Specific CD45 isoforms differentially regulate T cell receptor signaling

Role of alternative splicing in fish immunity

Alternative splicing serves as a pivotal source of complexity in the transcriptome and proteome, selectively connecting various coding elements to generate a diverse array of mRNAs. This process encodes multiple proteins with either similar or distinct functions, contributing significantly to the intricacies of cellular processes. The role of alternative splicing in mammalian immunity has been well studied. Remarkably, the immune system of fish shares substantial similarities with that of humans, and alternative splicing also emerges as a key player in the immune processes of fish. In this review, we offer an overview of alternative splicing and its associated functions in the immune processes of fish, and summarize the research progress on alternative splicing in the fish immunity. Furthermore, we review the impact of alternative splicing on the fish immune system's response to external stimuli. Finally, we present our perspectives on future directions in this field. Our aim is to provide valuable insights for the future investigations into the role of alternative splicing in immunity.

Strategies and rules for tuning TCR-derived therapy

Manipulation of T cells has revolutionized cancer immunotherapy. Notably, the use of T cells carrying engineered T cell receptors (TCR-T) offers a favourable therapeutic pathway, particularly in the treatment of solid tumours. However, major challenges such as limited clinical response efficacy, off-target effects and tumour immunosuppressive microenvironment have hindered the clinical translation of this approach. In this review, we mainly want to guide TCR-T investigators on several major issues they face in the treatment of solid tumours after obtaining specific TCR sequences: (1) whether we have to undergo affinity maturation or not, and what parameter we should use as a criterion for being more effective. (2) What modifications can be added to counteract the tumour inhibitory microenvironment to make our specific T cells to be more effective and what is the safety profile of such modifications? (3) What are the new forms and possibilities for TCR-T cell therapy in the future?

Tetraspanin CD53 controls T cell immunity through regulation of CD45RO stability, mobility, and function

T cells depend on the phosphatase CD45 to initiate T cell receptor signaling. Although the critical role of CD45 in T cells is established, the mechanisms controlling function and localization in the membrane are not well understood. Moreover, the regulation of specific CD45 isoforms in T cell signaling remains unresolved. By using unbiased mass spectrometry, we identify the tetraspanin CD53 as a partner of CD45 and show that CD53 controls CD45 function and T cell activation. CD53-negative T cells (Cd53^-/-) exhibit substantial proliferation defects, and Cd53^-/- mice show impaired tumor rejection and reduced IFNγ-producing T cells compared with wild-type mice. Investigation into the mechanism reveals that CD53 is required for CD45RO expression and mobility. In addition, CD53 is shown to stabilize CD45 on the membrane and is required for optimal phosphatase activity and subsequent Lck activation. Together, our findings reveal CD53 as a regulator of CD45 activity required for T cell immunity.

Alternative Splicing of Pre-mRNA in the Control of Immune Activity

The human immune response is a complex process that responds to numerous exogenous antigens in preventing infection by microorganisms, as well as to endogenous components in the surveillance of tumors and autoimmune diseases, and a great number of molecules are necessary to carry the functional complexity of immune activity. Alternative splicing of pre-mRNA plays an important role in immune cell development and regulation of immune activity through yielding diverse transcriptional isoforms to supplement the function of limited genes associated with the immune reaction. In addition, multiple factors have been identified as being involved in the control of alternative splicing at the cis, trans, or co-transcriptional level, and the aberrant splicing of RNA leads to the abnormal modulation of immune activity in infections, immune diseases, and tumors. In this review, we summarize the recent discoveries on the generation of immune-associated alternative splice variants, clinical disorders, and possible regulatory mechanisms. We also discuss the immune responses to the neoantigens produced by alternative splicing, and finally, we issue some alternative splicing and immunity correlated questions based on our knowledge.

Engagement of CD45 alters early signaling events in human T cells co-stimulated through TCR + CD28

Previously our lab has shown that co-stimulation of human T cells through different co-stimulatory molecules tune differentiation to different phenotypes. An open question is where in the signaling pathways induced by the co-stimulation do differences occur that contribute to outcome of differentiation. In this project, we investigate the early signaling process by comparing events that follow engagement of CD45 alone or in association with a co-stimulatory molecule: CD28. CD45 plays a crucial role to initiate T cell signaling by dephosphorylating a negatively regulatory tyrosine residue in Src family kinases such as Lck. First, we observed that engagement of CD45 alone induced signaling in T cells. We observed that TCR/CD3 stimulation with CD45 promoted prolonged Lck association with TCR/CD3 complex and Lck remained associated during TCR/CD3 + CD28 + CD45 stimulation as well. We concluded that Lck association is dependent on TCR/CD3 and CD45 engagement. Hence, CD45 altered early signaling events in T cells.

CD45 in human physiology and clinical medicine

CD45 is an evolutionary highly conserved receptor protein tyrosine phosphatase exclusively expressed on all nucleated cells of the hematopoietic system. It is characterized by the expression of several isoforms, specific to a certain cell type and the developmental or activation status of the cell. CD45 is one of the key players in the initiation of T cell receptor signaling by controlling the activation of the Src family protein-tyrosine kinases Lck and Fyn. CD45 deficiency results in T- and B-lymphocyte dysfunction in the form of severe combined immune deficiency. It also plays a significant role in autoimmune diseases and cancer as well as in infectious diseases including fungal infections. The knowledge collected on CD45 biology is rather vast, but it remains unclear whether all findings in rodent immune cells also apply to human CD45. This review focuses on human CD45 expression and function and provides an overview on its ligands and role in human pathology.

In vitro reconstitution of T cell receptor-mediated segregation of the CD45 phosphatase

T cell signaling initiates upon the binding of peptide-loaded MHC (pMHC) on an antigen-presenting cell to the T cell receptor (TCR) on a T cell. TCR phosphorylation in response to pMHC binding is accompanied by segregation of the transmembrane phosphatase CD45 away from TCR-pMHC complexes. The kinetic segregation hypothesis proposes that CD45 exclusion shifts the local kinase-phosphatase balance to favor TCR phosphorylation. Spatial partitioning may arise from the size difference between the large CD45 extracellular domain and the smaller TCR-pMHC complex, although parsing potential contributions of extracellular protein size, actin activity, and lipid domains is difficult in living cells. Here, we reconstitute segregation of CD45 from bound receptor-ligand pairs using purified proteins on model membranes. Using a model receptor-ligand pair (FRB-FKBP), we first test physical and computational predictions for protein organization at membrane interfaces. We then show that the TCR-pMHC interaction causes partial exclusion of CD45. Comparing two developmentally regulated isoforms of CD45, the larger R_ABC variant is excluded more rapidly and efficiently (∼50%) than the smaller R₀ isoform (∼20%), suggesting that CD45 isotypes could regulate signaling thresholds in different T cell subtypes. Similar to the sensitivity of T cell signaling, TCR-pMHC interactions with K_ds of ≤15 µM were needed to exclude CD45. We further show that the coreceptor PD-1 with its ligand PD-L1, immunotherapy targets that inhibit T cell signaling, also exclude CD45. These results demonstrate that the binding energies of physiological receptor-ligand pairs on the T cell are sufficient to create spatial organization at membrane-membrane interfaces.

