Molecular mimicry of the antigen receptor signalling motif by transmembrane proteins of the Epstein-Barr virus and the bovine leukaemia virus

Advances in Hodgkin lymphoma research

Hodgkin lymphoma (HL) has been and still is the most enigmatic lymphoid malignancy in humans. Since the first molecular analysis of isolated Hodgkin and Reed-Sternberg (HRS) tumor cells of classic HL 30 years ago, substantial advances in our understanding of HL have been made. This review describes the cellular origin of HL, summarizes the current knowledge about the genetic lesions in HRS cells, and highlights the role of Epstein-Barr virus (EBV) in HL pathogenesis. Moreover, the pathobiological roles of altered gene expression and deregulated signaling pathways are discussed and key aspects of the HL microenvironment are presented.

Comprehensive Analysis of Epstein-Barr Virus LMP2A-Specific CD8+ and CD4+ T Cell Responses Restricted to Each HLA Class I and II Allotype Within an Individual

Latent membrane protein 2A (LMP2A), a latent Ag commonly expressed in Epstein-Barr virus (EBV)-infected host cells, is a target for adoptive T cell therapy in EBV-associated malignancies. To define whether individual human leukocyte antigen (HLA) allotypes are used preferentially in EBV-specific T lymphocyte responses, LMP2A-specific CD8+ and CD4+ T cell responses in 50 healthy donors were analyzed by ELISPOT assay using artificial Ag-presenting cells expressing a single allotype. CD8+ T cell responses were significantly higher than CD4+ T cell responses. CD8+ T cell responses were ranked from highest to lowest in the order HLA-A, HLA-B, and HLA-C loci, and CD4+ T cell responses were ranked in the order HLA-DR, HLA-DP, and HLA-DQ loci. Among the 32 HLA class I and 56 HLA class II allotypes, 6 HLA-A, 7 HLA-B, 5 HLA-C, 10 HLA-DR, 2 HLA-DQ, and 2 HLA-DP allotypes showed T cell responses higher than 50 spot-forming cells (SFCs)/5×105 CD8+ or CD4+ T cells. Twenty-nine donors (58%) showed a high T cell response to at least one allotype of HLA class I or class II, and 4 donors (8%) had a high response to both HLA class I and class II allotypes. Interestingly, we observed an inverse correlation between the proportion of LMP2A-specific T cell responses and the frequency of HLA class I and II allotypes. These data demonstrate the allele dominance of LMP2A-specific T cell responses among HLA allotypes and their intra-individual dominance in response to only a few allotypes in an individual, which may provide useful information for genetic, pathogenic, and immunotherapeutic approaches to EBV-associated diseases.

Genotypes diversity of env gene of Bovine leukemia virus in Western Siberia

BACKGROUND

This study describes the biodiversity and properties of Bovine leukemia virus in Western Siberia. This paper explores the effect of different genotypes of the env gene of the cattle leukemia virus on hematological parameters of infected animals. The researchers focused on exploring the polymorphism of the env gene and, in doing so, discovered the new genotypes I_a and I_b, which differ from genotype I. Several hypotheses on the origin of the different genotypes in Siberia are discussed.

RESULTS

We obtained varying length of the restriction fragments for genotypes I_. Additionally using restrictase Hae III were received fragments was named genotype I_a, and genotype I_b. There are 2.57 ± 0.55% (20 out of 779) samples of genotype I_b which does not differ significantly from 1% (χ² = 2.46). Other genotypes were observed in the cattle of Siberia as wild type genotypes (their frequency varied from 17.84 to 32.73%). The maximum viral load was observed in animals with the II and IV viral genotypes (1000-1400 viral particles per 1000 healthy cells), and the minimum viral load was observed animals with genotype I_b (from 700 to 900 viral particles per 1000 healthy cells).

CONCLUSIONS

The probability of the direct introduction of genotype II from South America to Siberia is extremely small and it is more likely that the strain originated independently in an autonomous population with its distribution also occurring independently. A new variety of genotype I (I_b) was found, which can be both a neoplasm and a relict strain.

Epstein-Barr virus: more than 50 years old and still providing surprises

It is more than 50 years since the Epstein-Barr virus (EBV), the first human tumour virus, was discovered. EBV has subsequently been found to be associated with a diverse range of tumours of both lymphoid and epithelial origin. Progress in the molecular analysis of EBV has revealed fundamental mechanisms of more general relevance to the oncogenic process. This Timeline article highlights key milestones in the 50-year history of EBV and discusses how this virus provides a paradigm for exploiting insights at the molecular level in the diagnosis, treatment and prevention of cancer.

Latent Membrane Protein LMP2A Impairs Recognition of EBV-Infected Cells by CD8+ T Cells

The common pathogen Epstein-Barr virus (EBV) transforms normal human B cells and can cause cancer. Latent membrane protein 2A (LMP2A) of EBV supports activation and proliferation of infected B cells and is expressed in many types of EBV-associated cancer. It is not clear how latent EBV infection and cancer escape elimination by host immunity, and it is unknown whether LMP2A can influence the interaction of EBV-infected cells with the immune system. We infected primary B cells with EBV deleted for LMP2A, and established lymphoblastoid cell lines (LCLs). We found that CD8+ T cell clones showed higher reactivity against LMP2A-deficient LCLs compared to LCLs infected with complete EBV. We identified several potential mediators of this immunomodulatory effect. In the absence of LMP2A, expression of some EBV latent antigens was elevated, and cell surface expression of MHC class I was marginally increased. LMP2A-deficient LCLs produced lower amounts of IL-10, although this did not directly affect CD8+ T cell recognition. Deletion of LMP2A led to several changes in the cell surface immunophenotype of LCLs. Specifically, the agonistic NKG2D ligands MICA and ULBP4 were increased. Blocking experiments showed that NKG2D activation contributed to LCL recognition by CD8+ T cell clones. Our results demonstrate that LMP2A reduces the reactivity of CD8+ T cells against EBV-infected cells, and we identify several relevant mechanisms.

Epstein-Barr virus (EBV) is carried by most humans. It can cause several types of cancer. In healthy infected people, EBV persists for life in a "latent" state in white blood cells called B cells. For infected persons to remain healthy, it is crucial that they harbor CD8-positive "killer" T cells that recognize and destroy precancerous EBV-infected cells. However, this protection is imperfect, because the virus is not eliminated from the body, and the danger of EBV-associated cancer remains. How does the virus counteract CD8+ T cell control? Here we study the effects of latent membrane protein 2A (LMP2A), which is an important viral molecule because it is present in several types of EBV-associated cancers, and in latently infected cells in healthy people. We show that LMP2A counteracts the recognition of EBV-infected B cells by antiviral killer cells. We found a number of mechanisms that are relevant to this effect. Notably, LMP2A disturbs expression of molecules on B cells that interact with NKG2D, a molecule on the surface of CD8+ T cells that aids their activation. In this way, LMP2A weakens important immune responses against EBV. Similar mechanisms may operate in different types of LMP2A-expressing cancers caused by EBV.

