Human activation-induced cytidine deaminase is induced by IL-4 and negatively regulated by CD45: implication of CD45 as a Janus kinase phosphatase in antibody diversification

Th17-inducing dendritic cell vaccines stimulate effective CD4 T cell-dependent antitumor immunity in ovarian cancer that overcomes resistance to immune checkpoint blockade

BACKGROUND

Ovarian cancer (OC), a highly lethal cancer in women, has a 48% 5-year overall survival rate. Prior studies link the presence of IL-17 and Th17 T cells in the tumor microenvironment to improved survival in OC patients. To determine if Th17-inducing vaccines are therapeutically effective in OC, we created a murine model of Th17-inducing dendritic cell (DC) (Th17-DC) vaccination generated by stimulating IL-15 while blocking p38 MAPK in bone marrow-derived DCs, followed by antigen pulsing.

METHODS

ID8 tumor cells were injected intraperitoneally into mice. Mice were treated with Th17-DC or conventional DC (cDC) vaccine alone or with immune checkpoint blockade (ICB). Systemic immunity, tumor associated immunity, tumor size and survival were examined using a variety of experimental strategies.

RESULTS

Th17-DC vaccines increased Th17 T cells in the tumor microenvironment, reshaped the myeloid microenvironment, and improved mouse survival compared with cDC vaccines. ICB had limited efficacy in OC, but Th17-inducing DC vaccination sensitized it to anti-PD-1 ICB, resulting in durable progression-free survival by overcoming IL-10-mediated resistance. Th17-DC vaccine efficacy, alone or with ICB, was mediated by CD4 T cells, but not CD8 T cells.

CONCLUSIONS

These findings emphasize using biologically relevant immune modifiers, like Th17-DC vaccines, in OC treatment to reshape the tumor microenvironment and enhance clinical responses to ICB therapy.

The landscape of T and B lymphocytes interaction and synergistic effects of Th1 and Th2 type response in the involved tissue of IgG4-RD revealed by single cell transcriptome analysis

OBJECTIVES

To investigate the landscape of T-B cell interaction, immune receptor profiles and effects of different types of immune responses in the involved tissues of IgG4-RD.

METHODS

Single cell RNA sequencing, bulk sample RNA sequencing, immune receptor repertoire analysis (both BCR and TCR), multi-color flow cytometry, and in-vitro assays with model cells (e.g. EBV-immortalized B cells from IgG4-RD patient) and histologic methods were applied to investigate the immunopathological features of IgG4-RD from multiple aspects.

RESULTS

Ectopic germinal center formation was observed in IgG4-RD patients at advanced disease stage, and a large part of B cells in involved tissue were germinal center B cell-like. Germinal center reaction in IgG4-RD led to the irregularities of both TCR and BCR clones in the involved tissues, and limited clonal overlaps among different samples. Enhanced Th1- and Th2-type responses were observed in involved tissues of IgG4-RD and patients with both increased Th1- and Th2-type response related cell subsets possessed more severe inflammatory indices. Analyses to the origin of IGHG4 transcripts in IgG4-RD indicated that IgG4 could be switched from IgM directly, or from other IgG subclasses. In vitro assays with EBV-immortalized B cells, fibroblasts and epithelial cells revealed the effects of Th1-type and Th2-type responses on germinal center reaction, ectopic expression of MHC-II molecules, and formation of tertiary lymphoid structures.

CONCLUSIONS

Synergistic effects of Th1- and Th2-type responses were involved in the pathogenesis of IgG4-RD via their influences on both acute inflammatory processes and the chronicity and complexity of IgG4-RD.

Development of Tbet- and CD11c-expressing B cells in a viral infection requires T follicular helper cells outside of germinal centers

Tbet+CD11c+ B cells arise during type 1 pathogen challenge, aging, and autoimmunity in mice and humans. Here, we examined the developmental requirements of this B cell subset. In acute infection, T follicular helper (Tfh) cells, but not Th1 cells, drove Tbet+CD11c+ B cell generation through proximal delivery of help. Tbet+CD11c+ B cells developed prior to germinal center (GC) formation, exhibiting phenotypic and transcriptional profiles distinct from GC B cells. Fate tracking revealed that most Tbet+CD11c+ B cells developed independently of GC entry and cell-intrinsic Bcl6 expression. Tbet+CD11c+ and GC B cells exhibited minimal repertoire overlap, indicating distinct developmental pathways. As the infection resolved, Tbet+CD11c+ B cells localized to the marginal zone where splenic retention depended on integrins LFA-1 and VLA-4, forming a competitive memory subset that contributed to antibody production and secondary GC seeding upon rechallenge. Therefore, Tbet+CD11c+ B cells comprise a GC-independent memory subset capable of rapid and robust recall responses.

AID in Chronic Lymphocytic Leukemia: Induction and Action During Disease Progression

The enzyme activation-induced cytidine deaminase (AID) initiates somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin (Ig) genes, critical actions for an effective adaptive immune response. However, in addition to the benefits generated by its physiological roles, AID is an etiological factor for the development of human and murine leukemias and lymphomas. This review highlights the pathological role of AID and the consequences of its actions on the development, progression, and therapeutic refractoriness of chronic lymphocytic leukemia (CLL) as a model disease for mature lymphoid malignancies. First, we summarize pertinent aspects of the expression and function of AID in normal B lymphocytes. Then, we assess putative causes for AID expression in leukemic cells emphasizing the role of an activated microenvironment. Thirdly, we discuss the role of AID in lymphomagenesis, in light of recent data obtained by NGS analyses on the genomic landscape of leukemia and lymphomas, concentrating on the frequency of AID signatures in these cancers and correlating previously described tumor-gene drivers with the presence of AID off-target mutations. Finally, we discuss how these changes could affect tumor suppressor and proto-oncogene targets and how they could be associated with disease progression. Collectively, we hope that these sections will help to better understand the complex paradox between the physiological role of AID in adaptive immunity and its potential causative activity in B-cell malignancies.

PDGFA-associated protein 1 protects mature B lymphocytes from stress-induced cell death and promotes antibody gene diversification

The establishment of protective humoral immunity is dependent on the ability of mature B cells to undergo antibody gene diversification while adjusting to the physiological stressors induced by activation with the antigen. Mature B cells diversify their antibody genes by class switch recombination (CSR) and somatic hypermutation (SHM), which are both dependent on efficient induction of activation-induced cytidine deaminase (AID). Here, we identified PDGFA-associated protein 1 (Pdap1) as an essential regulator of cellular homeostasis in mature B cells. Pdap1 deficiency leads to sustained expression of the integrated stress response (ISR) effector activating transcription factor 4 (Atf4) and induction of the ISR transcriptional program, increased cell death, and defective AID expression. As a consequence, loss of Pdap1 reduces germinal center B cell formation and impairs CSR and SHM. Thus, Pdap1 protects mature B cells against chronic ISR activation and ensures efficient antibody diversification by promoting their survival and optimal function.

