The nuclease ARTEMIS is essential for the development of B and T lymphocytes. It is required for opening DNA hairpins generated during antigen receptor gene assembly from variable (V), diversity (D), ...and joining (J) subgenic elements (V(D)J recombination). As a member of the non-homologous end-joining pathway, it is also involved in repairing a subset of pathological DNA double strand breaks. Loss of ARTEMIS function therefore results in radiosensitive severe combined immunodeficiency (RS-SCID). The hairpin opening activity is dependent on the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), which can bind to and phosphorylate ARTEMIS. The ARTEMIS C terminus is dispensable for cellular V(D)J recombination and
nuclease assays with C-terminally truncated ARTEMIS showing DNA-PKcs-independent hairpin opening activity. Therefore, it has been postulated that ARTEMIS is regulated via autoinhibition by its C terminus. To obtain evidence for the autoinhibition model, we performed co-immunoprecipitation experiments with combinations of ARTEMIS mutants. We show that an N-terminal fragment comprising the catalytic domain can interact both with itself and with a C-terminal fragment. Amino acid exchanges N456A+S457A+E458Q in the C terminus of full-length ARTEMIS resulted in unmasking of the N terminus and in increased ARTEMIS activity in cellular V(D)J recombination assays. Mutations in ARTEMIS-deficient patients impaired the interaction with the C terminus and also affected protein stability. The interaction between the N- and C-terminal domains was not DNA-PKcs-dependent, and phosphomimetic mutations in the C-terminal domain did not result in unmasking of the catalytic domain. Our experiments provide strong evidence that a physical interaction between the C-terminal and catalytic domains mediates ARTEMIS autoinhibition.
To validate current donor selection strategies based on previous international studies, we retrospectively analyzed 2646 transplantations performed for hematologic malignancies in 28 German ...transplant centers. Donors and recipients were high resolution typed for HLA-A, -B, -C, -DRB1, and -DQB1. The highest mortality in overall survival analysis was seen for HLA-A, -B, and DRB1 mismatches. HLA-DQB1 mismatched cases showed a trend toward higher mortality, mostly due to HLA-DQB1 antigen disparities. HLA incompatibilities at >1 locus showed additive detrimental effects. HLA mismatching had no significant effect on relapse incidence and primary graft failure. Graft source had no impact on survival end points, neither in univariate nor in multivariate analysis. Higher patient age, advanced disease, transplantations before 2004, patient C2C2 killer cell immunoglobulin-like receptor (KIR)-ligand phenotype, and unavailability of a national donor adversely influenced outcomes in multivariate analysis. Our study confirms the association of HLA-A, -B, -C, and -DRB1 incompatibilities with adverse outcome in hematopoietic stem cell transplantation (HSCT). The relevance of HLA-DQB1 disparities in single mismatched transplantations remains unclear. Similar hazard ratios for allele and antigen mismatches (possibly with an exception for HLA-DQB1) highlight the importance of allele level typing and matching in HSCT. The number of incompatibilities and their type significantly impact survival.
•HLA mismatches at the allele and antigen level (possibly with the exception of HLA-DQB1) should be treated equally in donor selection.•HLA mismatches at >1 locus (including HLA-DQB1) have additive detrimental effects.
Childhood acute lymphoblastic leukemia (ALL) can often be traced to a pre-leukemic clone carrying a prenatal genetic lesion. Postnatally acquired mutations then drive clonal evolution toward overt ...leukemia. The enzymes RAG1-RAG2 and AID, which diversify immunoglobulin-encoding genes, are strictly segregated in developing cells during B lymphopoiesis and peripheral mature B cells, respectively. Here we identified small pre-BII cells as a natural subset with increased genetic vulnerability owing to concurrent activation of these enzymes. Consistent with epidemiological findings on childhood ALL etiology, susceptibility to genetic lesions during B lymphopoiesis at the transition from the large pre-BII cell stage to the small pre-BII cell stage was exacerbated by abnormal cytokine signaling and repetitive inflammatory stimuli. We demonstrated that AID and RAG1-RAG2 drove leukemic clonal evolution with repeated exposure to inflammatory stimuli, paralleling chronic infections in childhood.