K1 and K15 of Kaposi's Sarcoma-Associated Herpesvirus Are Partial Functional Homologues of Latent Membrane Protein 2A of Epstein-Barr Virus

The human herpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are associated with Hodgkin's lymphoma (HL) and Primary effusion lymphomas (PEL), respectively, which are B cell malignancies that originate from germinal center B cells. PEL cells but also a quarter of EBV-positive HL tumor cells do not express the genuine B cell receptor (BCR), a situation incompatible with survival of normal B cells. EBV encodes LMP2A, one of EBV's viral latent membrane proteins, which likely replaces the BCR's survival signaling in HL. Whether KSHV encodes a viral BCR mimic that contributes to oncogenesis is not known because an experimental model of KSHV-mediated B cell transformation is lacking. We addressed this uncertainty with mutant EBVs encoding the KSHV genes K1 or K15 in lieu of LMP2A and infected primary BCR-negative (BCR(-)) human B cells with them. We confirmed that the survival of BCR(-) B cells and their proliferation depended on an active LMP2A signal. Like LMP2A, the expression of K1 and K15 led to the survival of BCR(-) B cells prone to apoptosis, supported their proliferation, and regulated a similar set of cellular target genes. K1 and K15 encoded proteins appear to have noncomplementing, redundant functions in this model, but our findings suggest that both KSHV proteins can replace LMP2A's key activities contributing to the survival, activation and proliferation of BCR(-) PEL cells in vivo.

IMPORTANCE

Several herpesviruses encode oncogenes that are receptor-like proteins. Often, they are constitutively active providing important functions to the latently infected cells. LMP2A of Epstein-Barr virus (EBV) is such a receptor that mimics an activated B cell receptor, BCR. K1 and K15, related receptors of Kaposi's sarcoma-associated herpesvirus (KSHV) expressed in virus-associated tumors, have less obvious functions. We found in infection experiments that both viral receptors of KSHV can replace LMP2A and deliver functions similar to the endogenous BCR. K1, K15, and LMP2A also control the expression of a related set of cellular genes in primary human B cells, the target cells of EBV and KSHV. The observed phenotypes, as well as the known characteristics of these genes, argue for their contributions to cellular survival, B cell activation, and proliferation. Our findings provide one possible explanation for the tumorigenicity of KSHV, which poses a severe problem in immunocompromised patients.

Mechanisms of pathogenesis induced by bovine leukemia virus as a model for human T-cell leukemia virus

Bovine leukemia virus (BLV) and human T-cell leukemia virus type 1 (HTLV-1) make up a unique retrovirus family. Both viruses induce chronic lymphoproliferative diseases with BLV affecting the B-cell lineage and HTLV-1 affecting the T-cell lineage. The pathologies of BLV- and HTLV-induced infections are notably similar, with an absence of chronic viraemia and a long latency period. These viruses encode at least two regulatory proteins, namely, Tax and Rex, in the pX region located between the env gene and the 3′ long terminal repeat. The Tax protein is a key contributor to the oncogenic potential of the virus, and is also the key protein involved in viral replication. However, BLV infection is not sufficient for leukemogenesis, and additional events such as gene mutations must take place. In this review, we first summarize the similarities between the two viruses in terms of genomic organization, virology, and pathology. We then describe the current knowledge of the BLV model, which may also be relevant for the understanding of leukemogenesis caused by HTLV-1. In addition, we address our improved understanding of Tax functions through the newly identified BLV Tax mutants, which have a substitution between amino acids 240 and 265.

Epstein-Barr virus LMP2A signaling in statu nascendi mimics a B cell antigen receptor-like activation signal

Background

The latent membrane protein (LMP) 2A of Epstein-Barr virus (EBV) is expressed during different latency stages of EBV-infected B cells in which it triggers activation of cytoplasmic protein tyrosine kinases. Early studies revealed that an immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic N-terminus of LMP2A can trigger a transient increase of the cytosolic Ca²⁺ concentration similar to that observed in antigen-activated B cells when expressed as a chimeric transmembrane receptor. Even so, LMP2A was subsequently ascribed an inhibitory rather than an activating function because its expression seemed to partially inhibit B cell antigen receptor (BCR) signaling in EBV-transformed B cell lines. However, the analysis of LMP2A signaling has been hampered by the lack of cellular model systems in which LMP2A can be studied without the influence of other EBV-encoded factors.

Results

We have reanalyzed LMP2A signaling using B cells in which LMP2A is expressed in an inducible manner in the absence of any other EBV signaling protein. This allowed us for the first time to monitor LMP2A signaling in statu nascendi as it occurs during the EBV life cycle in vivo. We show that mere expression of LMP2A not only stimulated protein tyrosine kinases but also induced phospholipase C-γ2-mediated Ca²⁺ oscillations followed by activation of the extracellular signal-regulated kinase (Erk) mitogen-activated protein kinase pathway and induction of the lytic EBV gene bzlf1. Furthermore, expression of the constitutively phosphorylated LMP2A ITAM modulated rather than inhibited BCR-induced Ca²⁺ mobilization.

Conclusion

Our data establish that LMP2A expression has a function beyond the putative inhibition of the BCR by generating a ligand-independent cellular activation signal that may provide a molecular switch for different EBV life cycle stages and most probably contributes to EBV-associated lymphoproliferative disorders.

The biology of Hodgkin's lymphoma

Hodgkin's lymphoma was first described in 1832. The aetiology of this lymphoma, however, remained enigmatic for a long time. Only within the past 10 years has the B-cell nature of the pathognomonic Hodgkin and Reed-Sternberg (HRS) cells been revealed, along with several recurrent genetic lesions. The pathogenetic role for Epstein-Barr virus infection has also been substantiated. HRS cells in classical Hodgkin's lymphoma have several characteristics that are unusual for lymphoid tumour cells, and the Hodgkin's lymphoma microenvironment is dominated by an extensive mixed, potentially inflammatory cellular infiltrate. Understanding the contribution of all of these changes to the pathogenesis of this disease is essential for the development of novel therapies.

