Silencing of B cell receptor signals in human naive B cells

Targeting B Cells and Plasma Cells in Autoimmune Diseases

Success with B cell depletion using rituximab has proven the concept that B lineage cells represent a valid target for the treatment of autoimmune diseases, and has promoted the development of other B cell targeting agents. Present data confirm that B cell depletion is beneficial in various autoimmune disorders and also show that it can worsen the disease course in some patients. These findings suggest that B lineage cells not only produce pathogenic autoantibodies, but also significantly contribute to the regulation of inflammation. In this review, we will discuss the multiple pro- and anti-inflammatory roles of B lineage cells play in autoimmune diseases, in the context of recent findings using B lineage targeting therapies.

Deoxynivalenol- and zearalenone-contaminated feeds alter gene expression profiles in the livers of piglets

Objective

The Fusarium mycotoxins of deoxynivalenol (DON) and zerolenone (ZEN) cause health hazards for both humans and farm animals. Therefore, the main intention of this study was to reveal DON and ZEN effects on the mRNA expression of pro-inflammatory cytokines and other immune related genes in the liver of piglets.

Methods

In the present study, 15 six-week-old piglets were randomly assigned to the following three different dietary treatments for 4 weeks: control diet, diet containing 8 mg DON/kg feed, and diet containing 0.8 mg ZEN/kg feed. After 4 weeks, liver samples were collected and sequenced using RNA-Seq to investigate the effects of the mycotoxins on genes and gene networks associated with the immune systems of the piglets.

Results

Our analysis identified a total of 249 differentially expressed genes (DEGs), which included 99 upregulated and 150 downregulated genes in both the DON and ZEN dietary treatment groups. After biological pathway analysis, the DEGs were determined to be significantly enriched in gene ontology terms associated with many biological pathways, including immune response and cellular and metabolic processes. Consistent with inflammatory stimulation due to the mycotoxin-contaminated diet, the following Kyoto encyclopedia of genes and genomes pathways, which were related to disease and immune responses, were found to be enriched in the DEGs: allograft rejection pathway, cell adhesion molecules, graft-versus-host disease, autoimmune thyroid disease (AITD), type I diabetes mellitus, human T-cell leukemia lymphoma virus infection, and viral carcinogenesis. Genome-wide expression analysis revealed that DON and ZEN treatments downregulated the expression of the majority of the DEGs that were associated with inflammatory cytokines (interleukin 10 receptor, beta, chemokine [C-X-C motif] ligand 9), proliferation (insulin-like growth factor 1, major facilitator superfamily domain containing 2A, insulin-like growth factor binding protein 2, lipase G, and salt inducible kinase 1), and other immune response networks (paired immunoglobulin-like type 2 receptor beta, Src-like-adaptor-1 [SLA1], SLA3, SLA5, SLA7, claudin 4, nicotinamide N-methyltransferase, thyrotropin-releasing hormone degrading enzyme, ubiquitin D, histone H₂B type 1, and serum amyloid A).

Conclusion

In summary, our results demonstrated that high concentrations DON and ZEN disrupt immune-related processes in the liver.

CD30 in systemic mastocytosis

CD30 is a transmembrane receptor, normally not expressed by mast cells, which regulates proliferation/apoptosis and antibody responses. Aberrant expression of CD30 by mastocytosis mast cells and interaction with its ligand CD30L (CD153) appears to play an important role in the pathogenesis and clinical presentation of systemic mastocytosis. This article highlights the expression profile and role of CD30 and CD30L in physiologic and pathologic conditions, the applicability of CD30 as a marker for systemic mastocytosis, the consequences of mast cell-expressed CD30, and the possibility of future anti-CD30 based cytoreductive therapies.

High expression of CD244 and SAP regulated CD8 T cell responses of patients with HTLV-I associated neurologic disease

HTLV-I-specific CD8⁺ T cells have been characterized with high frequencies in peripheral blood and cerebrospinal fluid and production of proinflammatory cytokines, which contribute to central nervous system inflammation in HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). However, little is known about the differences in CD8⁺ T cell activation status between asymptomatic carrier (ACs) and patients with HAM/TSP. The expression of CD244, a signaling lymphocyte activation molecule (SLAM) family receptor, was significantly higher on CD8⁺ T cells in HTLV-I-infected patients, both ACs and patients with HAM/TSP, than those on healthy normal donors (NDs). Blockade of CD244 inhibited degranulation and IFN-γ production in CD8⁺ T cells of patients with HAM/TSP, suggesting that CD244 is associated with effector functions of CD8⁺ T cells in patients with HAM/TSP. Moreover, SLAM-associated protein (SAP) was overexpressed in patients with HAM/TSP compared to ACs and NDs. SAP expression in Tax-specific CTLs was correlated in the HTLV-I proviral DNA loads and the frequency of the cells in HTLV-I-infected patients. SAP knockdown by siRNA also inhibited IFN-γ production in CD8⁺ T cells of patients with HAM/TSP. Thus, the CD244/SAP pathway was involved in the active regulation of CD8⁺ T cells of patients with HAM/TSP, and may play roles in promoting inflammatory neurological disease.

Human T-lymphotropic virus type I (HTLV-I) is a retrovirus that persistently infects 20 million people worldwide. The majority of infected individuals are asymptomatic carriers of the virus, but 5–10% of infected people develop either adult T cell leukemia/lymphoma (ATL) or a chronic, progressive neurological disease termed HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HAM/TSP is characterized by central nervous system (CNS) inflammation including HTLV-I-specific CD8⁺ T cells where disease progression and pathogenesis is associated with a dysregulation of antigen-specific CD8⁺ T cells, although the mechanism of this dysregulation remains to be defined. Here we demonstrate that a signaling lymphocyte activation molecule (SLAM) family of receptors, CD244, was overexpressed on CD8⁺ T cells of HTLV-I-infected patients than those of healthy normal donors, and that the upregulation of the adaptor protein, SAP, in CD8⁺ T cells distinguished HTLV-I infected individuals with and without neurologic disease. Both CD244 and SAP were associated with effector functions (high expression of IFN-γ) of CD8⁺ T cells in patients with HAM/TSP. This finding has important implication for T cell-mediated pathogenesis in human chronic viral infection associated with imbalance of immune function.