Cytidine deamination-induced perpetual immunity to SAR-CoV-2 infection is a potential new therapeutic target

As the world is racing to develop perpetual immunity to the SARS-CoV-2 virus. The emergence of new viral strains, together with vaccination and reinfections, are all contributing to a long-term immunity against the deadly virus that has taken over the world since its introduction to humans in late December 2019. The discovery that more than 95 percent of people who recovered from COVID-19 had long-lasting immunity and that asymptomatic people have a different immune response to SARS-CoV-2 than symptomatic people has shifted attention to how our immune system initiates such diverse responses. These findings have provided reason to believe that SARS-CoV-2 days are numbered. Hundreds of research papers have been published on the causes of long-lasting immune responses and variations in the numbers of different immune cell types in COVID 19 survivors, but the main reason of these differences has still not been adequately identified. In this article, we focus on the activation-induced cytidine deaminase (AID), which initiates molecular processes that allow our immune system to generate antibodies against SARS-CoV-2. To establish lasting immunity to SARS-CoV-2, we suggest that AID could be the key to unlocking it.

Ibrutinib therapy downregulates AID enzyme and proliferative fractions in chronic lymphocytic leukemia

Activation-induced cytidine deaminase (AID) initiates somatic hypermutation and class switch recombination of the immunoglobulin genes. As a trade-off for its physiological function, AID also contributes to tumor development through its mutagenic activity. In chronic lymphocytic leukemia (CLL), AID is overexpressed in the proliferative fractions (PFs) of the malignant B lymphocytes, and its anomalous expression has been associated with a clinical poor outcome. Recent preclinical data suggested that ibrutinib and idelalisib, 2 clinically approved kinase inhibitors, increase AID expression and genomic instability in normal and neoplastic B cells. These results raise concerns about a potential mutagenic risk in patients receiving long-term therapy. To corroborate these findings in the clinical setting, we analyzed AID expression and PFs in a CLL cohort before and during ibrutinib treatment. We found that ibrutinib decreases the CLL PFs and, interestingly, also reduces AID expression, which correlates with dampened AKT and Janus Kinase 1 signaling. Moreover, although ibrutinib increases AID expression in a CLL cell line, it is unable to do so in primary CLL samples. Our results uncover a differential response to ibrutinib between cell lines and the CLL clone and imply that ibrutinib could differ from idelalisib in their potential to induce AID in treated patients. Possible reasons for the discrepancy between preclinical and clinical findings, and their effect on treatment safety, are discussed.

Factors regulating immunoglobulin production by normal and disease-associated plasma cells

Immunoglobulins are molecules produced by activated B cells and plasma cells in response to exposure to antigens. Upon antigen exposure, these molecules are secreted allowing the immune system to recognize and effectively respond to a myriad of pathogens. Immunoglobulin or antibody secreting cells are the mature form of B lymphocytes, which during their development undergo gene rearrangements and selection in the bone marrow ultimately leading to the generation of B cells, each expressing a single antigen-specific receptor/immunoglobulin molecule. Each individual immunoglobulin molecule has an affinity for a unique motif, or epitope, found on a given antigen. When presented with an antigen, activated B cells differentiate into either plasma cells (which secrete large amounts of antibody that is specific for the inducing antigen), or memory B cells (which are long-lived and elicit a stronger and faster response if the host is re-exposed to the same antigen). The secreted form of immunoglobulin, when bound to an antigen, serves as an effector molecule that directs other cells of the immune system to facilitate the neutralization of soluble antigen or the eradication of the antigen-expressing pathogen. This review will focus on the regulation of secreted immunoglobulin by long-lived normal or disease-associated plasma cells. Specifically, the focus will be on signaling and transcriptional events that regulate the development and homeostasis of long-lived immunoglobulin secreting plasma cells.

Local increase in IgE and class switch recombination to IgE in nasal polyps in chronic rhinosinusitis

BACKGROUND

Chronic rhinosinusitis with nasal polyps is generally characterized by local Th2 inflammation and is categorized into two subtypes in Japan: eosinophilic chronic rhinosinusitis (similar to chronic rhinosinusitis with nasal polyps in western countries) and non-eosinophilic chronic rhinosinusitis (characterized by Th1-dominant inflammation).

OBJECTIVE

To investigate local IgE production and class switch recombination to IgE in these two subtypes of chronic rhinosinusitis with nasal polyps.

METHODS

The identity of IgE-positive cells was determined using double-immunofluorescent staining for IgE and cell-type-specific molecular markers. To investigate the local class switch recombination to IgE and IgE synthesis in the mucosa, we performed real-time polymerase chain reaction to examine the mRNA expression of Th2 cytokines and class-switch-related molecules, including IL-4, IL-5, IL-13, ε germline gene transcripts, IgE mature transcript, IgG mature transcript, RAG1, RAG2 and activation-induced cytidine deaminase in eosinophilic polyps, non-eosinophilic polyps and controls.

RESULTS

The concentrations of total IgE and number of IgE-positive cells were significantly higher in the eosinophilic polyps compared with control and non-eosinophilic polyps. IgE-positive cells were predominantly mast cells in eosinophilic polyps and significantly correlated with the number of FcεR1-positive cells in the subepithelial layer. IL-5 and IL-13 mRNA and ε germline gene transcripts expression levels were significantly higher in eosinophilic polyps compared with control and non-eosinophilic polyps. In contrast, the number of plasma cells and the expression of IgG mature transcripts were increased in non-eosinophilic polyps compared with eosinophilic polyps. RAG2 mRNA was significantly increased in both eosinophilic and non-eosinophilic polyps compared with control mucosa.

CONCLUSION AND CLINICAL RELEVANCE

The current study suggests local class switching to IgE, production of IgE and IgE localization to the surface of mast cells in eosinophilic chronic rhinosinusitis in the Japanese population. The difference in the IgE-related profiles between eosinophilic chronic rhinosinusitis and non-eosinophilic chronic rhinosinusitis suggests heterogeneity in the pathogenesis of chronic rhinosinusitis with nasal polyps.

Lymphocyte-based model systems for allergy research: a historic overview

During the last decades, a multitude of studies applying distinct in vitro and in vivo model systems have contributed greatly to our better understanding of the initiation and regulation of inflammatory processes leading to allergic diseases. Over the years, it has become evident that among lymphocytes, not only IgE-producing B cells and allergy-orchestrating CD4(+) helper cells but also cytotoxic CD8(+) T cells, γδ-T cells and innate lymphoid cells, as well as regulatory lymphocytes, might critically shape the immune response towards usually innocuous allergens. In this review, we provide a historic overview of pioneering work leading to the establishment of important lymphocyte-based model systems for allergy research. Moreover, we contrast the original findings with our currently more refined knowledge to appreciate the actual validity of the respective models and to reassess the conclusions obtained from them. Conflicting studies and interpretations are identified and discussed. The tables are intended to provide an easy overview of the field not only for scientists newly entering the field but also for the broader readership interested in updating their knowledge. Along those lines, herein we discuss in vitro and in vivo approaches to the investigation of lymphocyte effector cell activation, polarization and regulation, and describe depletion and adoptive transfer models along with gene knockout and transgenic (tg) methodologies. In addition, novel attempts to establish humanized T cell antigen receptor tg mouse models for allergy research are described and discussed.

