CD45 and Src-family kinases: and now for something completely different

How the Discovery of the CD4/CD8-p56lck Complexes Changed Immunology and Immunotherapy

The past 25 years have seen enormous progress in uncovering the receptors and signaling mechanisms on T-cells that activate their various effecter functions. Until the late 1980s, most studies on T-cells had focused on the influx of calcium and the levels of cAMP/GMP in T-cells. My laboratory then uncovered the interaction of CD4 and CD8 co-receptors with the protein-tyrosine kinase p56lck which are now widely accepted as the initiators of the tyrosine phosphorylation cascade leading to T-cell activation. The finding explained how immune recognition receptors expressed by many immune cells, which lack intrinsic catalytic activity, can transduce activation signals via non-covalent association with non-receptor tyrosine kinases. The discovery also established the concept that a protein tyrosine phosphorylation cascade operated in T-cells. In this vein, we and others then showed that the CD4- and CD8-p56lck complexes phosphorylate the TCR complexes which led to the identification of other protein-tyrosine kinases such as ZAP-70 and an array of substrates that are now central to studies in T-cell immunity. Other receptors such as B-cell receptor, Fc receptors and others were also subsequently found to use src kinases to control cell growth. In T-cells, p56lck driven phosphorylation targets include co-receptors such as CD28 and CTLA-4 and immune cell-specific adaptor proteins such as LAT and SLP-76 which act to integrate signals proximal to surface receptors. CD4/CD8-p56lck regulated events in T-cells include intracellular calcium mobilization, integrin activation and the induction of transcription factors for gene expression. Lastly, the identification of the targets of p56lck in the TCR and CD28 provided the framework for the development of chimeric antigen receptor (CAR) therapy in the treatment of cancer. In this review, I outline a history of the development of events that led to the development of the "TCR signaling paradigm" and its implications to immunology and immunotherapy.

Roles of Zinc Signaling in the Immune System

Zinc (Zn) is an essential micronutrient for basic cell activities such as cell growth, differentiation, and survival. Zn deficiency depresses both innate and adaptive immune responses. However, the precise physiological mechanisms of the Zn-mediated regulation of the immune system have been largely unclear. Zn homeostasis is tightly controlled by the coordinated activity of Zn transporters and metallothioneins, which regulate the transport, distribution, and storage of Zn. There is growing evidence that Zn behaves like a signaling molecule, facilitating the transduction of a variety of signaling cascades in response to extracellular stimuli. In this review, we highlight the emerging functional roles of Zn and Zn transporters in immunity, focusing on how crosstalk between Zn and immune-related signaling guides the normal development and function of immune cells.

Protein tyrosine phosphatases as potential therapeutic targets

Protein tyrosine phosphorylation is a key regulatory process in virtually all aspects of cellular functions. Dysregulation of protein tyrosine phosphorylation is a major cause of human diseases, such as cancers, diabetes, autoimmune disorders, and neurological diseases. Indeed, protein tyrosine phosphorylation-mediated signaling events offer ample therapeutic targets, and drug discovery efforts to date have brought over two dozen kinase inhibitors to the clinic. Accordingly, protein tyrosine phosphatases (PTPs) are considered next-generation drug targets. For instance, PTP1B is a well-known targets of type 2 diabetes and obesity, and recent studies indicate that it is also a promising target for breast cancer. SHP2 is a bona-fide oncoprotein, mutations of which cause juvenile myelomonocytic leukemia, acute myeloid leukemia, and solid tumors. In addition, LYP is strongly associated with type 1 diabetes and many other autoimmune diseases. This review summarizes recent findings on several highly recognized PTP family drug targets, including PTP1B, Src homology phosphotyrosyl phosphatase 2(SHP2), lymphoid-specific tyrosine phosphatase (LYP), CD45, Fas associated phosphatase-1 (FAP-1), striatal enriched tyrosine phosphatases (STEP), mitogen-activated protein kinase/dual-specificity phosphatase 1 (MKP-1), phosphatases of regenerating liver-1 (PRL), low molecular weight PTPs (LMWPTP), and CDC25. Given that there are over 100 family members, we hope this review will serve as a road map for innovative drug discovery targeting PTPs.

SRC-family tyrosine kinases in oogenesis, oocyte maturation and fertilization: an evolutionary perspective

The oocyte is a highly specialized cell poised to respond to fertilization with a unique set of actions needed to recognize and incorporate a single sperm, complete meiosis, reprogram maternal and paternal genomes and assemble them into a unique zygotic genome, and finally initiate the mitotic cell cycle. Oocytes accomplish this diverse series of events through an array of signal transduction pathway components that include a characteristic collection of protein tyrosine kinases. The src-family protein kinases (SFKs) figure importantly in this signaling array and oocytes characteristically express certain SFKs at high levels to provide for the unique actions that the oocyte must perform. The SFKs typically exhibit a distinct pattern of subcellular localization in oocytes and perform critical functions in different subcellular compartments at different steps during oocyte maturation and fertilization. While many aspects of SFK signaling are conserved among oocytes from different species, significant differences exist in the extent to which src-family-mediated pathways are used by oocytes from species that fertilize externally vs those which are fertilized internally. The observation that several oocyte functions which require SFK signaling appear to represent common points of failure during assisted reproductive techniques in humans, highlights the importance of these signaling pathways for human reproductive health.

Novel tools to dissect the dynamic regulation of TCR signaling by the kinase Csk and the phosphatase CD45

Although the biochemical events induced by T-cell receptor (TCR) triggering have been well studied, both the mediators and function of basal signaling in T cells remain poorly understood. Furthermore, the precise mechanisms by which MHC-peptide interaction with the TCR disrupt the basal equilibrium to induce downstream signaling are also unclear. Here we describe novel approaches to understand the basal state of T cells and the mechanisms of TCR triggering by perturbing regulation of the Src family kinases (SFKs). The SFKs are critical proximal mediators of TCR signaling that are in turn tightly regulated by the tyrosine kinase Csk and the receptor-like tyrosine phosphatase CD45. We have developed a small-molecule analog-sensitive allele of Csk and an allelic series of mice in which expression of CD45 is varied across a broad range. Our studies have unmasked contributions of Csk and CD45 to maintain the basal state of T cells and also suggest that dynamic regulation of Csk may be involved in TCR triggering.

