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s Early B cell lymphopoiesis depends on E2A, Ebf1, Pax5 and Ikaros family members. In the present study, we used acute protein degradation in mice to identify direct target genes of these transcription factors in pro-B, small pre-B and immature B cells. E2A, Ebf1 and Pax5 predominantly function as transcriptional activators by inducing open chromatin at their target genes, have largely unique functions and are essential for early B cell maintenance. Ikaros and Aiolos act as dedicated repressors to cooperatively control early B cell development. The surrogate light-chain genes Igll1 and Vpreb1 are directly activated by Ebf1 and Pax5 in pro-B cells and directly repressed by Ikaros and Aiolos in small pre-B cells. Pax5 and E2A contribute to V(D)J recombination by activating Rag1 , Rag2 , Dntt , Irf4 and Irf8 . Similar to Pax5, Ebf1 also represses the cohesin-release factor gene Wapl to mediate prolonged loop extrusion across the Igh locus. In summary, in vivo protein degradation has provided unprecedented insight into the control of early B cell lymphopoiesis by five transcription factors. By use of a degron-mediated acute protein degradation model, Schwickert and colleagues are able to distinguish between direct and indirect gene targets of multiple transcription factors involved in early B cell development.

Survival of various immune cell populations has been proposed to preferentially rely on a particular anti-apoptotic BCL-2 family member, for example, naive T cells require BCL-2, while regulatory T cells require MCL-1. Here we examined the survival requirements of multiple immune cell subsets in vitro and in vivo , using both genetic and pharmacological approaches. Our findings support a model in which survival is determined by quantitative participation of multiple anti-apoptotic proteins rather than by a single anti-apoptotic protein. This model provides both an insight into how the sum of relative levels of anti-apoptotic proteins BCL-2, MCL-1 and A1 influence survival of T cells, B cells and dendritic cells, and a framework for ascertaining how these different immune cells can be optimally targeted in treatment of immunopathology, transplantation rejection or hematological cancers.

Mutations in the chloroquine resistance transporter (PfCRT) are the primary determinant of chloroquine (CQ) resistance in the malaria parasite Plasmodium falciparum . A number of distinct PfCRT haplotypes, containing between 4 and 10 mutations, have given rise to CQ resistance in different parts of the world. Here we present a detailed molecular analysis of the number of mutations (and the order of addition) required to confer CQ transport activity upon the PfCRT as well as a kinetic characterization of diverse forms of PfCRT. We measured the ability of more than 100 variants of PfCRT to transport CQ when expressed at the surface of Xenopus laevis oocytes. Multiple mutational pathways led to saturable CQ transport via PfCRT, but these could be separated into two main lineages. Moreover, the attainment of full activity followed a rigid process in which mutations had to be added in a specific order to avoid reductions in CQ transport activity. A minimum of two mutations sufficed for (low) CQ transport activity, and as few as four conferred full activity. The finding that diverse PfCRT variants are all limited in their capacity to transport CQ suggests that resistance could be overcome by reoptimizing the CQ dosage.

GM-CSF promotes myeloid differentiation of cultured bone marrow cells into cells of the granulocytic and monocytic lineage; the latter can further differentiate into monocytes/macrophages and dendritic cells. How GM-CSF selects for these different myeloid fates is unresolved. GM-CSF levels can change either iatrogenically (e.g., augmenting leukopoiesis after radiotherapy) or naturally (e.g., during infection or inflammation) resulting in different immunological outcomes. Therefore, we asked whether the dose of GM-CSF may regulate the development of three types of myeloid cells. Here, we showed that GM-CSF acted as a molecular rheostat where the quantity determined which cell type was favored; moreover, the cellular process by which this was achieved was different for each cell type. Thus, low quantities of GM-CSF promoted the granulocytic lineage, mainly through survival. High quantities promoted the monocytic lineage, mainly through proliferation, whereas moderate quantities promoted moDCs, mainly through differentiation. Finally, we demonstrated that monocytes/macrophages generated with different doses of GM-CSF differed in function. We contend that this selective effect of GM-CSF dose on myeloid differentiation and function should be taken into consideration during pathophysiological states that may alter GM-CSF levels and during GM-CSF agonistic or antagonistic therapy.

