An inactivating point mutation in the inhibitory wedge of CD45 causes lymphoproliferation and autoimmunity

Enforced Dimerization of CD45 by the Adenovirus E3/49K Protein Inhibits T Cell Receptor Signaling

Human adenoviruses (HAdVs) are widespread pathogens that generally cause mild infections in immunocompetent individuals but severe or even fatal diseases in immunocompromised patients. In order to counteract the host immune defenses, HAdVs encode various immunomodulatory proteins in the early transcription unit 3 (E3). The E3/49K protein is a highly glycosylated type I transmembrane protein uniquely expressed by species D HAdVs. Its N-terminal ectodomain sec49K is released by metalloprotease-mediated shedding at the cell surface and binds to the receptor-like protein tyrosine phosphatase CD45, a critical regulator of leukocyte activation and functions. It remained elusive which domains of CD45 and E3/49K are involved in the interaction and whether such an interaction can also occur on the cell surface with membrane-anchored full-length E3/49K. Here, we show that the two extracellular domains R1 and R2 of E3/49K bind to the same site in the domain d3 of CD45. This interaction enforces the dimerization of CD45, causing the inhibition of T cell receptor signaling. Intriguingly, the membrane-anchored E3/49K appears to be designed like a "molecular fishing rod" using an extended disordered region of E3/49K as a "fishing line" to bridge the distance between the plasma membrane of infected cells and the CD45 binding site on T cells to effectively position the domains R1 and R2 as baits for CD45 binding. This design strongly suggests that both secreted sec49K as well as membrane-anchored full-length E3/49K have immunomodulatory functions. The forced dimerization of CD45 may be applied as a therapeutic strategy in chronic inflammatory disorders and cancer. IMPORTANCE The battle between viruses and their hosts is an ongoing arms race. Whereas the host tries to detect and eliminate the virus, the latter counteracts such antiviral measures to replicate and spread. Adenoviruses have evolved various mechanisms to evade the human immune response. The E3/49K protein of species D adenoviruses mediates the inhibition of immune cell function via binding to the protein tyrosine phosphatase CD45. Here, we show that E3/49K triggers the dimerization of CD45 and thereby inhibits its phosphatase activity. Intriguingly, the membrane-anchored E3/49K seems to be designed like a "molecular fishing rod" with the two CD45 binding domains of E3/49K as baits positioned at the end of an extended disordered region reminiscent of a fishing line. The adenoviral strategy to inhibit CD45 activity by forced dimerization may be used for therapeutic intervention in autoimmune diseases or to prevent graft rejection after transplantation.

Shaping Heterogeneity of Naive CD8+ T Cell Pools

Immune diversification helps protect the host against a myriad of pathogens. CD8⁺ T cells are essential adaptive immune cells that inhibit the spread of pathogens by inducing apoptosis in infected host cells, ultimately ensuring complete elimination of infectious pathogens and suppressing disease development. Accordingly, numerous studies have been conducted to elucidate the mechanisms underlying CD8⁺ T cell activation, proliferation, and differentiation into effector and memory cells, and to identify various intrinsic and extrinsic factors regulating these processes. The current knowledge accumulated through these studies has led to a huge breakthrough in understanding the existence of heterogeneity in CD8⁺ T cell populations during immune response and the principles underlying this heterogeneity. As the heterogeneity in effector/memory phases has been extensively reviewed elsewhere, in the current review, we focus on CD8⁺ T cells in a "naïve" state, introducing recent studies dealing with the heterogeneity of naive CD8⁺ T cells and discussing the factors that contribute to such heterogeneity. We also discuss how this heterogeneity contributes to establishing the immense complexity of antigen-specific CD8⁺ T cell response.

Polygenic autoimmune disease risk alleles impacting B cell tolerance act in concert across shared molecular networks in mouse and in humans

Most B cells produced in the bone marrow have some level of autoreactivity. Despite efforts of central tolerance to eliminate these cells, many escape to periphery, where in healthy individuals, they are rendered functionally non-responsive to restimulation through their antigen receptor via a process termed anergy. Broad repertoire autoreactivity may reflect the chances of generating autoreactivity by stochastic use of germline immunoglobulin gene segments or active mechanisms may select autoreactive cells during egress to the naïve peripheral B cell pool. Likewise, it is unclear why in some individuals autoreactive B cell clones become activated and drive pathophysiologic changes in autoimmune diseases. Both of these remain central questions in the study of the immune system(s). In most individuals, autoimmune diseases arise from complex interplay of genetic risk factors and environmental influences. Advances in genome sequencing and increased statistical power from large autoimmune disease cohorts has led to identification of more than 200 autoimmune disease risk loci. It has been observed that autoantibodies are detectable in the serum years to decades prior to the diagnosis of autoimmune disease. Thus, current models hold that genetic defects in the pathways that control autoreactive B cell tolerance set genetic liability thresholds across multiple autoimmune diseases. Despite the fact these seminal concepts were developed in animal (especially murine) models of autoimmune disease, some perceive a disconnect between human risk alleles and those identified in murine models of autoimmune disease. Here, we synthesize the current state of the art in our understanding of human risk alleles in two prototypical autoimmune diseases – systemic lupus erythematosus (SLE) and type 1 diabetes (T1D) along with spontaneous murine disease models. We compare these risk networks to those reported in murine models of these diseases, focusing on pathways relevant to anergy and central tolerance. We highlight some differences between murine and human environmental and genetic factors that may impact autoimmune disease development and expression and may, in turn, explain some of this discrepancy. Finally, we show that there is substantial overlap between the molecular networks that define these disease states across species. Our synthesis and analysis of the current state of the field are consistent with the idea that the same molecular networks are perturbed in murine and human autoimmune disease. Based on these analyses, we anticipate that murine autoimmune disease models will continue to yield novel insights into how best to diagnose, prognose, prevent and treat human autoimmune diseases.