A role for the protein tyrosine phosphatase CD45 in macrophage adhesion through the regulation of paxillin degradation

CD45 is a protein tyrosine phosphatase expressed on all cells of hematopoietic origin that is known to regulate Src family kinases. In macrophages, the absence of CD45 has been linked to defects in adhesion, however the molecular mechanisms involved remain poorly defined. In this study, we show that bone marrow derived macrophages from CD45-deficient mice exhibit abnormal cell morphology and defective motility. These defects are accompanied by substantially decreased levels of the cytoskeletal-associated protein paxillin, without affecting the levels of other proteins. Degradation of paxillin in CD45-deficient macrophages is calpain-mediated, as treatment with a calpain inhibitor restores paxillin levels in these cells and enhances cell spreading. Inhibition of the tyrosine kinases proline-rich tyrosine kinase (Pyk2) and focal adhesion kinase (FAK), kinases that are capable of mediating tyrosine phosphorylation of paxillin, also restored paxillin levels, indicating a role for these kinases in the CD45-dependent regulation of paxillin. These data demonstrate that CD45 functions to regulate Pyk2/FAK activity, likely through the activity of Src family kinases, which in turn regulates the levels of paxillin to modulate macrophage adhesion and migration.

Differential polarization of C-terminal Src kinase between naive and antigen-experienced CD8+ T cells

In CD8(+) T cells, engagement of the TCR with agonist peptide:MHC molecules causes dynamic redistribution of surface molecules including the CD8 coreceptor to the immunological synapse. CD8 associates with the Src-family kinase (SFK) Lck, which, in turn, initiates the rapid tyrosine phosphorylation events that drive cellular activation. Compared with naive T cells, Ag-experienced CD8(+) T cells make shorter contacts with APC, are less dependent on costimulation, and are triggered by lower concentrations of Ag, yet the molecular basis of this more efficient response of memory T cells is not fully understood. In this article, we show differences between naive and Ag-experienced CD8(+) T cells in colocalization of the SFKs and their negative regulator, C-terminal Src kinase (Csk). In naive CD8(+) T cells, there was pronounced colocalization of SFKs and Csk at the site of TCR triggering, whereas in Ag-experienced cells, Csk displayed a bipolar distribution with a proportion of the molecules sequestered within a cytosolic area in the distal pole of the cell. The data show that there is differential redistribution of a key negative regulator away from the site of TCR engagement in Ag-experienced CD8(+) T cells, which might be associated with the more efficient responses of these cells on re-exposure to Ag.

The structural wedge domain of the receptor-like tyrosine phosphatase CD45 enforces B cell tolerance by regulating substrate specificity

CD45 is a receptor-like tyrosine phosphatase that positively regulates BCR signaling by dephosphorylating the inhibitory tyrosine of the Src family kinases. We showed previously that a single point mutation, E613R, introduced into the cytoplasmic membrane-proximal "wedge" domain of CD45 is sufficient to drive a lupus-like autoimmune disease on a susceptible genetic background. To clarify the molecular mechanism of this disease, we took advantage of a unique allelic series of mice in which the expression of CD45 is varied across a broad range. Although both E613R B cells and those with supraphysiologic CD45 expression exhibited hyperresponsive BCR signaling, they did so by opposite regulation of the Src family kinase Lyn. We demonstrated that the E613R allele of CD45 does not function as a hyper- or hypomorphic allele but rather alters the substrate specificity of CD45 for Lyn. Despite similarly enhancing BCR signaling, only B cells with supraphysiologic CD45 expression became anergic, whereas only mice harboring the E613R mutation developed frank autoimmunity on a susceptible genetic background. We showed that selective impairment of a Lyn-dependent negative-regulatory circuit in E613R B cells drove autoimmunity in E613R mice. This demonstrates that relaxing negative regulation of BCR signaling, rather than enhancing positive regulation, is critical for driving autoimmunity in this system.

Receptor type protein tyrosine phosphatases (RPTPs) - roles in signal transduction and human disease

Protein tyrosine phosphorylation is a fundamental regulatory mechanism controlling cell proliferation, differentiation, communication, and adhesion. Disruption of this key regulatory mechanism contributes to a variety of human diseases including cancer, diabetes, and auto-immune diseases. Net protein tyrosine phosphorylation is determined by the dynamic balance of the activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Mammals express many distinct PTKs and PTPs. Both of these families can be sub-divided into non-receptor and receptor subtypes. Receptor protein tyrosine kinases (RPTKs) comprise a large family of cell surface proteins that initiate intracellular tyrosine phosphorylation-dependent signal transduction in response to binding of extracellular ligands, such as growth factors and cytokines. Receptor-type protein tyrosine phosphatases (RPTPs) are enzymatic and functional counterparts of RPTKs. RPTPs are a family of integral cell surface proteins that possess intracellular PTP activity, and extracellular domains that have sequence homology to cell adhesion molecules. In comparison to extensively studied RPTKs, much less is known about RPTPs, especially regarding their substrate specificities, regulatory mechanisms, biological functions, and their roles in human diseases. Based on the structure of their extracellular domains, the RPTP family can be grouped into eight sub-families. This article will review one representative member from each RPTP sub-family.

Lck, Membrane Microdomains, and TCR Triggering Machinery: Defining the New Rules of Engagement

In spite of a comprehensive understanding of the schematics of T cell receptor (TCR) signaling, the mechanisms regulating compartmentalization of signaling molecules, their transient interactions, and rearrangement of membrane structures initiated upon TCR engagement remain an outstanding problem. These gaps in our knowledge are exemplified by recent data demonstrating that TCR triggering is largely dependent on a preactivated pool of Lck concentrated in T cells in a specific type of membrane microdomains. Our current model posits that in resting T cells all critical components of TCR triggering machinery including TCR/CD3, Lck, Fyn, CD45, PAG, and LAT are associated with distinct types of lipid-based microdomains which represent the smallest structural and functional units of membrane confinement able to negatively control enzymatic activities and substrate availability that is required for the initiation of TCR signaling. In addition, the microdomains based segregation spatially limits the interaction of components of TCR triggering machinery prior to the onset of TCR signaling and allows their rapid communication and signal amplification after TCR engagement, via the process of their coalescence. Microdomains mediated compartmentalization thus represents an essential membrane organizing principle in resting T cells. The integration of these structural and functional aspects of signaling into a unified model of TCR triggering will require a deeper understanding of membrane biology, novel interdisciplinary approaches and the generation of specific reagents. We believe that the fully integrated model of TCR signaling must be based on membrane structural network which provides a proper environment for regulatory processes controlling TCR triggering.

Control of genetically prescribed protein tyrosine kinase activities by environment-linked redox reactions

Recent observations on environment-linked control of genetically prescribed signaling systems for either cell activation or cell death have been reviewed with a focus on the regulation of activities of protein tyrosine kinases (PTKs). The environment-linked redox reactions seem to primarily affect cell surface receptors and cell membrane lipid rafts, and they induce generation of reactive oxygen species (ROS) in cells. ROS thus generated might upregulate the catalytic activities of PTKs through inactivating protein tyrosine phosphatases that dephosphorylate and inactivate autophosphorylated PTKs. Recent evidence has, however, demonstrated that ROS could also directly oxidize SH groups of genetically conserved specific cysteines on PTKs, sometimes producing disulfide-bonded dimers of PTK proteins, either for upregulation or downregulation of their catalytic activities. The basic role of the redox reaction/covalent bond-mediated modification of protein tertiary structure-linked noncovalent bond-oriented signaling systems in living organisms is discussed.

