PIM2 kinase has a pivotal role in plasmablast generation and plasma cell survival, opening up novel treatment options in myeloma.

The differentiation of B cells into plasmablasts (PBs) and then plasma cells (PCs) is associated with extensive cell reprogramming and new cell functions. By using specific inhibition strategies (including a novel morpholino RNA antisense approach), we found that early, sustained upregulation of the proviral integrations of Moloney virus 2 (PIM2) kinase is a pivotal event during human B-cell in vitro differentiation and then continues in mature normal and malignant PCs in the bone marrow. In particular, PIM2 sustained the G1/S transition by acting on CDC25A and p27Kip1 and hindering caspase 3-driven apoptosis through BAD phosphorylation and cytoplasmic stabilization of p21Cip1. In PCs, interleukin-6 triggered PIM2 expression, resulting in antiapoptotic effects on which malignant PCs were particularly dependent. In multiple myeloma, pan-PIM and myeloid cell leukemia-1 (MCL1) inhibitors displayed synergistic activity. Our results highlight a cell-autonomous function that links kinase activity to the newly acquired secretion ability of the PBs and the adaptability observed in both normal and malignant PCs. These findings should finally prompt the reconsideration of PIM2 as a therapeutic target in multiple myeloma.

Committed Human CD23-Negative Light-Zone Germinal Center B Cells Delineate Transcriptional Program Supporting Plasma Cell Differentiation.

B cell affinity maturation occurs in the germinal center (GC). Light-zone (LZ) GC B cells (BGC-cells) interact with follicular dendritic cells (FDCs) and compete for the limited, sequential help from T follicular helper cells needed to escape from apoptosis and complete their differentiation. The highest-affinity LZ BGC-cells enter the cell cycle and differentiate into PCs, following a dramatic epigenetic reorganization that induces transcriptome changes in general and the expression of the PRDM1 gene in particular. Human PC precursors are characterized by the loss of IL-4/STAT6 signaling and the absence of CD23 expression. Here, we studied the fate of human LZ BGC-cells as a function of their CD23 expression. We first showed that CD23 expression was restricted to the GC LZ, where it was primarily expressed by FDCs; less than 10% of tonsil LZ BGC-cells were positive. Sorted LZ BGC-cells left in culture and stimulated upregulated CD23 expression but were unable to differentiate into PCs - in contrast to cells that did not upregulate CD23 expression. An in-depth analysis (including single-cell gene expression) showed that stimulated CD23-negative LZ BGC-cells differentiated into plasmablasts and time course of gene expression changes delineates the transcriptional program that sustains PC differentiation. In particular, we identified a B cell proliferation signature supported by a transient MYC gene expression. Overall, the CD23 marker might be of value in answering questions about the differentiation of normal BGC-cells and allowed us to propose an instructive LZ BGC-cells maturation and fate model.

Early Emergence of Adaptive Mechanisms Sustaining Ig Production: Application to Antibody Therapy.

Antibody therapy, where artificially-produced immunoglobulins (Ig) are used to treat pathological conditions such as auto-immune diseases and cancers, is a very innovative and competitive field. Although substantial efforts have been made in recent years to obtain specific and efficient antibodies, there is still room for improvement especially when considering a precise tissular targeting or increasing antigen affinity. A better understanding of the cellular and molecular steps of terminal B cell differentiation, in which an antigen-activated B cell becomes an antibody secreting cell, may improve antibody therapy. In this review, we use our recently published data about human B cell differentiation, to show that the mechanisms necessary to adapt a metamorphosing B cell to its new secretory function appear quite early in the differentiation process i.e., at the pre-plasmablast stage. After characterizing the molecular pathways appearing at this stage, we will focus on recent findings about two main processes involved in antibody production: unfolded protein response (UPR) and endoplasmic reticulum (ER) stress. We'll show that many genes coding for factors involved in UPR and ER stress are induced at the pre-plasmablast stage, sustaining our hypothesis. Finally, we propose to use this recently acquired knowledge to improve productivity of industrialized therapeutic antibodies.

Plasmablasts derive from CD23- activated B cells after the extinction of IL-4/STAT6 signaling and IRF4 induction.