Cell origin of pyothorax-associated lymphoma: a lymphoma strongly associated with Epstein-Barr virus infection

Pyothorax-associated lymphoma (PAL) is an Epstein-Barr virus (EBV)-associated B cell lymphoma developing in the pleural cavity affected by chronic pyothorax. To clarify the cell origin of PAL, the expression of immunoglobulin heavy (IgH) and light chains in relation to somatic hypermutations (SHMs) of rearranged Ig heavy- and light-chain variable (IgV(H), IgV(L)) genes was examined using cell lines as well as clinical samples. SHMs without ongoing mutations of the IgV(H) gene were found in all PAL cell lines and clinical samples available for sequencing, indicating PAL to be derived from B cells at the postgerminal center (GC) stage of the differentiation process. They could be subdivided into post-GC cells with potentially productive IgV(H) genotypes (Group 1) and with sterile IgV(H) genotypes (Group 2). IgH expression was abrogated in Group 2 as expected and also in two cell lines in Group 1. DNA demethylation experiments with 5-aza-dC induced expression of IgH mRNA and protein in these cell lines. Most PAL cells were derived from crippled post-GC cells, which usually could not survive. Transformation of such B cells through EBV infection might provide a basis for the development of PAL with additional genetic changes.

The aberrant coexpression of several receptor tyrosine kinases is largely restricted to EBV-negative cases of classical Hodgkin's lymphoma

The Hodgkin-Reed/Sternberg (HRS) cells of classical Hodgkin's lymphoma (HL) aberrantly express up to 7 different receptor tyrosine kinases (RTK) with extensive heterogeneity regarding the number and combinations of expressed RTKs in individual cases and a more prominent coexpression in nodular-sclerosis (ns) than mixed-cellularity (mc) HL. To investigate whether RTK expression patterns are related to other pathogenetic mechanisms and clinical behaviour, we analysed a large collection of EBV(+) and EBV(-) cases of ns and mc subtype and cases with relapses for expression of the 7 RTKs. No specific relation of any RTK to a specific group of cases was observed. The analysis of average numbers of expressed RTKs per case as a measure for strength of overall RTK signalling revealed a relation with the histological subtype and the EBV-status. RTK coexpression was significantly higher in EBV(-) nsHL cases compared to both EBV(-) and EBV(+) mcHL cases. Among mcHL cases RTK coexpression was significantly higher in EBV(-) compared to EBV(+) cases. Coexpression of 3 and more RTKs was largely restricted to EBV(-) cases. The inverse correlation between strong RTK signalling and presence of EBV may indicate that RTK signalling can at least partially replace the role of EBV in HRS cell pathogenesis. For cases with aberrant coexpression of several RTKs inclusion of RTK inhibitors in therapy regimens may be a novel option.

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

In 1871, the observation of yellowish nodules in the enlarged spleen of a cow was considered to be the first reported case of bovine leukemia. The etiological agent of this lymphoproliferative disease, bovine leukemia virus (BLV), belongs to the deltaretrovirus genus which also includes the related human T-lymphotropic virus type 1 (HTLV-1). This review summarizes current knowledge of this viral system, which is important as a model for leukemogenesis. Recently, the BLV model has also cast light onto novel prospects for therapies of HTLV induced diseases, for which no satisfactory treatment exists so far.

Epstein-barr virus latent membrane protein 2B (LMP2B) modulates LMP2A activity

Latent membrane protein 2A (LMP2A) and LMP2B are viral proteins expressed during Epstein-Barr virus (EBV) latency in EBV-infected B cells both in cell culture and in vivo. LMP2A has important roles in modulating B-cell receptor (BCR) signal transduction by associating with the cellular tyrosine kinases Lyn and Syk via specific phosphotyrosine motifs found within the LMP2A N-terminal tail domain. LMP2A has been shown to alter normal BCR signal transduction in B cells by reducing levels of Lyn and by blocking tyrosine phosphorylation and calcium mobilization following BCR cross-linking. Although little is currently known about the function of LMP2B in B cells, the similarity in structure between LMP2A and LMP2B suggests that they may localize to the same cellular compartments. To investigate the function of LMP2B, B-cell lines expressing LMP2A, LMP2B, LMP2A/LMP2B, and the relevant vector controls were analyzed. As was previously shown, cells expressing LMP2A had a dramatic block in normal BCR signal transduction as measured by calcium mobilization and tyrosine phosphorylation. There was no effect on BCR signal transduction in cells expressing LMP2B. Interestingly, when LMP2B was expressed in conjunction with LMP2A, there was a restoration of normal BCR signal transduction upon BCR cross-linking. The expression of LMP2B did not alter the cellular localization of LMP2A but did bind to and prevent the phosphorylation of LMP2A. A restoration of Lyn levels, but not a change in LMP2A levels, was also observed in cells coexpressing LMP2B with LMP2A. From these results, we conclude that LMP2B modulates LMP2A activity.

Molecular biology of Hodgkin's and Reed/Sternberg cells in Hodgkin's lymphoma

Hodgkin's and Reed/Sternberg (HRS) cells, the tumour cells in classical Hodgkin's lymphoma (HL), represent transformed B cells in nearly all cases. The detection of destructive somatic mutations in the rearranged immunoglobulin (Ig) genes of HRS cells in classical HL indicated that they originate from preapoptotic germinal centre (GC) B cells that lost the capacity to express a high-affinity B-cell receptor (BCR). Several aberrantly activated signalling pathways and transcription factors have been identified that contribute to the rescue of HRS cells from apoptosis. Among the deregulated signalling pathways, activation of multiple receptor tyrosine kinases in HRS cells appears to be a specific feature of HL. In about 40% of cases of classical HL the HRS cells are infected by Epstein-Barr virus (EBV), indicating an important role of EBV in HL pathogenesis. Interestingly, nearly all cases of HL with destructive Ig gene mutations eliminating BCR expression (e.g. nonsense mutations) are EBV-positive, suggesting that EBV-encoded genes have a particular function to prevent apoptosis of HRS-cell precursors that acquired such crippling mutations. This idea is further supported by the recent demonstration that isolated human GC B cells harbouring crippled Ig genes can be rescued by EBV from cell death, giving rise to lymphoblastoid cell lines. The molecular analysis of composite Hodgkin's and non-Hodgkin's lymphomas indicated that many cases develop from a common GC B-cell precursor in a multistep transformation process with both shared and distinct oncogenic events.

Transformation of BCR-deficient germinal-center B cells by EBV supports a major role of the virus in the pathogenesis of Hodgkin and posttransplantation lymphomas

In classic Hodgkin lymphoma (HL) and posttransplantation lymphoproliferative disease (PTLD), 2 malignancies frequently associated with Epstein-Barr virus (EBV), the tumor cells often appear to derive from B-cell receptor (BCR)-deficient and therefore preapoptotic germinal center (GC) B cells. To test whether EBV can rescue BCR-less GC B cells, we infected human tonsillar CD77+ GC B cells in vitro with EBV. More than 60 monoclonal lymphoblastoid cell lines (LCLs) were established. Among these, 28 cell lines did not express surface immunoglobulin (sIg). Two of the sIg-negative cell lines carry obviously destructive mutations that have been introduced into originally functional V(H) gene rearrangements during the process of somatic hypermutation. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed that in most other lines the sIg deficiency was not simply the result of transcriptional down-regulation, but it was rather due to posttranscriptional defects. These findings strongly support the idea that EBV plays a central role in the pathogenesis of classic HL and PTLD by rescuing BCR-deficient, preapoptotic GC B cells from apoptosis, and that EBV infection renders the cells independent from survival signals normally supplied by a BCR. The monoclonal LCLs represent valuable models for early stages of lymphoma development in classic HL and PTLD.

