c-Cbl deficiency leads to diminished lymphocyte development and functions in an age-dependent manner

UBA1 dysfunction in VEXAS and cancer

UBA1, an X-linked gene, encodes one of the only two ubiquitin E1 enzymes, playing a pivotal role in initiating one of the most essential post-translational modifications. In late 2020, partial loss-of-function mutations in UBA1 within hematopoietic stem and progenitor cells were found to be responsible for VEXAS Syndrome, a previously unidentified hematoinflammatory disorder predominantly affecting older males. The condition is characterized by severe inflammation, cytopenias, and an association to hematologic malignancies. In this research perspective, we comprehensively review the molecular significance of UBA1 loss of function as well as advancements in VEXAS research over the past four years for each of the VEXAS manifestations - inflammation, cytopenias, clonality, and possible oncogenicity. Special attention is given to contrasting the M41 and non-M41 mutations, aiming to elucidate their differential effects and to identify targetable mechanisms responsible for each of the symptoms. Finally, we explore the therapeutic landscape for VEXAS Syndrome, discussing the efficacy and potential of clone-targeting drugs based on the pathobiology of VEXAS. This includes azacitidine, currently approved for myelodysplastic neoplasms (MDS), novel UBA1 inhibitors being developed for a broad spectrum of cancers, Protein Kinase R-like Endoplasmic Reticulum Kinase (PERK) inhibitors, and auranofin, a long-established drug for rheumatoid arthritis. This perspective bridges basic research to clinical symptoms and therapeutics.

Possible Mechanisms of Lymphopenia in Severe Tuberculosis

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (M. tuberculosis). In lymphopenia, T cells are typically characterized by progressive loss and a decrease in their count results. Lymphopenia can hinder immune responses and lead to systemic immunosuppression, which is strongly associated with mortality. Lymphopenia is a significant immunological abnormality in the majority of patients with severe and advanced TB, and its severity is linked to disease outcomes. However, the underlying mechanism remains unclear. Currently, the research on the pathogenesis of lymphopenia during M. tuberculosis infection mainly focuses on how it affects lymphocyte production, survival, or tissue redistribution. This includes impairing hematopoiesis, inhibiting T-cell proliferation, and inducing lymphocyte apoptosis. In this study, we have compiled the latest research on the possible mechanisms that may cause lymphopenia during M. tuberculosis infection. Lymphopenia may have serious consequences in severe TB patients. Additionally, we discuss in detail potential intervention strategies to prevent lymphopenia, which could help understand TB immunopathogenesis and achieve the goal of preventing and treating severe TB.

Cracking the Monoubiquitin Code of Genetic Diseases

Ubiquitination is a versatile and dynamic post-translational modification in which single ubiquitin molecules or polyubiquitin chains are attached to target proteins, giving rise to mono- or poly-ubiquitination, respectively. The majority of research in the ubiquitin field focused on degradative polyubiquitination, whereas more recent studies uncovered the role of single ubiquitin modification in important physiological processes. Monoubiquitination can modulate the stability, subcellular localization, binding properties, and activity of the target proteins. Understanding the function of monoubiquitination in normal physiology and pathology has important therapeutic implications, as alterations in the monoubiquitin pathway are found in a broad range of genetic diseases. This review highlights a link between monoubiquitin signaling and the pathogenesis of genetic disorders.

A20 deficiency in hematopoietic stem cells causes lymphopenia and myeloproliferation due to elevated Interferon-γ signals

Inflammation and inflammatory cytokines have been shown to exert both positive and negative effects on hematopoietic stem cells (HSCs) and hematopoiesis. While the significance of inflammation driven hematopoiesis has begun to unfold, molecular players that regulate this phenomenon remain largely unknown. In the present study, we identified A20 as a critical regulator of inflammation controlled hematopoietic cell fate decisions of HSCs. A20 deficiency in HSCs leads to increased differentiation of myeloid cells and myeloproliferation. Analysis of erythroid lineage cells of A20 deficient mice indicated a striking reduction of erythrocytes in the bone marrow (BM), but elevated numbers in the spleen. Loss of A20 in HSCs causes a severe blockade of B cell differentiation in the BM and absence of peripheral B cells in the spleen, liver and blood. T cell differentiation studies revealed a reduction of both T cell progenitors and differentiated T cells in the thymus and altered T cell numbers in the spleens of A20 mutant mice. Analysis of lineage committed progenitors of the myeloid, erythroid and lymphoid lineages specified an altered composition in the A20 deficient BM. Genetic studies identified that specific loss of A20 in the myeloid lineage cells results in myeloproliferation. Bone marrow transplantation studies and mixed bone marrow chimera studies suggested an involvement of inflammatory cytokines, particularly interferon (IFN)- γ, in the onset of myeloproliferation and lymphopenia of A20 deficient mice. Finally, ablation of IFNγ signals in A20 deficient mice rescued the hematopoietic defects. In essence, these studies highlight a previously unknown role for A20 in the restriction of inflammation driven pathologic hematopoiesis. We believe that our studies based on A20 mutant mice will be helpful in understanding the pathophysiology and in the treatment of patients with A20 (TNFAIP3) mutations.

