The BH3-only protein Bim is a critical initiator of apoptosis in hematopoietic cells. Bim is upregulated in response to growth factor withdrawal and in vitro studies have implicated the transcription factor Foxo3a as a critical inducer. To test the importance of this regulation in vivo, we generated mice with mutated Foxo-binding sites within the Bim promoters (Bim DFoxo/DFoxo ). Contrary to Bimdeficient mice, Bim DFoxo/DFoxo mice had a normal hematopoietic system. Moreover, cytokine-dependent haematopoietic cells from Bim DFoxo/DFoxo and wt mice died at similar rates. These results indicate that regulation of Bim by Foxo transcription factors is not critical for the killing of hematopoietic cells.

Graphical Abstract Highlights d Mice lacking coronin 1 show tolerance toward allografts d Costimulation from infected APCs breaks the tolerance to clear pathogens d Coronin 1-deficiency affects CAMKIV-mediated CREB activation in T cells d Prior transfer of coronin 1-deficient T cells promote tolerance toward allografts

Following thymic maturation, T cells egress as recent thymic emigrants to peripheral lymphoid organs where they undergo an additional maturation step to mature naive T cells that circulate through secondary lymphoid organs ready to be activated upon pathogenic challenges. Thymic maturation and peripheral T cell survival depend on several signaling cascades, but whether a dedicated mechanism exists that exclusively regulates homeostasis of mature naive T cells without affecting thymocytes and/or recent thymic emigrants remains unknown. In this article, we provide evidence for a specific and exclusive role of the WD repeat containing protein coronin 1 in the maintenance of naive T cells in peripheral lymphoid organs. We show that coronin 1 is dispensable for thymocyte survival and development, egress from the thymus, and survival of recent thymic emigrants. Importantly, coronin 1-deficient mice possessed comparable levels of peripheral T cells within the first 2 wk after birth but failed to populate the peripheral T cell compartment at later stages. Furthermore, dendritic cell- and IL-2/7-dependent T cell survival was found to be independent of coronin 1. Together, these results suggest the existence of a hitherto unrecognized coronin 1-dependent decision switch early during life that is responsible for peripheral naive T cell survival and homeostasis.

Dear Editor, A1/BFL-1 is the least studied pro-survival BCL-2 family member. This can be largely attributed to the lack of proper tools to study A1/BFL-1 function. Owing to the genomic organisation of the A1 locus in mice (three expressed A1 genes and one pseudo-gene, interspersed by unrelated genes) 1 a knockout is challenging. We generated shRNA transgenic mice in which all functional A1 isoforms were knocked down. In accordance with A1 mRNA expression studies, we found that A1 is critical for the development and survival of lymphocytes and granulocytes. 2 As the A1/ BFL-1 protein is regulated by ubiquitin-dependent proteasomal degradation, the A1 mRNA expression data may not truly reflect the A1/BFL-1 protein levels. Previous attempts to generate A1-specific antibodies have failed and commercially available antibodies do not reliably detect the endogenous protein.To generate A1-specific monoclonal antibodies, we immunised rats with a truncated/mutated A1 protein (delta-C20, P104K) 3 together with two KLH-conjugated peptides corresponding to central and C-terminal residues of the A1 protein (aa71-84; aa129-154). Screening by ELISA and western blotting identified one monoclonal antibody that detected overexpressed A1-a, A1-b and A1-d, and to a lesser extent overexpressed human homologue BFL-1 (data not shown and Figure 1a). To test whether this antibody could reliably detect endogenous A1, we used the mouse WEHI-231 B lymphoma cells, known to express high levels of this protein. 4 Western blotting revealed a single band of the molecular weight expected for A1 in untreated WEHI-231 cells (Figure 1b, first lane). Overexpressed A1 protein is highly unstable due to ubiquitin-dependent proteasomal degradation. 5 To further verify the specificity of the A1 antibody, we tested the impact of protein synthesis inhibition or proteasome inhibition on the protein detected in WEHI-231 cells. As expected, the protein synthesis inhibitor cyclohexamide (CHX) decreased the intensity of the protein band, whereas the proteasome inhibitor (MG132) increased it substantially (Figure 1b). Furthermore, we were able to show that this antibody can be used to immunoprecipitate endogenous A1 protein from lysates of WEHI-231 cells (Figure 1c). Next we examined whether this antibody could also detect endogenous A1 in primary mouse cells. In accordance with previous reports on A1 mRNA expression, 1 we could reliably detect A1 protein in haematopoietic tissues, such as the lymph nodes and spleen but not in the heart, kidney, liver or lungs (Figure 1d). Immunohistochemical staining using this antibody showed strong A1 protein staining within cell foci in the germinal centres of lymph nodes of non-immunised mice (Figure 1e). No staining with this antibody against A1 was observed in nonhaematopoietic tissues, such as the pancreas or the heart (data not shown). To further validate the specificity of this A1 antibody in primary cells, mouse spleen cells were treated with crosslinking IgM antibodies, a stimulus known to upregulate A1 mRNA levels in...