Pathogenesis of Hodgkin's lymphoma

In Hodgkin's lymphoma (HL), the B cell origin of the tumour cells, the Hodgkin and Reed-Sternberg (HRS) cells, has been disclosed by molecular single cell analysis about 10 yr ago. This finding formed the basis for various studies aimed to better understand the pathogenesis of this peculiar malignancy and the pathophysiology of the HRS cells. Work of our groups in this regard was focussed recently on two main topics, namely the study of differential gene expression in HRS cells and the pathogenesis of composite lymphomas. Composite lymphomas are combinations of HL and B cell non-Hodgkin lymphomas, that turned out to be often clonally related. By molecular analysis of several composite lymphomas for potential transforming events, we identified examples of both shared as well as distinct transforming events. Comparing gene expression profiles of HL-derived cell lines with the corresponding profiles from other B cell lymphomas and normal B cell subsets revealed a global down-regulation of the B cell-specific gene expression signature in HRS cells. Moreover, we identifed aberrant expression and activity of multiple receptor tyrosine kinases in HRS cells of classical and to a lesser extend lymphocyte predominant HL, which appears to be a unique feature of HL, and may offer novel strategies for treatment.

Epstein-Barr virus latent membrane protein 2A mimics B-cell receptor-dependent virus reactivation

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) shares protein motifs with the B-cell receptor that play a role in B-cell receptor signalling and has been shown to mimic an activated B-cell receptor by providing a survival signal for mature B cells in transgenic mice. Conversely, LMP2A has been reported not to support but to inhibit B-cell receptor signalling with respect to virus reactivation and to block lytic virus induction after anti-Ig treatment of EBV-infected B cells. To solve this apparent paradox, the role of LMP2A in lytic-cycle induction was re-examined in B cells conditionally immortalized by EBV. It was shown that, in the absence of other stimuli, LMP2A expression alone could lead to induction of the virus lytic cycle. Similarly to B-cell receptor stimulation by anti-Ig treatment, this LMP2A-mediated reactivation was dependent on the mitogen-activated protein kinase pathway and could be inhibited by the viral LMP1. Our data reinforce the notion that LMP2A is a functional homologue of the B-cell receptor, not only with respect to B-cell survival but also with respect to regulation of the lytic cycle.

Dileucine and YXXL motifs in the cytoplasmic tail of the bovine leukemia virus transmembrane envelope protein affect protein expression on the cell surface

Several retroviruses downmodulate the cell surface expression of envelope (Env) proteins through peptide sequences located in the cytoplasmic tail of the transmembrane (TM) subunit. We investigated whether cell surface expression of a chimeric protein containing the cytoplasmic domain of the TM protein (CTM) of bovine leukemia virus (BLV) was regulated by two membrane-proximal dileucine motifs or by tyrosine Y487 or Y498 in YXXL motifs. A chimeric protein composed of the extracellular and membrane-spanning portions of human CD8-alpha plus a wild-type (wt) BLV CTM was detectable on the surface of only 40% of the cells in which it was transiently expressed. Replacement of either dileucine pair with alanines increased the level of surface display of chimeric proteins. Nearly all cells became surface positive when both dileucine motifs were altered simultaneously and when either an N-terminal segment containing both dileucine motifs or a C-terminal segment containing all YXXL motifs was deleted. In contrast, replacement of Y487 or Y498 with alanine or phenylalanine enabled only small increases in surface display compared with wt levels. Chimeric proteins had similar stabilities but were downmodulated from the cell surface at three different rates. Point mutants segregated into each of the three groups of proteins categorized according to these different rates. Interestingly, Y487 mutants were downmodulated less efficiently than Y498 mutants, which behaved like wt. CD8-CTM chimeric proteins were phosphorylated on serine residues, but the native BLV Env protein was not phosphorylated either in transfected cells or in a lymphoid cell line constitutively producing BLV. Thus, both dileucine and YXXL motifs within the BLV CTM contribute to downmodulation of a protein containing this domain. Interactions with other proteins may influence surface exposure of Env protein complexes in virus-infected cells, assisting in viral evasion of adaptive immunity.

B cells under influence: transformation of B cells by Epstein-Barr virus

Epstein-Barr virus (EBV) is an extremely successful virus, infecting more than 90% of the human population worldwide. After primary infection, the virus persists for the life of the host, usually as a harmless passenger residing in B cells. However, EBV can transform B cells, which can result in the development of malignant lymphomas. Intriguingly, the three main types of EBV-associated B-cell lymphoma - that is, Burkitt lymphoma, Hodgkin lymphoma and post-transplant lymphomas - seem to derive from germinal-centre B cells or atypical survivors of the germinal-centre reaction in most, if not all, cases, indicating that EBV-infected germinal-centre B cells are at particular risk for malignant transformation.

Bovine leukemia virus gp30 transmembrane (TM) protein is not tyrosine phosphorylated: examining potential interactions with host tyrosine-mediated signaling

Bovine leukemia virus (BLV) causes persistent lymphocytosis, a preneoplastic, polyclonal expansion of B lymphocytes. The expansion increases viral transmission to new hosts, but the mechanisms of this expansion have not been determined. We hypothesized that BLV infection contributes to B-cell expansion by signaling initiated via viral transmembrane protein motifs undergoing tyrosine phosphorylation. Viral mimicry of host cell proteins is a well-demonstrated mechanism by which viruses may increase propagation or decrease recognition by the host immune system. The cytoplasmic tail of BLV transmembrane protein gp30 (TM) has multiple areas of homology to motifs of host cell signaling proteins, including two immunoreceptor tyrosine-based activation motifs (ITAMs) and two immunoreceptor tyrosine-based inhibition motifs (ITIMs), which are homologous to B-cell receptor and inhibitory co-receptor motifs. Signaling by these motifs in B cells typically relies on tyrosine phosphorylation, followed by interactions with Src-homology-2 (SH2) domains of nonreceptor protein tyrosine kinases or phosphatases. Phosphorylation of tyrosine residues in the cytoplasmic tail of TM was tested in four systems including ex vivo cultured peripheral blood mononuclear cells from BLV infected cows, BLV-expressing fetal lamb kidney cell and bat lung cell lines, and DT40 B cells transfected with a fusion of mouse extracellular CD8alpha and cytoplasmic TM. No phosphorylation of TM was detected in our experiments in any of the cell types utilized, or with various stimulation methods. Detection was attempted by immunoblotting for phosphotyrosines, or by metabolic labeling of cells. Thus BLV TM is not likely to modify host signal pathways through interactions between phosphorylated tyrosines of the ITAM or ITIM motifs and host-cell tyrosine kinases or phosphatases.