Roles of JAK2 in Aging, Inflammation, Hematopoiesis and Malignant Transformation

Clonal alterations in hematopoietic cells occur during aging and are often associated with the establishment of a subclinical inflammatory environment. Several age-related conditions and diseases may be initiated or promoted by these alterations. JAK2 mutations are among the most frequently mutated genes in blood cells during aging. The most common mutation within the JAK2 gene is JAK2-V617F that leads to constitutive activation of the kinase and thereby aberrant engagement of downstream signaling pathways. JAK2 mutations can act as central drivers of myeloproliferative neoplasia, a pre-leukemic and age-related malignancy. Likewise, hyperactive JAK-signaling is a hallmark of immune diseases and critically influences inflammation, coagulation and thrombosis. In this review we aim to summarize the current knowledge on JAK2 in clonal hematopoiesis during aging, the role of JAK-signaling in inflammation and lymphocyte biology and JAK2 function in age-related diseases and malignant transformation.

Slicer Endonuclease Argonaute 2 Is a Negative Regulator of Hematopoietic Stem Cell Quiescence

Hematopoietic stem cells (HSCs) are capable of both self-renewing throughout the lifetime of an organism and differentiating into all lineages of the blood system. A proper balance between quiescence and proliferation is critical for the self-renewal and functions of HSCs. The choice of HSCs to remain quiescent or to enter proliferation has been tightly regulated by a variety of cell intrinsic and extrinsic pathways. Identifying molecular players that control HSC quiescence and proliferation may lead to new treatment strategies and therapeutic interventions for hematologic disorders. To identify the functions of the slicer endonuclease Argonaute (Ago) 2 in the physiology of HSCs, we generated Ago2(Hem-KO) mice, that are deficient for Ago2 in HSCs and in their progeny. Analysis of Ago2(Hem-KO) mice indicated that a loss of Ago2 results in reduced HSC pool size and altered frequencies of hematopoietic progenitors. Ago2 deficient HSCs exhibit defective multilineage differentiation capacities and diminished repopulation abilities, in a cell intrinsic manner. Interestingly, Ago2 mutant HSCs remain largely quiescent and show reduced entry into cell cycle. Genome-wide transcriptome studies and gene set enrichment analysis revealed that Ago2 deficient HSCs downregulate the "HSC signature" and upregulate the "lineage signature." Moreover, our analysis on transcription factors (TFs) identified that a loss of Ago2 is sufficient to alter the "molecular signature" and "TF networks" that control the quiescent and proliferative states of HSCs. In essence, our study identified Ago2 as a key determinant of quiescence exit in HSCs. Stem Cells 2016;34:1343-1353.

Silencing autocrine death: a ubiquitin ligase that blocks activation-induced cell death in cutaneous T-cell lymphoma

Cutaneous T-cell lymphoma (CTCL) tumor cells lack the ability of activated T cells to undergo TCR/CD3-mediated activation-induced cell death (AICD). In this issue, the study reported by Wu et al. demonstrates that c-CBL (Casitas B-lineage Lymphoma proto-oncogene) is overexpressed in CTCL. When CTCL cells lose c-CBL, AICD is enhanced. Furthermore, combination therapy with methotrexate (a known demethylating agent for the CD95 gene) in combination with the loss of c-CBL increases CTCL cell death. Therefore, inhibition of c-CBL could represent a method of sensitizing lymphoma cells to enhance AICD. Armed with their novel data, the investigators envision combination therapies that target c-CBL to reactivate AICD in the malignant T cells whenever responsiveness to TCR/CD3 signaling is retained.

Lack of the ubiquitin-editing enzyme A20 results in loss of hematopoietic stem cell quiescence

Deficiency of the ubiquitin-editing enzyme A20 in the hematopoietic system causes anemia, lymphopenia, and postnatal lethality as the result of defective hematopoietic stem cell function.

A balance between quiescence and proliferation is critical for proper maintenance of the hematopoietic stem cell (HSC) pool. Although a lot is known about hematopoiesis, molecular mechanisms that control HSC quiescence remain largely unknown. The ubiquitin-editing enzyme A20 functions as a central regulator of inflammation and adaptive immunity. Here, we show that a deficiency of A20 in the hematopoietic system causes anemia, lymphopenia, and postnatal lethality. Lack of A20 in HSCs results in diminished pool size, impaired radioprotection, defective repopulation, and loss of quiescence. A20-deficient HSCs display increased IFN-γ signaling, caused by augmented NF-κB activation. Strikingly, deletion of both IFN-γ and A20 in hematopoietic cells results in partial rescue of the HSC phenotype. We anticipate that our experiments will facilitate the understanding of mechanisms through which A20-mediated inflammatory signals control HSC quiescence and functions.

IL-7: the global builder of the innate lymphoid network and beyond, one niche at a time

The development and homeostasis of adaptive and innate lymphocytes is dependent on the stromal cytokine IL-7. The initial priming of immune responses to pathogenic challenges is executed by innate lymphoid cells (ILCs) with programmed capacity to rapidly secrete effector cytokines. How ILCs are controlled by IL-7 in distinct anatomical locale has evolved into a more complex problem as IL-7 receptor is not only expressed on ILCs, but also on surrounding neighbors, including vascular endothelium and mesenchymal cells that compete for limiting IL-7. For the generation of γδ T and B cells IL-7 is required for the production of antigen receptors, and it is likely that IL-7 performs critical function in facilitating ILC effector programming in addition to its regulatory actions on cell survival and proliferation. Most of our current understanding of the highly calibrated regulatory circuits of IL-7 function and IL-7 receptor signaling has derived from studies of adaptive, conventional lymphocytes. Here we highlight recent advances in mapping the gene circuits and cellular interactions that regulate temporospatial activities of IL-7 in diverse macro and micro niches that have direct relevance to deciphering the sphere of impact of IL-7 on ILC differentiation.