Rapid and stable mobilization of CD8+ T cells by SARS-CoV-2 mRNA vaccine.

SARS-CoV-2 spike mRNA vaccines1-3 mediate protection from severe disease as early as ten days after prime vaccination3, when neutralizing antibodies are hardly detectable4-6. Vaccine-induced CD8+ T cells may therefore be the main mediators of protection at this early stage7,8. The details of their induction, comparison to natural infection, and association with other arms of vaccine-induced immunity remain, however, incompletely understood. Here we show on a single-epitope level that a stable and fully functional CD8+ T cell response is vigorously mobilized one week after prime vaccination with bnt162b2, when circulating CD4+ T cells and neutralizing antibodies are still weakly detectable. Boost vaccination induced a robust expansion that generated highly differentiated effector CD8+ T cells; however, neither the functional capacity nor the memory precursor T cell pool was affected. Compared with natural infection, vaccine-induced early memory T cells exhibited similar functional capacities but a different subset distribution. Our results indicate that CD8+ T cells are important effector cells, are expanded in the early protection window after prime vaccination, precede maturation of other effector arms of vaccine-induced immunity and are stably maintained after boost vaccination.

IL-7 receptor signaling drives human B-cell progenitor differentiation and expansion.

Although absence of interleukin-7 (IL-7) signaling completely abrogates T and B lymphopoiesis in mice, patients with severe combined immunodeficiency caused by mutations in the IL-7 receptor α chain (IL-7Rα) still generate peripheral blood B cells. Consequently, human B lymphopoiesis has been thought to be independent of IL-7 signaling. Using flow cytometric analysis and single-cell RNA sequencing of bone marrow samples from healthy controls and patients who are IL-7Rα deficient, in combination with in vitro modeling of human B-cell differentiation, we demonstrate that IL-7R signaling plays a crucial role in human B lymphopoiesis. IL-7 drives proliferation and expansion of early B-cell progenitors but not of pre-BII large cells and has a limited role in the prevention of cell death. Furthermore, IL-7 guides cell fate decisions by enhancing the expression of BACH2, EBF1, and PAX5, which jointly orchestrate the specification and commitment of early B-cell progenitors. In line with this observation, early B-cell progenitors of patients with IL-7Rα deficiency still expressed myeloid-specific genes. Collectively, our results unveil a previously unknown role for IL-7 signaling in promoting the B-lymphoid fate and expanding early human B-cell progenitors while defining important differences between mice and humans. Our results have implications for hematopoietic stem cell transplantation strategies in patients with T- B+ severe combined immunodeficiency and provide insights into the role of IL-7R signaling in leukemogenesis.

Defects in B-lymphopoiesis and B-cell maturation underlie prolonged B-cell depletion in ANCA-associated vasculitis.

OBJECTIVES: B-cell depletion time after rituximab (RTX) treatment is prolonged in antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) compared with other autoimmune diseases. We investigated central and peripheral B-cell development to identify the causes for the defect in B-cell reconstitution after RTX therapy. METHODS: We recruited 91 patients with AAV and performed deep phenotyping of the peripheral and bone marrow B-cell compartment by spectral flow and mass cytometry. B-cell development was studied by in vitro modelling and the role of BAFF receptor by quantitative PCR, western blot analysis and in vitro assays. RESULTS: Treatment-naïve patients with AAV showed low transitional B-cell numbers, suggesting impaired B-lymphopoiesis. We analysed bone marrow of treatment-naïve and RTX-treated patients with AAV and found reduced B-lymphoid precursors. In vitro modelling of B-lymphopoiesis from AAV haematopoietic stem cells showed intact, but slower and reduced immature B-cell development. In a subgroup of patients, after RTX treatment, the presence of transitional B cells did not translate in replenishment of naïve B cells, suggesting an impairment in peripheral B-cell maturation. We found low BAFF-receptor expression on B cells of RTX-treated patients with AAV, resulting in reduced survival in response to BAFF in vitro. CONCLUSIONS: Prolonged depletion of B cells in patients with AAV after RTX therapy indicates a B-cell defect that is unmasked by RTX treatment. Our data indicate that impaired bone marrow B-lymphopoiesis results in a delayed recovery of peripheral B cells that may be further aggravated by a survival defect of B cells. Our findings contribute to the understanding of AAV pathogenesis and may have clinical implications regarding RTX retreatment schedules and immunomonitoring after RTX therapy.

