Distinct subsets of multi-lymphoid progenitors support ontogeny-related changes in human lymphopoiesis.

Changes in lymphocyte production patterns occurring across human ontogeny remain poorly defined. In this study, we demonstrate that human lymphopoiesis is supported by three waves of embryonic, fetal, and postnatal multi-lymphoid progenitors (MLPs) differing in CD7 and CD10 expression and their output of CD127-/+ early lymphoid progenitors (ELPs). In addition, our results reveal that, like the fetal-to-adult switch in erythropoiesis, transition to postnatal life coincides with a shift from multilineage to B lineage-biased lymphopoiesis and an increase in production of CD127+ ELPs, which persists until puberty. A further developmental transition is observed in elderly individuals whereby B cell differentiation bypasses the CD127+ compartment and branches directly from CD10+ MLPs. Functional analyses indicate that these changes are determined at the level of hematopoietic stem cells. These findings provide insights for understanding identity and function of human MLPs and the establishment and maintenance of adaptative immunity.

Microenvironment tailors nTreg structure and function.

Natural regulatory T cells (nTregs) ensure the control of self-tolerance and are currently used in clinical trials to alleviate autoimmune diseases and graft-versus-host disease after hematopoietic stem cell transfer. Based on CD39/CD26 markers, blood nTreg analysis revealed the presence of five different cell subsets, each representing a distinct stage of maturation. Ex vivo added microenvironmental factors, including IL-2, TGFß, and PGE2, direct the conversion from naive precursor to immature memory and finally from immature to mature memory cells, the latest being a no-return stage. Phenotypic and genetic characteristics of the subsets illustrate the structural parental maturation between subsets, which further correlates with the expression of regulatory factors. Regarding nTreg functional plasticity, both maturation stage and microenvironmental cytokines condition nTreg activities, which include blockade of autoreactive immune cells by cell-cell contact, Th17 and IL-10 Tr1-like activities, or activation of TCR-stimulating dendritic cell tolerization. Importantly, blood nTreg CD39/CD26 profile remained constant over a 2-y period in healthy persons but varied from person to person. Preliminary data on patients with autoimmune diseases or acute myelogenous leukemia illustrate the potential use of the nTreg CD39/CD26 profile as a blood biomarker to monitor chronic inflammatory diseases. Finally, we confirmed that naive conventional CD4 T cells, TCR-stimulated under a tolerogenic conditioned medium, could be ex vivo reprogrammed to FOXP3 lineage Tregs, and further found that these cells were exclusively committed to suppressive function under all microenvironmental contexts.

Self-Restrained B Cells Arise following Membrane IgE Expression.

Among immunoglobulins (Igs), IgE can powerfully contribute to antimicrobial immunity and severe allergy despite its low abundance. IgE protein and gene structure resemble other Ig classes, making it unclear what constrains its production to thousand-fold lower levels. Whether class-switched B cell receptors (BCRs) differentially control B cell fate is debated, and study of the membrane (m)IgE class is hampered by its elusive in vivo expression. Here, we demonstrate a self-controlled mIgE+ B cell stage. Primary or transfected mIgE+ cells relocate the BCRs into spontaneously internalized lipid rafts, lose mobility to chemokines, and change morphology. We suggest that combined proapoptotic mechanisms possibly involving Hax1 prevent mIgE+ memory lymphocyte accumulation. By uncoupling in vivo IgE switching from cytokine and antigen stimuli, we show that these features are independent from B cell stimulation and instead result from mIgE expression per se. Consequently, few cells survive IgE class switching, which might ensure minimal long-term IgE memory upon differentiation into plasma cells.

Glycotranscriptome study reveals an enzymatic switch modulating glycosaminoglycan synthesis during B-cell development and activation.