A novel immune-type receptor of Japanese flounder (Paralichthys olivaceus) is expressed in both T and B lymphocytes

A cDNA encoding a novel immune-type receptor (NITR) of Japanese flounder, Paralichthys olivaceus, was isolated from a kidney cDNA library. The cDNA encoded 357 amino acid residues. The amino acid sequence identities between Japanese flounder NITR1 (poNITR1) and previously reported fish NITRs were approximately 30%-40%. The poNITR1 consisted of two extracellular immunoglobulin (Ig) domains (V and V/C2), a transmembrane domain, an immunoreceptor tyrosine-based inhibitor motif (ITIM) and an ITIM-like motif (itim) in the cytoplasmic region. Five potential N-link glycosylation sites (N-X-S/T) are present in the extracellular Ig domains. Seven cysteine (Cys) residues, which are conserved in previously reported NITRs were observed in the extracellular domain of poNITR1. The poNITR1 gene is composed of five exons and four introns spanning approximately 3.4kb. The poNITR1 transcripts were mainly detected in gill, head kidney, trunk kidney, intestine, while it was weakly detected in heart, liver, muscle, peripheral blood leucocytes, skin, spleen and stomach. However, poNITR1 gene expression was not detected in muscle or ovary. NITR gene expression was not induced by LPS or poly I:C. In situ hybridization revealed that Japanese flounder NITR is expressed in both TCR-alpha- and IgM-presenting cells.

Amino acid-coded tagging approaches in quantitative proteomics

To improve the efficiency, accuracy, reproducibility, throughput and proteome coverage of mass spectrometry-based quantitative approaches, both in vitro and in vivo tagging of particular amino acid residues of cellular proteins have been introduced to assist mass spectrometry for global-scale comparative studies of differentially expressed proteins/modifications between different biologically relevant cell states or cells at different pathological states. The basic features of these methods introduce pair-wise isotope signals of each individual peptide containing a particular type of tagged amino acid (amino acid-coded mass tagging) that originated from different cell states. In this review, the applications of major amino acid-coded mass tagging-based quantitative proteomics approaches, including isotope-coded affinity tag, isobaric tags for relative and absolute quantification (iTRAQ) and stable isotope labeling by amino acids in cell culture are summarized in the context of their respective strengths/weakness in identifying those differentially expressed or post-translational modified proteins regulated by particular cellular stress on a genomic scale in a high-throughput manner. Importantly, these gel-free, in-spectra quantitative mechanisms have been further explored to identify/characterize large-scale protein-protein interactions involving various functional pathways. Taken together, the information about quantitative proteome changes, including multiple regulated proteins and their interconnected relationships, will provide an important insight into the molecular mechanisms, where novel targets for diagnosis and therapeutic intervention will be identified.

SAP regulation of follicular helper CD4 T cell development and humoral immunity is independent of SLAM and Fyn kinase

Mutations in SH2D1A resulting in lack of SLAM-associated protein (SAP) expression cause the human genetic immunodeficiency X-linked lymphoproliferative disease. A severe block in germinal center development and lack of long-term humoral immunity is one of the most prominent phenotypes of SAP(-) mice. We show, in this study, that the germinal center block is due to an essential requirement for SAP expression in Ag-specific CD4 T cells to develop appropriate follicular helper T cell functions. It is unknown what signaling molecules are involved in regulation of SAP-dependent CD4 T cell help functions. SAP binds to the cytoplasmic tail of SLAM, and we show that SLAM is expressed on resting and activated CD4 T cells, as well as germinal center B cells. In addition, SAP can recruit Fyn kinase to SLAM. We have now examined the role(s) of the SLAM-SAP-Fyn signaling axis in in vivo CD4 T cell function and germinal center development. We observed normal germinal center development, long-lived plasma cell development, and Ab responses in SLAM(-/-) mice after a viral infection (lymphocytic choriomeningitis virus). In a separate series of experiments, we show that SAP is absolutely required in CD4 T cells to drive germinal center development, and that requirement does not depend on SAP-Fyn interactions, because CD4 T cells expressing SAP R78A are capable of supporting normal germinal center development. Therefore, a distinct SAP signaling pathway regulates follicular helper CD4 T cell differentiation, separate from the SLAM-SAP-Fyn signaling pathway regulating Th1/Th2 differentiation.

Exploring host-pathogen interactions at the epithelial surface: application of transcriptomics in lung biology

The epithelial surface of the airways is the largest barrier-forming interface between the human body and the outside world. It is now well recognized that, at this strategic position, airway epithelial cells play an eminent role in host defense by recognizing and responding to microbial exposure. Conversely, inhaled microorganisms also respond to contact with epithelial cells. Our understanding of this cross talk is limited, requiring sophisticated experimental approaches to analyze these complex interactions. High-throughput technologies, such as DNA microarray analysis and serial analysis of gene expression (SAGE), have been developed to screen for gene expression levels at large scale within single experiments. Since their introduction, these hypothesis-generating technologies have been widely used in diverse areas such as oncology and brain research. Successful application of these genomics-based technologies has also revealed novel insights in host-pathogen interactions in both the host and pathogen. This review aims to provide an overview of the SAGE and microarray technology illustrated by their application in the analysis of host-pathogen interactions. In particular, the interactions between epithelial cells in the human lungs and clinically relevant microorganisms are the central focus of this review.

Fc receptor-like molecules

Discovery of a large family of Fc receptor-like (FCRL) molecules, homologous to the well-known receptors for the Fc portion of immunoglobulin (FCR), has uncovered an impressive abundance of immunoglobulin superfamily (IgSF) genes in the human 1q21-23 chromosomal region and revealed significant diversity for these genes between humans and mice. The observation that FCRL representatives are members of an ancient multigene family that share a common ancestor with the classical FCR is underscored by their linked genomic locations, gene structure, shared extracellular domain composition, and utilization of common cytoplasmic tyrosine-based signaling elements. In contrast to the conventional FCR, however, FCRL molecules possess diverse extracellular frameworks, autonomous or dual signaling properties, and preferential B lineage expression. Most importantly, there is no strong evidence thus far to support a role for them as Ig-binding receptors. These characteristics, in addition to their identification in malignancies and autoimmune disorders, predict a fundamental role for these receptors as immunomodulatory agents in normal and subverted B lineage cells.

Src-like adaptor protein (SLAP) regulates B cell receptor levels in a c-Cbl-dependent manner

Src-like adaptor protein (SLAP) and c-Cbl recently have been shown to cooperate in regulating T cell receptor (TCR) levels in developing T cells. SLAP also is expressed in developing B cells, and its deficiency leads to alterations in B cell receptor (BCR) levels and B cell development. Hence, we hypothesized that SLAP and c-Cbl may cooperate during B cell development to regulate BCR levels. In mice deficient in both SLAP and c-Cbl, we found that B cell development is altered, suggesting that they function through intersecting pathways. To study the mechanism by which SLAP and c-Cbl alter BCR levels, we coexpressed them in a mature mouse B cell line (Bal-17). First we determined that SLAP associates with proximal components of the BCR complex after stimulation and internalization. Coexpression of SLAP and c-Cbl in Bal-17 led to decreased surface and total BCR levels. This decrease in BCR levels depended on intact Src homology 2 (SH2) and C-terminal domains of SLAP. In addition, a mutation in the SH2 domain of SLAP blocked its colocalization with c-Cbl and the BCR complex, whereas deletion of the C terminus did not affect its localization. Last, coexpression of SLAP and c-Cbl altered BCR complex recycling. This alteration in BCR complex recycling depended on enzymatically active c-Cbl and Src family kinases, as well as the intact SH2 and C-terminal domains of SLAP. These data suggest that SLAP has a conserved function in B and T cells by adapting c-Cbl to the antigen-receptor complex and targeting it for degradation.