Targeting MDM2 enhances antileukemia immunity after allogeneic transplantation via MHC-II and TRAIL-R1/2 upregulation.

Patients with acute myeloid leukemia (AML) often achieve remission after allogeneic hematopoietic cell transplantation (allo-HCT) but subsequently die of relapse driven by leukemia cells resistant to elimination by allogeneic T cells based on decreased major histocompatibility complex II (MHC-II) expression and apoptosis resistance. Here we demonstrate that mouse-double-minute-2 (MDM2) inhibition can counteract immune evasion of AML. MDM2 inhibition induced MHC class I and II expression in murine and human AML cells. Using xenografts of human AML and syngeneic mouse models of leukemia, we show that MDM2 inhibition enhanced cytotoxicity against leukemia cells and improved survival. MDM2 inhibition also led to increases in tumor necrosis factor-related apoptosis-inducing ligand receptor-1 and -2 (TRAIL-R1/2) on leukemia cells and higher frequencies of CD8+CD27lowPD-1lowTIM-3low T cells, with features of cytotoxicity (perforin+CD107a+TRAIL+) and longevity (bcl-2+IL-7R+). CD8+ T cells isolated from leukemia-bearing MDM2 inhibitor-treated allo-HCT recipients exhibited higher glycolytic activity and enrichment for nucleotides and their precursors compared with vehicle control subjects. T cells isolated from MDM2 inhibitor-treated AML-bearing mice eradicated leukemia in secondary AML-bearing recipients. Mechanistically, the MDM2 inhibitor-mediated effects were p53-dependent because p53 knockdown abolished TRAIL-R1/2 and MHC-II upregulation, whereas p53 binding to TRAILR1/2 promotors increased upon MDM2 inhibition. The observations in the mouse models were complemented by data from human individuals. Patient-derived AML cells exhibited increased TRAIL-R1/2 and MHC-II expression on MDM2 inhibition. In summary, we identified a targetable vulnerability of AML cells to allogeneic T-cell-mediated cytotoxicity through the restoration of p53-dependent TRAIL-R1/2 and MHC-II production via MDM2 inhibition.

Inborn errors of immunity and immunodeficiencies: Antibody-mediated pathology and autoimmunity as a consequence of impaired immune reactions.

B-cell tolerance to self-antigen is an active process that requires the temporal and spatial integration of signals of defined intensity. In common variable immune deficiency disorders, CTLA-4 deficiency, autoimmune lymphoproliferative syndrome, or in collagen VII deficiency, genetic defects in molecules regulating development, activation, maturation, and ECM composition alter the generation of B cells, resulting in immunodeficiency. Paradoxically, at the same time, the defective immune processes favor autoantibody production and immunopathology through impaired establishment of tolerance. The development of systemic autoimmunity in the framework of defective BCR signaling is relatively unusual in genetic mouse models. In sharp contrast, such reduced signaling in humans is clearly linked to pathological autoimmunity. The molecular mechanisms by which tolerance is broken in these settings are only starting to be explored resulting in novel therapeutic interventions. For instance, in CTLA-4 deficiency, homeostasis can be restored by CTLA-4 Ig treatment. Following this example, the identification of the molecular targets causing the reduced signals and their restoration is a visionary way to reestablish tolerance and develop novel therapeutic avenues for immunopathologies.

Mitochondrial DNA: a novel indicator of active inflammation in ANCA-associated vasculitides.