Envelope proteins containing single amino acid substitutions support a structural model of the receptor-binding domain of bovine leukemia virus surface protein

Functional domains of the strikingly conserved envelope (Env) glycoproteins of bovine leukemia virus (BLV) and its close relative, human T-cell leukemia virus type 1 (HTLV-1), are still being defined. We have used BLV Env protein variants to gain insights into the structure and function of this important determinant of viral infectivity. Each of 23 different single amino acid variants found in cDNA clones of env transcripts present after short-term culture of peripheral blood mononuclear cells from BLV-infected sheep was expressed in COS-1 cells and tested for the ability to mediate cell fusion and to be cleaved to surface (SU) and transmembrane (TM) protein subunits. Of 11 Env variants that failed to induce syncytia or did so poorly, 7 contained changes in amino acids identical or chemically conserved in the HTLV-1 Env protein. These seven included the four variants that showed aberrant proteolytic cleavage and poor cell surface expression, underscoring their importance for Env structure. Ten of 12 variants that retained wild-type syncytium-inducing ability clustered in the N-terminal half of BLV SU, which forms the putative receptor-binding domain (RBD). Several variants in the RBD showed evidence of subtle misfolding, as judged by reduced binding to monoclonal antibodies recognizing conformational epitopes F, G, and H formed by the N terminus of SU. We modeled the BLV RBD by aligning putative structural elements with known elements of the ecotropic Friend murine leukemia virus RBD monomer. All the variant RBD residues but one are exposed on the surface of this BLV model. These variants as well as function-altering, antibody-reactive residues defined by other investigators group on one face of the molecular model. They are strikingly absent from the opposite face, implying that it is likely to face inward in Env complexes. This surface might interact with the C-terminal domain of SU or with an adjacent monomer in the Env oligomer. This location suggests an orientation for the monomer of ecotropic Friend murine leukemia virus RBD.

Molecular biology of Hodgkin's lymphoma

Hodgkin's lymphoma (HL) is characterized by typical mononucleated Hodgkin and multinucleated Reed-Sternberg cells, which occur at low frequency in a mixed cellular infiltrate in the tumor tissue. Because of the rarity of these cells and their unusual immunophenotype, which is strikingly different from those of all normal hematopoietic cell types, the origin of these cells and their clonality have long been unclear. Single-cell studies of rearranged immunoglobulin genes showed that Hodgkin and Reed-Sternberg (HRS) cells represent clonal tumor-cell populations derived from germinal center B cells. In classical HL, the detection of obviously crippling immunoglobulin gene mutations in a fraction of the cases suggests that HRS cells may derive from germinal center B cells that have lost the capacity to be positively selected by antigen and that normally would have undergone apoptosis. In rare cases, HRS cells represent transformed T lymphocytes. The transforming events involved in malignant transformation of HRS cells are still largely unknown. Constitutive activation of the transcription factor NFkappaB, which can, for example, be induced through Epstein-Barr virus transformation of HRS cells or destructive somatic mutations of the inhibitor of NFkappaB, is likely to be a key event in HL pathogenesis. Significant progress has been made in our understanding of the cellular interactions in HL tissues, which are mainly mediated by a large variety of cytokines and chemokines.

Role of the proline-rich motif of bovine leukemia virus transmembrane protein gp30 in viral load and pathogenicity in sheep

The cytoplasmic tail of bovine leukemia virus (BLV) transmembrane protein gp30 has multiple amino acid motifs that mimic those present in signaling proteins associated with B-cell and T-cell receptors. The proline-rich motif of gp30, PX(2)PX(4-5)P, is analogous to the recognition site of Src homology 3 (SH3) domains of signaling molecules. Using site-directed mutagenesis of an infectious molecular clone of BLV, point mutations were introduced which changed three of the prolines of the motif to alanines. The influence of these mutations on the pathogenicity of BLV was studied in sheep which received either (i) plasmid DNA with provirus containing proline-to-alanine mutations (pppBLV), (ii) plasmid DNA with wild-type provirus (wtBLV), or (iii) transfection reagent alone. Although all of the BLV-injected animals seroconverted at approximately the same time, viral loads at later time points were high in five of five of the wtBLV group and two of five of the pppBLV group but low in three of five of the pppBLV group, as determined by semiquantitative PCR. Viral expression was lower in the pppBLV-transfected sheep, as measured by p24 antigen enzyme-linked immunosorbent assay in cultured cells, and serologic titers were lower. Thirty-one months after transfection, four of four wtBLV-transfected sheep had died of leukemia and lymphoma, and all five of the pppBLV-transfected sheep were clinically healthy and had normal peripheral blood lymphocyte counts. These data indicate that the proline-rich motif of gp30 is not required for viral infectivity but is important for high viral load in vivo, suggesting that SH3-mediated gp30 interactions are critical for viral pathogenesis following infection. Absence of interactions with the proline-rich motif may prevent or delay tumorigenesis in sheep.

Epstein-Barr virus latent membrane protein 2A (LMP2A) employs the SLP-65 signaling module

In latently infected B lymphocytes, the Epstein-Barr virus (EBV) suppresses signal transduction from the antigen receptor through expression of the integral latent membrane protein 2A (LMP2A). At the same time, LMP2A triggers B cell survival by a yet uncharacterized maintenance signal that is normally provided by the antigen receptor. The molecular mechanisms are unknown as LMP2A-regulated signaling cascades have not been described so far. Using a novel mouse model we have identified the intracellular adaptor protein Src homology 2 (SH2) domain-containing leukocyte protein (SLP)-65 as a critical downstream effector of LMP2A in vivo. Biochemical analysis of the underlying signaling pathways revealed that EBV infection causes constitutive tyrosine phosphorylation of one of the two SLP-65 isoforms and complex formation between SLP-65 and the protooncoprotein CrkL (CT10 regulator of kinase like). This leads to antigen receptor-independent phosphorylation of Cbl (Casitas B lineage lymphoma) and C3G. In contrast, phospholipase C-gamma2 (PLC-gamma2) activation is completely blocked. Our data show that in order to establish a latent EBV infection, LMP2A selectively activates or represses SLP-65-regulated signaling pathways.