Expression of CD45 isoforms correlates with differential proliferative responses of peripheral CD4+ and CD8+ T cells

CD4(+) T cells express IL-2 receptor complexes to the same level as CD8(+) T cells when the two T cell populations were stimulated simultaneously. However, the activation of downstream signaling molecules, such as Jaks, was increased in CD8(+) T cells. Although equivalent amounts of CD45, which acts as a Jak phosphatase, was expressed on the two T cell populations, those on the CD8(+) T cells have less protein tyrosine phosphatase activity than those on the CD4(+) T cells. Furthermore, we find that different CD45 isoforms dominate in the two populations; CD45RO on proliferating CD4(+) T cells and CD45RBC on proliferating CD8(+) T cells. In addition, NIH3T3 cells expressing the CD45RBC transgene had more phosphorylated Jak1 and grew faster than those with the CD45RO transgene. Thus, the expression of specific CD45 isoforms on T cells correlates with their proliferative response to IL-2, suggesting that controlling cells expressing specific CD45 isoforms could correct excessive or insufficient immune responses.

Mitogenic signals initiated via interleukin-6 receptor complexes in cooperation with other transmembrane molecules in myelomas

Cytokines exert multiple biological functions through binding to their specific receptors that triggers activation of intracellular signaling cascades. The cytokine-mediated signals may produce variable and even opposing effects on different cell types, depending on cellular context that is also dictated by the differentiation stage of the cell. Multiple myeloma (MM) is a monoclonal proliferative disorder of human plasma cells. Myeloma cells appear to include mixed subpopulations in accordance with the expression of their surface antigens, such as CD45. Although interleukin-6 (IL-6) is widely accepted as the most relevant growth factor for myeloma cells, only a few subpopulations of tumor cells, such as CD45(+) immature cells, proliferate in response to IL-6. The activation of both signal transducer and activator of transcription (STAT) 3 and extracellular signal-regulated kinase (ERK) 1/2 is not sufficient for IL-6-induced proliferation of myeloma cells that requires the src family kinase activation associated with a rapid translocation of CD45 to lipid rafts. The CD45 expression renders myeloma cells competent for not only mitogenic but also apoptotic stimuli, resulting in either proliferation or apoptosis of CD45(+) myeloma cells dependently upon the circumstantial stimuli. In contrast, in CD45(-) myeloma cells highly expressing IL-6 receptor alpha chain (IL-6Ralpha), IL-6Ralpha and insulin-like growth factor (IGF)-I receptors exist on plasma membrane in close proximity, facilitating efficient assembly of two receptors in response to IL-6. The synergistic effects of IL-6Ralpha on IGF-I receptor-mediated signals provide a novel insight into a Jak-independent IL-6 signaling mechanism of receptor cross talk in human myeloma cells. Furthermore, the signaling cross talk between the cytokine receptor, IL-6Ralpha/gp130 and the growth factor receptor tyrosine kinase, fibroblast growth factor receptor (FGFR) 3 appears in myeloma cells carrying t(4;14)(p16.3;q32). In this review we propose several mechanisms of the IL-6-induced cell proliferation that is strictly dependent upon the cellular context in myelomas.

Auxiliary splice factor U2AF26 and transcription factor Gfi1 cooperate directly in regulating CD45 alternative splicing

By alternative splicing, different isoforms of the transmembrane tyrosine phosphatase CD45 are generated that either enhance or limit T cell receptor signaling. We report here that CD45 alternative splicing is regulated by cooperative action of the splice factor U2AF26 and the transcription factor Gfi1. U2AF26 promoted formation of the less-active CD45RO by facilitating exon exclusion. Gfi1 antagonized that process by directly interacting with U2AF26, identifying a previously unknown link between a transcription factor and alternative splicing. The presence of Gfi1 led to formation of the more-active CD45RB, whereas loss of Gfi1 favored CD45RO production. We propose that the relative abundance of U2AF26 and Gfi1 determines the ratio of CD45 isoforms, thereby regulating T cell activation.

CD45: all is not yet crystal clear

CD45 has been recognized as an important player in regulating signalling in lymphocytes. However, compared with tyrosine kinases, phosphatases are still poorly understood in terms of the details of their specificity and regulation. Here, the recent progress in understanding the biology of the first recognized receptor tyrosine phosphatase, CD45, is reviewed.

A rapid translocation of CD45RO but not CD45RA to lipid rafts in IL-6-induced proliferation in myeloma

CD45, a receptor-type tyrosine phosphatase, is required for interleukin-6 (IL-6)-induced proliferation in human myeloma cells, which express the shortest isoform, CD45RO, but not the longest isoform, CD45RA. Here, we showed that IL-6 induced the translocation of CD45 to lipid rafts in an isoform-dependent manner. In myeloma cells, CD45RO was translocated to lipid rafts more rapidly than CD45RB, but exogenously expressed CD45RA was not translocated. When an IL-6Rα-transfected B-cell line was stimulated with IL-6, CD45RA was not translocated, although CD45RB was. We further confirmed that the translocated CD45 bound to IL-6Rα, Lyn, and flotillin-2, and this was followed by the dephosphorylation of the negative regulatory Tyr507 of Lyn. CD45 also bound to phosphoprotein associated with glycosphingolipid-enriched microdomains (PAGs), which were subsequently dephosphorylated, resulting in the release of C-terminal src kinase (Csk) from lipid rafts. Therefore, these results indicate that a rapid translocation of CD45RO to lipid rafts may be responsible for IL-6-induced proliferation, and that the change from CD45RA to CD45RO confers the ability to respond to IL-6 in human myeloma cells. (Blood. 2005;105:3295-3302)

Mutation of chicken anemia virus VP2 differentially affects serine/threonine and tyrosine protein phosphatase activities