OBJECTIVES: ANCA-associated vasculitis (AAV) includes granulomatosis with polyangiitis (GPA) and microscopic polyangiitis (MPA). ANCA triggers neutrophil extracellular trap formation, which releases either mitochondrial (mt) DNA or nuclear DNA (n) DNA, contributing to inflammation. Our aim was to prospectively examine the extent and nature of circulating DNA in AAV and the clinical utility of DNA quantification. METHODS: DNA was isolated from platelet-free plasma of consecutive GPA and MPA patients and healthy controls (HCs). mtDNA and nDNA copy numbers were quantified by PCR. Clinical data, including the BVAS, were collected. RESULTS: Ninety-two HCs (median age 51 years, 58.7% female) and 101 AAV patients (80 GPA, 21 MPA, median age 64 years, 50.5% female, BVAS range: 0-30) were included. Median mtDNA copies were 13-fold higher in patients with AAV than in HCs; nDNA concentrations did not differ. Patients with active AAV (BVAS > 0) had 4-fold higher median mtDNA copies than patients in remission (P = 0.03). mtDNA, unlike nDNA, correlated with BVAS (r = 0.30, P = 0.002) and was associated with AAV activity at multivariable analysis. Receiver operating characteristic curve analysis indicated that mtDNA quantification differentiates patients with active AAV (BVAS > 0) from HCs with 96.1% sensitivity and 98.9% specificity (area under the curve 0.99). In 27 AAV patients with follow-up, mtDNA changes but not CRP or ANCA-titres correlated with BVAS changes (r = 0.56, P = 0.002). CONCLUSIONS: mtDNA, unlike nDNA, is elevated in the plasma of AAV patients and may contribute to systemic inflammation. mtDNA could be superior to established biomarkers in the laboratory monitoring of AAV activity.

Nonpermissive bone marrow environment impairs early B-cell development in common variable immunodeficiency.

Common variable immunodeficiency (CVID) is a disease characterized by increased susceptibility to infections, hypogammaglobulinemia, and immune dysregulation. Although CVID is thought to be a disorder of the peripheral B-cell compartment, in 25% of patients, early B-cell development in the bone marrow is impaired. Because poor B-cell reconstitution after hematopoietic stem cell transplantation has been observed, we hypothesized that in some patients the bone marrow environment is not permissive to B-cell development. Studying the differentiation dynamics of bone marrow-derived CD34+ cells into immature B cells in vitro allowed us to distinguish patients with B-cell intrinsic defects and patients with a nonpermissive bone marrow environment. In the former, immature B cells did not develop and in the latter CD34+ cells differentiated into immature cells in vitro, but less efficiently in vivo. In a further group of patients, the uncommitted precursors were unable to support the constant development of B cells in vitro, indicating a possible low frequency or exhaustion of the precursor population. Hematopoietic stem cell transplantation would result in normal B-cell repopulation in case of intrinsic B-cell defect, but in defective B-cell repopulation in a nonpermissive environment. Our study points to the importance of the bone marrow niche in the pathogenesis of CVID.

Tumor Extracellular Matrix Stiffness Promptly Modulates the Phenotype and Gene Expression of Infiltrating T Lymphocytes.

The immune system is a fine modulator of the tumor biology supporting or inhibiting its progression, growth, invasion and conveys the pharmacological treatment effect. Tumors, on their side, have developed escaping mechanisms from the immune system action ranging from the direct secretion of biochemical signals to an indirect reaction, in which the cellular actors of the tumor microenvironment (TME) collaborate to mechanically condition the extracellular matrix (ECM) making it inhospitable to immune cells. TME is composed of several cell lines besides cancer cells, including tumor-associated macrophages, cancer-associated fibroblasts, CD4+ and CD8+ lymphocytes, and innate immunity cells. These populations interface with each other to prepare a conservative response, capable of evading the defense mechanisms implemented by the host's immune system. The presence or absence, in particular, of cytotoxic CD8+ cells in the vicinity of the main tumor mass, is able to predict, respectively, the success or failure of drug therapy. Among various mechanisms of immunescaping, in this study, we characterized the modulation of the phenotypic profile of CD4+ and CD8+ cells in resting and activated states, in response to the mechanical pressure exerted by a three-dimensional in vitro system, able to recapitulate the rheological and stiffness properties of the tumor ECM.