A role for host activation-induced cytidine deaminase in innate immune defense against KSHV

Activation-induced cytidine deaminase (AID) is specifically induced in germinal center B cells to carry out somatic hypermutation and class-switch recombination, two processes responsible for antibody diversification. Because of its mutagenic potential, AID expression and activity are tightly regulated to minimize unwanted DNA damage. Surprisingly, AID expression has been observed ectopically during pathogenic infections. However, the function of AID outside of the germinal centers remains largely uncharacterized. In this study, we demonstrate that infection of human primary naïve B cells with Kaposi's sarcoma-associated herpesvirus (KSHV) rapidly induces AID expression in a cell intrinsic manner. We find that infected cells are marked for elimination by Natural Killer cells through upregulation of NKG2D ligands via the DNA damage pathway, a pathway triggered by AID. Moreover, without having a measurable effect on KSHV latency, AID impinges directly on the viral fitness by inhibiting lytic reactivation and reducing infectivity of KSHV virions. Importantly, we uncover two KSHV-encoded microRNAs that directly regulate AID abundance, further reinforcing the role for AID in the antiviral response. Together our findings reveal additional functions for AID in innate immune defense against KSHV with implications for a broader involvement in innate immunity to other pathogens.

Immune responses to pathogens rely heavily on the ability of B cells to generate a unique set of antibodies that can bind and eliminate the pathogen. Activation-induced cytidine deaminase (AID) is the enzyme specifically upregulated in activated B cells to diversify the antibody repertoire by introducing mutations within the antibody coding genes. Curiously, AID expression has been observed outside of activated B cells upon infection with a number of viral and bacterial pathogens. However, in such cases AID function is poorly characterized and often deemed inappropriate since its mutagenic activity can put the cell at risk for oncogenic transformation. In this study, we investigate the expression of AID in response to infection with an oncogenic human pathogen Kaposi's sarcoma-associated herpesvirus (KSHV) and the antibody-independent immune defense it exerts. We show that AID marks infected cells for elimination by natural killer (NK) cells and directly impinges on viral fitness. Furthermore, we uncover novel viral immune evasion strategies employed by KSHV to counteract AID. Together, our findings demonstrate a protective role for AID in the response to infection with an oncogenic virus such as KSHV and suggest that AID may actually limit transformation rather than serve as its culprit.

Activation-induced cytidine deaminase (AID) linking immunity, chronic inflammation, and cancer

Activation-induced cytidine deaminase (AID) is critically involved in class switch recombination and somatic hypermutation of Ig loci resulting in diversification of antibodies repertoire and production of high-affinity antibodies and as such represents a physiological tool to introduce DNA alterations. These processes take place within germinal centers of secondary lymphoid organs. Under physiological conditions, AID is expressed predominantly in activated B lymphocytes. Because of the mutagenic and recombinogenic potential of AID, its expression and activity is tightly regulated on different levels to minimize the risk of unwanted DNA damage. However, chronic inflammation and, probably, combination of other not-yet-identified factors are able to create a microenvironment sufficient for triggering an aberrant AID expression in B cells and, importantly, in non-B-cell background. Under these circumstances, AID may target also non-Ig genes, including cancer-related genes as oncogenes, tumor suppressor genes, and genomic stability genes, and modulate both genetic and epigenetic information. Despite ongoing progress, the complete understanding of fundamental aspects is still lacking as (1) what are the crucial factors triggering an aberrant AID expression/activity including the impact of Th2-driven inflammation and (2) to what extent may aberrant AID in human non-B cells lead to abnormal cell state associated with an increased rate of genomic alterations as point mutations, small insertions or deletions, and/or recurrent chromosomal translocations during solid tumor development and progression.

Activation-induced cytidine deaminase in antibody diversification and chromosome translocation

DNA damage, rearrangement, and mutation of the human genome are the basis of carcinogenesis and thought to be avoided at all costs. An exception is the adaptive immune system where lymphocytes utilize programmed DNA damage to effect antigen receptor diversification. Both B and T lymphocytes diversify their antigen receptors through RAG1/2 mediated recombination, but B cells undergo two additional processes--somatic hypermutation (SHM) and class-switch recombination (CSR), both initiated by activation-induced cytidine deaminase (AID). AID deaminates cytidines in DNA resulting in U:G mismatches that are processed into point mutations in SHM or double-strand breaks in CSR. Although AID activity is focused at Immunoglobulin (Ig) gene loci, it also targets a wide array of non-Ig genes including oncogenes associated with lymphomas. Here, we review the molecular basis of AID regulation, targeting, and initiation of CSR and SHM, as well as AID's role in generating chromosome translocations that contribute to lymphomagenesis.

Activation of PI3K/AKT and ERK MAPK signal pathways is required for the induction of lytic cycle replication of Kaposi's sarcoma-associated herpesvirus by herpes simplex virus type 1

Background

Kaposi's sarcoma-associated herpesvirus (KSHV) is causally linked to several acquired immunodeficiency syndrome-related malignancies, including Kaposi's sarcoma (KS), primary effusion lymphoma (PEL) and a subset of multicentric Castleman's disease. Regulation of viral lytic replication is critical to the initiation and progression of KS. Recently, we reported that herpes simplex virus type 1 (HSV-1) was an important cofactor that activated lytic cycle replication of KSHV. Here, we further investigated the possible signal pathways involved in HSV-1-induced reactivation of KSHV.

Results

By transfecting a series of dominant negative mutants and protein expressing constructs and using pharmacologic inhibitors, we found that either Janus kinase 1 (JAK1)/signal transducer and activator of transcription 3 (STAT3) or JAK1/STAT6 signaling failed to regulate HSV-1-induced KSHV replication. However, HSV-1 infection of BCBL-1 cells activated phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB, also called AKT) pathway and inactivated phosphatase and tensin homologue deleted on chromosome ten (PTEN) and glycogen synthase kinase-3β (GSK-3β). PTEN/PI3K/AKT/GSK-3β pathway was found to be involved in HSV-1-induced KSHV reactivation. Additionally, extracellular signal-regulated protein kinase (ERK) mitogen-activated protein kinase (MAPK) pathway also partially contributed to HSV-1-induced KSHV replication.

Conclusions

HSV-1 infection stimulated PI3K/AKT and ERK MAPK signaling pathways that in turn contributed to KSHV reactivation, which provided further insights into the molecular mechanism controlling KSHV lytic replication, particularly in the context of HSV-1 and KSHV co-infection.

Local production of IgE in the respiratory mucosa and the concept of entopy: does allergy exist in nonallergic rhinitis?

OBJECTIVE

To review research regarding locally produced IgE and its impact on patients with chronic rhinitis.

DATA SOURCES

PubMed search with the following keywords: entopy, local IgE, nonallergic rhinitis, idiopathic rhinitis, vasomotor rhinitis, and allergic rhinitis.

STUDY SELECTION

Articles were selected based on their relevance to entopy and locally produced IgE and its clinical effect and relationship to idiopathic rhinitis (IR).

RESULTS

Local IgE has been found in a variety of tissues, including nasal and bronchial mucosa. IgE is produced in these local tissues and not simply the product of migration to the tissue from regional lymphoid tissue or blood. Local IgE has been identified in most of both atopic and nonatopic asthmatic patients and allergic rhinitis patients. Up to 40% of patients with IR and a positive nasal provocation test result have evidence of locally produced IgE, which has been coined entopy. Although patients with allergic rhinitis and IR show similar inflammatory patterns with increased activated mast cells, eosinophils, and T-cell subsets in some studies, other studies on IR patients are conflicting with regard to both inflammation and allergen-specific nasal provocation test results.