Novel dual-specificity protein phosphatases (DSPs), which catalyse the removal of phosphate from both phosphotyrosine and phosphoserine/phosphothreonine substrates, have recently been identified in two viruses within the familyCircoviridae. Viral protein 2 (VP2) of chicken anemia virus (CAV) and ORF2 of TT virus have been shown to possess DSP activityin vitro. CAV VP2 is unusual in possessing two vicinal cysteines within the protein phosphatase signature motif. The first cysteine residue (C95) within the motif has been identified by mutagenesis as the essential catalytic cysteine. In this study, it was shown that virus mutated at this residue displayed a marked inhibition of growth, with titres reduced 104-fold, and reduced cytopathogenic effect in cell culture, indicating that viral DSP activity may be significant during infection. As with virus mutated at the first cysteine residue, mutation of the second cysteine (C97) within the motif resulted in a marked reduction in viral growth and attenuation of cytopathogenicity in infected cell cultures. However, mutagenesis of this second cysteine only reduced phosphotyrosine phosphatase activity to 70 % of that of wild-type VP2, but increased phosphoserine/phosphothreonine phosphatase activity by as much as 700 %. The differential effect of the C97S mutation on VP2 activity does not appear to have parallels in other DSPs and suggests a unique role for the second cysteine in the function of these viral proteins, particularlyin vivo.

Epitope-specific crosslinking of CD45 down-regulates membrane-associated tyrosine phosphatase activity and triggers early signalling events in human activated T cells

CD45 engagement by monoclonal antibodies on human activated T cells triggers tumour necrosis factor-alpha (TNF-alpha) gene transcription in an epitope-specific manner. To dissect the early signalling events leading to TNF-alpha gene expression, we established that CD45 crosslinking resulted in tyrosine phosphorylation of p56lck, ZAP-70, CD3-zeta, LAT and Vav. This was accompanied by down-regulation of membrane-associated protein tyrosine phosphatase activity in the absence of demonstration of enhanced p56lck, p72syk and ZAP-70 kinase activity, which remained constitutive. These early events eventually triggered an intracellular Ca(2+) rise and phosphoinositide turnover. We conclude that down-regulation of membrane-associated tyrosine phosphatase activity by CD45 extracytoplasmic domain multimerization led, in an epitope-specific fashion, to unopposed tyrosine kinase activity and to the activation of the T-cell receptor/CD3 complex signalling cascade, resulting in TNF-alpha gene expression. This model strongly suggests that CD45 extracytoplasmic tail multimerization may contribute to the modulation T-cell functions.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

Chicken anemia virus VP2 is a novel dual specificity protein phosphatase

The function of viral protein 2 (VP2) of the immunosuppressive circovirus chicken anemia virus (CAV) has not yet been established. We show that the CAV VP2 amino acid sequence has some similarity to a number of eukaryotic, receptor, protein-tyrosine phosphatase (PTPase) alpha proteins as well as to a cluster of human TT viruses within the Sanban group. To investigate if CAV VP2 functions as a PTPase, purified glutathione S-transferase (GST)-VP2 fusion protein was assayed for PTPase activity using the generalized peptide substrates ENDpYINASL and DADEpYLIPQQG (where pY represents phosphotyrosine), with free phosphate detected using the malachite green colorimetric assay. CAV GST-VP2 was shown to catalyze dephosphorylation of both substrates. CAV GST-VP2 PTPase activity for the ENDpYINASL substrate had a V(max) of 14,925 units/mg.min and a K(m) of 18.88 microm. Optimal activity was observed between pH 6 and 7, and activity was specifically inhibited by 0.01 mm orthovanadate. We also show that the ORF2 sequence of the CAV-related human virus TT-like minivirus (TLMV) possessed PTPase activity and steady state kinetics equivalent to CAV GST-VP2 when expressed as a GST fusion protein. To establish whether these viral proteins were dual specificity protein phosphatases, the CAV GST-VP2 and TLMV GST-ORF2 fusion proteins were also assayed for serine/threonine phosphatase (S/T PPase) activity using the generalized peptide substrate RRApTVA, with free phosphate detected using the malachite green colorimetric assay. Both CAV GST-VP2 and TLMV GST-ORF2 fusion proteins possessed S/T PPase activity, which was specifically inhibited by 50 mm sodium fluoride. CAV GST-VP2 exhibited S/T PPase activity with a V(max) of 28,600 units/mg.min and a K(m) of 76 microm. Mutagenesis of residue Cys(95) to serine in CAV GST-VP2 abrogated both PTPase and S/T PPase activity, identifying it as the catalytic cysteine within the proposed signature motif. These studies thus show that the circoviruses CAV and TLMV encode dual specificity protein phosphatases (DSP) with an unusual signature motif that may play a role in intracellular signaling during viral replication. This is the first DSP gene to be identified in a small viral genome.

Sweet 'n' sour: the impact of differential glycosylation on T cell responses

The fate and functional activity of T lymphocytes depend largely on the precise timing of gene expression and protein production. However, it is clear that post-translational modification of proteins affects their functional properties. Although modifications such as phosphorylation have been intensely studied by immunologists, less attention has been paid to the impact that changes in glycosylation have on protein function. However, there is considerable evidence that glycosylation plays a key role in immune regulation. We will focus here on examples in which differential glycosylation affects the development, survival or reactivity of T cells.

Negative regulation of CD45 by differential homodimerization of the alternatively spliced isoforms

The regulation of receptor-like protein tyrosine phosphatases (RPTPs) is not well understood. Although CD45 can be negatively regulated by dimerization, how dimerization is modulated is unclear. Here we show that various isoforms of CD45 differentially homodimerize in T cells. The dimerization is modulated by the sialylation and O-glycosylation of alternatively spliced CD45 exons in the extracellular domain. Thus, the smallest isoform, CD45RO--which undergoes the least extracellular sialylation and O-glycosylation--homodimerizes with the highest efficiency, resulting in decreased signaling via the T cell receptor. Because CD45 is required for T cell activation, our findings may reveal a mechanism that contributes to the termination of the primary T cell response. Our results not only demonstrate the biological significance of alternative splicing in the immune system, but also suggest a model for regulating RPTP dimerization and function.