Intrinsic properties of human and murine memory B cells

The central question of how the immune system responds in a qualitatively and quantitatively better way upon re-exposure to a pathogen is largely unanswered. Both the increased frequency of antigen-specific memory cells and the intrinsic properties that memory cells acquire after antigen experience could contribute to the faster and more robust responses seen after repeated exposure to antigen. In the case of the memory B-cell response, it has been difficult to discern the individual contributions of these two effects. However, because of recent advances in identifying memory B cells, there is an increasing understanding of the intrinsic properties of these cells. The current insights into the unique properties of memory B cells and the progress that has been made in understanding how these affect secondary responses in both the human and the mouse systems are discussed. In addition, we compare the various advantages and disadvantages inherent in each of these systems, in terms of studying the intrinsic properties of memory B cells, and introduce the details of the system that we have developed using conventional heavy chain transgenic (Tgic) mice, which addresses some of the drawbacks of traditional memory models.

Hypogammaglobulinemia and exacerbated CD8 T-cell–mediated immunopathology in SAP-deficient mice with chronic LCMV infection mimics human XLP disease

The human genetic disease X-linked lymphoproliferative disease (XLP), which is caused by mutations in SH2D1A/SAP that encode SLAM-associated protein (SAP), is characterized by an inability to control Epstein-Barr virus (EBV) and hypogammaglobulinemia. It is unclear which aspects of XLP disease are specific to herpesvirus infection and which reflect general immunologic functions performed by SAP. We examined SAP– mice during a chronic LCMV infection, specifically to address the following question: Which SAP deficiency immunologic problems are general, and which are EBV specific? Illness, weight loss, and prolonged viral replication were much more severe in SAP– mice. Aggressive immunopathology was observed. This inability to control chronic LCMV was associated with both CD8 T-cell and B-cell response defects. Importantly, we demonstrate that SAP– CD8 T cells are the primary cause of the immunopathology and clinical illness, because depletion of CD8 T cells blocked disease. This is the first direct demonstration of SAP– CD8 T-cell–mediated immunopathology, confirming 30 years of XLP clinical observations and indirect experimentation. In addition, germinal center formation was extremely defective in chronically infected SAP– animals, and hypogammaglobulinemia was observed. These findings in a chronic viral infection mouse model recapitulate key features of human XLP and clarify SAP's critical role regulating both cellular and humoral immunity.

A new memory CD27-IgG+ B cell population in peripheral blood expressing VH genes with low frequency of somatic mutation

In humans, up to 40% of peripheral B cells express CD27 and have hypermutated variable regions in their Ig genes. The CD27+ B cells are considered to be derived from germinal center following specific antigenic stimulation. Actually, somatic hypermutation in Ig genes and CD27 expression are hallmarks of memory B cells. However, the blood IgM+ IgD+ CD27+ B cells were recently associated to splenic marginal zone B cells and proposed to be a subset distinct from germinal center-derived memory B cells showing premutated Igs. The results presented herein further weaken this bona fide association because B cells expressing surface IgG, but not CD27, were found in human blood. Representing 1-4% of all peripheral B cells and approximately 25% of the IgG+ blood B cells, this population expressed mutated IgG genes showing antigenic selection characteristics but with lower mutation frequencies than that of CD27+ IgG+ B cells. However, their morphology and phenotype were similar to that of CD27+ IgG+ cells. Interestingly, the proportion of IgG2 over IgG3 transcripts was opposite in CD27- IgG+ and CD27+ IgG+ cells, suggesting distinct functions or origins. Overall, these findings extend the memory B cell reservoir beyond the CD27+ compartment and could provide further insights into B cell disorders of unknown etiology.

Deciphering CD30 ligand biology and its role in humoral immunity

Ligands and receptors in the tumour necrosis factor (TNF) and tumour necrosis factor receptor (TNFR) superfamilies have been the subject of extensive investigation over the past 10-15 years. For certain TNFR family members, such as Fas and CD40, some of the consequences of receptor ligation were predicted before the identification and cloning of their corresponding ligands through in vitro functional studies using agonistic receptor-specific antibodies. For other members of the TNFR family, including CD30, cross-linking the receptor with specific antibodies failed to yield many clues about the functional significance of the relevant ligand-receptor interactions. In many instances, the subsequent availability of TNF family ligands in the form of recombinant protein facilitated the determination of biological consequences of interactions with their relevant receptor in both in vitro and in vivo settings. In the case of CD30 ligand (CD30L; CD153), definition of its biological role remained frustratingly elusive. Early functional studies using CD30L+ cells or agonistic CD30-specific antibodies logically focused attention on cell types that had been shown to express CD30, namely certain lymphoid malignancies and subsets of activated T cells. However, it was not immediately clear how the reported activities from these in vitro studies relate to the biological activity of CD30L in the more complex whole animal setting. Recently, results from in vivo models involving CD30 or CD30L gene disruption, CD30L overexpression, or pharmacological blockade of CD30/CD30L interactions have begun to provide clues about the role played by CD30L in immunological processes. In this review we consider the reported biology of CD30L and focus on results from several recent studies that point to an important role for CD30/CD30L interactions in humoral immune responses.

Rapid, reliable and inexpensive quality assessment of biotinylated cRNA

The interpretation of oligonucleotide array experiments depends on the quality of the target cRNA used. cRNA target quality is assessed by quantitative analysis of the representation of 5' and 3' sequences of control genes using commercially available Test arrays. The Test array provides an economically priced means of determining the quality of labeled target prior to analysis on whole genome expression arrays. This manuscript validates the use of a duplex RT-PCR assay as a faster (6 h) and less expensive (<10 US dollars) but equally accurate alternative to the Test arrays in determining biotinylated cRNA quality. Forty-one different cRNA samples were hybridized to HG-U133A microarrays from Affymetrix. Ten cRNA samples with a beta-actin 3'/5' ratio >6 were chosen and classified as degraded cRNAs, and 31 samples with a beta-actin 3'/5' ratio <6 were selected as good quality cRNAs. Blinded samples were then used for the RT-PCR assay. After gel electrophoresis, optical densities of the amplified 3' and 5' fragments of beta-actin were measured and the 3'/5' ratio was calculated. There was a strong correlation (r(2) = 0.6802) between the array and the RT-PCR beta-actin 3'/5' ratios. Moreover, the RT-PCR 3'/5' ratio was significantly different (P < 0.0001) between undegraded (mean +/- SD, 0.34 +/- 0.09) and degraded (1.71 +/- 0.83) samples. None of the other parameters analyzed, such as i) the starting amount of RNA, ii) RNA quality assessed using the Bioanalyzer Chip technology, or iii) the concentration and OD260/OD280 ratio of the purified biotinylated cRNA, correlated with cRNA quality.