Lck and the nature of the T cell receptor trigger

Exactly how ligand binding 'triggers' T cell receptor (TCR) phosphorylation is unclear. It has been proposed that ligand engagement by the TCR somehow activates the Src kinase Lck, which in turn phosphorylates the receptor. Recent data, however, suggest instead that a significant fraction of the Lck in resting T cells is already activated and that the proportion of active Lck does not change during the early stages of T cell activation. We argue that, caveats notwithstanding, these new observations offer support for the 'kinetic-segregation' model of TCR triggering, which involves spatial reorganization of signalling proteins upon ligand binding and requires a fraction of Lck to be active in resting T cells.

The nonphagocytic NADPH oxidase Duox1 mediates a positive feedback loop during T cell receptor signaling

Production of reactive oxygen species, often by NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidases, plays a role in the signaling responses of cells to many receptor stimuli. Here, we describe the function of the calcium-dependent, nonphagocytic NADPH oxidase Duox1 in primary human CD4(+) T cells and cultured T cell lines. Duox1 bound to inositol 1,4,5-trisphosphate receptor 1 and was required for early T cell receptor (TCR)-stimulated production of hydrogen peroxide (H(2)O(2)) through a pathway that was dependent on TCR-proximal kinases. Transient or stable knockdown of Duox1 inhibited TCR signaling, especially phosphorylation of tyrosine-319 of zeta chain-associated protein kinase of 70 kilodaltons (ZAP-70), store-operated entry of calcium ions (Ca(2+)), and activation of extracellular signal-regulated kinase. The production of cytokines was also inhibited by knockdown of Duox1. Duox1-mediated inactivation of Src homology 2 domain-containing protein tyrosine phosphatase 2 promoted the phosphorylation of ZAP-70 and its association with the Src family tyrosine kinase Lck and the CD3zeta chain of the TCR complex. Thus, we suggest that activation of Duox1, downstream of proximal TCR signals, generates H(2)O(2) that acts in a positive feedback loop to enhance and sustain further TCR signaling.

CD45-Csk phosphatase-kinase titration uncouples basal and inducible T cell receptor signaling during thymic development

The kinase-phosphatase pair Csk and CD45 reciprocally regulate phosphorylation of the inhibitory tyrosine of the Src family kinases Lck and Fyn. T cell receptor (TCR) signaling and thymic development require CD45 expression but proceed constitutively in the absence of Csk. Here, we show that relative titration of CD45 and Csk expression reveals distinct regulation of basal and inducible TCR signaling during thymic development. Low CD45 expression is sufficient to rescue inducible TCR signaling and positive selection, whereas high expression is required to reconstitute basal TCR signaling and beta selection. CD45 has a dual positive and negative regulatory role during inducible but not basal TCR signaling. By contrast, Csk titration regulates basal but not inducible signaling. High physiologic expression of CD45 is thus required for two reasons-to downmodulate inducible TCR signaling during positive selection and to counteract Csk during basal TCR signaling.

Involvement of tyrosine phosphatase CD45 in apoptosis

CD45 is a transmembrane molecule with phosphatase activity expressed in all nucleated haematopoietic cells and plays a major role in immune cells. It is a protein tyrosine phosphatase that is essential for antigen-receptor-mediated signal transduction by regulating Src family members that initiate TCR signaling. CD45 is being attributed a new emerging role as an apoptosis regulator. Cross-linking of the extracellular portion of the CD45 by monoclonal antibodies and by galectin-1, can induce apoptosis in T and B cells. Interestingly, this phosphatase has also been involved in nuclear apoptosis induced by mitochondrial perturbing agents. Furthermore, it is involved in apoptosis induced by HIV-1. CD45 defect is implicated in various diseases such as severe-combined immunodeficiency disease (SCID), acquired immunodeficiency syndrome (AIDS), lymphoma and multiple myelomas. The understanding of the mechanisms by which CD45 regulates apoptosis would be very useful in disease treatment.

Modulation of immune cell signalling by the leukocyte common tyrosine phosphatase, CD45

CD45 is a leukocyte specific transmembrane glycoprotein and a receptor-like protein tyrosine phosphatase (PTP). CD45 can be expressed as several alternatively spliced isoforms that differ in the extracellular domain. The isoforms are regulated in a cell type and activation state-dependent manner, yet their function has remained elusive. The Src family kinase members Lck and Lyn are key substrates for CD45 in T and B lymphocytes, respectively. CD45 lowers the threshold of antigen receptor signalling, which impacts T and B cell activation and development. CD45 also regulates antigen triggered Fc receptor signalling in mast cells and Toll-like receptor (TLR) signalling in dendritic cells, thus broadening the role of CD45 to other recognition receptors involved in adaptive and innate immunity. In addition, CD45 can affect immune cell adhesion and migration and can modulate cytokine production and signalling. Here we review what is known about the substrate specificity and regulation of CD45 and summarise its effect on immune cell signalling pathways, from its established role in T and B antigen receptor signalling to its emerging role regulating innate immune cell recognition and cytokine production.

The Biology of CD45 and its Use as a Therapeutic Target

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

SHP-1 and SHP-2 in T cells: two phosphatases functioning at many levels

Tyrosine phosphorylation and dephosphorylation of proteins play a critical role for many T-cell functions. The opposing actions of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) determine the level of tyrosine phosphorylation at any time. It is well accepted that PTKs are essential during T-cell signaling; however, the role and importance of PTPs are much less known and appreciated. Both transmembrane and cytoplasmic tyrosine phosphatases have been identified in T cells and shown to regulate T-cell responses. This review focuses on the roles of the two cytoplasmic PTPs, the Src-homology 2 domain (SH2)-containing SHP-1 and SHP-2, in T-cell signaling, development, differentiation, and function.