Apoptosis mediated through CD45 is independent of its phosphatase activity and association with leukocyte phosphatase-associated phosphoprotein

Besides the well-recognized role of CD45 as a major player in TCR signaling, we and others have demonstrated that cross-linking of CD45 with mAbs can induce cell death in T lymphocytes. To investigate the role of CD45 phosphatase activity in apoptosis induction, we expressed either wild-type or phosphatase-dead CD45 molecules in a CD45-deficient BW5147 T cell line. We show here that the phosphatase activity of CD45 was not required for apoptosis triggering after cross-linking of the molecule. It is noteworthy that a revertant of the CD45-negative BW5147 cell line, expressing a truncated form of CD45 lacking most of the cytoplasmic domain, was also susceptible to CD45-mediated death. Moreover, we also demonstrate that leukocyte phosphatase-associated phosphoprotein expression is totally dispensable for CD45-mediated apoptosis to occur. Taken together, these results strongly suggest a role for the extracellular and/or the transmembrane portion of CD45 in apoptosis signaling, which contrasts with the previously reported functions for CD45 in T lymphocytes.

Aging-dependent exclusion of antigen-inexperienced cells from the peripheral B cell repertoire

Aging is accompanied by greatly reduced B cell production in the bone marrow, yet peripheral B cell numbers do not decline. We hypothesize that this may reflect filling of the peripheral pool with B cells that are long-lived as a consequence of specificity for, and chronic stimulation by, environmental Ags. To begin to explore this possibility, we analyzed the effects of aging on B cell population dynamics in the anti-H2(k/b) 3-83 mu-delta Ig-transgenic mouse. We predicted that, because they presumably do not bind environmental Ags, B cells bearing the transgenic receptor may be lost in aged animals. As seen in nontransgenic animals, total splenic B cell numbers remained constant with age in the Ig-transgenic animals despite reduced B cell production. Importantly, although the few newly produced B cells in the bone marrow of aged mice are 3-83 positive, the peripheral compartment of these mice is dominated by B cells that express endogenous Ig genes rather than the transgenes. This population includes large numbers of marginal zone-like and CD21(low/-)CD23(low/-)IgM(low) B cells, as well as elevated numbers of CD5+ B cells. Many of these cells express only non-B220 CD45 isoforms, suggesting that they may be memory cells. A significant proportion of aged transgenic animals produce autoantibodies that are reactive with ssDNA, dsDNA, or histones. Results support the hypothesis that, in the face of severely reduced production with age, B cells are selected based on reactivity to environmental Ags, accumulate, and display activated phenotypes. Cells bearing 3-83-transgenic receptors are excluded from this population due to their specificity. Beyond their importance in aging, these findings define a novel form of receptor revision in which B cells are selected rather than deleted based on Ag reactivity.

The role of intron sequences in high level expression from CD45 cDNA constructs

Consistent expression from CD45 cDNA constructs has proven difficult to achieve. Through the use of new CD45 cDNA constructs and reporter genes, the role 5', 3', and intron sequences play in CD45 expression was determined. The CD45 polyadenylation signal sequence was fully functional in a beta-galactosidase reporter construct. Furthermore, the CD45 3'-untranslated region and downstream sequences were shown to contain no negative regulatory elements. Several new CD45 cDNA constructs were designed that contain either the cytomegalovirus promoter, the leukocyte function-associated antigen (LFA-1; CD11a) promoter, or various CD45 5' regions. Neither the cytomegalovirus nor the LFA-1 promoter was capable of generating detectable levels of expression in constructs with CD45 cDNA. However, when CD45 intron sequences between exons 3 and 9 were inserted in the cDNA construct to generate a CD45 minigene, the LFA-1 promoter was able to drive reproducible, significant expression of CD45. CD45 minigenes using the CD45 5' sequences up to 19 kilobases upstream of the transcriptional start produced very little protein. The LFA-1 CD45 minigene construct produced correct cell type-specific isoforms when expressed in T and B lymphocyte lines. Therefore, we conclude that the regulation of CD45 expression and cell type-specific splicing requires elements within the intron sequences.

Structural and functional analysis of the human CD45 gene (PTPRC) upstream region: evidence for a functional promoter within the first intron of the gene

Expression of the leucocyte common antigen (CD45) in mammals is restricted to the nucleated lineages of haematopoietic cells. It appears in early progenitors in the bone marrow and is expressed at the surface of these cells throughout their differentiation. However, at least in T cells, the pattern of expression switches between different isoforms during the successive stages of differentiation in the thymus and after activation in the periphery. In order to understand the mechanisms controlling the transcription of the human CD45 gene, 2.7 kbp of the 5'-flanking region were sequenced and analysed for their ability to direct expression of a reporter gene. The only region with promoter activity was localized within the first intron of the gene. This promoter shows no tissue specificity but could be enhanced by a heterologous enhancer. Mobility shift assays showed complex but specific protein binding. The sequence in this region lacks similarity with known promoters or initiators but is highly conserved in evolution. No transcription initiation could be detected within or downstream of this region, suggesting that this might be a new type of RNA polymerase II promoter able to drive transcription from an upstream sequence. An additional exon was also found upstream of exon 1. The two exons 1 (1a and 1b) are mutually exclusive and both are spliced to exon 2. This makes the structure of the 5' region of the human CD45 gene identical to its mouse homologue.

SC35 plays a role in T cell development and alternative splicing of CD45

Molecular diversity via alternative splicing is important for cellular function and development. SR proteins are strong candidate regulators of alternative splicing because they can modulate splice site selection. However, endogenous substrates for SR proteins are largely unknown, and their roles as splicing regulators in vertebrate development are unclear. Here we report that Cre-mediated conditional deletion of the prototypical SR protein SC35 in the thymus causes a defect in T cell maturation. Deletion of SC35 alters alternative splicing of CD45, a receptor tyrosine phosphatase known to be regulated by differential splicing during thymocyte development and activation. This study establishes a model to address the function of SR proteins in physiological settings and reveals a critical role of SC35 in a T cell-specific regulated splicing pathway.

Biosynthesis and function of beta 1,6 branched mucin-type glycans

The contribution of carbohydrate structure to biomolecular, cellular, and organismal function is well-established, but has not yet received the attention it deserves, perhaps due to the complexity of the structures involved and to a lack of simple experimental methods for relating structure and function. In particular, beta1,6 GlcNAc branching plays a key functional role in processes ranging from inflammation and immune system function to tumor cell metastasis. For instance, synthesis of the core 2 beta1,6 branched structure in the mucin glycan chain by C2GnT enables the expression of functional structures at the termini of polylactosamine chains, such as blood group antigens and sialyl Lewis x. Also, IGnT can create multiple branches on the polylactosamine chain, which may serve as a mechanism for amplifying the functional potency of cell surface glycoproteins and glycolipids. The family of enzymes which creates beta1,6 branched structure in mucin glycans is proving to be quite complex, since multiple isoforms appear to exist for these enzymes, and some of the enzymes are adept at forming more than one type of beta1,6 branched structure, as in the case of C2GnT-M. Furthermore, the enzymes do not appear to be restricted to acting on mucin-type acceptor structures, but are able to act on glycolipid structures as well. Much remains to be learned regarding the specific biological niche filled by each of these enzymes and how their activities complement one another, as well as the manner in which the activities of these enzymes are regulated in the cell.