Autoimmune lymphoproliferative syndrome (ALPS) is a human disorder characterized by defective Fas signaling, resulting in chronic benign lymphoproliferation and accumulation of TCRαβ+ CD4− CD8− ...double-negative T (DNT) cells. Although their phenotype resembles that of terminally differentiated or exhausted T cells, lack of KLRG1, high eomesodermin, and marginal T-bet expression point instead to a long-lived memory state with potent proliferative capacity. Here we show that despite their terminally differentiated phenotype, human ALPS DNT cells exhibit substantial mitotic activity in vivo. Notably, hyperproliferation of ALPS DNT cells is associated with increased basal and activation-induced phosphorylation of serine-threonine kinases Akt and mechanistic target of rapamycin (mTOR). The mTOR inhibitor rapamycin abrogated survival and proliferation of ALPS DNT cells, but not of CD4+ or CD8+ T cells in vitro. In vivo, mTOR inhibition reduced proliferation and abnormal differentiation by DNT cells. Importantly, increased mitotic activity and hyperactive mTOR signaling was also observed in recently defined CD4+ or CD8+ precursor DNT cells, and mTOR inhibition specifically reduced these cells in vivo, indicating abnormal programming of Fas-deficient T cells before the DNT stage. Thus, our results identify the mTOR pathway as a major regulator of lymphoproliferation and aberrant differentiation in ALPS.
•ALPS DNT cells and their putative precursors reveal high proliferative activity in vivo, which is associated with hyperactive mTOR signaling.•Rapamycin therapy controls mitotic activity and abnormal differentiation of ALPS DNT cells and reduces CD4+ or CD8+ precursor DNT cells.
Non-homologous DNA end-joining (NHEJ)--the main pathway for repairing double-stranded DNA breaks--functions throughout the cell cycle to repair such lesions. Defects in NHEJ result in marked ...sensitivity to ionizing radiation and ablation of lymphocytes, which rely on NHEJ to complete the rearrangement of antigen-receptor genes. NHEJ is typically imprecise, a characteristic that is useful for immune diversification in lymphocytes, but which might also contribute to some of the genetic changes that underlie cancer and ageing.
We report 5 individuals in 3 unrelated families with severe thrombocytopenia progressing to trilineage bone marrow failure (BMF). Four of the children received hematopoietic stem cell transplants and ...all showed poor graft function with persistent severe cytopenias even after repeated transplants with different donors. Exome and targeted sequencing identified mutations in the gene encoding thrombopoietin (THPO): THPO R99W, homozygous in affected children in 2 families, and THPO R157X, homozygous in the affected child in the third family. Both mutations result in a lack of THPO in the patients' serum. For the 2 surviving patients, improvement in trilineage hematopoiesis was achieved following treatment with a THPO receptor agonist. These studies demonstrate that biallelic loss-of-function mutations in THPO cause BMF, which is unresponsive to transplant due to a hematopoietic cell-extrinsic mechanism. These studies provide further support for the critical role of the MPL-THPO pathway in hematopoiesis and highlight the importance of accurate genetic diagnosis to inform treatment decisions for BMF.
•Germ line biallelic loss-of-function THPO mutations cause BMF.•Marrow failure due to THPO mutations is characterized by poor graft function after transplantation but responds to THPO receptor agonists.
Severe combined immunodeficiency (SCID) comprises a heterogeneous group of heritable deficiencies of humoral and cell-mediated immunity. Many patients with SCID have lymphocyte-activation defects ...that remain uncharacterized.
We performed genetic studies in four patients, from four families of Northern Cree ancestry, who had clinical characteristics of SCID, including early onset of severe viral, bacterial, and fungal infections despite normal B-cell and T-cell counts. Genomewide homozygosity mapping was used to identify a candidate region, which was found on chromosome 8; all genes within this interval were sequenced. Immune-cell populations, signal transduction on activation, and effector functions were studied.
The patients had hypogammaglobulinemia or agammaglobulinemia, and their peripheral-blood B cells and T cells were almost exclusively of naive phenotype. Regulatory T cells and γδ T cells were absent. All patients carried a homozygous duplication--c.1292dupG in exon 13 of IKBKB, which encodes IκB kinase 2 (IKK2, also known as IKKβ)--leading to loss of expression of IKK2, a component of the IKK-nuclear factor κB (NF-κB) pathway. Immune cells from the patients had impaired responses to stimulation through T-cell receptors, B-cell receptors, toll-like receptors, inflammatory cytokine receptors, and mitogens.
A form of human SCID is characterized by normal lymphocyte development despite a loss of IKK2 function. IKK2 deficiency results in an impaired response to activation stimuli in a variety of immune cells, leading to clinically relevant impairment of adaptive and innate immunity. Although Ikk2 deficiency is lethal in mouse embryos, our observations suggest a more restricted, unique role of IKK2-NF-κB signaling in humans. (Funded by the German Federal Ministry of Education and Research and others.).