B-cell fate and responses are modulated by soluble mediators and direct cellular interactions. Migration properties also vary during differentiation, commitment and activation. In many cells, modulation of responses to stimuli involves cell surface glycans, whose architecture depends on the simultaneous expression of multiple enzymes. By looking at the glycosylation-related gene expression patterns among B-cell populations, we determined in this study that the strongest variations were observed for CSGalNAcT-1 and EXTL1. These are enzymes involved in the biosynthesis of alternative forms of glycosaminoglycans (GAGs), namely chondroitin sulfate and heparan sulfate, respectively. These two enzymes showed inverse fluctuations in progenitors, resting B cells and activated B cells, suggesting a developmentally regulated switch between chondroitin and heparan sulfate synthesis. To explore whether these variations contributed to optimal B-cell differentiation, we overexpressed EXTL1 in the B-cell lineage of transgenic mice, yielding a partial differentiation blockade at the pro-B to pre-B transition. In the periphery, this defect was almost fully compensated for in vivo, with normal-size B-cell compartments and normal serum immunoglobulin levels in the transgenic EXTL1 mice. The peripheral B cells from EXTL1 transgenics were only affected with regard to their in vitro responses to polyclonal activation, showing reduced proliferation. Together the data suggest that despite their low amounts in lymphocytes, the heparan sulfate chains decorating the endogenous GAGs appear to be regulators of B-cell physiology.

Reciprocal polarization of T and B cells at the immunological synapse.

Cognate interactions between T and B lymphocytes lead to the formation of the immunological synapse (IS) where bidirectional activation signals are exchanged. Although the molecular architecture and the function of the IS have been studied extensively on the T cell side, little is known about events occurring during synapse formation in Ag-presenting B cells. We investigated the impact of BCR and TLR signaling on human B cell activation and on the T and B cell side of the IS. On the T cell side, we observed that T cells polarized toward both naive and previously activated B cells. Nevertheless, when T cells interacted with different B cells simultaneously, T cells selectively polarized their secretory machinery toward preactivated B cells. Furthermore, both naive and preactivated B cells reoriented their microtubule-organizing center toward the synaptic T cell during cognate interactions. This phenomenon was rapid and not dependent on T cell secretory activity. Interestingly, not only the microtubule-organizing center but also the Golgi apparatus and Lamp-3(+) and MHC class II(+) vesicles all repositioned beneath the IS, suggesting that the entire endocytic/exocytic B cell compartment was reoriented toward the T cell. Taken together, our results show that the B cell activation status fine-tunes T cell polarization responses and reveal the capacity of naive and activated B cells to polarize toward T cells during cognate interactions.

Distribution, function, and prognostic value of cytotoxic T lymphocytes in follicular lymphoma: a 3-D tissue-imaging study.

CD8+ CTLs are thought to play a role in the control of follicular lymphoma (FL). Yet, the link between CTL tissue distribution, activation status, ability to kill FL cells in vivo, and disease progression is still elusive. Pretreatment lymph nodes from FL patients were analyzed by IHC (n = 80) or by 3-color confocal microscopy (n = 10). IHC revealed a rich infiltrate of CD8+ granzyme B+ (GrzB) cells in FL interfollicular spaces. Accordingly, confocal microscopy showed an increased number of CD3+CD8+GrzB+ CTLs and a brighter GrzB staining in individual CTL in FL samples compared with reactive lymph nodes. CTLs did not penetrate tumor nodules. In 3-dimensional (3-D) image reconstructions, CTLs were detected at the FL follicle border where they formed lytic synapse-like structures with FL B cells and with apoptotic cells, suggesting an in situ cytotoxic function. Finally, although GrzB expression in CTLs did not correlate with risk factors, high GrzB content correlated with prolonged progression free-survival (PFS) after rituximab-combined chemotherapy. Our results show the recruitment of armed CTLs with a tumor-controlling potential into FL lymph nodes and suggest that CTL-associated GrzB expression could influence PFS in FL patients having received rituximab-combined chemotherapy.