The ST3Gal-I sialyltransferase controls CD8+ T lymphocyte homeostasis by modulating O-glycan biosynthesis

T lymphocyte activation evokes distinct changes in cell surface O-glycans. CD8+ T cells undergo an elimination of sialic acid on core 1 O-glycans and an induction of core 2 O-glycans until either apoptotic death or differentiation into memory cells. We find that the ST3Gal-I sialyltransferase is required for core 1 O-glycan sialylation and its deficiency induces core 2 O-glycan biosynthesis. Apoptosis ensues with the loss of peripheral CD8+ T cells in the absence of immune stimulation. Cell surface ligation of the ST3Gal-I substrate CD43 recapitulates this phenotype by a caspase 3-independent mechanism. Control of core 1 O-glycan sialylation in T lymphocytes by ST3Gal-I comprises a homeostatic mechanism that eliminates CD8+ T cells by apoptosis while facilitating the production of viable CD8+ memory T cells.

CD4+ CD45RA+ and CD4+ CD45RO+ T cells differ in their TCR-associated signaling responses

Peripheral CD4+ T cells can be divided into two different functional populations based on the expression of distinct isoforms of the surface molecule CD45. We have investigated the differences in the proximal signaling induced by anti-CD3 monoclonal antibody in purified populations of "naive" CD45RA+ and "memory" CD45RO+ human CD4+ T cells. Expression of cell surface CD3, CD4 and CD28 was comparable between RA+ and RO+ cells. However, TCR-directed stimulation in the form of anti-CD3 produced markedly different patterns of intracellular signaling. Greater inositol triphosphate generation occurred in naive cells and the rise in intracellular free calcium was also substantially greater in naive than in memory cells. Cells with the naive phenotype were considerably more active in TCR-dependent tyrosine phosphorylation, both at an overall level and specifically in terms of TCR-zeta and ZAP-70 phosphorylation. Despite these differences in phosphorylation, the amounts of TCR-zeta, ZAP-70 and Ick were equivalent between the two subsets. These findings suggest that the TCR-dependent signaling is differentially regulated in naive and memory CD4+ T cells. This may be due to differences in the way that the two isoforms of the CD45 phosphatase regulate the activity of proximal kinases in the TCR signaling pathway, and could be an important means by which the unique functions of differentiated T cell populations are maintained.

SF2 and SRp55 regulation of CD45 exon 4 skipping during T cell activation

CD45 is an alternatively spliced membrane phosphatase required for T cell activation. Exons 4, 5 and 6 can be included or skipped from spliced mRNA resulting in several protein isoforms that include or exclude epitopes referred to as RA, RB or RC, respectively. T cells reciprocally express CD45RA or CD45RO (lacking all three exons), corresponding to naive versus memory T cells. Overexpression of the alternative splicing regulators, SF2 or SWAP, induces skipping of CD45 exon 4 in transfected COS cells. We show here that the arginine/serine-rich domain of SWAP and the RNA recognition motifs of SF2 are required for skipping of CD45 exon 4. Unlike SWAP, SF2 specifically regulated alternative splicing of CD45 exon 4, having no effect on a non-regulated exon of CD45 (exon 9). Like SF2 and SWAP, the SR proteins SC35, SRp40 and SRp75, but not SRp55 also induced CD45 exon 4 skipping. In contrast, antisense inhibition of SRp55 induced exon 4 skipping. SF2 and SRp55 proteins were not detectable or expressed at a very low level in freshly isolated CD45RA+ and CD45RO+ T cells. Activation of CD45RA+ T cells shifted CD45 expression from CD45RA to CD45RO, and induced a large increase in expression of both SF2 and SRp55. Thus, SF2 at least in part determines splicing of CD45 exon 4 during T cell activation. SRp55, SR protein phosphorylation, or other splicing factors likely regulate CD45 splicing in CD45RO+ memory T cells. The different SR proteins expressed by memory and activated T cells emphasize the different phenotypes of these cell types that both express CD45RO.

Overexpression of branched O-linked oligosaccharides on T cell surface glycoproteins impairs humoral immune responses in transgenic mice

The aberrant expression of core 2 O-glycans on T cell surface glycoproteins has been associated with various immunodeficient syndromes such as Wiskott-Aldrich syndrome and AIDS. To determine the effect of this aberrant expression of core 2 O-glycans on immune responses, we previously generated transgenic mice overexpressing core 2 beta-1,6-N-acetylglucosaminyltransferase (C2GnT) in T cells, and demonstrated that T cell primary immune responses mediated through interaction between T cells and antigen-presenting cells are impaired in the transgenic mice (Tsuboi, S., and Fukuda, M. (1997) EMBO J. 16, 6364-6373). In this study, we determined whether overexpression of core 2 oligosaccharides on T cells leads to impaired humoral immune responses by B cells using the same transgenic mice. When T cells were activated, both T and B cells from the transgenic and control mice expressed an equivalent amount of CD40L and CD40, which are, respectively, the receptor and counter-receptor for the interaction between T and B cells. However, activated T cells from the transgenic mice induced B cell proliferation less efficiently than those from control mice, regardless of whether B cells were isolated from control or the transgenic mice. This suggests that overexpression of core 2 O-glycans on T cell surface glycoproteins renders T cell-B cell interaction inefficient. Moreover, in the transgenic mice both immunoglobulin isotype switching and germinal center formation were also impaired. Taken together, these results indicate that aberrant expression of core 2 O-glycans on T cell surface glycoproteins results in impaired humoral immune responses due to an impaired interaction between T and B cells.