CD45, CD148, and Lyp/Pep: critical phosphatases regulating Src family kinase signaling networks in immune cells

Reciprocal regulation of tyrosine phosphorylation by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) is central to normal immune cell function. Disruption of the equilibrium between PTK and PTP activity can result in immunodeficiency, autoimmunity, or malignancy. Src family kinases (SFKs) play a central role in both immune cell function and disease due to their proximal position in numerous signal transduction cascades including those emanating from integrin, T and B-cell antigen receptors, Fc, growth factor, and cytokine receptors. Given that tight regulation of SFKs activity is critical for appropriate responses to stimulation of these various signaling pathways, it is perhaps not surprising that multiple PTPs are involved in their regulation. Here, we focus on the role of three phosphatases, CD45, CD148, and LYP/PEP, which are critical regulators of SFKs in hematopoietic cells. We review our current understanding of their structures, expression, functions in different hematopoietic cell subsets, regulation, and putative roles in disease. Finally, we discuss remaining questions that must be addressed if we are to have a clearer understanding of the coordinated regulation of tyrosine phosphorylation and signaling networks in hematopoietic cells and how they could potentially be manipulated therapeutically in disease.

CD45 down-regulates Lck-mediated CD44 signaling and modulates actin rearrangement in T cells

The tyrosine phosphatase CD45 dephosphorylates the negative regulatory tyrosine of the Src family kinase Lck and plays a positive role in TCR signaling. In this study we demonstrate a negative regulatory role for CD45 in CD44 signaling leading to actin rearrangement and cell spreading in activated thymocytes and T cells. In BW5147 T cells, CD44 ligation led to CD44 and Lck clustering, which generated a reduced tyrosine phosphorylation signal in CD45(+) T cells and a more sustained, robust tyrosine phosphorylation signal in CD45(-) T cells. This signal resulted in F-actin ring formation and round spreading in the CD45(+) cells and polarized, elongated cell spreading in CD45(-) cells. The enhanced signal in the CD45(-) cells was consistent with enhanced Lck Y394 phosphorylation compared with the CD45(+) cells where CD45 was recruited to the CD44 clusters. This enhanced Src family kinase-dependent activity in the CD45(-) cells led to PI3K and phospholipase C activation, both of which were required for elongated cell spreading. We conclude that CD45 induces the dephosphorylation of Lck at Y394, thereby preventing sustained Lck activation and propose that the amplitude of the Src family kinase-dependent signal regulates the outcome of CD44-mediated signaling to the actin cytoskeleton and T cell spreading.

The lectin Jacalin induces human B-lymphocyte apoptosis through glycosylation-dependent interaction with CD45

It has been well established that CD45 is a key receptor-type protein tyrosine phosphatase (PTPase) regulating Src-family protein tyrosine kinase (Src-PTK) in T and B lymphocytes. However, precisely how CD45 exerts its effect in these lymphocytes remains controversial. We recently reported that Jacalin, an alpha-O-glycoside of the disaccharide Thomsen-Friedenreich antigen-specific lectin from jackfruit seeds, caused marked T-cell activation in response to T-cell receptor ligation and CD28 costimulation by binding to CD45. On extending the reported research, we found that CD45 and isoforms are major Jacalin receptors on B lymphocytes, and that the glycosylation of CD45 is involved in the interaction of Jacalin with the PTPase. In contrast to Jacalin-stimulated T-cell activation, we found that Jacalin induced human B-lymphocyte apoptosis, resulting in calcium mobilization and calpain activation, suggesting that the calcium-calpain pathway may mediate the Jacalin-induced apoptosis. Importantly, the apoptosis was effectively blocked by a specific CD45 PTPase inhibitor, indicating that Jacalin induces human B-lymphocyte apoptosis through CD45 triggering. Furthermore, we found that Jacalin significantly increased the C-terminal inhibitory tyrosine (Tyr507) phosphorylation of Src-PTK Lyn, one of the major substrates of CD45 PTPase, and this effect was also observed on incubation of B lymphocytes with the specific CD45 PTPase inhibitor, suggesting that Jacalin stimulation results in increasing C-terminal tyrosine phosphorylation of the kinase through inhibition of CD45 tyrosine phosphatase activity in human B lymphocytes. Therefore, the down-modulation of Lyn kinase may play a role in the regulation of B-lymphocyte viability.

Extracellular ligation-dependent CD45RB enzymatic activity negatively regulates lipid raft signal transduction

CD45 is the most prominent membrane protein on lymphocytes. The function and regulation of this protein tyrosine phosphatase remain largely obscure, mainly because of the lack of a known ligand, and it still remains unknown whether such tyrosine phosphatases are subject to extracellular control at all. We report that an anti-CD45RB antibody (Ab) that prevents rejection and induces tolerance activates CD45RB tyrosine phosphatase enzymatic activity in T lymphocytes, allowing us to directly monitor the effects of increased CD45RB activity on signal transduction. Using both kinase substrate peptide arrays as well as conventional biochemistry, we also provide evidence of the various kinases involved in bringing about the inhibitory effect of this Ab on CD3-induced T-cell receptor signaling. Furthermore, we report that activated CD45RB translocates to lipid rafts and interferes with lipid raft localization and activation state of CD45 substrate Lck. Thus, these findings indeed prove that CD45 is subject to extracellular control and also define a novel mechanism by which receptor tyrosine phosphatases control lymphocyte biology and provide further insight into the intracellular signaling pathways effected by anti-CD45RB monoclonal Ab treatment.

Why is there so much CD45 on T cells?

The balance between kinases and phosphatases is crucial for regulating lymphocyte signaling. In this issue, McNeill et al. (2007) show that the transmembrane phosphatase CD45 has a role as both positive and negative regulator of T cell signaling.

The role of integrins and coreceptors in refining thresholds for B-cell responses

Despite compelling evidence that a large proportion of antigens encountered in vivo by B cells are membrane bound, the general view is that B cells are mainly activated by soluble antigens. This notion may have been biased somewhat over the years because the high affinity of the B-cell receptor (BCR) for soluble intact ligands allows efficient B-cell stimulation in vitro. In vivo, however, even soluble antigens are likely to be deposited on the surface of antigen-presenting cells, either by complement or Fc receptors in the form of immune complexes, thus becoming more potent stimulators of B-cell activation. In this framework, the BCR works in a complex environment of integrins and coreceptors, as well as the B-cell cytoskeleton. Over the last few years, we have focused on B-cell membrane-bound antigen recognition. Here, we discuss some of our findings in the context of what is currently known in this exciting new field.