Latent membrane protein 2A of Epstein-Barr virus binds WW domain E3 protein-ubiquitin ligases that ubiquitinate B-cell tyrosine kinases

The latent membrane protein (LMP) 2A of Epstein-Barr virus (EBV) is implicated in the maintenance of viral latency and appears to function in part by inhibiting B-cell receptor (BCR) signaling. The N-terminal cytoplasmic region of LMP2A has multiple tyrosine residues that upon phosphorylation bind the SH2 domains of the Syk tyrosine kinase and the Src family kinase Lyn. The LMP2A N-terminal region also has two conserved PPPPY motifs. Here we show that the PPPPY motifs of LMP2A bind multiple WW domains of E3 protein-ubiquitin ligases of the Nedd4 family, including AIP4 and KIAA0439, and demonstrate that AIP4 and KIAA0439 form physiological complexes with LMP2A in EBV-positive B cells. In addition to a C2 domain and four WW domains, these proteins have a C-terminal Hect catalytic domain implicated in the ubiquitination of target proteins. LMP2A enhances Lyn and Syk ubiquitination in vivo in a fashion that depends on the activity of Nedd4 family members and correlates with destabilization of the Lyn tyrosine kinase. These results suggest that LMP2A serves as a molecular scaffold to recruit both B-cell tyrosine kinases and C2/WW/Hect domain E3 protein-ubiquitin ligases. This may promote Lyn and Syk ubiquitination in a fashion that contributes to a block in B-cell signaling. LMP2A may potentiate a normal mechanism by which Nedd4 family E3 enzymes regulate B-cell signaling.

Deregulated signal transduction by the K1 gene product of Kaposi's sarcoma-associated herpesvirus

The Kaposi's sarcoma (KS)-associated herpesvirus is a lymphotropic virus strongly implicated in the pathogenesis of KS and several lymphoproliferative disorders. The KS-associated herpesvirus K1 gene encodes a transmembrane protein bearing a functional immunoreceptor tyrosine-based activation motif (ITAM)-like sequence; it previously has been proposed to be important in viral tumorigenesis because its expression can trigger cell proliferation in vitro and in vivo. Here we show that expression of the full-length K1 protein can initiate calcium-dependent signal transduction in B cells; however, unlike other ITAM-based signal transduction events, K1 signaling occurs constitutively, in the absence of exogenous crosslinking ligands. This property is caused by its cysteine-rich ectodomain, which when transferred to other consensus ITAMs induces constitutive signaling. Although ITAM-based signaling by K1 involves classical syk and phospholipase C gamma2 activation, both ITAM- and syk-independent signaling pathways are activated by K1 expression. These studies indicate that K1 is a deregulated signaling molecule with pleitropic effects that may explain its known growth deregulatory properties.

Bovine leukemia virus transmembrane protein gp30 physically associates with the down-regulatory phosphatase SHP-1

In B lymphocytes, the down-regulatory phosphatase SHP-1 associates with CD22 and CD32b (also known as FcgammaRIIB) and acts as a critical negative regulator of B-cell receptor signaling. Bovine leukemia virus, a retrovirus of the HTLV/BLV group, causes persistently increased numbers of peripheral blood B lymphocytes, known as persistent lymphocytosis (PL) and, in some animals, progression to B-cell leukemia and/or lymphoma. Here, we show that SHP-1 associates with the bovine leukemia virus transmembrane protein, gp30. This interaction is either direct or indirect. The interaction is dependent on tyrosine phosphorylation, and the interaction increases after cell stimulation with sodium pervanadate. The gp30-SHP-1 interaction is seen in all of the BLV-infected, PL animals tested, but is not seen in uninfected animals or in most BLV-infected, non-PL animals, which do not express significant quantities of gp30. However, one BLV-infected, non-PL animal expressed large quantities of gp30, yet no gp30-SHP-1 interaction was detected, suggesting that there may be other factors in cells from the PL animals that facilitate the gp30-SHP-1 interaction. The association of gp30 and SHP-1 suggests the hypothesis that gp30 may act as a decoy to sequester SHP-1, resulting in up-regulation of B-cell receptor signaling. The implication of this could be a novel mechanism of viral activation of lymphocytes by removal of a down-regulatory phosphatase.

Conservative mutations in the immunosuppressive region of the bovine leukemia virus transmembrane protein affect fusion but not infectivity in vivo

Many retroviruses, including bovine leukemia virus (BLV), contain a highly conserved region located about 40 amino acids downstream from the fusion peptide within the sequence of the external domain of the transmembrane (TM) protein. This region is notably thought to be involved in the presentation of the NH2-terminal peptide to allow cell fusion. By using hydrophobic cluster analysis and by analogy with the influenza A hemagglutinin structures, the core of the TM structure including this particular region was predicted to consist, in the BLV and other retroviral envelope proteins, of an alpha-helix followed by a loop region, both docked against a subsequent alpha-helix that forms a triple-stranded coiled coil. The loop region could undergo, as in hemagglutinin, a major refolding into an alpha-helix integrating the coiled coil structure and putting the fusion peptide to one tip of the molecule. Based on this model, we have identified amino acids that may be essential to the BLV TM structure, and a series of mutations were introduced in the BLV env gene of an infectious molecular clone. A first series of mutations was designed to disturb the coiled coil structure (substitutions with proline residues), whereas others would maintain the general TM structure. When expressed by Semliki Forest virus recombinants, all the mutated envelope proteins were stable and efficiently synthesized in baby hamster kidney cells. Both proline-substituted and conservative mutants were strongly affected in their capacity to fuse to CC81 indicator cells. In addition, it appeared that the integrity of the TM coiled coil structure is essential for envelope protein multimerization, as analyzed by metrizamide gradient centrifugation. Finally, to gain insight into the role of this coiled coil in the infectious potential of BLV in vivo, the mutated TM genes were introduced in an infectious and pathogenic molecular clone and injected into sheep. It appeared that only the conservative mutations (A60V and A64S) allowed maintenance of viral infectivity in vivo. Since these mutations destroyed the ability to induce syncytia, we conclude that efficient fusion capacity of the recombinant envelopes is not a prerequisite for the infectious potential of BLV in vivo. Viral propagation of these mutants was strongly affected in some of the infected sheep. However, the proviral loads within half of the infected animals (2 out of 2 for A60V and 1 out of 4 for A64S) were close to the wild-type levels. In these sheep, it thus appears that the A60V and A64S mutants propagate efficiently despite being unable to induce syncytia in cell culture.