Gammopathy with IgA mesangial deposition provides a monoclonal model of IgA nephritogenicity and offers new insights into its molecular mechanisms.

BACKGROUND: Henoch-Schönlein purpura (HSP) and IgA nephropathy (IgAN) are characterized by mesangial deposition of polyclonal IgA eventually showing aberrant glycosylation, affinity for mesangial cells and/or co-precipitation with antigen, bacterial peptides, autoantibodies or soluble receptors. IgA were also suggested to be negatively charged and predominantly of λ type but rarely in a monoclonal form. METHODS: A gammopathy case with HSP provided us with a unique molecularly defined nephritogenic IgA1λ. Immunological analysis, biological activities, glycosylation analysis and finally IgA sequence were determined. RESULTS: Compared to IgA1 from healthy subjects or IgAN patients, IgA1 CAT showed hyposialylation but no hypogalactosylation, in agreement with underexpression of sialyltransferase genes by the plasma cell clone. IgA variable domains had low pIs with negatively charged complementarity-determining regions. Weak reactivity appeared against the cationic autoantigen lactoferrin, which was, however, absent from kidney deposits. Deposition also occurred in mice upon injection of only the polymeric form of IgA1 CAT, despite whether or not co-injected with lactoferrin. CONCLUSIONS: This monoclonal model of IgA nephritogenicity strongly suggests that beside hinge region glycosylation, V domains play a role in IgA stability and pathogenicity and supports the hypothesis that responses against cationic epitopes from pathogens or autoantigens may select negatively charged complementarity-determining regions prone either to bind charged structures of the mesangium or to promote by themselves IgA aggregation and deposition.

Activation of the ancestral polarity regulator protein kinase C zeta at the immunological synapse drives polarization of Th cell secretory machinery toward APCs.

A key feature in T lymphocyte biology is that Th cells rapidly polarize their secretory machinery toward cognate APCs. The molecular mechanisms of these dynamic Th cell responses and their impact on APC biology remain to be elucidated. In this study, we demonstrate that protein kinase Czeta (PKCzeta) is rapidly activated at the immunological synapse (IS) in human Th cells interacting with cognate dendritic cells (DCs) and that a functional PKCzeta is required for the polarization of Th cell secretory machinery toward DCs. We also show that PKCzeta-dependent Th cell polarization allows dedicated delivery of IFN-gamma and CD40L at the IS and is required for the activation of cognate DCs to IL-12 production. PKCzeta synaptic activation is a low-threshold phenomenon and, in Th cells interacting with multiple DCs, selectively occurs at the IS formed with the DCs offering the strongest stimulus leading to dedicated Th cell polarization. Our results identify the PKCzeta signaling pathway as a key component of the Th cell polarization machinery and provide a molecular basis for T cell-dedicated activation of cognate DCs.

Multiple RNA surveillance mechanisms cooperate to reduce the amount of nonfunctional Ig kappa transcripts.

Random V(D)J junctions ensure that the diversity of the Ig primary repertoire is adapted to the vast heterogeneity of Ags. In two-thirds of cases, recombination between variable segments induces a frameshift in the open reading frame and generates a premature termination codon. In B cells harboring biallelic V(D)J rearrangement of Ig genes, transcription is known to occur on both the functional and nonfunctional alleles, generating considerable amounts of primary transcripts with out-of-frame V regions. In this study, we analyzed in cell lines and primary B cells the RNA surveillance of nonfunctional Igkappa transcripts arising from nonproductive rearrangement. We demonstrated that splicing inhibition, nonsense-mediated decay and nonsense-altered splicing each have an individual partial effect that together associate into an efficient surveillance machinery, downregulating nonfunctional Igkappa mRNA. Moreover, we provide evidence that the RNA surveillance efficiency increases throughout B cell development. Whereas splicing inhibition remains constant in most cell lines, differences in nonsense-mediated decay and nonsense-altered splicing are responsible for the higher RNA surveillance observed in plasma cells. Altogether, these data show that nonfunctionally rearranged alleles are subjected to active transcription but that multiple RNA surveillance mechanisms eradicate up to 90% of out-of-frame Igkappa mRNA.