CONCLUSION

The concept of local allergy in IR patients is both intriguing and controversial. Studies have reported conflicting results, and currently there is no single best test to evaluate for entopy. It is known that there are a large number of IR patients for whom current treatment regimens are suboptimal. Therefore, further research elucidating the mechanisms of IR and the concept of localized IgE are needed to optimally diagnose this condition and treat this group of patients.

The Biology of CD45 and its Use as a Therapeutic Target

All mature hemopoietic lineage cells, with exclusion of platelets and mature erythrocytes, share the surface expression of a transmembrane phosphatase, the CD45 molecule. It is also present on hemopoietic stem cells and most leukemic clones and therefore presents as an appropriate target for immunotherapy with anti-CD45 antibodies. This short review details the biology of CD45 and its recent targeting for both treatment of malignant disorders and tolerance induction. In particular, the question of potential stem cell depletion for induction of central tolerance or depletion of malignant hemopoietic cells is addressed. Mechanisms underlying the effects downstream of CD45 binding to the cell surface are discussed.

Cytokine-secreting follicular T cells shape the antibody repertoire

High-affinity antibodies are critical for host protection and underlie successful vaccines. The generation of such antibodies requires T cell-dependent help, which mediates germinal center reactions in which mutation and selection of B cells occurs. Using an interleukin 4-reporter system, we show here that CD4(+) follicular helper T cells constituted essentially all of the cytokine-secreting T cells in lymph nodes and were functionally distinct from T cells secreting the same cytokine in peripheral tissues. Follicular helper T cells with different cytokine profiles could be isolated as conjugates with B cells undergoing cytokine-specific immunoglobulin class switching with evidence of somatic hypermutation. Our findings support a model in which B cells compete for cytokines produced by follicular helper T cells that shape the affinity and isotype of the antibody response.

Key developmental transitions in human germinal center B cells are revealed by differential CD45RB expression

We previously reported that RO(+) expression correlated with increased mutation, activation, and selection among human germinal center (GC) B cells. Here, we subdivided human tonsillar B cells, including IgD(-)CD38(+) GC B cells, into different fractions based on RB expression. Although each subset contained RB(+) cells, when used as an intrasubset marker, differential RB expression effectively discriminated between phenotypically distinct cells. For example, RB(+) GC B cells were enriched for activated cells with lower AID expression. RB inversely correlated with mutation frequency, demonstrating a key difference between RB- and RO-expressing GC B cells. Reduced RB expression during the transition from pre-GC (IgM(+)IgD(+)CD38(+)CD27(-)) to GCB cells was followed by a dramatic increase during the GC-to-plasmablast (IgD(-)CD38(++)CD27(+)) and memory (IgD(-)CD38(-)CD27(+)) transition. Interestingly, RB(+) GC B cells showed increased signs of terminal differentiation toward CD27(+) post-GC early plasmablast (increased CD38 and RO) or early memory (decreased CD38 and RO) B cells. We propose that as in T cells, differential RB expression directly correlates with development- and function-based transitions in tonsillar B cells. Application of this RB:RO system should advance our understanding of normal B-cell development and facilitate the isolation of more discrete B-cell populations with potentially different propensities in disease pathogenesis.

Aging down-regulates the transcription factor E2A, activation-induced cytidine deaminase, and Ig class switch in human B cells

Elderly humans have compromised humoral and cellular immune responses, which lead to reduced protection to infectious agents and to vaccines. Currently, available vaccines suboptimally protect the elderly population. The capacity to class switch the Ig H chain is critical to the effectiveness of humoral immune responses in mice and humans. We have previously shown in mice that the E2A-encoded transcription factor E47, which regulates many B cell functions, is down-regulated in old splenic B cells. This leads to a reduction in the activation-induced cytidine deaminase (AID), which is known to induce class switch recombination and Ig somatic hypermutation. The old activated murine B cells also have less AID and less switched Abs. We have extended our study here to investigate whether aging also affects Ab production and E47 and AID expression in B cells isolated from the peripheral blood of human subjects (18-86 years). Our results obtained with activated CD19(+) B cells show that the expression of E47, AID, and Iggamma1 circle transcripts progressively decrease with age. We also show an age-related decline in the percentage of switch memory B cells (IgG(+)/IgA(+)), an increase in that of naive B cells (IgG(-)/IgA(-)/CD27(-)) for most individuals, and no decrease in that of IgM memory cells in peripheral blood, consistent with our data on the decrease seen in class switch recombination in vitro. Our results provide a possible molecular mechanism for a B cell intrinsic defect in the humoral immune response with aging and suggest avenues for improvement of vaccine response in elderly humans.

B-cell receptor activation inhibits AID expression through calmodulin inhibition of E-proteins

Upon encountering antigens, B-lymphocytes can adapt to produce a highly specific and potent antibody response. Somatic hypermutation, which introduces point mutations in the variable regions of antibody genes, can increase the affinity for antigen, and antibody effector functions can be altered by class switch recombination (CSR), which changes the expressed constant region exons. Activation-induced cytidine deaminase (AID) is the mutagenic antibody diversification enzyme that is essential for both somatic hypermutation and CSR. The mutagenic AID enzyme has to be tightly controlled. Here, we show that engagement of the membrane-bound antibodies of the B-cell receptor (BCR), which signals that good antibody affinity has been reached, inhibits AID gene expression and that calcium (Ca(2+)) signaling is essential for this inhibition. Moreover, we show that overexpression of the Ca(2+) sensor protein calmodulin inhibits AID gene expression, and that the transcription factor E2A is required for regulation of the AID gene by the BCR. E2A mutated in the binding site for calmodulin, and thus showing calmodulin-resistant DNA binding, makes AID expression resistant to the inhibition through BCR activation. Thus, BCR activation inhibits AID gene expression through Ca(2+)/calmodulin inhibition of E2A.

Activation-induced deaminase-mediated class switch recombination is blocked by anti-IgM signaling in a phosphatidylinositol 3-kinase-dependent fashion

Activation-induced deaminase (AID) is expressed in activated B lymphocytes and initiates somatic hypermutation and class switch recombination. To determine if different stimuli affect the expression and function of AID, we monitored AID activity in murine B cells stimulated ex vivo with various ligands. AID was rapidly expressed at both the RNA and protein levels following stimulation with LPS, LPS plus IL-4, and anti-CD40 plus IL-4, but was delayed after stimulation with anti-IgM plus IL-4. By day 4, AID was expressed in all groups; however, cells stimulated with anti-IgM plus IL-4 did not undergo switch recombination. These cells expressed normal levels of gamma 1 germline transcripts, implying that the gamma 1 switch region was accessible. Furthermore, switching was suppressed by the addition of anti-IgM to cells stimulated with LPS plus IL-4 or anti-CD40 plus IL-4, even though AID was expressed. The lack of class switch recombination could be reversed by inhibition of phosphatidylinositol 3-kinase (PI3K). This suggests that activation through the B cell receptor induces PI3K, which interferes with the function of AID.