Epstein-Barr virus latent membrane protein 2 associates with and is a substrate for mitogen-activated protein kinase

The latent membrane protein 2 (LMP2) of Epstein-Barr virus interferes with B-lymphocyte signal transduction through the immunoglobulin (Ig) receptor. Two isoforms of LMP2 exist and differ only in that one isoform (LMP2a) contains an N-terminal cytoplasmic domain that the other isoform does not. LMP2a is a phosphoprotein that is phosphorylated on tyrosines and serines in the cytoplasmic domain. GST1-119, a glutathione S-transferase (GST) fusion protein containing the 119 amino acids of the cytoplasmic domain, affinity precipitated serine kinase activity from BJAB cell extracts. The affinity-precipitated kinase phosphorylated LMP2a sequences, and kinase activity was increased following induction. Probing of Western immunoblots of affinity-precipitated proteins showed that the Erk1 form of mitogen-activated protein kinase (MAPK) was present. Purified MAPK phosphorylated GST fusion proteins containing the cytoplasmic domain of LMP2a and mutational analyses were used to identify S15 and S102 as the sites of in vitro phosphorylation. A polyclonal rabbit antiserum was prepared against a maltose binding protein-LMP2a cytoplasmic domain fusion protein (MBP1-119) and used to immunoprecipitate LMP2a from the in vitro-immortalized lymphoblastoid B-cell line B95-8CR. LMP2a immunoprecipitates from B95-8CR contained MAPK as a coprecipitated protein. Cross-linking surface Ig on B95-8CR cells failed to induce MAPK activity within the cells. Treatment of B95-8CR with phorbol myristate acetate (PMA) was able to bypass the Ig receptor block and activate MAPK activity. Phosphorylation of LMP2a on serine residues increased after PMA induction. The possible role for LMP2a serine phosphorylation by MAPK in the control of latency is discussed.

Signaling difference between class IgM and IgD antigen receptors

Most mature B lymphocytes coexpress two classes of antigen receptor, IgM and IgD. The differences in the signal transduction from the two receptors are still a matter of controversy. We have analyzed B-cell lines expressing IgM or IgD antigen receptors with the same antigen specificity. Cross-linking of these receptors with either antigen or class-specific antibodies results in the activation of protein tyrosine kinases and the phosphorylation of the same substrate proteins. The kinetic and intensity of phosphorylation, however, was quite different between the two receptors when they were cross-linked by antigen. In membrane IgM-expressing cells, the substrate phosphorylation reached a maximum after one minute and diminished after 60 minutes, whereas in the membrane IgD-expressing cells, the substrate phosphorylation increased further over time, reaching its maximum at 60 minutes and persisting longer than 240 minutes after exposure to antigen. Recently prolonged signaling has been found to be responsible for signaling differences between tyrosine kinase receptors using otherwise similar signaling routes. Thus, the duration of a signal may be an important biological feature of signal-transducing cascades.

The YXXL signalling motifs of the bovine leukemia virus transmembrane protein are required for in vivo infection and maintenance of high viral loads

The bovine leukemia virus (BLV) transmembrane protein (gp30) contains three YXXL motifs at its carboxyterminal end. Two of these motifs have been implicated in vitro in signal transduction pathways from the external to the intracellular compartment. In order to analyze the biological relevance of these motifs in vivo, recombinant BLV proviruses were constructed. A mutation of the tyrosine residue of the second YXXL motif completely destroyed the infectious potential of the virus in sheep. In contrast, the tyrosine of the first motif appeared to be dispensable for infectivity. However, the propagation of the recombinant virus within the animal was greatly impaired (as demonstrated by PCR and enzyme-linked immunosorbent assay). These recombinant BLVs thus exhibit an attenuated phenotype. Altogether, our data demonstrate the importance of the YXXL motifs of the BLV transmembrane protein for in vivo infection and viral propagation.

A comparison of the interaction of Shc and the tyrosine kinase ZAP-70 with the T cell antigen receptor zeta chain tyrosine-based activation motif

Tyrosine-based activation motifs (TAMs) define a conserved signaling sequence, EX2YX2L/IX7YX2L/I, that couples the T cell antigen receptor to protein tyrosine kinases and adapter molecules. The present study shows that phosphorylation of both tyrosines within the motif is required for high affinity binding of the tyrosine kinase ZAP-70 whereas phosphorylation of the single COOH-terminal tyrosine within the motif is optimal for the binding of the adapter Shc. There were also quantitative differences in the ZAP-70 and Shc association with the zeta 1-TAM since nM concentrations of the doubly phosphorylated zeta 1-TAM are sufficient for ZAP-70 recruitment whereas micromolar levels of singly phosphorylated TAMs are necessary for Shc binding. Shc is tyrosine phosphorylated in antigen receptor-activated T cells and can potentially form a complex with the adapter molecule Grb2 and could thus recruit the Ras guanine nucleotide exchange protein Sos into the antigen receptor complex. The present data show that Grb2 can bind to the phosphorylated TAM, but this binding is independent of Shc and there is no formation of zeta 1-TAM.Shc.Grb2.Sos complexes in antigen receptor-activated cells. Accordingly, Shc function should not be considered in the context of Grb2/Sos recruitment to the T cell antigen receptor complex.

Integral membrane protein 2 of Epstein-Barr virus regulates reactivation from latency through dominant negative effects on protein-tyrosine kinases

An Epstein-Barr virus-encoded protein, LMP2, blocks the effects of surface immunoglobulin (slg) cross-linking on calcium mobilization and on lytic reactivation of EBV in latently infected and growth-transformed primary human B lymphocytes. In wild-type EBV-transformed cells, LMP2 is constitutively tyrosine phosphorylated and is associated with Lyn and Syk protein-tyrosine kinases (PTKs). Baseline Lyn PTK activity is substantially reduced, and slg cross-linking fails to activate Lyn, Syk, Pl3-K, PLC gamma 2, Vav, Shc, and MAPK. Syk, Pl3-K, PLC gamma 2, and Vav are constitutively tyrosine phosphorylated, and their tyrosine phosphorylation does not change following slg cross-linking. In contrast, cross-linking slg on cells transformed by LMP2 null mutant EBV recombinants triggers the same protein tyrosine kinase cascade as in noninfected B lymphocytes. These data are consistent with a model in which LMP2 is a constitutive dominant negative modulator of slg receptor signaling through its effects on Lyn, Syk, or regulators of these kinases.

Function of B-cell antigen receptor of different classes

Most mature B lymphocytes co-express two classes of antigen receptor, IgM and IgD. The differences in the signal transduction from the 2 receptors are still a matter of controversy. We have analysed B-cell lines expressing IgM or IgD antigen receptors with the same antigen specificity. Cross-linking of these receptors with either antigen or class-specific antibodies results in the activation of protein tyrosine kinases and the phosphorylation of the same substrate proteins. The kinetics and intensity of phosphorylation, however, were quite different between the 2 receptors when they were cross-linked by antigen. In membrane IgM-expressing cells, the substrate phosphorylation reached a maximum already after 1 min and diminished after 60 min whereas in the membrane IgD-expressing cells, the substrate phosphorylation increases further over time, reached its maximum at 60 min and persisted longer than 240 min after exposure to antigen. Recently prolonged signaling has been found to be responsible for signaling differences between tyrosine kinase receptors using otherwise similar signaling routes. Thus, the duration of a signal may be an important biological feature of signal transducing cascades.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.