Successful treatment of extensive calcifications and acute pulmonary involvement in dermatomyositis with the Janus-Kinase inhibitor tofacitinib - A report of two cases.

INTRODUCTION: Dermatomyositis (DM) can be complicated by calcinosis and interstitial lung disease (ILD). Calcinosis can be severely debilitating or life-threatening and to date there is no treatment with proven efficacy. In DM type I interferon contributes to pathophysiology by inducing the expression of proinflammatory cytokines and the JAK-STAT (signal transducer and activator of transcription) pathway may be involved in the regulation of mitochondrial calcium store release, a process potentially important for calcification in DM. JAK-inhibition may therefore be an attractive therapy in DM complicated by calcifications. METHODS AND RESULTS: We report on the fast and persistent response of extensive and rapidly progressive DM-associated calcifications in two patients treated with the JAK-inhibitor tofacitinib. During the 28-week observation period in both patients no new calcifications formed and existing calcifications were either regressive or stable. Furthermore, concomitant life-threatening DM-associated ILD (acute fibrinous and organizing pneumonia; AFOP) in one patient rapidly responded to tofacitinib monotherapy. Both patients were able to taper concomitant glucocorticoids. Tofacitinib was well tolerated and safe. CONCLUSIONS: The results of our study support the role of JAK/STAT signaling in the development of calcinosis and ILD in DM. Tofacitinib may be an effective and safe treatment for calcinosis in DM and potentially for other connective tissue disease complicated by calcinosis.

Abatacept modulates CD80 and CD86 expression and memory formation in human B-cells.

BACKGROUND: Cytotoxic T lymphocyte antigen-4 (CTLA-4) limits T-cell activation and is expressed on T-regulatory cells. Human CTLA-4 deficiency results in severe immune dysregulation. Abatacept (CTLA-4 Ig) is approved for the treatment of rheumatoid arthritis (RA) and its mechanism of action is attributed to effects on T-cells. It is known that CTLA-4 modulates the expression of its ligands CD80 and CD86 on antigen presenting cells (APC) by transendocytosis. As B-cells express CD80/CD86 and function as APC, we hypothesize that B-cells are a direct target of abatacept. OBJECTIVES: To investigate direct effects of abatacept on human B-lymphocytes in vitro and in RA patients. METHODS: The effect of abatacept on healthy donor B-cells' phenotype, activation and CD80/CD86 expression was studied in vitro. Nine abatacept-treated RA patients were studied. Seven of these were followed up to 24 months, and two up to 12 months only and treatment response, immunoglobulins, ACPA, RF concentrations, B-cell phenotype and ACPA-specific switched memory B-cell frequency were assessed. RESULTS: B-cell development was unaffected by abatacept. Abatacept treatment resulted in a dose-dependent decrease of CD80/CD86 expression on B-cells in vitro, which was due to dynamin-dependent internalization. RA patients treated with abatacept showed a progressive decrease in plasmablasts and serum IgG. While ACPA-titers only moderately declined, the frequency of ACPA-specific switched memory B-cells significantly decreased. CONCLUSIONS: Abatacept directly targets B-cells by reducing CD80/CD86 expression. Impairment of antigen presentation and T-cell activation may result in altered B-cell selection, providing a new therapeutic mechanism and a base for abatacept use in B-cell mediated autoimmunity.

Plasma cell deficiency in human subjects with heterozygous mutations in Sec61 translocon alpha 1 subunit (SEC61A1).

