonetheless, LEC-mediated deletion of tyrosinase-specific TCD8 is solely dependent on the PD-1:PD-L1 pathway. Based on radioresistance and the fact that LEC express the highest levels of any LN resident cell, we conclude that PD-L1 expressed by LEC is necessary and sufficient for tyrosinase-specific TCD8 deletion. Presentation of tyrosinase by LEC leads to deletion by inducing more rapid, high-level PD-1 expression on TCD8 than that induced by professional APC. This is due to lack of costimulation by LEC, and is correctable with agonistic anti-4-1BB and OX-40. Apoptosis of tyrosinase-specific TCD8 induced by PD-1 is a consequence of failure to upregulate the IL-2 receptor. Rescue of tyrosinase-specific TCD8 from LEC-mediated deletion by interference with PD-1:PD-L1 or provision of costimulatory signals leads to the induction of autoimmune vitiligo. As LEC express numerous PTA, this mechanism may be of general importance in the development of autoimmune diseases.