BCR-ABL1 induces aberrant splicing of IKAROS and lineage infidelity in pre-B lymphoblastic leukemia cells

Pre-B lymphoblastic leukemia cells carrying a BCR-ABL1 gene rearrangement exhibit an undifferentiated phenotype. Comparing the genome-wide gene expression profiles of normal B-cell subsets and BCR-ABL1+ pre-B lymphoblastic leukemia cells by SAGE, the leukemia cells show loss of B lymphoid identity and aberrant expression of myeloid lineage-specific molecules. Consistent with this, BCR-ABL1+ pre-B lymphoblastic leukemia cells exhibit defective expression of IKAROS, a transcription factor needed for early lymphoid lineage commitment. As shown by inducible expression of BCR-ABL1 in human and murine B-cell precursor cell lines, BCR-ABL1 induces the expression of a dominant-negative IKAROS splice variant, termed IK6. Comparing matched leukemia sample pairs from patients before and during therapy with the BCR-ABL1 kinase inhibitor STI571 (Imatinib), inhibition of BCR-ABL1 partially corrected aberrant expression of IK6 and lineage infidelity of the leukemia cells. To elucidate the contribution of IK6 to lineage infidelity in BCR-ABL1+ cell lines, IK6 expression was silenced by RNA interference. Upon inhibition of IK6, BCR-ABL1+ leukemia cells partially restored B lymphoid lineage commitment. Therefore, we propose that BCR-ABL1 induces aberrant splicing of IKAROS, which interferes with lineage identity and differentiation of pre-B lymphoblastic leukemia cells.

Role of MHC Class II on Memory B Cells in Post-Germinal Center B Cell Homeostasis and Memory Response

We investigated the role of B cell Ag presentation in homeostasis of the memory B cell compartment in a mouse model where a conditional allele for the beta-chain of MHC class II (MHC-II) is deleted in the vast majority of all B cells by cd19 promoter-mediated expression of Cre recombinase (IA-B mice). Upon T cell-dependent immunization, a small number of MHC-II(+) B cells in IA-B mice dramatically expanded and restored normal albeit delayed levels of germinal center (GC) B cells with an affinity-enhancing somatic mutation to Ag. IA-B mice also established normal levels of MHC-II(+) memory B cells, which, however, subsequently lost MHC-II expression by ongoing deletion of the conditional iab allele without significant loss in their number. Furthermore, in vivo Ag restimulation of MHC-II(-) memory B cells of IA-B mice failed to cause differentiation into plasma cells (PCs), even in the presence of Ag-specific CD4(+) T cells. In addition, both numbers and Ag-specific affinity of long-lived PCs during the late post-GC phase, as well as post-GC serum affinity maturation, were significantly reduced in IA-B mice. These results support a notion that MHC-II-dependent T cell help during post-GC phase is not absolutely required for the maintenance of memory B cell frequency but is important for their differentiation into PCs and for the establishment of the long-lived PC compartment.

Regulation of the germinal center gene program by interferon (IFN) regulatory factor 8/IFN consensus sequence-binding protein

Interferon (IFN) consensus sequence-binding protein/IFN regulatory factor 8 (IRF8) is a transcription factor that regulates the differentiation and function of macrophages, granulocytes, and dendritic cells through activation or repression of target genes. Although IRF8 is also expressed in lymphocytes, its roles in B cell and T cell maturation or function are ill defined, and few transcriptional targets are known. Gene expression profiling of human tonsillar B cells and mouse B cell lymphomas showed that IRF8 transcripts were expressed at highest levels in centroblasts, either from secondary lymphoid tissue or transformed cells. In addition, staining for IRF8 was most intense in tonsillar germinal center (GC) dark-zone centroblasts. To discover B cell genes regulated by IRF8, we transfected purified primary tonsillar B cells with enhanced green fluorescent protein-tagged IRF8, generated small interfering RNA knockdowns of IRF8 expression in a mouse B cell lymphoma cell line, and examined the effects of a null mutation of IRF8 on B cells. Each approach identified activation-induced cytidine deaminase (AICDA) and BCL6 as targets of transcriptional activation. Chromatin immunoprecipitation studies demonstrated in vivo occupancy of 5' sequences of both genes by IRF8 protein. These results suggest previously unappreciated roles for IRF8 in the transcriptional regulation of B cell GC reactions that include direct regulation of AICDA and BCL6.

Selective generation of functional somatically mutated IgM+CD27+, but not Ig isotype-switched, memory B cells in X-linked lymphoproliferative disease

Individuals with X-linked lymphoproliferative disease (XLP) display defects in B cell differentiation in vivo. Specifically, XLP patients do not generate a normal number of CD27 memory B cells, and those few that are present are IgM. Recent studies have suggested that IgMCD27 B cells are not true memory cells, but rather B cells that guard against T cell-independent pathogens. Here we show that human XLP IgMCD27 B cells resemble normal memory B cells both morphologically and phenotypically. Additionally, IgMCD27 B cells exhibited functional characteristics of normal memory B cells, including the ability to secrete more Ig than naive B cells in response to both T cell-dependent and -independent stimuli. Analysis of spleens from XLP patients revealed a paucity of germinal centers (GCs), and the rare GCs detected were poorly formed. Despite this, Ig variable region genes expressed by XLP IgMCD27 B cells had undergone somatic hypermutation to an extent comparable to that of normal memory B cells. These findings reveal a differential requirement for the generation of IgM and Ig isotype-switched memory B cells, with the latter only being generated by fully formed GCs. Production of affinity-matured IgM by IgMCD27 B cells may protect against pathogens to which a normal immune response is elicited in XLP patients.

Src-Like Adaptor Protein Regulates B Cell Development and Function

The avidity of BCRs and TCRs influences signal strength during processes of lymphocyte development. Avidity is determined by both the intrinsic affinity for Ag and surface levels of the Ag receptor. The Src-like adaptor protein (SLAP) is a regulator of TCR levels on thymocytes, and its deficiency alters thymocyte development. We hypothesized that SLAP, which is expressed in B cells, also is important in regulating BCR levels, signal strength, and B cell development. To test this hypothesis, we analyzed the B cell compartment in SLAP-deficient mice. We found increased splenic B cell numbers and decreased surface IgM levels on mature, splenic B cells deficient in SLAP. Immature bone marrow and splenic B cells from BCR-transgenic, SLAP-deficient mice were found to express higher surface levels of IgM. In contrast, mature splenic B cells from BCR-transgenic mice expressed decreased levels of surface BCR associated with decreased calcium flux and activation-induced markers, compared with controls. These data suggest that SLAP regulates BCR levels and signal strength during lymphocyte development.