BACKGROUND: Primary antibody deficiencies (PADs) are the most frequent primary immunodeficiencies in human subjects. The genetic causes of PADs are largely unknown. Sec61 translocon alpha 1 subunit (SEC61A1) is the major subunit of the Sec61 complex, which is the main polypeptide-conducting channel in the endoplasmic reticulum membrane. SEC61A1 is a target gene of spliced X-box binding protein 1 and strongly induced during plasma cell (PC) differentiation. OBJECTIVE: We identified a novel genetic defect and studied its pathologic mechanism in 11 patients from 2 unrelated families with PADs. METHODS: Whole-exome and targeted sequencing were conducted to identify novel genetic mutations. Functional studies were carried out ex vivo in primary cells of patients and in vitro in different cell lines to assess the effect of SEC61A1 mutations on B-cell differentiation and survival. RESULTS: We investigated 2 families with patients with hypogammaglobulinemia, severe recurrent respiratory tract infections, and normal peripheral B- and T-cell subpopulations. On in vitro stimulation, B cells showed an intrinsic deficiency to develop into PCs. Genetic analysis and targeted sequencing identified novel heterozygous missense (c.254T>A, p.V85D) and nonsense (c.1325G>T, p.E381*) mutations in SEC61A1, segregating with the disease phenotype. SEC61A1-V85D was deficient in cotranslational protein translocation, and it disturbed the cellular calcium homeostasis in HeLa cells. Moreover, SEC61A1-V85D triggered the terminal unfolded protein response in multiple myeloma cell lines. CONCLUSION: We describe a monogenic defect leading to a specific PC deficiency in human subjects, expanding our knowledge about the pathogenesis of antibody deficiencies.

Disturbed B-lymphocyte selection in autoimmune lymphoproliferative syndrome.

Fas is a transmembrane receptor involved in the maintenance of tolerance and immune homeostasis. In murine models, it has been shown to be essential for deletion of autoreactive B cells in the germinal center. The role of Fas in human B-cell selection and in development of autoimmunity in patients carrying FAS mutations is unclear. We analyzed patients with either a somatic FAS mutation or a germline FAS mutation and somatic loss-of-heterozygosity, which allows comparing the fate of B cells with impaired vs normal Fas signaling within the same individual. Class-switched memory B cells showed: accumulation of FAS-mutated B cells; failure to enrich single V, D, J genes and single V-D, D-J gene combinations of the B-cell receptor variable region; increased frequency of variable regions with higher content of positively charged amino acids; and longer CDR3 and maintenance of polyreactive specificities. Importantly, Fas-deficient switched memory B cells showed increased rates of somatic hypermutation. Our data uncover a defect in B-cell selection in patients with FAS mutations, which has implications for the understanding of the pathogenesis of autoimmunity and lymphomagenesis of autoimmune lymphoproliferative syndrome.

Disturbed canonical nuclear factor of κ light chain signaling in B cells of patients with common variable immunodeficiency.

BACKGROUND: Most patients with common variable immunodeficiency (CVID) present with severely reduced switched memory B-cell counts, and some display an increase of CD21low B-cell counts (CVID 21low), whereas others do not (CVID 21norm). Altered B-cell receptor (BCR) signaling might contribute to the defective memory formation observed in patients with CVID. OBJECTIVE: We sought to investigate canonical nuclear factor of κ light chain (NF-κB) signaling in B cells from patients with CVID as a central pathway in B-cell differentiation. METHODS: Degradation of inhibitor of κBα (IκBα) and p65 phosphorylation, nuclear translocation of p65, and regulation of target genes and cell function were investigated after different modes of B-cell stimulation. RESULTS: BCR-mediated canonical NF-κB signaling was impaired in all mature naive CVID-derived B cells. This impairment was more profound in naive B cells from CVID 21low patients than CVID 21norm patients and most pronounced in CD21low B cells. The signaling defect translated into reduced induction of Bcl-xL and IκBα, 2 bona fide target genes of the canonical NF-κB pathway. CD40 ligand- and Toll-like receptor 9-mediated signaling were less strongly altered. Signaling in CD21low B cells but not CD21+ B cells of patients with HIV was similarly affected. CONCLUSION: Combined with the previous description of disturbed Ca2+ signaling, the discovery of NF-κB signaling defects, especially in CVID 21low patients, suggests a broad underlying signaling defect affecting especially BCR-derived signals. Given the immune phenotype of monogenic defects affecting Ca2+ and NF-κB signaling, the latter is more likely to contribute to the humoral deficiency. The strongly disturbed BCR signaling of CD21low B cells is characteristic for this cell type and independent of the underlying disease.