Receptor-Specific Allelic Exclusion of TCRVα-Chains During Development

Expression of a single Ag receptor on lymphocytes is maintained via allelic exclusion that generates cells with a clonal receptor repertoire. We show in normal mice and mice expressing functionally rearranged TCRαβ transgenes that allelic exclusion at the TCRα locus is not operational in immature thymocytes, whereas most mature T cells express a single TCRVα-chain. TCRVα allelic exclusion in mature thymocytes is regulated through a CD45 tyrosine phosphatase-mediated signal during positive selection. Using functional and genetic systems for selection of immature double TCRVα+ thymocytes, we show that peptide-specific ligand recognition provides the signal for allelic exclusion, i.e., mature T cells maintain expression of the ligand-specific TCRVα-chain, but lose the nonfunctional receptor. Whereas activation of TCRVβ-chains or CD3ε leads to receptor internalization, TCRVα ligation promotes retention of the TCR on the cell surface. Although both TCRVα- and TCRVβ-chains trigger phosphotyrosine signaling, only the TCRVβ-chain mediates membrane recruitment of the GTPase dynamin. These data indicate that TCRVα-directed signals for positive selection control allelic exclusion in T cells, and that developmental signals can select for single receptor usage.

The role of CD45 and CD45-associated molecules in T cell activation

CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56lck and p59fyn of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

Differential accumulation of DPTP61F alternative transcripts: regulation of a protein tyrosine phosphatase by segmentation genes

DPTP61F is a non-receptor protein tyrosine phosphatase that is expressed during Drosophila oogenesis and embryogenesis. DPTP61F transcripts are alternatively spliced to produce two isoforms of the protein which are targeted to different subcellular locations. DPTP61Fn accumulates in the nucleus, and DPTP61Fm associates with the membranes of the reticular network and the mitochondria. We have examined the spatial and temporal expression of the two alternative transcripts of dptp61F during Drosophila embryogenesis. Our observations indicate that the two isoforms are expressed in distinct patterns. The DPTP61Fn transcript is expressed in the mesoderm and neuroblast layer during germband extension and later in the gut epithelia. In comparison, the transcript encoding DPTP61Fm accumulates in 16 segmentally repeated stripes in the ectoderm during germband extension. These stripes are flanked by, and adjacent to, the domains of engrailed and wingless gene expression in the anterior/posterior axis. In stage 10 embryos, the domains of DPTP61Fm transcript accumulation are wedge shaped and roughly coincide with the area lateral to the denticle belts that will give rise to naked cuticle. The DPTP61Fm transcript is also expressed later in embryogenesis in the central nervous system. The segmental modulation of DPTP61Fm transcript accumulation in the A/P axis of the germband is regulated by the pair-rule genes, and the intrasegmental pattern of transcript accumulation is regulated by the segment polarity genes.

9-O-Acetylation of sialomucins: a novel marker of murine CD4 T cells that is regulated during maturation and activation

Terminal sialic acids on cell surface glycoconjugates can carry 9-O-acetyl esters. For technical reasons, it has previously been difficult to determine their precise distribution on different cell types. Using a recombinant soluble form of the Influenza C virus hemagglutinin-esterase as a probe for 9-O-acetylated sialic acids, we demonstrate here their preferential expression on the CD4 T cell lineage in normal B10.A mouse lymphoid organs. Of total thymocytes, 8-10% carry 9-O-acetylation; the great majority of these are the more mature PNA-, HSA-, and TCRhi medullary cells. While low levels of 9-O-acetylation are seen on some CD4/CD8 double positive (DP) and CD8 single positive (SP) cells, high levels are present primarily on 80- 85% of CD4 SP cells. Correlation with CD4 and CD8 levels suggests that 9-O-acetylation appears as an early differentiation marker as cells mature from the DP to the CD4 SP phenotype. This high degree of 9-O-acetylation is also present on 90-95% of peripheral spleen and lymph node CD4 T cells. In contrast, only a small minority of CD8 T cells and B cells show such levels of 9-O-acetylation. Among mature peripheral CD4 T lymphocytes, the highly O-acetylated cells are Mel 14(hi), CD44(lo), and CD45R(exon B)hi, features typical of naive cells. Digestions with trypsin and O-sialoglycoprotease (OSGPase) and ELISA studies of lipid extracts indicate that the 9-O-acetylated sialic acids on peripheral CD4 T cells are predominantly on O-linked mucintype glycoproteins and to a lesser degree, on sialylated glycolipids (gangliosides). In contrast, sialic acids on mucin type molecules of CD8 T cells are not O-acetylated; instead these molecules mask the recognition of O-acetylated gangliosides that seem to be present at similar levels as on CD4 cells. The 9-O-acetylated gangliosides on mouse T cells are not bound by CD60 antibodies, which recognize O-acetylated gangliosides in human T cells. Tethering 9-O-acetylated mucins with the Influenza C probe with or without secondary cross-linking did not cause activation of CD4 T cells. However, activation by other stimuli including TCR ligation is associated with a substantial decrease in surface 9-O-acetylation, primarily in the mucin glycoprotein component. Thus, 9-O-acetylation of sialic acids on cell surface mucins is a novel marker on CD4 T cells that appears on maturation and is modulated downwards upon activation.

Role of phosphatases in lymphocyte activation

Many lymphocyte signaling pathways are regulated by protein tyrosyl phosphorylation, which is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Substantial progress has been made in defining the functions of lymphocyte PTPs. Individual PTPs can enhance or diminish cell signaling levels. The transmembrane PTP CD45 is a key positive element in multiple lymphocyte signaling pathways in vivo. New insights into the function of individual CD45 isoforms have emerged. Anti-CD45 antibodies with potent immunosuppressant activity have been identified, suggesting that CD45 may be a propitious target for drug design. Progress has also been made in elucidating the function and targets of specific nontransmembrane PTPs, particularly those with Src homology 2 domains.

An analysis of alternatively spliced CD45 mRNA transcripts during T cell maturation in humans

CD45 is a transmembrane protein tyrosine phosphatase found on nucleated hematopoietic cells. In humans, multiple protein isoforms of CD45 are produced by alternative mRNA splicing of exons 4, 5, and 6 coding for the extracellular portion. We measured all eight possible CD45 mRNA transcripts using RT-PCR in human thymocytes and T cell lines. We report that only six mRNA transcripts are present in T cells. The high mw CD45 mRNA transcripts containing exon 4 correlated with the stage of T cell maturation: abundant high mw transcripts (30.7% of all CD45 mRNA transcripts) were present in immature, CD3-4-8 triple-negative thymocytes which decreased (7.7%) in intermediate, CD4+8+ double-positive (DP) thymocytes and then increased (13.8% or 16.8%) in mature, CD4+8- or CD4-8+ single-positive thymocytes. In addition, there was a complex variation in the spliced mRNA transcripts coding for CD45R(O), CD45R(B), CD45R(BC), CD45R(AB), and CD45R(ABC) protein isoforms. High mw CD45 mRNA transcripts accumulated immediately prior to an important physiologic event such as thymocyte expansion. In addition, we identified linkage between RNA splicing of exons 5 and 6, and splicing of exon 5 only and exons 4, 5, and 6 in FACS-purified CD4+ and CD8+ thymocytes. These data support the developmental regulation of alternatively spliced CD45 mRNA transcripts and suggest that specific CD45 isoforms may play an important role at critical stages of T cell development.