Stage-dependent gene expression profiles during natural killer cell development

Natural killer (NK) cells develop from hematopoietic stem cells (HSCs) in the bone marrow. To understand the molecular regulation of NK cell development, serial analysis of gene expression (SAGE) was applied to HSCs, NK precursor (pNK) cells, and mature NK cells (mNK) cultured without or with OP9 stromal cells. From 170,464 total individual tags from four SAGE libraries, 35,385 unique genes were identified. A set of genes was expressed in a stage-specific manner: 15 genes in HSCs, 30 genes in pNK cells, and 27 genes in mNK cells. Among them, lipoprotein lipase induced NK cell maturation and cytotoxic activity. Identification of genome-wide profiles of gene expression in different stages of NK cell development affords us a fundamental basis for defining the molecular network during NK cell development.

Deficiency of Bruton's tyrosine kinase in B cell precursor leukemia cells

Bruton's tyrosine kinase (BTK) deficiency results in a differentiation block at the pre-B cell stage. Likewise, acute lymphoblastic leukemia cells are typically arrested at early stages of B cell development. We therefore investigated BTK function in B cell precursor leukemia cells carrying a BCR-ABL1, E2A-PBX1, MLL-AF4, TEL-AML1, or TEL-PDGFRB gene rearrangement. Although somatic mutations of the BTK gene are rare in B cell precursor leukemia cells, we identified kinase-deficient splice variants of BTK throughout all leukemia subtypes. Unlike infant leukemia cells carrying an MLL-AF4 gene rearrangement, where expression of full-length BTK was detectable in only four of eight primary cases, in leukemia cells harboring other fusion genes full-length BTK was typically coexpressed with kinase-deficient variants. As shown by overexpression experiments, kinase-deficient splice variants can act as a dominant-negative BTK in that they suppress BTK-dependent differentiation and pre-B cell receptor responsiveness of the leukemia cells. On the other hand, induced expression of full-length BTK rendered the leukemia cells particularly sensitive to apoptosis. Comparing BTK expression in surviving or preapoptotic leukemia cells after 10-Gy gamma radiation, we observed selective survival of leukemia cells that exhibit expression of dominant-negative BTK forms. These findings indicate that lack of BTK expression or expression of dominant-negative splice variants in B cell precursor leukemia cells can (i) inhibit differentiation beyond the pre-B cell stage and (ii) protect from radiation-induced apoptosis.

Mimicry of a constitutively active pre-B cell receptor in acute lymphoblastic leukemia cells

Pre-B cells undergo apoptosis unless they are rescued by pre-B cell receptor-dependent survival signals. We previously showed that the BCR-ABL1 kinase that is expressed in pre-B lymphoblastic leukemia bypasses selection for pre-B cell receptor-dependent survival signals. Investigating possible interference of BCR-ABL1 with pre-B cell receptor signaling, we found that neither SYK nor SLP65 can be phosphorylated in response to pre-B cell receptor engagement. Instead, Bruton's tyrosine kinase (BTK) is constitutively phosphorylated by BCR-ABL1. Activated BTK is essential for survival signals that otherwise would arise from the pre-B cell receptor, including activation of PLCgamma1, autonomous Ca2+ signaling, STAT5-phosphorylation, and up-regulation of BCLX(L). Inhibition of BTK activity specifically induces apoptosis in BCR-ABL1+ leukemia cells to a similar extent as inhibition of BCR-ABL1 kinase activity itself. However, BCR-ABL1 cannot directly bind to full-length BTK. Instead, BCR-ABL1 induces the expression of a truncated splice variant of BTK that acts as a linker between the two kinases. As opposed to full-length BTK, truncated BTK lacks kinase activity yet can bind to BCR-ABL1 through its SRC-homology domain 3. Acting as a linker, truncated BTK enables BCR-ABL1-dependent activation of full-length BTK, which initiates downstream survival signals and mimics a constitutively active pre-B cell receptor.

Novel Role of the Ras Cascade in Memory B Cell Response

Engagement of the B cell antigen receptor (BCR) triggers the Ras cascade, but the biological role of the latter in B cell response is unknown. Here, we report that in T cell-dependent response, the role of the Ras cascade is confined to memory B cells and possibly marginal zone B cells. When Ras-dependent BCR signaling was impaired, the generation of IgG germinal center B cells was unaffected but the recruitment of high-affinity cells into the memory compartment and terminal differentiation were inhibited. Furthermore, inhibition of MEK activity consistently impaired antibody production by IgG memory B cells (but not naïve B cells) in vitro. Notably, this impairment was countered by overexpression of Bcl-2. Thus, our data suggest that upon antigen stimulation, memory B cells are susceptible to apoptosis but can be rescued via an antiapoptotic effect mediated through the Ras cascade.

Molecular and cellular pathogenesis of X-linked lymphoproliferative disease

X-linked lymphoproliferative disease (XLP) is an inherited immune defect caused by mutations in the Src homology 2 domain-containing gene 1A, which encodes the adapter protein, signaling lymphocytic activation molecule (SLAM)-associated protein (SAP). SAP is expressed in T cells, natural killer (NK) cells, and NKT cells, where it binds to the cytoplasmic domain of the surface receptor SLAM (CD150) and the related receptors, 2B4 (CD244), CD84, Ly9 (CD229), NK-T-B-antigen, and CD2-like receptor-activating cytotoxic T cells. SAP also binds to the Src family tyrosine kinase Fyn and recruits it to SLAM, which leads to the generation of downstream phosphotyrosine signals. While the roles of the SLAM family receptors are only beginning to be understood, experiments suggest that these molecules regulate important aspects of lymphocyte function, such as proliferation, cytokine secretion, cytotoxicity, and antibody production. Thus, in XLP patients who lack functional SAP, the SLAM family receptors may not signal properly. This property likely contributes to the phenotypes of XLP, including fulminant infectious mononucleosis, lymphoma, and hypogammaglobulinemia. Further studies of SAP and the SLAM family receptors will provide insights into XLP and elucidate the signaling events regulating lymphocyte ontogeny and function.

In Vivo Dual-Tagging Proteomic Approach in Studying Signaling Pathways in Immune Response

Up to date, few successes have been achieved to identify the signaling molecules directly from immune cells due to their low-abundance and dynamic nature. Here, we designed an in vivo dual-tagging quantitative approach that integrated epitope-tagging which allows single affinity purification of the natural complexes formed at real-time, and amino acid-coded mass tagging (AACT) that assists mass spectrometry-based quantitative measurement, to identify the specific components of a signaling complex formed in macrophage cells upon lipopolysaccharide (LPS) stimulation. The sensitivity and accuracy of this quantitative method are significantly higher than those of tandem affinity purification, because the multiple step of purifications are avoided to preserve weakly interacting molecules. We identified a number of proteins that interact with MyD88, a critical adaptor protein in innate immune response, in macrophages upon stimulation. Among those newly identified MyD88-interacting partners, FLAP-1 was found to be an activator of NF-kappaB, the key transcription factor in immune response. This integrated approach provides global information on the functional link between MyD88 and other proteins in transducing the TLR-mediated signal and is generally applicable to in vivo analyses of other signaling pathways.