The terminal differentiation of B cells into antibody-secreting cells (ASCs) is a critical component of adaptive immune responses. However, it is a very sensitive process, and dysfunctions lead to a variety of lymphoproliferative neoplasias including germinal center-derived lymphomas. To better characterize the late genomic events that drive the ASC differentiation of human primary naive B cells, we used our in vitro differentiation system and a combination of RNA sequencing and Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC sequencing). We discovered 2 mechanisms that drive human terminal B-cell differentiation. First, after an initial response to interleukin-4 (IL-4), cells that were committed to an ASC fate downregulated the CD23 marker and IL-4 signaling, whereas cells that maintained IL-4 signaling did not differentiate. Second, human CD23- cells also increased IRF4 protein to levels required for ASC differentiation, but they did that independently of the ubiquitin-mediated degradation process previously described in mice. Finally, we showed that CD23- cells carried the imprint of their previous activated B-cell status, were precursors of plasmablasts, and had a phenotype similar to that of in vivo preplasmablasts. Altogether, our results provide an unprecedented genomic characterization of the fate decision between activated B cells and plasmablasts, which provides new insights into the pathological mechanisms that drive lymphoma biology.

Integrative Analysis of Cell Crosstalk within Follicular Lymphoma Cell Niche: Towards a Definition of the FL Supportive Synapse.

Follicular lymphoma (FL), the most frequent indolent non-Hodgkin's B cell lymphoma, is considered as a prototypical centrocyte-derived lymphoma, dependent on a specific microenvironment mimicking the normal germinal center (GC). In agreement, several FL genetic alterations affect the crosstalk between malignant B cells and surrounding cells, including stromal cells and follicular helper T cells (Tfh). In our study, we sought to deconvolute this complex FL supportive synapse by comparing the transcriptomic profiles of GC B cells, Tfh, and stromal cells, isolated from normal versus FL tissues, in order to identify tumor-specific pathways. In particular, we highlighted a high expression of IL-6 and IL-7 in FL B cells that could favor the activation of FL Tfh overexpressing IFNG, able in turn to stimulate FL B cells without triggering MHC (major histocompatibility) class II expression. Moreover, the glycoprotein clusterin was found up-regulated in FL stromal cells and could promote FL B cell adhesion. Finally, besides its expression on Tfh, CD200 was found overexpressed on tumor B cells and could contribute to the induction of the immunosuppressive enzyme indoleamine-2,3 dioxygenase by CD200R-expressing dendritic cells. Altogether our findings led us to outline the contribution of major signals provided by the FL microenvironment and their interactions with malignant FL B cells.

The hydroxymethylome of multiple myeloma identifies FAM72D as a 1q21 marker linked to proliferation.

Cell identity relies on the cross-talk between genetics and epigenetics and their impact on gene expression. Oxidation of 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC) is the first step of an active DNA demethylation process occurring mainly at enhancers and gene bodies and, as such, participates in processes governing cell identity in normal and pathological conditions. Although genetic alterations are well documented in multiple myeloma (MM), epigenetic alterations associated with this disease have not yet been thoroughly analyzed. To gain insight into the biology of MM, genome-wide 5hmC profiles were obtained and showed that regions enriched in this modified base overlap with MM enhancers and super enhancers and are close to highly expressed genes. Through the definition of a MM-specific 5hmC signature, we identified FAM72D as a poor prognostic gene located on 1q21, a region amplified in high risk myeloma. We further uncovered that FAM72D functions as part of the FOXM1 transcription factor network controlling cell proliferation and survival and we evidenced an increased sensitivity of cells expressing high levels of FOXM1 and FAM72 to epigenetic drugs targeting histone deacetylases and DNA methyltransferases.

Locus suicide recombination actively occurs on the functionally rearranged IgH allele in B-cells from inflamed human lymphoid tissues.

B-cell activation yields abundant cell death in parallel to clonal amplification and remodeling of immunoglobulin (Ig) genes by activation-induced deaminase (AID). AID promotes affinity maturation of Ig variable regions and class switch recombination (CSR) in mature B lymphocytes. In the IgH locus, these processes are under control of the 3' regulatory region (3'RR) super-enhancer, a region demonstrated in the mouse to be both transcribed and itself targeted by AID-mediated recombination. Alternatively to CSR, IgH deletions joining Sµ to "like-switch" DNA repeats that flank the 3' super-enhancer can thus accomplish so-called "locus suicide recombination" (LSR) in mouse B-cells. Using an optimized LSR-seq high throughput method, we now show that AID-mediated LSR is evolutionarily conserved and also actively occurs in humans, providing an activation-induced cell death pathway in multiple conditions of B-cell activation. LSR either focuses on the functional IgH allele or is bi-allelic, and its signature is mainly detected when LSR is ongoing while it vanishes from fully differentiated plasma cells or from "resting" blood memory B-cells. Highly diversified breakpoints are distributed either within the upstream (3'RR1) or downstream (3'RR2) copies of the IgH 3' super-enhancer and all conditions activating CSR in vitro also seem to trigger LSR although TLR ligation appeared the most efficient. Molecular analysis of breakpoints and junctions confirms that LSR is AID-dependent and reveals junctional sequences somehow similar to CSR junctions but with increased usage of microhomologies.

