IkappaBalpha is required for marginal zone B cell lineage development

Bruton's Tyrosine Kinase Synergizes with Notch2 To Govern Marginal Zone B Cells in Nonobese Diabetic Mice

Expansion of autoimmune-prone marginal zone (MZ) B cells has been implicated in type 1 diabetes. To test disease contributions of MZ B cells in NOD mice, Notch2 haploinsufficiency (Notch2(+/-)) was introduced but failed to eliminate the MZ, as it does in C57BL/6 mice. Notch2(+/-)/NOD have MZ B cell numbers similar to those of wild-type C57BL/6, yet still develop diabetes. To test whether BCR signaling supports Notch2(+/-)/NOD MZ B cells, Bruton's tyrosine kinase (Btk) deficiency was introduced. Surprisingly, MZ B cells failed to develop in Btk-deficient Notch2(+/-)/NOD mice. Expression of Notch2 and its transcriptional target, Hes5, was increased in NOD MZ B cells compared with C57BL/6 MZ B cells. Btk deficiency reduced Notch2(+/-) signaling exclusively in NOD B cells, suggesting that BCR signaling enhances Notch2 signaling in this autoimmune model. The role of BCR signaling was further investigated using an anti-insulin transgenic (Tg) BCR (125Tg). Anti-insulin B cells in 125Tg/Notch2(+/-)/NOD mice populate an enlarged MZ, suggesting that low-level BCR signaling overcomes reliance on Notch2. Tracking clonotypes of anti-insulin B cells in H chain-only VH125Tg/NOD mice showed that BTK-dependent selection into the MZ depends on strength of antigenic binding, whereas Notch2-mediated selection does not. Importantly, anti-insulin B cell numbers were reduced by Btk deficiency, but not Notch2 haploinsufficiency. These studies show that 1) Notch2 haploinsufficiency limits NOD MZ B cell expansion without preventing type 1 diabetes, 2) BTK supports the Notch2 pathway in NOD MZ B cells, and 3) autoreactive NOD B cell survival relies on BTK more than Notch2, regardless of MZ location, which may have important implications for disease-intervention strategies.

The endocytic adaptor Eps15 controls marginal zone B cell numbers

Eps15 is an endocytic adaptor protein involved in clathrin and non-clathrin mediated endocytosis. In Caenorhabditis elegans and Drosophila melanogaster lack of Eps15 leads to defects in synaptic vesicle recycling and synapse formation. We generated Eps15-KO mice to investigate its function in mammals. Eps15-KO mice are born at the expected Mendelian ratio and are fertile. Using a large-scale phenotype screen covering more than 300 parameters correlated to human disease, we found that Eps15-KO mice did not show any sign of disease or neural deficits. Instead, altered blood parameters pointed to an immunological defect. By competitive bone marrow transplantation we demonstrated that Eps15-KO hematopoietic precursor cells were more efficient than the WT counterparts in repopulating B220⁺ bone marrow cells, CD19⁻ thymocytes and splenic marginal zone (MZ) B cells. Eps15-KO mice showed a 2-fold increase in MZ B cell numbers when compared with controls. Using reverse bone marrow transplantation, we found that Eps15 regulates MZ B cell numbers in a cell autonomous manner. FACS analysis showed that although MZ B cells were increased in Eps15-KO mice, transitional and pre-MZ B cell numbers were unaffected. The increase in MZ B cell numbers in Eps15 KO mice was not dependent on altered BCR signaling or Notch activity. In conclusion, in mammals, the endocytic adaptor protein Eps15 is a regulator of B-cell lymphopoiesis.

Eukaryotic initiation factor 2alpha phosphorylation is required for B-cell maturation and function in mice

BACKGROUND

The control of translation initiation is a crucial component in the regulation of gene expression. The eukaryotic initiation factor 2α (eIF2α) mediates binding of the initiator transfer-messenger-RNA to the AUG initiation codon, and thus controls a rate-limiting step in translation initiation. Phosphorylation of eIF2α at serine 51 is linked to cellular stress response and attenuates translation initiation. The biochemistry of translation inhibition mediated by eIF2α phosphorylation is well characterized, yet the physiological importance in hematopoiesis remains only partially known.

DESIGN AND METHODS

Using hematopoietic stem cells carrying a non-phosphorylatable mutant form of eIF2α (eIF2αAA), we examined the efficiency of reconstitution in wild-type and B-cell-deficient microMT C57BL/6 recipients in two independent models.

RESULTS

We provide evidence that phosphorylation-deficient eIF2α mutant hematopoietic stem cells may repopulate lethally irradiated mice but have a defect in the development and maintenance of newly formed B cells in the bone marrow and of naïve follicular B cells in the periphery. The mature B-cell compartment is markedly reduced in bone marrow, spleen and peripheral blood, and B-cell receptor-mediated proliferation in vitro and serum immunoglobulin secretion in vivo are impaired.

CONCLUSIONS

The data suggest that regulation of translation through eIF2α phosphorylation is dispensable in hematopoietic reconstitution but essential during late B-cell development.