Primary testicular diffuse large B-cell lymphomas have activated B-cell-like subtype characteristics

Diffuse large B-cell lymphomas (DLBCLs) constitute a heterogeneous group of lymphomas in which germinal centre B-cell-like and activated B-cell-like subtypes can be discerned based on pathology, clinical presentation, and gene expression patterns. Testicular DLBCLs form an immune-privileged site-related subgroup of DLBCLs with an unfavourable prognosis. In the present study, cDNA microarray analysis, immunohistochemistry for CD10, Bcl6 and MUM1, and somatic hypermutation analysis of the immunoglobulin heavy chain gene rearrangements were used to determine the subtype of primary testicular DLBCL. Immunohistochemistry revealed 14/22 testicular DLBCLs with an activated B-cell-like immunophenotype and 8/22 with an ambiguous immunophenotype co-expressing CD10 and high levels of MUM1. cDNA microarray analysis of these 22 and four additional cases showed a uniform activated B-cell-like gene expression pattern for both immunophenotypes. Somatic hypermutation analysis of immunoglobulin heavy chain genes showed a very high mutation load in seven cases tested, but intraclonal heterogeneity was found at low level in only one of these cases. It is concluded that primary testicular DLBCLs have uniform activated B-cell-like subtype characteristics despite a number of cases showing an ambiguous immunophenotype.

Redox-sensitive modulation of CD45 expression in pancreatic acinar cells during acute pancreatitis

CD45, a transmembrane protein tyrosine phosphatase required for signal transduction in leukocytes, has recently been found in pancreatic acinar cells. We have investigated the relationship between kinetic expression of CD45 on acinar cells during acute pancreatitis (AP) and the ability of these cells to produce tumour necrosis factor-alpha (TNF-alpha) through mechanisms sensitive to the cellular redox state. Flow cytometric analysis showed a significant decrease in the constitutive expression of CD45 in acinar cells from six hours onwards after inducing AP by bile-pancreatic duct obstruction (BPDO) in parallel with a significant increase in acinar TNF-alpha production. Changes in protein expression on the acinar cell surface preceded CD45 mRNA down-regulation, which was not found until 12 hours after BPDO. N-Acetylcysteine treatment delayed and reduced the down-regulation of CD45 expression induced by AP and prevented acinar cells from producing TNF-alpha. Our results show that CD45 expression is down-regulated in acinar cells during acute pancreatitis by redox-sensitive mechanisms, and they support the notion that CD45 negatively controls the production of cytokines in pancreatic acinar cells.

Role of AID in tumorigenesis

A hallmark of mature B-cell lymphomas is reciprocal chromosomal translocations involving the Ig locus and a proto-oncogene, which usually result in the deregulated, constitutive expression of the translocated gene. In addition to such translocations, proto-oncogenes are frequently hypermutated in germinal center (GC)-derived B-cell lymphomas. Although aberrant, mistargeted class switch recombination (CSR) and somatic hypermutation (SHM) events have long been suspected of causing chromosomal translocations and mutations in oncogenes, and thus of playing a critical role in the pathogenesis of most B-cell lymphomas, the molecular basis for such deregulation of CSR and SHM is only beginning to be elucidated by recent genetic approaches. The tumorigenic ability of activation-induced cytidine deaminase (AID), a key enzyme that initiates CSR and SHM, was revealed in studies on AID transgenic mice. In addition, experiments with AID-deficient mice clearly showed that AID is required not only for the c-myc/IgH translocation but also for the malignant progression of translocation-bearing lymphoma precursor cells, probably by introducing additional genetic hits. Normally, AID expression is only transiently and specifically induced in activated B cells in GCs. However, recent studies indicate that AID can be induced directly in B cells outside the GCs by various pathogens, including transforming viruses associated with human malignancies. Indeed, AID expression is not restricted to GC-derived B-cell lymphomas, but is also found in other types of B-cell lymphoma and even in nonlymphoid tumors, suggesting that ectopically expressed AID is involved in tumorigenesis and disease progression in a wide variety of cell types.

CD45RO: A Marker for BCR-mediated Selection

We previously showed that IgH sequence alone minimally influenced germinal centre (GC) B-cell survival fate. As end-stage effector B cells are typically more mutated than founder GC B cells, we worked to develop an assay that would enrich for populations of GC B cells with progressively increasing numbers of somatic mutations, which could potentially be used as an indicator of positive selection. We targeted CD45 as it has been shown to influence activation-induced cytidine deaminase (AID) expression. In this study, anti-CD77 and anti-CD45RO (RO) were used to subdivide CD19(+)IgD(-)CD38(+)CD77(+) centroblasts (CB) and CD19(+)IgD(-)CD38(+)CD77(-) centrocytes (CC) into three contiguous RO fractions (RO(-), RO(+/-) and RO(+)) and assessed whether mutation frequency and characteristics associated with selection varied with respect to increasing RO expression. Here, we show that the average number of mutations per IgV(H)4 transcript increased concordantly with RO for CC, but not for CB. CC also exhibited an RO-associated increase in replacement mutations. Comparative analysis of clonally related sequences revealed that increased mutations were not due to the exclusive persistence of surface RO on highly mutated cells. RO-expressing CC and CB pools showed increased signs of activation (CD69(+)) and were enriched for surface Ig(+) cells. BCR-crosslinking induced a significant increase in surface RO on total tonsillar and GC B cells, which collectively suggests that the RO-associated increase in mutations is attributable, at least in part, to the cycling of cells that may have recently undergone BCR-mediated selection, or are potentially in developmental transition between CC and CB stages.

CD45RO enriches for activated, highly mutated human germinal center B cells

To date, there is no consensus regarding the influence of different CD45 isoforms during peripheral B-cell development. Examining correlations between surface CD45RO expression and various physiologic processes ongoing during the germinal center (GC) reaction, we hypothesized that GC B cells, like T cells, that up-regulate surface RO should progressively acquire phenotypes commonly associated with activated, differentiating lymphocytes. GC B cells (IgD(-)CD38(+)) were subdivided into 3 surface CD45RO fractions: RO(-), RO(+/-), and RO(+). We show here that the average number of mutations per IgV(H) transcript increased in direct correlation with surface RO levels. Conjunctional use of RO and CD69 further delineated low/moderately and highly mutated fractions. Activation-induced cytidine deaminase (AID) mRNA was slightly reduced among RO(+) GC B cells, suggesting that higher mutation averages are unlikely due to elevated somatic mutation activity. Instead, RO(+) GC B cells were negative for Annexin V, comprised mostly (93%) of CD77(-) centrocytes, and were enriched for CD69(+) cells. Collectively, RO(+) GC B cells occupy what seems to be a specialized niche comprised mostly of centrocytes that may be in transition between activation states. These findings are among the first to sort GC B cells into populations enriched for live mutated cells solely using a single extracellular marker.

IL-4-induced AID expression and its relevance to IgA class switch recombination

Activation-induced cytidine deaminase (AID) is an inducible gene that plays a critical role in Ig class switch recombination and somatic hypermutation in B cells. We explored the mechanisms by which IL-4 induces AID expression in mouse B cells. IL-4 increased AID expression and over-expression of Stat6 further augmented IL-4-induced promoter activity. The involvement of Stat6 in the promoter activity was confirmed using ChIP assays and site-directed mutagenesis. Treatment with H89, a PKA inhibitor, markedly decreased IL-4-induced AID expression, and over-expression of CREB enhanced it. These results indicate that Stat6 and PKA/CREB are involved in IL-4-induced AID expression. The relevance of these signal transducing molecules was verified using the TGFbeta1-induced IgA isotype switching model. Our results indicate that IL-4, through Stat6 and PKA/CREB, induces AID expression leading to Ig isotype switching event.