Defective B cell responses in the absence of SH2D1A

More than half of patients with X-linked lympho-proliferative disease, which is caused by a defect in the intracellular adapter protein SH2D1A, suffer from an extreme susceptibility to Epstein-Barr virus. One-third of these patients, however, develop dysgammaglobulenemia without an episode of severe mononucleosis. Here we show that in SH2D1A(-/-) mice, both primary and secondary responses of all Ig subclasses are severely impaired in response to specific antigens. Because germinal centers were absent in SH2D1A(-/-) mice upon primary immunization, and because SH2D1A was detectable in wt germinal center B cells, we examined whether SH2D1A(-/-) B cell functions were impaired. Using the adoptive cotransfer of B lymphocytes from hapten-primed SH2D1A(-/-) mice with CD4(+) T cells from primed wt mice into irradiated wt mice provided evidence that signal transduction events controlled by SH2D1A are essential for B cell activities resulting in antigen specific IgG production. Defects in naive SH2D1A(-/-) B cells became evident upon cotransfer with non-primed wt CD4(+) cells into Rag2(-/-) recipients. Thus, both defective T and B cells exist in the absence of SH2D1A, which may explain the progressive dysgammaglobulinemia in a subset of X-linked lympho-proliferative disease patients without involvement of Epstein-Barr virus.

The molecular requirements for LAT-mediated differentiation and the role of LAT in limiting pre-B cell expansion

Successful recombination of the heavy-chain locus in developing B cells results in the expression of the pre-BCR, which induces the proliferation and expansion of pre-B cells. To avoid uncontrolled proliferation, pre-BCR signals transmitted via the adaptor protein SLP-65 (SH2-domain-containing leukocyte protein of 65 kDa) lead to the down-regulation of pre-BCR expression and to pre-B cell differentiation. Here, we show that, similarly to SLP-65, the adaptor protein LAT (linker for activation of T cells) limits pre-B cell proliferation and reduces the potential of a tumorgenic pre-B cell line to develop leukemia in immune-deficient mice. We further show that the four distal tyrosines are required for LAT activity in pre-B cells. Mutation at Y136 completely abolishes LAT activity, whereas single point-mutations at Y175, Y195 or Y235 impair, but do not block, LAT-induced pre-B cell differentiation. As LAT is also expressed in human pre-B cells, our results suggest that LAT cooperates with SLP-65 to promote the differentiation and control the proliferation of both murine and human pre-B cells.

Tracing the pre-B to immature B cell transition in human leukemia cells reveals a coordinated sequence of primary and secondary IGK gene rearrangement, IGK deletion, and IGL gene rearrangement

The BCR-ABL1 kinase expressed in acute lymphoblastic leukemia (ALL) drives malignant transformation of pre-B cells and prevents further development. We studied whether inhibition of BCR-ABL1 kinase activity using STI571 can relieve this differentiation block. STI571 treatment of leukemia patients induced expression of the Ig L chain-associated transcription factors IRF4 and SPIB, up-regulation of RAG1 and RAG2, Ckappa and Clambda germline transcription, and rearrangement of Ig kappa L chain (IGK) and Ig lambda L chain (IGL) genes. However, STI571-treated pre-B ALL cells expressed lambda L, but almost no kappa L chains. This could be explained by STI571-induced rearrangement of the kappa-deleting element (KDE), which can delete productively rearranged Vkappa-Jkappa joints. Amplifying double-strand breaks at recombination signal sequences within the IGK, KDE, and IGL loci revealed a coordinated sequence of rearrangement events induced by STI571: recombination of IGK gene segments was already initiated within 1 h after STI571 treatment, followed by KDE-mediated deletion of Vkappa-Jkappa joints 6 h later and, ultimately, IGL gene rearrangement after 12 h. Consistently, up-regulation of Ckappa and Clambda germline transcripts, indicating opening of IGK and IGL loci, was detected after 1 and 6 h for IGK and IGL, respectively. Continued activity of the recombination machinery induced secondary IGK gene rearrangements, which shifted preferential usage of upstream located Jkappa- to downstream Jkappa-gene segments. Thus, inhibition of BCR-ABL1 in pre-B ALL cells 1) recapitulates early B cell development, 2) directly shows that IGK, KDE, and IGL genes are rearranged in sequential order, and 3) provides a model for Ig L chain gene regulation in the human.

Autocrine- and paracrine-activated receptor tyrosine kinases in classic Hodgkin lymphoma

The pathogenesis of Hodgkin lymphoma (HL) is still largely unknown. Based on a search for footprints of pathogenetic mechanisms in global RNA expression data of Hodgkin/Reed-Sternberg (HRS) cell lines, we analyzed the expression and activation of 6 receptor tyrosine kinases (RTKs) in classic HL. Immunohistochemistry revealed that the RTKs platelet-derived growth factor receptor A (PDGFRA), DDR2, EPHB1, RON, TRKB, and TRKA were each expressed in HRS cells in 30% to 75% of patients. These RTKs were not expressed in normal B cells, the origin of HRS cells, or in most B-cell non-Hodgkin lymphoma (NHL). In the majority of patients at least one RTK was expressed, and in most patients several RTKs were coexpressed, most prominently in Hodgkin lymphoma of the nodular sclerosis subtype. Phosphotyrosine-specific antibodies revealed exemplarily the activation of PDGFRA and TRKA/B and an elevation of cellular phosphotyrosine content. Immunohistochemistry for RTK ligands indicated that DDR2 and TRKA are likely activated in a paracrine fashion, whereas PDGFRA and EPHB1 seem to be activated by autocrine loops. Activating mutations were not detected in cDNA encoding the RTKs in HRS cell lines. These findings show the unprecedented coexpression of multiple RTKs in a tumor and indicate that aberrant RTK signaling is an important factor in HL pathogenesis and that it may be a novel therapeutic target.

Zizimin2: a novel, DOCK180-related Cdc42 guanine nucleotide exchange factor expressed predominantly in lymphocytes

A novel superfamily of guanine nucleotide exchange factors for Rho GTPases includes DOCK180 and zizimin1. The zizimin subfamily includes three genes of which only zizimin1 has been cloned. We report here the cloning of zizimin2, identified in a screen for genes enriched in germinal center B cells. Zizimin2 and zizimin1 have similar primary structures and both proteins bound and activated Cdc42 but not the Cdc42-related proteins TC10 or TCL. Their tissue distributions are distinct, however, with zizimin2 expressed predominantly in lymphocytes and an opposite pattern for zizimin1. Zizimin3 was also analyzed and showed distinct GTPase specificity and tissue distribution.