Mutations in the Artemis protein in humans result in hypersensitivity to DNA double-strand break-inducing agents and absence of B and T lymphocytes (radiosensitive severe combined immune deficiency ...RS-SCID). Here, we report that Artemis forms a complex with the 469 kDa DNA-dependent protein kinase (DNA-PK
cs) in the absence of DNA. The purified Artemis protein alone possesses single-strand-specific 5′ to 3′ exonuclease activity. Upon complex formation, DNA-PK
cs phosphorylates Artemis, and Artemis acquires endonucleolytic activity on 5′ and 3′ overhangs, as well as hairpins. Finally, the Artemis:DNA-PK
cs complex can open hairpins generated by the RAG complex. Thus, DNA-PK
cs regulates Artemis by both phosphorylation and complex formation to permit enzymatic activities that are critical for the hairpin-opening step of V(D)J recombination and for the 5′ and 3′ overhang processing in nonhomologous DNA end joining.
Autoinflammatory diseases are a heterogenous group of disorders defined by fever and systemic inflammation suggesting involvement of genes regulating innate immune responses. Patients with homozygous ...loss‐of‐function variants in the OTU‐deubiquitinase OTULIN suffer from neonatal‐onset OTULIN‐related autoinflammatory syndrome (ORAS) characterized by fever, panniculitis, diarrhea, and arthritis. Here, we describe an atypical form of ORAS with distinct clinical manifestation of the disease caused by two new compound heterozygous variants (c.258G>A (p.M86I)/c.500G>C (p.W167S)) in the OTULIN gene in a 7‐year‐old affected by a life‐threatening autoinflammatory episode with sterile abscess formation. On the molecular level, we find binding of OTULIN to linear ubiquitin to be compromised by both variants; however, protein stability and catalytic activity is most affected by OTULIN variant p.W167S. These molecular changes together lead to increased levels of linear ubiquitin linkages in patient‐derived cells triggering the disease. Our data indicate that the spectrum of ORAS patients is more diverse than previously thought and, thus, supposedly asymptomatic individuals might also be affected. Based on our results, we propose to subdivide the ORAS into classical and atypical entities.
Synopsis
The OTULIN‐Related Autoinflammatory Syndrome (ORAS) is known to be caused by homozygous variants in the OTULIN gene. This study discovers that two new compound‐heterozygous variants in OTULIN are associated with an atypical, but potentially fatal, late‐onset form of ORAS.
Atypical ORAS is characterized by multiorgan abscess formation and bears the risk of being clinically inapparent.
OTULIN variants p.M86I and p.W167S disrupt OTULIN function leading to increased levels of linear ubiquitin linkages in patient‐derived fibroblasts and B cells.
Molecularly, atypical ORAS resembles classical ORAS with enhanced TNF production in monocytes, but diminished TNF‐induced gene activation and sensitization to TNF‐mediated cell death (in the presence of cycloheximide) in fibroblasts.
Anti‐TNF therapy with Adalimumab resulted in normalization of the patient's inflammatory plasma protein profile suggesting that patients with atypical ORAS might benefit from TNF‐blocking agents as seen in patients with classical ORAS.
The OTULIN‐Related Autoinflammatory Syndrome (ORAS) is known to be caused by homozygous variants in the OTULIN gene. This study discovers that two new compound‐heterozygous variants in OTULIN are associated with an atypical, but potentially fatal, late‐onset form of ORAS.
Although negative selection of developing B cells in the periphery is well described, yet poorly understood, evidence of naive B cell positive selection remains elusive. Using 2 humanized mouse ...models, we demonstrate that there was strong skewing of the expressed immunoglobulin repertoire upon transit into the peripheral naive B cell pool. This positive selection of expanded naive B cells in humanized mice resembled that observed in healthy human donors and was independent of autologous thymic tissue. In contrast, negative selection of autoreactive B cells required thymus-derived Tregs and MHC class II-restricted self-antigen presentation by B cells. Indeed, both defective MHC class II expression on B cells of patients with rare bare lymphocyte syndrome and prevention of self-antigen presentation via HLA-DM inhibition in humanized mice resulted in the production of autoreactive naive B cells. These latter observations suggest that Tregs repressed autoreactive naive B cells continuously produced by the bone marrow. Thus, a model emerged, in which both positive and negative selection shaped the human naive B cell repertoire and that each process was mediated by fundamentally different molecular and cellular mechanisms.
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