DCLRE1C (ARTEMIS) mutations causing phenotypes ranging from atypical severe combined immunodeficiency to mere antibody deficiency.

Null mutations in genes involved in V(D)J recombination cause a block in B- and T-cell development, clinically presenting as severe combined immunodeficiency (SCID). Hypomorphic mutations in the non-homologous end-joining gene DCLRE1C (encoding ARTEMIS) have been described to cause atypical SCID, Omenn syndrome, Hyper IgM syndrome and inflammatory bowel disease-all with severely impaired T-cell immunity. By whole-exome sequencing, we investigated the molecular defect in a consanguineous family with three children clinically diagnosed with antibody deficiency. We identified perfectly segregating homozygous variants in DCLRE1C in three index patients with recurrent respiratory tract infections, very low B-cell numbers and serum IgA levels. In patients, decreased colony survival after irradiation, impaired proliferative response and reduced counts of naïve T cells were observed in addition to a restricted T-cell receptor repertoire, increased palindromic nucleotides in the complementarity determining regions 3 and long stretches of microhomology at switch junctions. Defective V(D)J recombination was complemented by wild-type ARTEMIS protein in vitro. Subsequently, homozygous or compound heterozygous DCLRE1C mutations were identified in nine patients from the same geographic region. We demonstrate that DCLRE1C mutations can cause a phenotype presenting as only antibody deficiency. This novel association broadens the clinical spectrum associated with ARTEMIS mutations. Clinicians should consider the possibility that an immunodeficiency with a clinically mild initial presentation could be a combined immunodeficiency, so as to provide appropriate care for affected patients.

Impact of tofacitinib treatment on human B-cells in vitro and in vivo.

B-cells are pivotal to the pathogenesis of rheumatoid arthritis and tofacitinib, a JAK inhibitor, is effective and safe in its treatment. Tofacitinib interferes with signal transduction via cytokine receptors using the common γ-chain. Despite extensive data on T-lymphocytes, the impact of tofacitinib on B-lymphocytes is poorly understood. In this study we assessed the effect of tofacitinib on B-lymphocyte differentiation and function. Tofacitinib treatment strongly impaired in vitro plasmablast development, immunoglobulin secretion and induction of B-cell fate determining transcription factors, Blimp-1, Xbp-1, and IRF-4, in naïve B-cells. Interestingly, class switch and activation-induced cytidine deaminase (AICDA) induction was only slightly reduced in activated naïve B-cells. The effect of tofacitinib on plasmablast formation, immunoglobulin secretion and proliferation was less profound, when peripheral blood B-cells, including not only naïve but also memory B-cells, were stimulated. In line with these in vitro results, the relative distribution of B-cell populations remained stable in tofacitinib treated patients. Nevertheless, a temporary increase in absolute B-cell numbers was observed 6-8 weeks after start of treatment. In addition, B-cells isolated from tofacitinib treated patients responded rapidly to in vitro activation. We demonstrate that tofacitinib has a direct impact on human naïve B-lymphocytes, independently from its effect on T-lymphocytes, by impairing their development into plasmablasts and immunoglobulin secretion. The major effect of tofacitinib on naïve B-lymphocyte development points to the potential inability of tofacitinib-treated patients to respond to novel antigens, and suggests planning vaccination strategies prior to tofacitinib treatment.

Transplantation from a symptomatic carrier sister restores host defenses but does not prevent colitis in NEMO deficiency.