The SAP family of adaptors in immune regulation

SAP and EAT-2 define a new class of adaptor proteins composed almost exclusively of a Src homology 2 (SH2) domain. By way of their SH2 domain, SAP-like adaptors interact with tyrosine-based motifs in the cytoplasmic region of SLAM-related receptors, a family of immune cell-specific molecules involved in immunoregulation. Recent findings indicate that SAP is required for the functions of SLAM family receptors, as a consequence of its ability to promote recruitment of Src-related protein tyrosine kinase FynT and allow SLAM-related receptors to transduce tyrosine phosphorylation signals. SAP is mutated in X-linked lymphoproliferative (XLP) syndrome, a rare inherited human disease characterized by an deregulated immune response to Epstein-Barr virus infection. Several lines of evidence indicate that defects in the activities of SLAM-related receptors caused by SAP deficiency account for the immune dysfunctions associated with XLP.

The BCR-ABL1 kinase bypasses selection for the expression of a pre-B cell receptor in pre-B acute lymphoblastic leukemia cells

The BCR-ABL1 kinase expressed in acute lymphoblastic leukemia (ALL) drives malignant transformation of human pre–B cells. Comparing genome-wide gene expression profiles of BCR-ABL1⁺ pre–B ALL and normal bone marrow pre–B cells by serial analysis of gene expression, many genes involved in pre–B cell receptor signaling are silenced in the leukemia cells. Although normal pre–B cells are selected for the expression of a functional pre–B cell receptor, BCR-ABL1⁺ ALL cells mostly do not harbor a productively rearranged IGH allele. In these cases, we identified traces of secondary V_H gene rearrangements, which may have rendered an initially productive V_H region gene nonfunctional. Even BCR-ABL1⁺ ALL cells harboring a functional V_H region gene are unresponsive to pre–B cell receptor engagement and exhibit autonomous oscillatory Ca²⁺ signaling activity. Conversely, leukemia subclones surviving inhibition of BCR-ABL1 by STI571 restore responsiveness to antigen receptor engagement and differentiate into immature B cells expressing immunoglobulin light chains. BCR-ABL1 kinase activity is linked to defective pre–B cell receptor signaling and the expression of a truncated isoform of the pre–B cell receptor–associated linker molecule SLP65. Also in primary leukemia cells, truncated SLP65 is expressed before but not after treatment of the patients with STI571. We conclude that inhibition of BCR-ABL1 reconstitutes selection for leukemia cells expressing a functional (pre–) B cell receptor.

Differential expression of SAP and EAT-2-binding leukocyte cell-surface molecules CD84, CD150 (SLAM), CD229 (Ly9) and CD244 (2B4)

The CD150 (SLAM) family consists of nine leukocyte cell-surface proteins involved in lymphocyte activation that belong to the immunoglobulin (Ig) superfamily. Six members of this family--CD84, CD150 (SLAM), CD229 (Ly9), CD244 (2B4), NTB-A, and CS1--associate with adapter proteins--SLAM-associated protein (SAP) and EAT-2. SAP is a short intracellular molecule that is mutated in humans with X-linked lymphoproliferative disease. Flow cytometric analysis of the expression of CD84, CD150, CD229, and CD244 cell-surface receptors on several leukocyte and lymphocyte subsets was performed. CD84 and CD150 were present on thymocytes, mature T cells and antigen-presenting cells. The expression of CD84 and CD150 was high on memory T cells. CD150 expression was strongly up-regulated after cell activation. In contrast to CD84, CD150 was absent on resting monocytes and immature dendritic cells (DCs). CD229 presented a pattern of expression restricted to lymphocytes. CD244 was preferentially expressed on natural killer cells, CD8(+) effector cells, resting monocytes, basophils, and eosinophils. We describe a broader distribution of CD84, CD150, CD229, and CD244 than previously reported and show that they are differentially expressed on hematopoietic cells. The heterogeneous expression of these receptors indicates that these molecules may play non-redundant functions in the regulation of both innate and adaptive immune responses.

Synergistic B Cell Activation by CD40 and the B Cell Antigen Receptor

Optimal activation of B-lymphocytes depends both upon expression of various cell surface receptors and adequate integration of signaling pathways. This requires signals generated upon recognition of antigen by the B lymphocyte antigen receptor (BCR) as well as additional signals provided by cognate interaction with T helper cells, including the CD40-CD154 interaction. Engagement of both the BCR and CD40 results in synergistic activation of B cells. Previous studies identified tumor necrosis factor receptor-associated factor (TRAF)-2 and TRAF3 in the CD40-signaling pathway together with BCR-activated protein kinase D (PKD) as important cooperative factors in this synergy. To better understand the role of these factors in bridging the BCR and CD40 signaling pathways, BCR signal regulation of TRAF function was examined. Results show that phosphorylation of TRAF2 is increased upon BCR but not CD40 engagement and that of the potentially phosphorylated residues of TRAF2, tyrosine 484 is crucial for BCR-CD40 synergy. Additionally, wild type or constitutively active Bruton's tyrosine kinase (Btk) enhanced, whereas the xid mutant form of Btk prevented, BCR-CD40 synergy. These effects were dependent upon TRAF2 and PKD activity. These findings suggest a model in which Btk contributes to the enhancement of the CD40 response by TRAF2 in a PKD-dependent manner.

The early transcription factor GATA-2 is expressed in classical Hodgkin's lymphoma

Hodgkin/Reed-Sternberg (HRS) cells of classical Hodgkin's lymphoma (cHL) are thought to be derived from germinal centre B-cells in almost all cases. However, expression profiling has revealed that HRS cells do not show a germinal centre B-cell-like phenotype. Although the nature of this aberrant phenotype and the underlying molecular mechanisms remain largely unknown, it has been reported that the activity of NOTCH1 plays an important role in the growth and survival of HRS cells. In some leukaemic cell lines, the effect of Notch signalling is mediated by the early transcription factor GATA-2. This and the fact that HRS cells lack expression of PU.1, which can repress Gata-2, led to an investigation of GATA-2 expression in HRS cells. GATA-2 expression was found in all the cHL-derived cell lines studied, but not in a Burkitt lymphoma-derived cell line. In addition, 50% of biopsies from patients with cHL contained GATA-2-expressing HRS cells. In contrast, neither normal germinal centre B-cells nor malignant cells of nodular lymphocyte-predominant Hodgkin's lymphoma, Burkitt lymphoma or diffuse large B-cell lymphoma expressed GATA-2. Thus, GATA-2 expression was found specifically in HRS cells of cHL, suggesting that GATA-2 is important in establishing the abnormal B-cell phenotype of HRS cells.