It has long been assumed that p53 suppresses tumor development through induction of apoptosis, possibly with contributions by cell cycle arrest and cell senescence 1 , 2 . However, combined deficiency in these three processes does not result in spontaneous tumor formation as observed upon loss of p53, suggesting the existence of additional mechanisms that are critical mediators of p53-dependent tumor suppression function 3 – 5 . To define such mechanisms, we performed in vivo shRNA screens targeting p53-regulated genes in sensitized genetic backgrounds. We found that knockdown of Zmat3, Ctsf and Cav1 , promoted lymphoma/leukemia development only when PUMA and p21, the critical effectors of p53-driven apoptosis, cell cycle arrest and senescence, were also absent. Notably, loss of the DNA repair gene Mlh1 caused lymphoma in a wild-type background, and its enforced expression was able to delay tumor development driven by loss of p53. Further examination of direct p53 target genes implicated in DNA repair showed that knockdown of Mlh1 , Msh2 , Rnf144b , Cav1 and Ddit4 accelerated MYC-driven lymphoma development to a similar extent as knockdown of p53. Collectively, these findings demonstrate that extensive functional overlap of several p53-regulated processes safeguards against cancer and that coordination of DNA repair appears to be an important process by which p53 suppresses tumor development. In vivo shRNA screens in sensitized genetic backgrounds identify p53-activated target genes involved in DNA repair that enable its tumor suppressor function.

The pro-survival proteins of the BCL-2 family regulate the survival of all cells, and genetic deletion models for these proteins have revealed which specific BCL-2 family member(s) is/are critical for the survival of particular cell types. A1 is a pro-survival BCL-2-like protein that is expressed predominantly in haematopoietic cells, and here we describe the characterisation of a novel mouse strain that lacks all three functional isoforms of A1 (A1-a, A1-b and A1-d). Surprisingly, complete loss of A1 caused only minor defects, with significant, although relatively small, decreases in γδ TCR T cells, antigen-experienced conventional as well as regulatory CD4 T cells and conventional dendritic cells (cDCs). When examining these cell types in tissue culture, only cDC survival was significantly impaired by the loss of A1. Therefore, A1 appears to be a surprisingly redundant pro-survival protein in the haematopoietic system and other tissues, suggesting that its targeting in cancer may be readily tolerated.

Inflammation is a natural defence mechanism of the body to protect against pathogens. It is induced by immune cells, such as macrophages and neutrophils, which are rapidly recruited to the site of infection, mediating host defence. The processes for eliminating inflammatory cells after pathogen clearance are critical in preventing sustained inflammation, which can instigate diverse pathologies. During chronic inflammation, the excessive and uncontrollable activity of the immune system can cause extensive tissue damage. New therapies aimed at preventing this over-activity of the immune system could have major clinical benefits. Here, we investigated the role of the pro-survival Bcl-2 family member A1 in the survival of inflammatory cells under normal and inflammatory conditions using murine models of lung and peritoneal inflammation. Despite the robust upregulation of A1 protein levels in wild-type cells upon induction of inflammation, the survival of inflammatory cells was not impacted in A1-deficient mice compared to wild-type controls. These findings indicate that A1 does not play a major role in immune cell homoeostasis during inflammation and therefore does not constitute an attractive therapeutic target for such morbidities.

The physiological role of the pro-survival BCL-2 family member A1 has been debated for a long time. Strong mRNA induction in T cells on T cell receptor (TCR)-engagement suggested a major role of A1 in the survival of activated T cells. However, the investigation of the physiological roles of A1 was complicated by the quadruplication of the A1 gene locus in mice, making A1 gene targeting very difficult. Here, we used the recently generated A1 −/− mouse model to examine the role of A1 in T cell immunity. We confirmed rapid and strong induction of A1 protein in response to TCR/CD3 stimulation in CD4 + as well as CD8 + T cells. Surprisingly, on infection with the acute influenza HKx31 or the lymphocytic choriomeningitis virus docile strains mice lacking A1 did not show any impairment in the expansion, survival, or effector function of cytotoxic T cells. Furthermore, the ability of A1 −/− mice to generate antigen-specific memory T cells or to provide adequate CD4-dependent help to B cells was not impaired. These results suggest functional redundancy of A1 with other pro-survival BCL-2 family members in the control of T cell-dependent immune responses.