Differential function of PTPalpha and PTPalpha Y789F in T cells and regulation of PTPalpha phosphorylation at Tyr-789 by CD45

CD45 is a major membrane protein tyrosine phosphatase (PTP) expressed in T cells where it regulates the activity of Lck, a Src family kinase important for T cell receptor-mediated activation. PTPalpha is a more widely expressed transmembrane PTP that has been shown to regulate the Src family kinases, Src and Fyn, and is also present in T cells. Here, PTPalpha was phosphorylated at Tyr-789 in CD45(-) T cells but not in CD45(+) T cells suggesting that CD45 could regulate the phosphorylation of PTPalpha at this site. Furthermore, CD45 could directly dephosphorylate PTPalpha in vitro. Expression of PTPalpha and PTPalpha-Y789F in T cells revealed that the mutant had a reduced ability to decrease Fyn and Cbp phosphorylation, to regulate the kinase activity of Fyn, and to restore T cell receptor-induced signaling events when compared with PTPalpha. Conversely, this mutant had an increased ability to prevent Pyk2 phosphorylation and CD44-mediated cell spreading when compared with PTPalpha. These data demonstrate distinct activities of PTPalpha and PTPalpha-Y789F in T cells and identify CD45 as a regulator of PTPalpha phosphorylation at tyrosine 789 in T cells.

The juxtamembrane wedge negatively regulates CD45 function in B cells

CD45 is a receptor-like protein tyrosine phosphatase highly expressed on all nucleated hematopoietic cells. We previously generated mice containing a point mutation in the juxtamembrane wedge of CD45. Demonstrating the critical negative regulatory function of the wedge, the CD45 E613R mutation led to a lymphoproliferative disorder (LPD) and a lupus-like autoimmune syndrome. Here we show the central role of B cells in this phenotype. Genetic elimination of B cells, but not T cells, ablates the LPD. In contrast to CD45-deficient B cells, the E613R mutation generates hyperresponsive B cells. Comparison of CD45-deficient and CD45 E613R mice reveals dichotomous effects of these mutations on B cell development. Together, the results support a role for CD45 as a rheostat, with both positive and negative regulatory functions, that fine-tunes the signal transduction threshold at multiple checkpoints in B cell development.

Regulation of Ly49D/DAP12 Signal Transduction by Src-Family Kinases and CD45

Activating, DAP12-coupled members of the Ly-49 family of NK cell receptors help control viral infections in mice. However, the kinases and/or phosphatases mediating tyrosine phosphorylation of Ly-49D-associated DAP12 have not been elucidated. In this study, we show for the first time that Src family tyrosine kinases are physically and functionally associated with Ly-49D/DAP12 signaling in murine NK cells. Specifically, we demonstrate the following: 1) inhibition of Src family kinases suppresses DAP12 phosphorylation and downstream DAP12 signals; 2) both Fyn and Lck are capable of phosphorylating DAP12; and 3) both kinases coimmunoprecipitate with the Ly-49D/DAP12 complex in NK cells. Although we detect enhanced phosphorylation of Fyn upon Ly-49D cross-linking in NK cells, Ly-49D-mediated events in both Fyn-/- and Fyn/Lck-/- mice appear normal, reinforcing the theme of redundancy in the ability of Src family kinases to initiate activation events. In contrast to disruption of specific Src family enzymes, Ly-49D/DAP12-mediated calcium mobilization and cytokine production by CD45 null NK cells are defective. Although others have ascribed the effects of CD45 mutation solely on the suppression of Src family activity, we demonstrate in this study that DAP12 is hyperphosphorylated in CD45 null NK cells, resulting in uncoordinated tyrosine-mediated signaling upon Ly-49D ligation. Therefore, although our data are consistent with a Src kinase activity proximally within DAP12 signaling, DAP12 also appears to be a substrate of CD45, suggesting a more complex role for this phosphatase than has been reported previously.

SHP-1 inhibits LPS-mediated TNF and iNOS production in murine macrophages

Several lines of evidence have suggested that protein tyrosine phosphatases, including CD45 and SHP-1, regulate macrophage activation. Macrophages from mice lacking SHP-1 (motheaten mice) are hyper-responsive to many stimuli, suggesting that SHP-1 may negatively regulate macrophage activation. Herein we report that the repressible/inducible over-expression of wild-type SHP-1 in a subclone of RAW 264.7 macrophages (RAW-TT10 cells) inhibited both TNF secretion and iNOS protein accumulation in response to stimulation with lipopolysaccharide (LPS) and recombinant murine interferon-gamma and led to diminished LPS-mediated tyrosine phosphorylation of vav1. In contrast, expression of a truncated SHP-1 construct previously shown to interfere with endogenous SHP-1 function modestly augmented LPS-mediated TNF and iNOS production and did not inhibit vav1 tyrosine phosphorylation. Taken together, these data provide the first direct evidence that SHP-1 inhibits macrophage activation by LPS and suggest that this effect may be mediated in part by dephosphorylation of vav1.

Protein Tyrosine Phosphatase α Regulates Fyn Activity and Cbp/PAG Phosphorylation in Thymocyte Lipid Rafts

A role for the receptor protein tyrosine phosphatase alpha (PTPalpha) in immune cell function and regulation of Src family kinases was investigated using thymocytes from PTPalpha-deficient mice. PTPalpha-null thymocytes develop normally, but unstimulated PTPalpha-/- cells exhibit increased tyrosine phosphorylation of specific proteins, increased Fyn activity, and hyperphosphorylation of Cbp/PAG that promotes its association with C-terminal Src kinase. Elevated Fyn activity in the absence of PTPalpha is due to enhanced phosphorylation of Fyn tyrosines 528 and 417. Some PTPalpha is localized in lipid rafts of thymocytes, and raft-associated Fyn is specifically activated in PTPalpha-/- cells. PTPalpha is not a Cbp/PAG phosphatase, because it is not required for Cbp/PAG dephosphorylation in unstimulated or anti-CD3-stimulated thymocytes. Together, our results indicate that PTPalpha, likely located in lipid rafts, regulates the activity of raft Fyn. In the absence of PTPalpha this population of Fyn is activated and phosphorylates Cbp/PAG to enhance association with C-terminal Src kinase. Although TCR-mediated tyrosine phosphorylation was apparently unaffected by the absence of PTPalpha, the long-term proliferative response of PTPalpha-/- thymocytes was reduced. These findings indicate that PTPalpha is a component of the complex Src family tyrosine kinase regulatory network in thymocytes and is required to suppress Fyn activity in unstimulated cells in a manner that is not compensated for by the major T cell PTP and SFK regulator, CD45.