The SAP and SLAM families in immune responses and X-linked lymphoproliferative disease

SAP (signalling lymphocytic activation molecule (SLAM)-associated protein) is a T- and natural killer (NK)-cell-specific protein containing a single SH2 domain encoded by a gene that is defective or absent in patients with X-linked lymphoproliferative syndrome (XLP). The SH2 domain of SAP binds with high affinity to the cytoplasmic tail of the haematopoietic cell-surface glycoprotein SLAM and five related receptors. SAP regulates signal transduction of the SLAM-family receptors by recruiting SRC kinases. Similarly, the SAP-related proteins EAT2A and EAT2B are thought to control signal transduction that is initiated by SLAM-related receptors in professional antigen-presenting cells. In this review, we discuss recent findings on the structure and function of proteins of the SAP and SLAM families.

Paired activating and inhibitory immunoglobulin-like receptors, MAIR-I and MAIR-II, regulate mast cell and macrophage activation

Immune responses are regulated by opposing positive and negative signals triggered by the interaction of activating and inhibitory cell surface receptors with their ligands. Here, we describe novel paired activating and inhibitory immunoglobulin-like receptors, designated myeloid-associated immunoglobulin-like receptor (MAIR) I and MAIR-II, whose extracellular domains are highly conserved by each other. MAIR-I, expressed on the majority of myeloid cells, including macrophages, granulocytes, mast cells, and dendritic cells, contains the tyrosine-based sorting motif and the immunoreceptor tyrosine-based inhibitory motif-like sequences in the cytoplasmic domain and mediates endocytosis of the receptor and inhibition of IgE-mediated degranulation from mast cells. On the other hand, MAIR-II, expressed on subsets of peritoneal macrophages and B cells, associates with the immunoreceptor tyrosine-based activation motif-bearing adaptor DAP12 and stimulates proinflammatory cytokines and chemokine secretions from macrophages. Thus, MAIR-I and MAIR-II play important regulatory roles in cell signaling and immune responses.

Gene expression profiling of plasma cells and plasmablasts: toward a better understanding of the late stages of B-cell differentiation

Plasma cells (PCs), the end point of B-cell differentiation, are a heterogeneous cell compartment comprising several cell subsets from short-lived highly proliferative plasmablasts to long-lived nondividing fully mature PCs. Whereas the major transcription factors driving the differentiation of B cells to PCs were recently identified, the subtle genetic changes that underlie the transition from plasmablasts to mature PCs are poorly understood. We recently described an in vitro model making it possible to obtain a large number of cells with the morphologic, phenotypic, and functional characteristics of normal polyclonal plasmablastic cells (PPCs). Using Affymetrix microarrays we compared the gene expression profiles of these PPCs with those of mature PCs isolated from tonsils (TPCs) and bone marrow (BMPCs), and with those of B cells purified from peripheral blood (PBB cells) and tonsils (TBCs). Unsupervised principal component analysis clearly distinguished the 5 cell populations on the basis of their differentiation and proliferation status. Detailed statistical analysis allowed the identification of 85 PC genes and 40 B-cell genes, overexpressed, respectively, in the 3 PC subsets or in the 2 B-cell subsets. In addition, several signaling molecules and antiapoptotic proteins were found to be induced in BMPCs compared with PPCs and could be involved in the accumulation and prolonged survival of BMPCs in close contact with specialized stromal microenvironment. These data should help to better understand the molecular events that regulate commitment to a PC fate, mediate PC maintenance in survival niches, and could facilitate PC immortalization in plasma cell dyscrasias.

Epstein-Barr virus and the B cell: a secret romance

Infection by the Epstein-Barr virus (EBV) in immunocompetent individuals seems mainly confined to antigen-experienced memory B cells. However, a recent report shows that EBV(+) post-transplant lymphoproliferative disease might arise not only from memory B cells but also from nai;ve and germinal center (GC) B cells. Intriguingly, some of the EBV-positive B-cell clones seem to carry non-functional Ig-V-region genes as a result of deleterious somatic mutations acquired during the GC reaction. Given that such GC B cells are destined to die by apoptosis in the absence of EBV, these findings suggest that transformation by EBV might bypass negative selection of B cells within GCs.

T lymphoid differentiation in human bone marrow

The unique role of the thymus in the development of T cells was established >4 decades ago. To elucidate how uncommitted lymphoid progenitor cells are instructed to migrate from bone marrow to the thymus to undergo T lymphoid differentiation, we generated and analyzed a genome-wide gene expression profile of CD7+ CD10+ human bone marrow T cell lineage precursors (TLPs) by using the serial analysis of gene expression technique. Unexpectedly, the serial analysis of gene expression profile identified a high number of (pre-) T cell receptor antigen (TCR)-related transcripts in bone marrow TLPs. To determine the configuration of the TCRbeta locus in these cells at a quantitative level, we sorted and analyzed bone marrow TLPs from five donors by single-cell PCR. Similar proportions of TLPs harbored TCRbeta germ-line alleles, D-J, or V-DJ gene rearrangements. Thus, bone marrow TLPs are heterogenous with respect to TCRbeta rearrangement status, suggesting an active recombination machinery that is consistent with the expression of RAG1, RAG2, and TdT in this population. As a hallmark of ongoing TCRbeta V-DJ rearrangement, we could amplify broken-ended recombination-signal sequence DNA intermediates from bone marrow TLPs, but not from mature T cells by ligation-mediated PCR. Approximately half of the TCRbeta rearrangements were compatible with the expression of a functional pre-TCR, which is in agreement with surface expression of pre-Talpha on bone marrow TLPs as shown by confocal laser microscopy and flow cytometry. At a frequency <0.5% of mononucleated cells in human bone marrow, this population is rare, yet it exemplifies T lymphoid differentiation in the human already before immigration into the thymus.

Transcriptional analysis of the B cell germinal center reaction

The germinal center (GC) reaction is crucial for T cell-dependent immune responses and is targeted by B cell lymphomagenesis. Here we analyzed the transcriptional changes that occur in B cells during GC transit (naive B cells --> centroblasts --> centrocytes --> memory B cells) by gene expression profiling. Naive B cells, characterized by the expression of cell cycle-inhibitory and antiapoptotic genes, become centroblasts by inducing an atypical proliferation program lacking c-Myc expression, switching to a proapoptotic program, and down-regulating cytokine, chemokine, and adhesion receptors. The transition from GC to memory cells is characterized by a return to a phenotype similar to that of naive cells except for an apoptotic program primed for both death and survival and for changes in the expression of cell surface receptors including IL-2 receptor beta. These results provide insights into the dynamics of the GC reaction and represent the basis for the analysis of B cell malignancies.