Class switch recombination to IgE in the bronchial mucosa of atopic and nonatopic patients with asthma

BACKGROUND

Class switching from IgM/IgG/IgA to IgE is required for B cells to express IgE. This requires class switch recombination in the Ig heavy-chain gene locus. It is generally believed that class switch recombination occurs in lymphoid tissue, but it was recently shown that class switching to IgE occurs in the nasal mucosa in allergic rhinitis.

OBJECTIVE

We aimed to determine whether class switching to IgE also occurs in the bronchial mucosa in asthma, and to look for possible differences/similarities between atopic and nonatopic asthma.

METHODS

We have used RT-PCR to examine epsilon immunoglobulin heavy-chain germline gene transcripts (GLTs; epsilonGLTs), epsilon circle transcripts (CTs; Ivarepsilon-Cmu CT or Ivarepsilon-Cgamma CT), and mRNA encoding the heavy chain of IgE (epsilon mRNA) and activation-induced cytidine deaminase (AID) in bronchial biopsies from atopic patients with asthma, nonatopic patients with asthma, atopic controls without asthma, and nonatopic controls without asthma (10 subjects in each group).

RESULTS

The varepsilonGLT and AID mRNA were detectable in the bronchial mucosa of subjects in all 4 groups. In contrast, Iepsilon-Cmu CT, Ivarepsilon-Cgamma CT, and epsilon mRNA were detectable in the bronchial mucosa of the majority of both atopic and nonatopic patients with asthma, but rarely in the controls without asthma.

CONCLUSION

The bronchial mucosa is a site primed in all individuals for class switching to IgE, because of B-cell expression of epsilonGLT and AID mRNA. However, it is only in patients with asthma, regardless of atopic status, that class switching to IgE occurs.

CLINICAL IMPLICATIONS

Our findings reveal prospects for local targeting of the Ig class switch mechanism in the management of atopic and nonatopic asthma.

Gut IgA class switch recombination in the absence of CD40 does not occur in the lamina propria and is independent of germinal centers

Conflicting findings have recently been presented as to the sites and sources of B cells that undergo class switch recombination (CSR) to IgA in the gut. In this study we provide compelling evidence in CD40(-/-) mice demonstrating that IgA CSR can be independent of CD40 signaling and germinal center formation and does not occur in the gut lamina propria (LP) itself. We found that CD40(-/-) mice had near normal levels of gut total IgA despite lacking germinal centers and completely failing to raise specific responses against the T cell-dependent Ags cholera toxin and keyhole limpet hemocyanin. The Peyer's patches in CD40(-/-) mice expressed unexpectedly high levels of activation-induced cytidine deaminase mRNA and germline alpha transcripts, but few postswitch circular DNA transcripts, arguing against significant IgA CSR. Moreover and more surprisingly, wild-type mice exhibited no to low IgA CSR in mesenteric lymph nodes or isolated lymphoid follicles. Importantly, both strains failed to demonstrate any of the molecular markers for IgA CSR in the gut LP itself. Whereas all of the classical sites for IgA CSR in the GALT in CD40(-/-) mice appeared severely compromised for IgA CSR, B cells in the peritoneal cavity demonstrated the expression of activation-induced cytidine deaminase mRNA comparable to that of wild-type mice. However, peritoneal cavity B cells in both strains expressed intermediate levels of the germinal center marker GL7 and exhibited no germline alpha transcripts, and only three of 51 mice analyzed showed the presence of postswitch circular DNA transcripts. Taken together, these findings strongly argue for alternative inductive sites for gut IgA CSR against T cell-independent Ags outside of the GALT and the nonorganized LP.

Class switch recombination: a comparison between mouse and human

Humans and mice separated more than 60 million years ago. Since then, evolution has led to a multitude of changes in their genomic sequences. The divergence of genes has resulted in differences both in the innate and adaptive immune systems. In this chapter, we focus on species difference with regard to immunoglobulin class switch recombination (CSR). We have compared the immunoglobulin constant region gene loci from human and mouse, with an emphasis on the switch regions, germ line transcription promoters, and 3' enhancers. We have also compared pathways/factors that are involved in CSR. Although there are remarkable similarities in the cellular machinery involved in CSR, there are also a number of unique features in each species.

The Role of Activation‐Induced Deaminase in Antibody Diversification and Chromosome Translocations

Although B and T lymphocytes are similar in many respects including diversification of their antigen receptor genes by V(D)J recombination, 95% of all lymphomas diagnosed in the western world are of B-cell origin. Many of these are derived from mature B cells [Kuppers, R. (2005). Mechanisms of B-cell lymphoma pathogenesis. Nat. Rev. Cancer 5, 251-262] and display hallmark chromosome translocations involving immunoglobulin genes and a proto-oncogene partner whose expression becomes deregulated as a result of the translocation reaction [Kuppers, R. (2005). Mechanisms of B-cell lymphoma pathogenesis. Nat. Rev. Cancer 5, 251-262; Kuppers, R., and Dalla-Favera, R. (2001). Mechanisms of chromosomal translocations in B cell lymphomas. Oncogene 20, 5580-5594]. These translocations are essential to the etiology of B-cell neoplasms. Here we will review how the B-cell specific molecular events required for immunoglobulin class switch recombination are initiated and how they contribute to chromosome translocations in vivo.

Intracellular Tat of human immunodeficiency virus type 1 activates lytic cycle replication of Kaposi's sarcoma-associated herpesvirus: role of JAK/STAT signaling

Human immunodeficiency virus type 1 (HIV-1) infection significantly increases the risk of Kaposi's sarcoma (KS) occurrence in individuals infected with Kaposi's sarcoma-associated herpesvirus (KSHV). KSHV infection appears to be necessary but not sufficient for KS development without other cofactors. However, factors that facilitate KSHV to cause KS have not been well defined. Previously, we determined that human herpesvirus 6 was one of the cofactors that activated lytic cycle replication of KSHV. Here, we demonstrate that the Tat protein of HIV-1 is a potentially important factor in the pathogenesis of KS, as determined by production of lytic phase mRNA transcripts and viral proteins in BCBL-1 cells. Mechanistic studies showed ectopic expression of Tat induced the production of human interleukin-6 (huIL-6) and its receptor (huIL-6Ra) and activated STAT3 signaling. Neutralization of huIL-6 or huIL-6R or inhibition of STAT3 signaling enhanced the replication. In addition, IL-4/STAT6 signaling also partially contributed to Tat-induced KSHV replication. These findings suggest that Tat may participate in KS pathogenesis by inducing KSHV replication and increasing KSHV viral load. These data also suggest that JAK/STAT signaling may be of therapeutic value in AIDS-related KS patients.