Signaling thresholds and interclonal competition in preimmune B-cell selection

The need to eliminate autoreactive B cells must be checked against the need for a diverse B-cell repertoire. Protein tyrosine phosphatases SHP1 and CD45 act antagonistically within B cells to set the threshold level of antigen-receptor engagement required for B-cell elimination. The fate of B cells binding weak autoantigens is also regulated by interclonal competition. In the presence of a normal diverse repertoire of competitor B cells, the autoantigen-binding cells are excluded from follicles in spleen and lymph nodes and undergo rapid cell death. In the absence of interclonal competition, the autoreactive cells enter the follicular microenvironment and survive. A model in which B cells compete for access to limiting follicular niches in order to survive is discussed.

Different nuclear signals are activated by the B cell receptor during positive versus negative signaling

It is not known how immunogenic versus tolerogenic cellular responses are signaled by receptors such as the B cell antigen receptor (BCR). Here we compare BCR signaling in naive cells that respond positively to foreign antigen and self-tolerant cells that respond negatively to self-antigen. In naive cells, foreign antigen triggered a large biphasic calcium response and activated nuclear signals through NF-AT, NF-kappa B, JNK, and ERK/pp90rsk. In tolerant B cells, self-antigen stimulated low calcium oscillations and activated NF-AT and ERK/pp90rsk but not NF-kappa B or JNK. Self-reactive B cells lacking the phosphatase CD45 did not exhibit calcium oscillations or ERK/pp90rsk activation, nor did they repond negatively to self-antigen. These data reveal striking biochemical differences in BCR signaling to the nucleus during positive selection by foreign antigens and negative selection by self-antigens.

Regulation of cell signaling by the protein tyrosine phosphatases, CD45 and SHP-1

An equilibrium between positive and negative regulation of immunoreceptor signaling leads to the proper execution of lymphocyte activation. Tyrosine phosphorylation is the initial event in antigen receptor-induced lymphocyte activation. It is generally accepted that protein tyrosine kinases are involved in positive regulation, whereas protein tyrosine phosphatases are important for the negative regulation of tyrosine phosphorylation-dependent processes. However, the interaction between protein tyrosine kinases and protein tyrosine phosphatases is complex. This article discusses the role of two protein tyrosine phosphatases. CD45 and SHP-1, in the regulation of immunoreceptor signaling. SHP-1 acts as a negative regulator for several immunoreceptors, including those for T- and B-cell antigen receptors. The major role of CD45 is in the positive regulation of T- and B-cell antigen receptor signaling.

Differential signaling by lymphocyte antigen receptors

Studies performed during the past several years make plain that ligand occupancy of antigen receptors need not necessarily provoke identical responses in all instances. For example, ligation of antigen receptors may stimulate a proliferative response, induce a state of unresponsiveness to subsequent stimulation (anergy), or induce apoptosis. How does a single type of transmembrane receptor induce these very heterogeneous cellular responses? In the following pages, we outline evidence supporting the view that the nature of the ligand/receptor interaction directs the physical recruitment of signaling pathways differentially inside the lymphocyte and hence defines the nature of the subsequent immune response. We begin by providing a functional categorization of antigen receptor components, considering the ways in which these components interact with the known set of signal transduction pathways, and then review the evidence suggesting that differential signaling through the TCR is achieved by qualitative differences in the effector pathways recruited by TCR, perhaps reflecting the time required to bring complicated signal transduction elements into proximity within the cell. The time-constant of the interaction between antigen and receptor in this way determines, at least in part, the nature of the resulting response. Finally, although our review focuses substantially on T cell receptor signaling, we have included a less detailed description of B cell receptor signaling as well, simply to emphasize the parallels that exist in these two closely related systems.

The activation status of ovine CD45R+ and CD45R- efferent lymph T cells after orf virus reinfection

The dynamics and activation status of CD4+ and CD8+ T-cells differentially expressing the CD45R (220 kDa) antigen were studied in prefemoral efferent lymph draining the site of cutaneous reinfection with orf virus. CD4+, CD45R+ lymphoblasts preceded CD4+, CD45R- lymphoblasts during the first 48 h after reinfection. Thereafter, the output of both total and blast-transformed CD4+, CD45R- T-cells increased in proportion to the CD4+, CD45R+ cells for the duration of the virus reinfection. Output of CD8+, CD45R+ T-cells exceeded that of the CD8+, CD45R+ cells both before and after reinfection. However, within the lymphoblast population, CD8+, CD45R+ and CD8+, CD45R- T-cells increased and decreased in parallel. CD4+, CD45R- and CD8+, CD45R- T-cells produced interleukin-2, interferon-gamma and granulocyte-macrophage colony-stimulating factor after culture for 24 h without exogenous restimulation, whereas CD4+, CD45R+ T-cells produced only interleukin-2. The results show that although both CD45R+ and CD45R- alpha beta receptor+ T-cell subsets are activated as a consequence of virus reinfection in vivo, it is the CD45R- subset that predominates in the later stages of reinfection and is the principal cellular source of lymphokines in the efferent lymph.

Protein tyrosine phosphatases in T-cell development, apoptosis and signalling

The study of phosphatases was viewed as a rather esoteric subject for immunologists until eight years ago, when the discovery that CD45 is a protein tyrosine phosphatase (PTPase) began to make the topic respectable. Now, as reviewed by Julie Frearson and Denis Alexander, PTPases are increasingly being shown to play key roles in the molecular physiology of haematopoietic cells and some have been shown to regulate critical events in T-cell development and signalling.