Expression of CD45 Lacking the Catalytic Protein Tyrosine Phosphatase Domain Modulates Lck Phosphorylation and T Cell Activation

The function of the second protein tyrosine phosphatase domain (D2) in two-domain protein tyrosine phosphatases (PTP) is not well understood. In CD45, D2 can interact with the catalytic domain (D1) and stabilize its activity. Although D2 itself has no detectable catalytic activity, it can bind substrate and may influence the substrate specificity of CD45. To further explore the function of D2 in T cells, a full-length construct of CD45 lacking the D1 catalytic domain (CD45RABC-D2) was expressed in CD45+ and CD45- Jurkat T cells. In CD45- Jurkat T cells, CD45RABC-D2 associated with Lck but, unlike its active counterpart CD45RABC, did not restore the induction of tyrosine phosphorylation or CD69 expression upon T cell receptor (TCR) stimulation. Expression of CD45RABC-D2 in CD45+ Jurkat T cells resulted in its association with Lck, increased the phosphorylation state of Lck, and reduced T cell activation. TCR-induced tyrosine phosphorylation was delayed, and although MAPK phosphorylation and CD69 expression were not significantly affected, the calcium signal and IL2 production were severely reduced. This indicates that the non-catalytic domains of CD45 can interact with Lck in T cells. CD45RABC-D2 acts as a dominant negative resulting in an increase in Lck phosphorylation and a preferential loss of the calcium signaling pathway, but not the MAPK pathway, upon TCR signaling. This finding suggests that, in addition to their established roles in the initiation of TCR signaling, CD45 and Lck may also influence the type of TCR signal generated.

Lyn tyrosine kinase: accentuating the positive and the negative

Lyn, one of several Src-family tyrosine kinases in immune cells, is noted for its ability to negatively regulate signaling pathways through phosphorylation of inhibitory receptors, enzymes, and adaptors. Somewhat paradoxically, it is also a key mediator in several pathways of B cell activation, such as CD19 and CD180. Whether Lyn functions to promote or inhibit immune cell activation depends on the stimulus and the developmental state, meaning that the consequences of Lyn activity are context dependent. The importance of regulating Lyn activity is exemplified by the pathological conditions that develop in both lyn-/- and lyn gain-of-function mice (lynup/up), including lethal antibody-mediated autoimmune diseases and myeloid neoplasia. Here, we review the outcomes of altered Lyn activity within the framework of B cell development and differentiation and the circumstances that appear to dictate the outcome.

CD45: direct and indirect government of immune regulation

The protein tyrosine phosphatase (PTP) CD45 is abundantly expressed on all nucleated hematopoietic cells and is critical for classical antigen receptor signalling indicated by the arrested development of B and T cells in mice deficient for CD45. Despite its clear role in positive regulation of signalling through the activation of the Src family of tyrosine kinases, many reports have shown CD45 to also negatively regulate this process. Given such a dichotomy in CD45 function and a poor understanding of the mechanism underlying the phenotype of CD45(-/-) lymphocytes, we considered it timely to review the existing data and attempt to determine whether aspects of the CD45(-/-) phenotype result from excessive positive or negative kinase activity and the target molecules that may mediate such effects.

Redox control of catalytic activities of membrane-associated protein tyrosine kinases

Protein tyrosine kinases (PTKs) play key roles in starting the signal transduction network for cellular development and functions. A number of both receptor-type and non-receptor-type PTKs, which are normally at a resting state, are initially activated in association with functions of the cell membrane and membrane rafts. Results of recent studies have suggested that these membrane-associated mechanisms for activation of PTKs consist of the two steps that are under redox control. The first step is activation of cell surface receptors through chemical crosslinkage or aggregation of receptors and membrane rafts, which leads to production of reactive oxygen species (ROS) as second messengers of intracellular signal transduction. The second step involves chemical modification of PTKs at the highly conserved cysteine in the MXXCW motif as a global switch for starting the tyrosine phosphorylation-dependent local switch for activation of the catalytic activity of the enzyme.

CD45 isoforms in T cell signalling and development

The CD45 phosphotyrosine phosphatase is expressed on T cells as multiple isoforms due to alternative splicing. The panoply of isoforms expressed is tightly regulated during T cell development and on mature peripheral T cell subsets following activation. We describe the analysis of comparative CD45 isoform expression levels on thymic and T cell subsets from the C57BL/6 mouse. Only four isoforms were expressed at significant protein levels: CD45R0, CD45RB, CD45RBC and CD45RABC, although trace amounts of others may be present. The expression of CD45RBC was about nine-fold higher on CD8(+) than on CD4(+) peripheral T cells, whereas CD45R0 expression was higher on CD4(+) T cells. We provide a general overview of the current models that have been proposed to explain the molecular actions of the different CD45 isoforms. Achieving a thorough understanding of the biological reasons for the existence and tight regulation of CD45 isoform expression in immune cells remains one of the outstanding challenges in the CD45 research field.

CD45-associated protein inhibits CD45 dimerization and up-regulates its protein tyrosine phosphatase activity

CD45, a receptor-like protein tyrosine phosphatase (PTP), plays an essential role in lymphocyte development and immune responses. Recent evidence suggests that dimerization of CD45 down-regulates its function. However, the mechanisms by which CD45 dimerization is regulated remain unclear, and there is no direct evidence that the PTP activity of CD45 dimers is less than that of monomers. CD45 in lymphocytes associates with CD45-AP (CD45-associated protein). Here we show that T cells from CD45-AP-null mice have a much higher level of CD45 dimers than those of wild-type mice, suggesting that CD45-AP inhibits CD45 dimer formation. This was confirmed with the use of a novel CD45-AP-null T-cell line, ALST-1, that we established from a spontaneous thymic tumor found in a CD45-AP-null mouse. Transfected CD45-AP inhibited CD45 dimer formation in ALST-1 cells in proportion to the amount of CD45-AP expressed. Finally, with the use of microsomal fractions from both mouse thymocytes and ALST-1 transfectants, the PTP activity of CD45 was found to be significantly lower in CD45-AP-negative cells than in CD45-AP-positive cells. Therefore, our results support a model in which binding of CD45-AP to inactive CD45 dimers converts them to active monomers. (Blood. 2004;103:3440-3447)

Dynamic regulation of Src-family kinases by CD45 in B cells

CD45 is a key protein tyrosine phosphatase regulating Src-family protein tyrosine kinases (Src-PTKs) in lymphocytes; precisely how it exerts its effect remains controversial, however. We previously demonstrated that CD45 negatively regulates Lyn in the WEHI-231 B-cell line. Here we show that negative regulation by CD45 is physiologically significant in B cells and that some CD45 is constitutively associated with glycolipid-enriched microdomains (GEMs), where it inhibits Src-PTKs by dephosphorylating both the negative and the positive regulatory sites. Upon B-cell receptor (BCR) ligation, however, CD45 dissociates from GEMs within 30 seconds, inducing phosphorylation of 2 regulatory sites and activation of Src-PTKs, but subsequently reassociates with the GEMs within 15 minutes. Disruption of GEMs with methyl-β-cyclodextrin results in abrogation of BCR-induced apoptosis in WEHI-231 cells, suggesting GEMs are critical to signals leading to the fate determination. We propose that the primary function of CD45 is inhibition of Src-PTKs and that the level of Src-PTK activation and the B-cell fate are determined in part by dynamic behavior of CD45 with respect to GEMs.