Distinct patterns of cytokine regulation of APOBEC3G expression and activity in primary lymphocytes, macrophages, and dendritic cells

Human APOBEC3G (A3G), a deoxycytidine deaminase, is a broadly acting antiretroviral factor expressed in a variety of cells. Mitogen activation of CD4 T cells enhances A3G expression and leads to recruitment of low molecular mass (LMM) A3G, which functions as a post-entry human immunodeficiency virus (HIV) restriction factor, into enzymatically inactive, high molecular mass (HMM) RNA-protein complexes that include Staufen RNA-transporting granules. We now report that interleukin-2 (IL-2), IL-15 and, to a lesser extent, IL-7 enhance the expression of A3G in peripheral blood lymphocytes and that this effect is blocked by inhibitors of the JAK and MAPK signaling pathways. In mixed cultures of CD4+ T cells containing either HMM or LMM A3G, HIV preferentially infected cells containing HMM A3G. A3G shifted into a HMM complex when IL-2, -7, or -15 was added to resting T cells, likely explaining how cytokine treatment renders resting CD4+ T cells permissive to HIV infection. Similarly, poly(I:C)/tumor necrosis factor-alpha-induced maturation of dendritic cells was associated with a sharp increase in A3G expression; however, this induction led to the accumulation of LMM A3G. Together, these results highlight the distinct inductive effects of select cytokines on A3G gene expression and A3G complex assembly that occur in natural cellular targets of HIV infection.

Regulation of IgE homeostasis, and the identification of potential targets for therapeutic intervention

Atopic allergies have increased during the past 20-30 years in frequency quite dramatically and in many countries have reached almost epidemic proportions. Allergies have thereby become one of the major medical issues of the western world. Immunoglobulin E (IgE) is here a central player. IgE is the Ig class that is present in the lowest concentration in human plasma. IgG is, for example, 10 000 to 1 million times more abundant than IgE. However, despite of its low plasma levels IgE is a very important inducer of inflammation, due to its interaction with high-affinity receptors on mast cell and basophils. IgE has been conserved as a single active gene in all placental mammals studied, and the expression of this gene is under a very stringent control, most likely due to its very potent inflammatory characteristics. IgE expression is being regulated at many levels: by cytokines, switch region length, positive and negatively acting transcription factors and suppressors of cytokine signaling (SOCS). In addition, the plasma half-life differs markedly for IgG and IgE, with 21 and 2.5 days, respectively. This review summarizes the rapid progress in our understanding of the complex network of regulatory mechanisms acting on IgE and also how this new information may help us in our efforts to control IgE-mediated inflammatory conditions.

Activation-induced deaminase: light and dark sides

Activation-induced deaminase (AID) is required for class switch recombination (CSR) and somatic hypermutation (SHM), which are responsible for secondary diversification of antibodies in germinal centers. AID initiates these processes by deamination of cytosines on the immunoglobulin (Ig) locus, a potentially mutagenic activity. AID expression is restricted to germinal-center B cells, but the mechanisms that regulate its target specificity are not completely understood. Here, we review the most recent findings on the regulation of AID targeting and discuss how AID activity on non-Ig genes is relevant to the generation of chromosome translocations and to lymphomagenesis.

Association of Ig/BCL6 translocations with germinal center B lymphocytes in human lymphoid tissues: implications for malignant transformation

Chromosomal translocations (CTs) between immunoglobulin (Ig) genes and the BCL6 proto-oncogene are frequently associated with diffuse large B-cell lymphomas (DLBCLs) and follicular lymphomas (FLs) and are implicated in the development of these lymphomas. However, whether Ig/BCL6 translocation per se is sufficient to drive malignant transformation is not clear. To understand the biology of Ig/BCL6-translocated cells prior to their malignant transformation, we developed a system capable of detecting 1 to 3 Igmu/BCL6 CT cells in 1 million mixed cells through the detection of chimeric Imu-BCL6E2 and BCL6E1-Cmu1 transcripts that reflect reciprocal Igmu/BCL6 translocations. The chimeric transcripts that existed in the vast majority of normal lymphoid tissues are due to Igmu/BCL6 CT and were not generated from trans-splicing. Both Imu-BCL6E2 and BCL6E1-Cmu1 transcripts were coexpressed in the same cell populations. The Ig/BCL6 recombination junctions themselves were isolated from B-cell subpopulations expressing the Imu-BCL6 transcripts. The appearance of Igmu/BCL6 CT was associated with cells expressing germinal center but not naive B-cell markers. This study shows that Ig/BCL6 translocations occur in germinal center-stage B cells in healthy humans, and that Ig/BCL6 CTs per se are not likely sufficient to cause the malignant transformation in the context of human B cells.

IL-21 induces differentiation of human naive and memory B cells into antibody-secreting plasma cells

IL-21 is a type I cytokine that influences the function of T cells, NK cells, and B cells. In this study, we report that IL-21 plays a major role in stimulating the differentiation of human B cells. When human B cells were stimulated through the BCR, IL-21 induced minimal proliferation, IgD down-modulation, and small numbers of plasma cells. In contrast, after CD40 engagement, IL-21 induced extensive proliferation, class switch recombination (CSR), and plasma cell differentiation. Upon cross-linking both BCR and CD40, IL-21 induced the largest numbers of plasma cells. IL-21 drove both postswitch memory cells as well as poorly responsive naive cord blood B cells to differentiate into plasma cells. The effect of IL-21 was more potent than the combination of IL-2 and IL-10, especially when responsiveness of cord blood B cells was examined. IL-21 costimulation potently induced the expression of both B lymphocyte-induced maturation protein-1 (BLIMP-1) and activation-induced cytidine deaminase as well as the production of large amounts of IgG from B cells. Despite the induction of activation-induced cytidine deaminase and CSR, IL-21 did not induce somatic hypermutation. Finally, IL-2 enhanced the effects of IL-21, whereas IL-4 inhibited IL-21-induced plasma cell differentiation. Taken together, our data show that IL-21 plays a central role in CSR and plasma cell differentiation during T cell-dependent B cell responses.

Nuclear and cytoplasmic AID in extrafollicular and germinal center B cells

Activation-induced cytidine deaminase (AID) is necessary for immunoglobulin somatic hypermutation (SHM) and class switch recombination (CSR) in T-dependent immune response in germinal centers (GCs). The structural similarity of AID with RNA-editing enzymes and its largely cytoplasmic location have fueled controversial views of its mode of interaction with DNA. We show that AID, a mature B-cell-restricted cytoplasmic antigen, is relocated into the nucleus in 2.5% of CDKN1B(-), CCNB1(-) GC cells. The GC dark zone and the outer zone (OZ), but not the light zone, contain nuclear and cytoplasmic AID(+) blasts. AID(+) cells in the OZ are in contact with T cells and CD23(-) follicular dendritic cells. In addition, AID is expressed in extrafollicular large proliferating B cells, 14% of which have nuclear AID. GC and extrafollicular AID(+) cells express E47 but not the inhibiting BHLH protein Id2. Outside the GC, AID(+) B cells are in contact with T cells and show partial evidence of CD40 plus bcr stimulation-dependent signature (CCL22, JunB, cMYC, CD30) but lack early and late plasma cell markers. The distribution of nuclear AID is consistent with the topography of SHM and CSR inside the GC and in extrafollicular activated B cells.