Epitope-specific engagement of the protein tyrosine phosphatase CD45 induces tumor necrosis factor-alpha gene expression via transcriptional mechanisms

The common leukocyte antigen CD45 plays a central role in T cell activation in coupling the T cell receptor (TCR) to the phosphatidylinositol pathway via interactions with TCR-associated protein tyrosine kinases lck and fyn. We here demonstrate that engagement of CD45 by monoclonal antibodies (mAb) on activated T cells induces tumor necrosis factor (TNF)-alpha as well as TNF-beta, interleukin (IL)-2 and IL-3 gene expression. When human alloreactive T cells are stimulated with mAb 4B2, which recognizes a determinant common to all CD45 isoforms, a vigorous production of TNF-alpha mRNA was detected, which peaked 2 h later. Anti-CD45 mAb cross-linking was required. In contrast, neither mAb 10G10, which recognizes an epitope distinct from the one recognized by mAb 4B2, nor mAb UCHL-1, a CD45RO-specific antibody, induced any significant increase in TNF-alpha transcription. Nuclear run-on transcription assays demonstrated that CD45 cross-linking caused transcriptional activation of the TNF-alpha gene. De novo protein synthesis was not required, since incubation with cycloheximide (CHX) did not block transcriptional activation. CHX in contrast up-regulated TNF-alpha gene expression and increased transcript half-life, an effect that was under control of post-transcriptional mechanisms. Engagement of CD45 by itself did not affect transcript stability. CD45 ligation resulted in TNF-alpha secretion. These results indicate that in addition to its role in TCR/CD3-mediated T cell activation, CD45, in an epitope-specific manner, may act as a primary signaling molecule, leading to the transcriptional regulation and secretion of a major pro-inflammatory cytokine.

Regulation of B-lymphocyte negative and positive selection by tyrosine phosphatase CD45

Elimination of self-reactive B cells must be balanced against the need for B-cell diversity for antibody responses to pathogens. To analyse factors that determine the extent of B-cell negative selection, we crossed CD45-deficient mice with mice carrying immunoglobulin transgenes specific for hen egg lysozyme (HEL). CD45 positively regulates antigen-receptor signalling and CD45-deficient HEL-specific B cells gave diminished signalling in response to HEL. Significantly, few mature CD45-/- B cells accumulated, despite normal immature B-cell production. Circulating HEL autoantigen mediates negative selection of mature CD45+/+ HEL-binding B cells but, in striking contrast, the autoantigen positively selected CD45-/- HEL-binding B cells, promoting their accumulation as long-lived IgD(hi) cells. These findings are consistent with a signal-threshold model for B-cell selection and demonstrate that changes in antigen receptor signalling can cause high-affinity self-reactive B cells to be actively retained instead of eliminated, thus revealing a potential mechanism for inherited susceptibility to autoimmune disease.

Increase in the specific activity of p50csk in proliferating T cells correlates with decreased specific activity of p56lck and p59fyn and reduced phosphorylation of CD3 subunits

Depending on their prior antigen recognition history, mature T cells respond with different functional outcomes to T cell receptor (TCR) stimulation. These functional outcomes include proliferation, anergy and cell death. The biochemical basis underlying differential responses by mature T cells at different stages of their developmental pathway to TCR stimulation remains to be determined. We have previously shown that proliferating but not naive T cells were susceptible to apoptosis after TCR stimulation and that the tyrosine phosphorylation of TCR zeta, CD3 gamma, and CD3 epsilon in proliferating T cells was decreased after TCR stimulation. In this study. We determined whether differences in phosphorylation between naive and proliferating T cells were due to altered regulation of p56lck (Lck) or p59fyn (Fyn) by their positive or negative regulators, CD45 or p5Ocsk (Csk), respectively. We found that Lck was expressed at the same level and had the same phosphotyrosine content in naive and proliferating T cells. However, its autophosphorylation activity was lower in proliferating cells, corresponding to a 2-fold decrease in its specific kinase activity. Similarly, the specific kinase activity of Fyn was also decreased by about 2-fold in proliferating T cells. In contrast, although Csk was expressed at the same level in both cell types its specific kinase activity was increased by 6-fold in proliferating T cells. The tyrosine phosphatase CD45, a positive regulator of src-family kinases, was overexpressed by 3- to 6-fold in proliferating cells. However, the specific activity of CD45 in naive and proliferating T cells was the same. Therefore, although the protein expression level of CD45 was increased in proliferating T cells it only partially compensated for the hyperactivity of Csk resulting in a 2-fold reduction in the specific activity of Lck and Fyn in proliferating T cells.

Detection of restricted isoform expression and tyrosine phosphatase activity of CD45 in murine dendritic cells

CD45 is a cell surface transmembrane tyrosine phosphatase. It is expressed as distinct protein isoforms via alternative splicing of exons 4, 5 and 6. In T and B lymphocytes, CD45 is thought to play a critical role in antigen-dependent signaling through their respective antigen receptor complexes. However, the isoform expression and enzymatic activity of CD45 in other leukocytes remains largely unknown. Here, we examine the isoform expression and phosphatase activity of CD45 in murine dendritic cells (DC). Flow cytometric double-labeling analysis and biochemical analysis of purified splenic DC CD45 demonstrate that DC express both the CD45RB and CD45R0 isoforms. Flow cytometric analyses of freshly isolated splenic DC and thymic DC also indicate the expression of CD45RB and CD45R0 on these DC populations. In addition, we find that purified splenic DC CD45 possesses a high level of intrinsic tyrosine phosphatase activity. These data therefore establish the restricted isoform expression pattern of CD45 in murine DC and demonstrate that cells lacking specific antigen receptor complexes have active tyrosine phosphatase activity associated with CD45.

Characterization of the interaction between CD45 and CD45-AP

CD45, a leukocyte-specific transmembrane protein tyrosine phosphatase, is required for critical signal transduction pathways in immune responses. To elucidate the molecular interactions of CD45 with other proteins involved in CD45-mediated signal transduction pathways, we have recently cloned a 30-kDa phosphorylated protein, CD45-AP, which specifically associates with CD45. Binding analysis employing several deleted or chimeric forms of CD45-AP and CD45 demonstrated that the potential transmembrane segment of CD45-AP bound to the transmembrane portion of CD45. CD45-AP was found in particulate fractions of lymphocytes along with CD45, indicating that it is likely to be a transmembrane protein. In addition, CD45-AP was resistant to proteolysis by tosylphenylalanyl chloromethyl ketone-treated trypsin applied to intact cells. This is consistent with the most likely membrane orientation of CD45-AP predicted from the amino acid sequence, that is, only a short amino-terminal segment of CD45-AP is extracellular. We propose that CD45-AP interacts with CD45 at the plasma membrane and that the bulk of CD45-AP located in the cytoplasm act as an adapter which directs the interaction between CD45 and other molecules involved in CD45-mediated signal transduction pathways.