Pan-HDAC Inhibitors Restore PRDM1 Response to IL21 in CREBBP-Mutated Follicular Lymphoma.

PURPOSE: Follicular lymphoma arises from a germinal center B-cell proliferation supported by a bidirectional crosstalk with tumor microenvironment, in particular with follicular helper T cells (Tfh). We explored the relation that exists between the differentiation arrest of follicular lymphoma cells and loss-of-function of CREBBP acetyltransferase.Experimental Design: The study used human primary cells obtained from either follicular lymphoma tumors characterized for somatic mutations, or inflamed tonsils for normal germinal center B cells. Transcriptome and functional analyses were done to decipher the B- and T-cell crosstalk. Responses were assessed by flow cytometry and molecular biology including ChIP-qPCR approaches. RESULTS: Conversely to normal B cells, follicular lymphoma cells are unable to upregulate the transcription repressor, PRDM1, required for plasma cell differentiation. This defect occurs although the follicular lymphoma microenvironment is enriched in the potent inducer of PRDM1 and IL21, highly produced by Tfhs. In follicular lymphoma carrying CREBBP loss-of-function mutations, we found a lack of IL21-mediated PRDM1 response associated with an abnormal increased enrichment of the BCL6 protein repressor in PRDM1 gene. Moreover, in these follicular lymphoma cells, pan-HDAC inhibitor, vorinostat, restored their PRDM1 response to IL21 by lowering BCL6 bound to PRDM1. This finding was reinforced by our exploration of patients with follicular lymphoma treated with another pan-HDAC inhibitor. Patients showed an increase of plasma cell identity genes, mainly PRDM1 and XBP1, which underline the progression of follicular lymphoma B cells in the differentiation process. CONCLUSIONS: Our data uncover a new mechanism by which pan-HDAC inhibitors may act positively to treat patients with follicular lymphoma through the induction of the expression of plasma cell genes.

IL-2 imprints human naive B cell fate towards plasma cell through ERK/ELK1-mediated BACH2 repression.

Plasma cell differentiation is a tightly regulated process that requires appropriate T cell helps to reach the induction threshold. To further understand mechanisms by which T cell inputs regulate B cell fate decision, we investigate the minimal IL-2 stimulation for triggering human plasma cell differentiation in vitro. Here we show that the timed repression of BACH2 through IL-2-mediated ERK/ELK1 signalling pathway directs plasma cell lineage commitment. Enforced BACH2 repression in activated B cells unlocks the plasma cell transcriptional program and induces their differentiation into immunoglobulin M-secreting cells. RNA-seq and ChIP-seq results further identify BACH2 target genes involved in this process. An active regulatory region within the BACH2 super-enhancer, under ELK1 control and differentially regulated upon B-cell activation and cellular divisions, helps integrate IL-2 signal. Our study thus provides insights into the temporal regulation of BACH2 and its targets for controlling the differentiation of human naive B cells.

The IGH locus relocalizes to a "recombination compartment" in the perinucleolar region of differentiating B-lymphocytes.

The immunoglobulin heavy chain (IGH) gene loci are subject to specific recombination events during B-cell differentiation including somatic hypermutation and class switch recombination which mark the end of immunoglobulin gene maturation in germinal centers of secondary lymph nodes. These two events rely on the activity of activation-induced cytidine deaminase (AID) which requires DNA double strand breaks be created, a potential danger to the cell. Applying 3D-fluorescence in situ hybridization coupled with immunofluorescence staining to a previously described experimental system recapitulating normal B-cell differentiation ex vivo, we have kinetically analyzed the radial positioning of the two IGH gene loci as well as their proximity with the nucleolus, heterochromatin and γH2AX foci. Our observations are consistent with the proposal that these IGH gene rearrangements take place in a specific perinucleolar "recombination compartment" where AID could be sequestered thus limiting the extent of its potentially deleterious off-target effects.

Distinct Patterns of Colocalization of the CCND1 and CMYC Genes With Their Potential Translocation Partner IGH at Successive Stages of B-Cell Differentiation.