Cytokine gene expression profile distinguishes CD4+/CD57+ T cells of the nodular lymphocyte predominance type of Hodgkin's lymphoma from their tonsillar counterparts

Little is known about the cytokine profile of nodular lymphocyte predominance Hodgkin's lymphoma (NLPHL) and the significance of the characteristic rosetting CD4(+)/CD57(+) T cells. We analysed the T lymphocyte populations isolated from lymph node suspensions from five patients with NLPHL, two with follicular hyperplasia and progressive transformation of germinal centres (PTGC), three with classical Hodgkin's lymphoma (CHL) and five with hyperplasia of the tonsil. We sorted the T cells based on expression of CD3, CD4 and CD57 by flow cytometry and evaluated the cytokine mRNA profiles of the T cells with quantitative RT-PCR. NLPHL cases were as rich in T cells as the CHL cases, but all NLPHL cases had a much higher frequency of CD4(+)/CD57(+) T cells. In contrast to the CD4(+)/CD57(+) T cells from tonsils, IL2 and IL4 mRNAs were consistently absent from the CD4(+)/CD57(+) T cells of NLPHL. Even after stimulation, no IL4 transcripts could be detected in the CD4(+)/CD57(+) T cells of NLPHL. On the other hand, IFNgamma transcripts were elevated in NLPHL and PTGC T cell subsets as compared to tonsillar T cell subsets. IL13 mRNA was exclusively produced by the T cells of CHL cases, indicating that IL13 may be a key cytokine in CHL. In conclusion, elevated levels of CD4(+)/CD57(+) T cells are characteristic of NLPHL and these T cells display a distinct cytokine mRNA profile.

CD45 expression on rat acinar cells: Involvement in pro-inflammatory cytokine production

CD45 transduces activation signals in inflammatory cells. We investigate CD45 expression on pancreatic acinar cells and examine its role in the inflammatory response which these cells have also shown under certain circumstances. Similar CD45 mRNA levels were found in acinar cells and leukocytes (positive control). Flow cytometric and immunohistochemical analysis showed a heterogeneous CD45 distribution on acinar cells. Activation of acinar cells by incubation with pancreatitis-associated ascitic fluid as evidencied by TNF-alpha production resulted in a decreased CD45 expression, suggesting that CD45 acts as a negative regulator of cytokine production. As a validation of this finding in vivo, a decrease in the acinar CD45 expression in parallel with an increased ability to produce TNF-alpha was found in rats with acute pancreatitis. Our data show that CD45 is constitutively expressed in acinar cells and suggest that it plays an important role in negatively regulating cytokine production.

CD45RA and RO isoforms have distinct effects on cytokine- and B-cell-receptor-mediated signalling in human B cells

The common leucocyte antigen, CD45, is widely expressed on the surface of lymphocytes. In T and B cells, CD45 has an important role in the early events of receptor signalling. However, the role of various CD45 isoforms in B-cell receptor (BCR)- and cytokine-induced signalling and proliferation is still unclear. In the present study, we establish two follicular lymphoma cell lines expressing either CD45RA (HF28RA) or CD45R0 (HF28R0) isoforms. It was observed that the two isoforms had distinct effects on BCR- or cytokine-induced cellular proliferation. BCR stimulation significantly increased the proliferation of HF28R0 cells, in contrast to a decreased proliferation of HF28RA cells. Moreover, proliferation of HF28R0 cells significantly increased after the addition of interleukin-2 (IL-2), IL-4, IL-6, IL-10, IL-12, IL-13, IL-15, interferon-gamma and tumour necrosis factor-alpha cytokines, whereas most of these cytokines significantly inhibited the proliferation of HF28RA cells. In addition, the cell lines had their individual cytokine mRNA expression profiles after BCR stimulation. We also analysed the effect of CD45 isoforms on intracellular signalling after BCR stimulation. It was found out that the kinetics of ERK1/2 MAP kinase phosphorylation was clearly faster in HF28R0 than in HF28RA cells. The phosphorylation of other analysed MAP kinases or PTKs was very similar in the cell lines.

Aberrant somatic hypermutation and expression of activation-induced cytidine deaminase mRNA in mediastinal large B-cell lymphoma

To determine the possible role of aberrant somatic hypermutation (ASHM) and activation-induced cytidine deaminase (AID) expression in the pathogenesis of mediastinal large B-cell lymphoma (MBL), the mutational status of genes affected by ASHM, including c-MYC, PAX-5 and RhoH, was analysed, and the expression level of AID mRNA in tumour specimens from six patients with MBL was determined. Mutations in one or more genes and high expression of AID mRNA were detected in all the six cases of MBL. These results suggest that ASHM and AID expression may have a role in the pathogenesis of MBL.

Allergen drives class switching to IgE in the nasal mucosa in allergic rhinitis

IgE-expressing B cells are over 1000 times more frequent in the nasal B cell than the peripheral blood B cell population. We have investigated the provenance of these B cells in the nasal mucosa in allergic rhinitis. It is generally accepted that expression of activation-induced cytidine deaminase and class switch recombination (CSR) occur in lymphoid tissue, implying that IgE-committed B cells must migrate through the circulation to the nasal mucosa. Our detection of mRNA for activation-induced cytidine, multiple germline gene transcripts, and epsilon circle transcripts in the nasal mucosa of allergic, in contrast to nonallergic control subjects, however, indicates that local CSR occurs in allergic rhinitis. The germline gene transcripts and epsilon circle transcripts in grass pollen-allergic subjects are up-regulated during the season and also when biopsies from allergic subjects are incubated with the allergen ex vivo. These results demonstrate that allergen stimulates local CSR to IgE, revealing a potential target for topical therapies in allergic rhinitis.

Inhibition of granulocyte-macrophage colony-stimulating factor signaling and microglial proliferation by anti-CD45RO: role of Hck tyrosine kinase and phosphatidylinositol 3-kinase/Akt

Increasing evidence suggests that CD45, a transmembrane protein tyrosine phosphatase, is an important modulator of macrophage activation. Microglia, resident brain macrophages, express CD45 and proliferate under pathologic conditions. In this study, we examined the role of CD45 in modulating GM-CSF-induced proliferation and signal transduction in primary human microglial cultures. Soluble, but not immobilized anti-CD45RO induced tyrosine phosphatase activity and inhibited GM-CSF-induced microglial proliferation. Microglial proliferation was also inhibited by PP2 (Src inhibitor), LY294002 (PI3K inhibitor), and U0126 (MEK inhibitor). GM-CSF induced phosphorylation of Jak2, Stat5, Hck (the myeloid-restricted Src kinase), Akt, Stat3, and Erk MAPKs in microglia. Of these, anti-CD45RO inhibited phosphorylation of Hck and Akt, and PP2 inhibited phosphorylation of Hck and Akt. In a macrophage cell line stably overexpressing wild-type or kinase-inactive Hck, GM-CSF increased proliferation of the control (empty vector) and wild-type but not kinase-inactive cells, and this was inhibited by anti-CD45RO. Together, these results demonstrate that, in macrophages, Hck tyrosine kinase is activated by GM-CSF, and that Hck plays a pivotal role in cell proliferation and survival by activating the PI3K/Akt pathway. Ab-mediated activation of macrophage and microglial CD45 tyrosine phosphatase may have therapeutic implications for CNS inflammatory diseases.