Mechanistic studies of a signaling pathway activated by the organic dimerizer FK1012

BACKGROUND

The T-cell receptor (TCR) signaling pathway is initiated by regulated association of TCR chains, including the zeta chain. A recently reported method for inducing the dimerization or oligomerization of targeted proteins in cells used the TCR pathway as a test system. In cells transfected with cDNA encoding MZF3E, a chimeric receptor comprising the intracellular domain of the zeta chain and three copies of FK506-binding protein (FKBP), low concentrations of a synthetic dimer of the natural product FK506 (FK1012) activated the expression of reporter genes. We set out to examine the signaling pathway initiated by FK1012.

RESULTS

We characterized the effect of FK1012 on MZF3E and a second chimeric receptor, MZF1E, which contains the zeta chain and one copy of FKBP. Only MZF3E gave FK1012-activated signaling, as shown by an increase in the kinase activity associating with MZF3E, and the appearance of specific phosphotyrosine-containing proteins. Signaling required localization of MZF3E to the inner plasma membrane, and activation of gene transcription in response to FK1012 was dependent on the protein phosphatase calcineurin and the transcriptional activator NF-AT. Some signaling events in the pathway had different kinetics when activated by MZF3E instead of the TCR, however. An unexpected requirement for the prolonged activation of calcineurin was observed.

CONCLUSIONS

Synthetic dimerizers can be used to gain control over cellular processes that require the association of specific intracellular proteins. The TCR signaling pathway was selected as an initial test system; we show here that one can indeed activate this signaling pathway by inducing the oligomerization of the cytoplasmic tail of the zeta chain with the cell-permeable reagent FK1012.

Transmembrane signalling by the B-cell antigen receptor

Crosslinking the B-cell antigen receptor is sufficient to generate intracellular signals. Recent work has shown that this signalling capability can be ascribed to the presence of the alpha and beta sheath proteins within the antigen receptor that couple it to signal transduction pathways. However, a variety of other transmembrane proteins, including CD19, CD21, CD22, CD32 and CD45, can also associate with the receptor and we are beginning to understand how they may act in concert with it to efficiently regulate B lymphocyte activity in response to antigen.

Role of tyrosine kinases in lymphocyte activation

Interaction of T- and B-cell antigen receptors with cytoplasmic non-receptor tyrosine protein kinases is critical to the activation of lymphocytes by antigen. Both the src-family tyrosine protein kinases Lck, Fyn, Lyn and Blk and the syk-family tyrosine protein kinases Syk and ZAP-70 play a role in lymphocyte activation. The antigen receptors are coupled to this cluster of kinases by the cytoplasmic tails of the gamma, delta, epsilon, zeta, and eta subunits of the T-cell receptor, and the Ig-alpha and Ig-beta subunits of the B-cell receptor. Each of these proteins contains one or more 'tyrosine based activation motifs', with the amino acid sequence D/EX7D/EXXYXXL/IX7YXXL/I. This motif appears to allow binding of one or more src-like kinases, via their unique amino termini, before the onset of lymphocyte activation. Invariant tyrosines in the motif become phosphorylated following the triggering of lymphocyte activation, and this modification induces the binding of the src- and syk-family tyrosine protein kinases, and potentially other signalling molecules, through SH2 domains to the antigen receptors.

The hetero-oligomeric antigen receptor complex and its coupling to cytoplasmic effectors

T-cell and B-cell antigen receptors are representative of a family of multisubunit receptors that utilize Src-family kinases as proximal cytoplasmic effectors in signal transduction. Recent studies have shown that distinct receptor subunits mediate ligand and effector interactions and demonstrate that physical interaction with effectors, and their activation, is a function of a 26 amino acid motif found in multiple receptor subunits. Further, receptor ligation induces tyrosine phosphorylation of this motif, and this initiates SH2-mediated association and activation of Src-family kinases and, apparently, ZAP70 kinases. Finally, this association triggers SH3-mediated binding of Lyn and Fyn to PI3-K, resulting in PI3-K activation. An integrated model of signal transduction is presented.

Signal transduction by lymphocyte antigen receptors

Despite the differences in the antigens that they recognize and in the effector functions they carry out, B and T lymphocytes utilize remarkably similar signal transduction components to initiate responses. They both use oligomeric receptors that contain distinct recognition and signal transduction subunits. Antigen receptors on both cells interact with at least two distinct families of PTKs via common sequence motifs, ARAMs, in the cytoplasmic tails of their invariant chains, which have likely evolved from a common evolutionary precursor. Coreceptors appear to serve to increase the sensitivity of both of these receptor systems through events that influence ligand binding and signal transduction. The critical role of tyrosine phosphorylation of downstream signaling components, such as phospholipase C, is the net result of changes in the balance of the action of antigen receptor-regulated PTKs and PTPases. The identification of downstream effectors, including calcineurin and Ras, that regulate cellular responses, such as lymphokine gene expression, promises the future possibility of connecting the complex pathway from the plasma membrane to the nucleus in lymphocytes. Insight gained from studies of the signaling pathways downstream of TCR and BCR stimulation is likely to contribute significantly to future understanding of mechanisms responsible for lymphocyte differentiation and for the discrimination of self from nonself in developing and mature cells.

B-cell activation by wild type and mutant Ig-beta cytoplasmic domains

In B lymphocytes, the cytoplasmic domains of the membrane immunoglobulin-associated heterodimeric Ig-alpha and Ig-beta proteins link membrane immunoglobulin to intracellular signalling molecules. We constructed chimeric genes encoding the extracellular and transmembrane domain of human CD8 alpha and the cytoplasmic domain of Ig-alpha or Ig-beta and examined the ability of the chimeric proteins to induce signalling in the murine B-cell lymphoma A20. Crosslinking of CD8/Ig-alpha or CD8/Ig-beta induced both calcium mobilization and protein tyrosine phosphorylation, although induction by CD8/Ig-alpha was somewhat stronger. We also carried out mutagenesis of residues within the "Reth" motif of the CD8/Ig-beta cytoplasmic domain and determined the effects of these mutations on signalling in the murine B-cell hybridoma LK 35.2. Mutants in which alanine was substituted for glutamine 202, threonine 205, and isoleucine 209 retained the ability to induce protein tyrosine phosphorylation and calcium mobilization. In contrast, substitution of alanine for leucine 198 abrogated these responses, suggesting a critical role for this residue in interaction with cytoplasmic signalling proteins.