Subdomain X of the kinase domain of Lck binds CD45 and facilitates dephosphorylation

CD45 is a transmembrane, two-domain protein-tyrosine phosphatase expressed exclusively in nucleated hematopoietic cells. The Src family kinase, Lck, is a major CD45 substrate in T cells and CD45 dephosphorylation of Lck is important for both T cell development and activation. However, how the substrate specificity of phosphatases such as CD45 is achieved is not well understood. Analysis of the interaction between the cytoplasmic domain of CD45 and its substrate, Lck, revealed that the active, membrane-proximal phosphatase domain of CD45 (CD45-D1) bound to the phosphorylated Lck kinase domain, the SH2 domain, and the unique N-terminal region of Lck. The second, inactive phosphatase domain (CD45-D2) bound only to the kinase domain of Lck. CD45-D2 was unable to bind phosphotyrosine, and its interaction with the kinase domain of Lck was independent of tyrosine phosphorylation. The binding of CD45-D2 was localized to subdomain X (SD10) of Lck. CD45-D2 bound similarly to Src family kinases but bound Csk to a lesser extent and did not bind significantly to the less related kinase, Erk1. CD45 dephosphorylated Lck and Src at similar rates but dephosphorylated Csk and Erk1 at lower rates. Replacement of Erk1 SD10 with that of Lck resulted in the binding of CD45-D2 and the conversion of Erk1 to a more efficient CD45 substrate. This demonstrates a role for CD45-D2 in binding substrate and identifies the SD10 region in Lck as a novel site involved in substrate recognition.

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.

The noncatalytic domains of Lck regulate its dephosphorylation by CD45

The Src-family tyrosine kinase, Lck, contains two key regulatory phosphotyrosine residues, tyrosine 394 (Tyr-394) and tyrosine 505 (Tyr-505), both of which can be dephosphorylated by CD45. Here, the interaction of CD45 with its substrate, Lck, was determined to be complex, involving multiple interactions with both the catalytic and noncatalytic regions of Lck. CD45 preferentially dephosphorylated Tyr-394 over Tyr-505 in Lck. This was not due to sequence specificity surrounding the phosphotyrosine, but was due to the noncatalytic domains of Lck. The interactions with the noncatalytic domains of Lck and CD45 enhanced the dephosphorylation of Tyr-394 whereas intramolecular interactions within Lck reduced, but did not abolish, the dephosphorylation of Tyr-505. This demonstrates that the noncatalytic domains of Lck regulate the dephosphorylation of both Tyr-394 and Tyr-505 by CD45.

HIV-1 infection is facilitated in T cells by decreasing p56lck protein tyrosine kinase activity

Several studies have suggested an important role for the protein tyrosine kinase p56lck (Lck) in HIV infection; however, the exact nature of this role remains unclear. Using a series of well characterized Jurkat-derived cell lines having a wide range of Lck kinase activity, our results showed that, while the entry of HIV-1 into these cell lines was similar, the kinetics of virus production by these cells were very different. Cells expressing a kinase-inactive Lck showed accelerated viral replication, whereas, cells expressing Lck with normal or elevated enzymatic activity showed a delay in virus replication that was proportional to the initial level of endogenous Lck activity. The cell line having the highest initial Lck kinase activity showed the slowest rate of productive HIV-1 infection. Analysis of 2-LTR circles revealed that this inhibitory effect of Lck was not due to inhibition of reverse transcription of HIV-1 genome or migration of the proviral DNA into the nuclei. This affect of Lck was confirmed in additional studies that used either the S1T cell line lacking completely Lck or where the Lck activity was altered in Jurkat cells prior to infection. S1T cells showed a 3- to 12-fold increase in the level of infection compared to Jurkat cells despite similar CD4 and chemokine coreceptor expression and cell doubling times. Pretreatment of Jurkat with an antisense lck oligodeoxynucleotide inhibited the synthesis of functional Lck and facilitated the viral replication by the cells as did expressing a dominant-negative mutant Lck which increased the productive infection>3-fold. Conversely, whereas IL-16 had no affect on productive infection in S1T cells that lack Lck, IL-16 pretreatment of Jurkat cells resulted in an immediate (within 5 min) and sustained and gradual (over 5 h) increase in Lck activity that resulted in a reduction of HIV-1 replication that paralleled the increasing Lck kinase activity. These results show that the enzymatic activity of Lck kinase can affect viral replication, that a lack of, or decreased Lck activity facilitates viral replication. Conversely, Lck can mediate a delay in HIV-1 infection that is proportional to the initial endogenous Lck enzyme activity.

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

Activation-induced cytidine deaminase (AID) plays critical roles in Ig class switch recombination and V(H) gene somatic hypermutation. We investigated the role of IL-4 in AID mRNA induction, the signaling transduction involved in IL-4-mediated AID induction, and the effect of CD45 on IL-4-dependent AID expression in human B cells. IL-4 was able to induce AID expression in human primary B cells and B cell lines, and IL-4-induced AID expression was further enhanced by CD40 signaling. IL-4-dependent AID induction was inhibited by a dominant-negative STAT6, indicating that IL-4 induced AID expression via the Janus kinase (JAK)/STAT6 signaling pathway. Moreover, triggering of CD45 with anti-CD45 Abs can inhibit IL-4-induced AID expression, and this CD45-mediated AID inhibition correlated with the ability of anti-CD45 to suppress IL-4-activated JAK1, JAK3, and STAT6 phosphorylations. Thus, in humans, IL-4 alone is sufficient to drive AID expression, and CD40 signaling is required for optimal AID production; IL-4-induced AID expression is mediated via the JAK/STAT signaling pathway, and can be negatively regulated by the JAK phosphatase activity of CD45. This study indicates that the JAK phosphatase activity of CD45 can be induced by anti-CD45 Ab treatment, and this principle may find clinical application in modulation of JAK activation in immune-mediated diseases.

A CD45 minigene restores regulated isoform expression and immune function in CD45-deficient mice: therapeutic implications for human CD45-null severe combined immunodeficiency

Transgenic mice have been generated that carry a CD45 minigene under control of the human leukocyte function-associated antigen (LFA-1, CD11a) promoter. CD45-null mice carrying the transgene exhibit the lymphocyte lineage-specific isoform expression patterns of wild-type mice. Furthermore, these mice have normal thymocyte development and peripheral T-cell numbers. The proliferative ability of T cells in response to mitogens and antigen also is regained, as is B-cell responsiveness to anti-IgM. The antibody response to antigen is also restored and is similar to that of normal mice. Therefore, introduction of a functional CD45 minigene is sufficient to overcome the principal severe combined immunodeficiency (SCID)-associated defects and represents a potential route to a gene therapy for human CD45-deficent SCID.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

Ezrin is a substrate for Lck in T cells

We evaluated the role of Lck tyrosine kinase, an early effector of T cell activation, in regulation of the membrane-cytoskeleton linker protein ezrin. Ezrin was constitutively tyrosine phosphorylated in wild-type and CD45-deficient Jurkat T cells, but not in Lck-deficient cells. However, phosphorylation was evident in cells, in which Lck activity had been restored by transfection. Phosphorylation was reduced by the Src family kinase inhibitor PP2 and increased by the tyrosine phosphatase inhibitor pervanadate, implying continuous tyrosine phosphorylation and dephosphorylation. Lck phosphorylated ezrin in vitro, and the major phosphotyrosine was identified as Y145. These results identify ezrin as the first cytoskeletal substrate for Lck.