Premature replacement of mu with alpha immunoglobulin chains impairs lymphopoiesis and mucosal homing but promotes plasma cell maturation.

Sequentially along B cell differentiation, the different classes of membrane Ig heavy chains associate with the Ig alpha/Ig beta heterodimer within the B cell receptor (BCR). Whether each Ig class conveys specific signals adapted to the corresponding differentiation stage remains debated. We investigated the impact of the forced expression of an IgA-class receptor throughout murine B cell differentiation by knocking in the human C alpha Ig gene in place of the S mu region. Despite expression of a functional BCR, homozygous mutant mice showed a partial developmental blockade at the pro-B/pre-BI and large pre-BII cell stages, with decreased numbers of small pre-BII cells. Beyond this stage, peripheral B cell compartments of reduced size developed and allowed specific antibody responses, whereas mature cells showed constitutive activation and a strong commitment to plasma cell differentiation. Secreted IgA correctly assembled into polymers, associated with the murine J chain, and was transported into secretions. In heterozygous mutants, cells expressing the IgA allele competed poorly with those expressing IgM from the wild-type allele and were almost undetectable among peripheral B lymphocytes, notably in gut-associated lymphoid tissues. Our data indicate that the IgM BCR is more efficient in driving early B cell education and in mucosal site targeting, whereas the IgA BCR appears particularly suited to promoting activation and differentiation of effector plasma cells.

[Transgenesis and humanization of murine antibodies]. / Transgenèse animale et humanisation des anticorps : des souris pour des hommes.

The properties of monoclonal antibodies explain why they are such a successful class of therapeutic molecules. However, pionneered initial antibodies were of murine origin and triggered an immune response which limited the therapeutic potential of the antibody and generated deleterious effects. Consequently, tremendous efforts have been developped to engineer these murine Ig by introducing human sequences in vitro, or in vivo by humanization of murine antibodies, leading to chimeric immunoglobulins, and more recently generation of fully human antibodies in transgenic mice with a more or less diversified V repertoire. These approaches have led to the development of an increasing number of these chimeric or humanized monoclonal antibodies entering pharmaceutical pipelines.

RNA surveillance down-regulates expression of nonfunctional kappa alleles and detects premature termination within the last kappa exon.

Random V(D)J junctions would generate nonfunctional and/or out-of-frame sequences in about two-thirds of cases and result in abundant transcripts encoding truncated proteins. Although allelic exclusion at the DNA recombination level ensures that a single allele is functional, the frequent biallelic rearrangements need additional mechanisms to down-regulate aberrant transcripts in those cells with both a functionally and a nonfunctionally rearranged allele. The process of nonsense-mediated decay targets aberrantly rearranged Ig heavy-chain transcripts, but the situation of light-chain mRNAs is more complex, because they do not meet the usual requirements for nonsense-mediated decay and most often lack a spliceable intron downstream of the premature termination. We studied immunoglobulin heavy-chain -/- pro-B cells in which light chain genes get rearranged and expressed in the absence of any selection for the assembly of a functional B cell receptor. Using this model, we show that the whole kappa locus is accessible in pro-B cells and allows the assembly of a broad spectrum of VkappaJkappa segments, most of which are out-of-frame. This model provides an evaluation of the in vivo efficiency of RNA surveillance toward aberrant kappa mRNAs produced in pro-B cells. Our data show that nonfunctional kappa transcripts are excluded from the mature mRNA pool not only by detecting termination in an upstream exon but also by detecting changes in the position of termination within the last exon. Similar mechanisms efficiently down-regulate nonfunctional kappa transcripts arising in normal mature B cells due to the biallelic transcription of rearranged kappa genes.