The immunoglobulin heavy chain (IGH) locus is submitted to intra-chromosomal DNA breakages and rearrangements during normal B cell differentiation that create a risk for illegitimate inter-chromosomal translocations leading to a variety of B-cell malignancies. In most Burkitt's and Mantle Cell lymphomas, specific chromosomal translocations juxtapose the IGH locus with a CMYC or Cyclin D1 (CCND1) gene, respectively. 3D-fluorescence in situ hybridization was performed on normal peripheral B lymphocytes induced to mature in vitro from a naive state to the stage where they undergo somatic hypermutation (SHM) and class switch recombination (CSR). The CCND1 genes were found very close to the IGH locus in naive B cells and further away after maturation. In contrast, the CMYC alleles became localized closer to an IGH locus at the stage of SHM/CSR. The colocalization observed between the two oncogenes and the IGH locus at successive stages of B-cell differentiation occurred in the immediate vicinity of the nucleolus, consistent with the known localization of the RAGs and AID enzymes whose function has been demonstrated in IGH physiological rearrangements. We propose that the chromosomal events leading to Mantle Cell lymphoma and Burkitt's lymphoma are favored by the colocalization of CCND1 and CMYC with IGH at the time the concerned B cells undergo VDJ recombination or SHM/CSR, respectively. J. Cell. Biochem. 117: 1506-1510, 2016. © 2016 Wiley Periodicals, Inc.

Cell-Cycle-Dependent Reconfiguration of the DNA Methylome during Terminal Differentiation of Human B Cells into Plasma Cells.

Molecular mechanisms underlying terminal differentiation of B cells into plasma cells are major determinants of adaptive immunity but remain only partially understood. Here we present the transcriptional and epigenomic landscapes of cell subsets arising from activation of human naive B cells and differentiation into plasmablasts. Cell proliferation of activated B cells was linked to a slight decrease in DNA methylation levels, but followed by a committal step in which an S phase-synchronized differentiation switch was associated with an extensive DNA demethylation and local acquisition of 5-hydroxymethylcytosine at enhancers and genes related to plasma cell identity. Downregulation of both TGF-?1/SMAD3 signaling and p53 pathway supported this final step, allowing the emergence of a CD23-negative subpopulation in transition from B cells to plasma cells. Remarkably, hydroxymethylation of PRDM1, a gene essential for plasma cell fate, was coupled to progression in S phase, revealing an intricate connection among cell cycle, DNA (hydroxy)methylation, and cell fate determination.

Whole-genome fingerprint of the DNA methylome during human B cell differentiation.

We analyzed the DNA methylome of ten subpopulations spanning the entire B cell differentiation program by whole-genome bisulfite sequencing and high-density microarrays. We observed that non-CpG methylation disappeared upon B cell commitment, whereas CpG methylation changed extensively during B cell maturation, showing an accumulative pattern and affecting around 30% of all measured CpG sites. Early differentiation stages mainly displayed enhancer demethylation, which was associated with upregulation of key B cell transcription factors and affected multiple genes involved in B cell biology. Late differentiation stages, in contrast, showed extensive demethylation of heterochromatin and methylation gain at Polycomb-repressed areas, and genes with apparent functional impact in B cells were not affected. This signature, which has previously been linked to aging and cancer, was particularly widespread in mature cells with an extended lifespan. Comparing B cell neoplasms with their normal counterparts, we determined that they frequently acquire methylation changes in regions already undergoing dynamic methylation during normal B cell differentiation.

IL-2 requirement for human plasma cell generation: coupling differentiation and proliferation by enhancing MAPK-ERK signaling.

Mature B cell differentiation involves a well-established transcription factor cascade. However, the temporal dynamics of cell signaling pathways regulating transcription factor network and coordinating cell proliferation and differentiation remain poorly defined. To gain insight into the molecular processes and extrinsic cues required for B cell differentiation, we set up a controlled primary culture system to differentiate human naive B cells into plasma cells (PCs). We identified T cell-produced IL-2 to be critically involved in ERK1/2-triggered PC differentiation. IL-2 drove activated B cell differentiation toward PC independently of its proliferation and survival functions. Indeed, IL-2 potentiated ERK activation and subsequent BACH2 and IRF8 downregulation, sustaining BLIMP1 expression, the master regulator for PC differentiation. Inhibition of the MAPK-ERK pathway, unlike STAT5 signaling, impaired IL-2-induced PC differentiation and rescued the expression profile of BACH2 and IRF8. These results identify IL-2 as a crucial early input in mature B cell fate commitment.