The protein-tyrosine phosphatase CD45 reaches the cell surface via golgi-dependent and -independent pathways

CD45 is a receptor protein-tyrosine phosphatase essential for T cell development and lymphocyte activation. It is highly glycosylated, with multiple isoforms and glycoforms expressed on the cell surface depending on the cell type and stage of differentiation. Interestingly, we found two pools of newly synthesized CD45 expressed on plasma membrane, one of which arrived by 5 min after synthesis. The remaining pool of CD45 was fully glycosylated and began to arrive at the cell surface at approximately 15 min. The rapidly expressed population of CD45 possessed exclusively endoglycosidase H-sensitive N-linked carbohydrate. Additionally, this rapidly expressed pool of CD45 appeared on the cell surface in a brefeldin A (BFA)-insensitive manner, suggesting that it reached the cell surface independent of the Golgi complex. The remaining CD45 trafficked through the Golgi complex, and transport proceeded via a BFA-sensitive mechanism. These data suggest that CD45 is able to reach the cell surface via two distinct routes. The first is a conventional Golgi-dependent pathway that allows fully processed CD45 to be expressed. The second utilizes an ill defined mechanism that is independent of the Golgi, is BFA-resistant, and allows for the expression of CD45 with immature carbohydrate on the cell surface.

Dynamic association of CD45 with detergent-insoluble microdomains in T lymphocytes

The receptor-like protein tyrosine phosphatase CD45 is essential for TCR signal transduction. Substrates of CD45 include the protein tyrosine kinases p56(lck) and p59(fyn), both of which have been shown to be enriched in detergent-insoluble microdomains. Here we find that there is a cholesterol-dependent association between CD45 and the raft-associated protein linker for activation of T cells, suggesting that CD45 and linker for activation of T cells may colocalize in lipid rafts. Consistent with this observation, we find that approximately 5% of total CD45 can be detected in Triton X-100-insoluble buoyant fractions of sucrose gradients, demonstrating that CD45 is not excluded from lipid rafts. Upon stimulation of T cells with anti-CD3, there is a reduction in the amount of CD45 found associating with lipid rafts. Our data suggest that CD45 is present in lipid rafts in T cells before activation, perhaps to activate raft-associated p56(lck), allowing membrane-proximal signaling events to proceed. Furthermore, the reduction in CD45 content of lipid rafts after CD3 stimulation may serve to limit the amounts of activated p56(lck) in rafts and thus possibly the duration of T cell responses.

Staging and resetting T cell activation in SMACs

During the productive interaction of T cells with antigen-presenting cells (APCs), engaged receptors, including the T cell antigen receptors and their associated tyrosine kinases, assemble into spatially segregated supramolecular activation clusters (SMACs) at the area of cell contact. Here, we studied intracellular signaling in SMACs by three-dimensional immunofluorescence microscopic localization of CD3, CD45, talin, phosphotyrosine, Lck and phosphorylated ZAP-70 in T cell-APC conjugates. Two distinct phases of spatial-temporal activation, one before and one after SMAC formation, which were separated by a brief state of inactivation caused by CD45, were observed at the T cell-APC contact area. We propose that pre-SMAC signals are sufficient to activate cell adhesion, but not productive T cell responses, which require orchestrated signaling in SMACs.

Principles of mucin architecture: structural studies on synthetic glycopeptides bearing clustered mono-, di-, tri-, and hexasaccharide glycodomains

The structural characteristics of a mucin glycopeptide motif derived from the N-terminal fragment STTAV of the cell surface glycoprotein CD43 have been investigated by NMR. In this study, a series of molecules prepared by total synthesis were examined, consisting of the peptide itself, three glycopeptides having clustered sites of alpha-O-glycosylation on the serine and threonine side chains with the Tn, TF, and STF carbohydrate antigens, respectively, and one with the beta-O-linked TF antigen. Additionally, a glycopeptide having the sequence SSSAVAV, triglycosylated with the Le(y) epitope, was investigated. NMR data for the tri-STF-STTAV glycopeptide were used to solve the structure of this construct through restrained molecular dynamics calculations. The calculations revealed a defined conformation for the glycopeptide core rooted in the interaction of the peptide and the first N-acetylgalactosamine residue. The similarity of the NMR data for each of the alpha-O-linked glycopeptides demonstrates that this structure persists for each construct and that the mode of attachment of the first sugar and the peptide is paramount in establishing the organization of the core. The core provides a common framework on which a variety of glycans may be displayed. Remarkably, while there is a profound organizational effect on the peptide backbone with the alpha-linked glycans, attachment via a beta-linkage has little apparent consequence.

CD45 controls interleukin-4-mediated IgE class switch recombination in human B cells through its function as a Janus kinase phosphatase

CD45 plays a critical regulatory role in receptor signaling through its protein tyrosine phosphatase and Janus kinase (JAK) phosphatase activities. To investigate whether CD45 also plays a regulatory role in Ig class switching in human B cells, we examined the effects of CD45 triggering on Ig class switching to IgE and its relationship with CD45 JAK phosphatase activity. Anti-CD45 triggering of CD45 significantly inhibited interleukin-4 + anti-CD40-induced switch recombination in a switch recombination vector assay in stably transfected Ramos 2G6 human B cells, as well as Ig epsilon germ-line transcription and Smu-Sepsilon switch recombination in primary human B cells. These negative regulatory effects on Ig class switching were concomitant with the ability of CD45 to dephosphorylate the induced phosphorylation of JAK1, JAK3, and signal transducer and activator of transcription 6, but not on stress-activated/mitogen-activated protein kinases. We also showed that phosphorylated JAK1 and JAK3 were directly dephosphorylated by recombinant CD45 in vitro. These results indicate that CD45 is able to function as JAK phosphatase in human B cells and that this activity is directly associated with the negative regulation of the class switch recombination to IgE. CD45 may be an appropriate target drug for modulating IgE in allergic diseases.