NF-κB essential modulator (NEMO) deficiency causes ectodermal dysplasia with immunodeficiency in males, while manifesting as incontinentia pigmenti in heterozygous females. We report a family with NEMO deficiency, in which a female carrier displayed skewed X-inactivation favoring the mutant NEMO allele associated with symptoms of Behçet's disease. Hematopoietic stem cell transplantation of an affected boy from this donor reconstituted an immune system with retained skewed X-inactivation. After transplantation no more severe infections occurred, indicating that an active wild-type NEMO allele in only 10% of immune cells restores host defense. Yet he developed inflammatory bowel disease (IBD). While gut infiltrating immune cells stained strongly for nuclear p65 indicating restored NEMO function, this was not the case in intestinal epithelial cells - in contrast to cells from conventional IBD patients. These results extend murine observations that epithelial NEMO-deficiency suffices to cause IBD. High anti-TNF doses controlled the intestinal inflammation and symptoms of Behçet's disease.

Deficiency of innate and acquired immunity caused by an IKBKB mutation.

BACKGROUND: 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. METHODS: 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. RESULTS: 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 IKKß)--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. CONCLUSIONS: 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.).

Defective anti-polysaccharide response and splenic marginal zone disorganization in ALPS patients.

Autoimmune lymphoproliferative syndrome (ALPS) caused by impaired FAS-mediated apoptosis of lymphocytes is characterized by lymphoproliferation, autoimmunity, but also an increased risk of invasive bacterial infection, notably following splenectomy. We surveyed a cohort of 100 ALPS patients (including 33 splenectomized) and found that 12 (10 splenectomized) had experienced 23 invasive bacterial infections mainly caused by Streptococcus pneumoniae. This vulnerability was associated with evidence of defective B-cell function characterized by low serum immunoglobulin (Ig) M, low IgM antibody production in response to S pneumoniae following nonconjugated immunization, and low blood memory B-cells counts (including marginal zone [MZ] B-cell counts). This immunodeficiency strongly correlated with intensity of lymphoproliferation. Spleen sections from 9 ALPS patients revealed double-negative T-cell (DN-T) infiltration of the MZ, which was depleted of B cells. MZ in ALPS patients contained an abnormally thick layer of MAdCAM-1((+)) stromal cells and an excess of DN-Ts. DN-Ts were shown to express MAdCAM-1 ligand, the α4ß7 integrin. These observations suggest that accumulating DN-Ts are trapped within stromal cell meshwork and interfere with correct localization of MZ B cells. Similar observations were made in spleens of fas-deficient mice. Our data revealed an unexpected mechanism by which ALPS results in anti-polysaccharide IgM antibody production-specific defect. Splenectomy should be avoided.

A feeder-free differentiation system identifies autonomously proliferating B cell precursors in human bone marrow.

The peripheral B cell compartment is maintained by homeostatic proliferation and through replenishment by bone marrow precursors. Because hematopoietic stem cells cycle at a slow rate, replenishment must involve replication of precursor B cells. To study proliferation of early human B cell progenitors, we established a feeder cell-free in vitro system allowing the development of B cells from CD34(+) hematopoietic stem cells up to the stage of immature IgM(+) B cells. We found that pro-B and pre-B cells generated in vitro can proliferate autonomously and persist up to 7 wk in culture in the absence of signals induced by exogenously added cytokines. Nevertheless, addition of IL-7 enhanced pre-B cell expansion and inhibited maturation into IgM(+) B cells. The B cell precursor subsets replicating in vitro were highly similar to the bone marrow B cell precursors cycling in vivo. The autonomous proliferation of B cell precursor subsets in vitro and their long-term persistence implies that proliferation during pro-B and pre-B cell stages plays an important role in the homeostasis of the peripheral B cell compartment. Our in vitro culture can be used to study defects in B cell development or in reconstitution of the B cell pool after depletion and chemotherapy.