Mesenchymal stromal cells orchestrate follicular lymphoma cell niche through the CCL2-dependent recruitment and polarization of monocytes.

Accumulating evidence indicates that infiltrating stromal cells contribute directly and indirectly to tumor growth in a wide range of cancers. In follicular lymphoma (FL), malignant B cells are found admixed with heterogeneous lymphoid-like stromal cells within invaded lymph nodes and BM. In addition, mesenchymal stromal cells (MSCs) support in vitro FL B-cell survival, in particular after their engagement toward lymphoid differentiation. We show here that BM-MSCs obtained from patients with FL (FL-MSCs) display a specific gene expression profile compared with MSCs obtained from healthy age-matched donors (HD-MSCs). This FL-MSC signature is significantly enriched for genes associated with a lymphoid-like commitment. Interestingly, CCL2 could be detected at a high level within the FL-cell niche, is up-regulated in HD-MSCs by coculture with malignant B cells, and is overexpressed by FL-MSCs, in agreement with their capacity to recruit monocytes more efficiently than HD-MSCs. Moreover, FL-MSCs and macrophages cooperate to sustain malignant B-cell growth, whereas FL-MSCs drive monocyte differentiation toward a proangiogenic and lipopolysaccharide-unresponsive phenotype close to that of tumor-associated macrophages. Altogether, these results highlight the complex role of FL stromal cells that promote direct tumor B-cell growth and orchestrate FL-cell niche, thus emerging as a potential therapeutic target in this disease.

Characterization of a transitional preplasmablast population in the process of human B cell to plasma cell differentiation.

The early steps of differentiation of human B cells into plasma cells are poorly known. We report a transitional population of CD20(low/-)CD38(-) preplasmablasts along differentiation of human memory B cells into plasma cells in vitro. Preplasmablasts lack documented B cell or plasma cell (CD20, CD38, and CD138) markers, express CD30 and IL-6R, and secrete Igs at a weaker level than do plasmablasts or plasma cells. These preplasmablasts further differentiate into CD20(-)CD38(high)CD138(-) plasmablasts and then CD20(-)CD38(high)CD138(+) plasma cells. Preplasmablasts were fully characterized in terms of whole genome transcriptome profiling and phenotype. Preplasmablasts coexpress B and plasma cell transcription factors, but at a reduced level compared with B cells, plasmablasts, or plasma cells. They express the unspliced form of XBP1 mRNA mainly, whereas plasmablasts and plasma cells express essentially the spliced form. An in vivo counterpart (CD19(+)CD20(low/-)CD38(-)IL-6R(+) cells) of in vitro-generated preplasmablasts could be detected in human lymph nodes (0.06% of CD19(+) cells) and tonsils (0.05% of CD19(+) cells). An open access "B to Plasma Cell Atlas," which makes it possible to interrogate gene expression in the process of B cell to plasma cell differentiation, is provided. Taken together, our findings show the existence of a transitional preplasmablast population using an in vitro model of plasma cell generation and of its in vivo counterpart in various lymphoid tissues.

CXCR4 expression functionally discriminates centroblasts versus centrocytes within human germinal center B cells.

The human germinal center is a highly dynamic structure where B cells conduct their terminal differentiation and traffic following chemokine gradients. The rapidly dividing centroblasts and the nondividing centrocytes represent the two major B cell subsets present in germinal center and also the most common normal counterparts for a majority of lymphomas. CD77 expression was previously associated to proliferating centroblasts undergoing somatic hypermutation, but data from transcriptional studies demonstrate that CD77 is not a reliable marker to discriminate human centroblasts from centrocytes. Herein we were able for the first time to separate these two subpopulations based on the expression of the chemokine receptor CXCR4 allowing their characterization. Phenotypic and functional features were especially explored, giving an accurate definition of CXCR4(+) centroblasts compared with CXCR4(-) centrocytes. We show that CXCR4(+) and CXCR4(-) germinal center B cells present a clear dichotomy in terms of proliferation, transcription factor expression, Ig production, and somatic hypermutation regulation. Microarray analysis identified an extensive gene list segregating these B cells, including highly relevant genes according to previous knowledge. By gene set enrichment analysis we demonstrated that the centroblastic gene expression signature was significantly enriched in Burkitt's lymphomas. Collectively, our findings show that CXCR4 expression can properly separate human centroblasts from centrocytes and offer now the possibility to have purified normal counterparts of mature B cell-derived malignancies.

Histamine and prostaglandin E up-regulate the production of Th2-attracting chemokines (CCL17 and CCL22) and down-regulate IFN-gamma-induced CXCL10 production by immature human dendritic cells.

Effector memory T helper 2 (Th2) cells that accumulate in target organs (i.e. skin or bronchial mucosa) have a central role in the pathogenesis of allergic disorders. To date, the factors that selectively trigger local production of Th2-attracting chemokines remain poorly understood. In mucosa, at the sites of allergen entry, immature dendritic cells (DC) are in close contact with mast cells. Histamine and prostaglandin E2 (PGE2) are two mediators released by allergen-activated mast cells that favour the polarization of maturing DC into Th2-polarizing cells. We analysed here the effects of histamine and PGE2 on the prototypic, Th2-(CCL17, CCL22) versus Th1-(CXCL10) chemokine production by human DC. We report that histamine and PGE2 dose-dependently up-regulate CCL17 and CCL22 by monocyte-derived immature DC. These effects were potentiated by tumour necrosis factor-alpha, still observed in the presence of the Th1-cytokine interferon-gamma (IFN-gamma) and abolished by the immunomodulatory cytokine interleukin-10. In addition, histamine and PGE2 down-regulated IFN-gamma-induced CXCL10 production by monocyte-derived DC. These properties of histamine and PGE2 were observed at the transcriptional level and were mediated mainly through H2 receptors for histamine and through EP2 and EP4 receptors for PGE2. Finally, histamine and PGE2 also up-regulated CCL17 and CCL22 and decreased IFN-gamma-induced CXCL10 production by purified human myeloid DC. In conclusion, these data show that, in addition to polarizing DC into mature cells that promote naïve T-cell differentiation into Th2 cells, histamine and PGE2 may act on immature DC to trigger local Th2 cell recruitment through a selective control of Th1/Th2-attracting chemokine production, thereby contributing to maintain a microenvironment favourable to persistent immunoglobulin E synthesis.

Direct stimulation of human T cells via TLR5 and TLR7/8: flagellin and R-848 up-regulate proliferation and IFN-gamma production by memory CD4+ T cells.

TLRs are involved in innate cell activation by conserved structures expressed by microorganisms. Human T cells express the mRNA encoding most of TLRs. Therefore, we tested whether some TLR ligands may modulate the function of highly purified human CD4+ T lymphocytes. We report that, in the absence of APCs, flagellin (a TLR5 ligand) and R-848 (a TLR7/8 ligand) synergized with suboptimal concentrations of TCR-dependent (anti-CD3 mAb) or -independent stimuli (anti-CD2 mAbs or IL-2) to up-regulate proliferation and IFN-gamma, IL-8, and IL-10 but not IL-4 production by human CD4+ T cells. No effect of poly(I:C) and LPS, ligands for TLR3 and TLR4, respectively, was detected. We also observed that CD4+CD45RO+ memory T cell responses to TLR ligands were more potent than those observed with CD4+CD45RA+ naive T cells. Moreover, among the memory T cells, CCR7- effector cells were more sensitive to TLR ligands than CCR7+ central memory cells. These data demonstrate for the first time a direct effect of TLR5 and TLR7/8 ligands on human T cells, and highlight an innate arm in T cell functions. They also suggest that some components from invading microorganisms may directly stimulate effector memory T cells located in tissues by up-regulating cytokine and chemokine production.

Broad antiretroviral defence by human APOBEC3G through lethal editing of nascent reverse transcripts.

Viral replication usually requires that innate intracellular lines of defence be overcome, a task usually accomplished by specialized viral gene products. The virion infectivity factor (Vif) protein of human immunodeficiency virus (HIV) is required during the late stages of viral production to counter the antiviral activity of APOBEC3G (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G; also known as CEM15), a protein expressed notably in human T lymphocytes. When produced in the presence of APOBEC3G, vif-defective virus is non-infectious. APOBEC3G is closely related to APOBEC1, the central component of an RNA-editing complex that deaminates a cytosine residue in apoB messenger RNA. APOBEC family members also have potent DNA mutator activity through dC deamination; however, whether the editing potential of APOBEC3G has any relevance to HIV inhibition is unknown. Here, we demonstrate that it does, as APOBEC3G exerts its antiviral effect during reverse transcription to trigger G-to-A hypermutation in the nascent retroviral DNA. We also find that APOBEC3G can act on a broad range of retroviruses in addition to HIV, suggesting that hypermutation by editing is a general innate defence mechanism against this important group of pathogens.