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.

The three as: Alternative splicing, alternative polyadenylation and their impact on apoptosis in immune function

The latest advances in next-generation sequencing studies and transcriptomic profiling over the past decade have highlighted a surprising frequency of genes regulated by RNA processing mechanisms in the immune system. In particular, two control steps in mRNA maturation, namely alternative splicing and alternative polyadenylation, are now recognized to occur in the vast majority of human genes. Both have the potential to alter the identity of the encoded protein, as well as control protein abundance or even protein localization or association with other factors. In this review, we will provide a summary of the general mechanisms by which alternative splicing (AS) and alternative polyadenylation (APA) occur, their regulation within cells of the immune system, and their impact on immunobiology. In particular, we will focus on how control of apoptosis by AS and APA is used to tune cell fate during an immune response.

Modulation of TCR Signaling by Tyrosine Phosphatases: From Autoimmunity to Immunotherapy

Early TCR signaling is dependent on rapid phosphorylation and dephosphorylation of multiple signaling and adaptor proteins, leading to T cell activation. This process is tightly regulated by an intricate web of interactions between kinases and phosphatases. A number of tyrosine phosphatases have been shown to modulate T cell responses and thus alter T cell fate by negatively regulating early TCR signaling. Mutations in some of these enzymes are associated with enhanced predisposition to autoimmunity in humans, and mouse models deficient in orthologous genes often show T cell hyper-activation. Therefore, phosphatases are emerging as potential targets in situations where it is desirable to enhance T cell responses, such as immune responses to tumors. In this review, we summarize the current knowledge about tyrosine phosphatases that regulate early TCR signaling and discuss their involvement in autoimmunity and their potential as targets for tumor immunotherapy.

The role of competing mechanisms on Lck regulation

Lck is a Src-related protein tyrosine kinase that associates with CD4 and CD8 molecules and is essential to T cell development and T cell activation. Regulatory mechanisms of Lck are diverse and controversy exists regarding the importance of each mechanism. The balance of phosphorylation at the inhibitory and activating Tyr residues is maintained by a balance between CD45 and Csk and is dependent upon intact intracellular trafficking machinery. Current evidence shows that lipid-binding changes depending on Lck conformation and that phosphorylation-induced conformational changes in Lck modulate its kinase activity potentially through regulation of Lck clustering at the plasma membrane. Downstream regulators such as ZAP-70 mediate negative feedback that is dependent on Tyr¹⁹² phosphorylation. This review examines the diverse regulation of Lck in detail, highlighting the role of each mechanism on maintaining an appropriate amount of Lck in each conformational state, thus allowing for an efficient, appropriate, and controlled amount of T cell activation following TCR stimulation.

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.

Pathogenic roles of B lymphocytes in systemic sclerosis

Systemic sclerosis (SSc) is a collagen disease characterized by autoimmunity and excessive extracellular matrix deposition in the skin and visceral organs. Although the pathogenic relationship between systemic autoimmunity and the clinical manifestations of SSc remains unknown, SSc patients show a variety of abnormal immune activation including the production of disease-specific autoantibodies and cytokine production. Many recent studies have demonstrated that immune cells, including T cells, B cells, and macrophages, have a variety of immunological abnormalities in SSc. So far, several groups and our group reported that B cells play a critical role in systemic autoimmunity and disease expression through various functions, such as cytokine production, lymphoid organogenesis, and induction of other immune cell activation in addition to autoantibody production. Recent studies show that B cells from SSc patients demonstrate an up-regulated CD19 expression, a crucial regulator of B cell activation, which induces chronic hyper-reactivity of memory B cells and SSc-specific autoantibody production and also causes fibrosis of several organs. Furthermore, in SSc-model mice, such as tight-skin mice, bleomycin-induced SSc model mice, and DNA topoisomerase I and complete Freund's adjuvant-induced SSc model mice, have abnormal B cell activation which associates with skin and lung fibrosis. Indeed, B cell depletion therapy using anti-CD20 Ab, Rituximab, is considered to one potential beneficial treatment for patients with SSc. However, there is no direct evidence which can explain how B cells, especially autoantigen-reactive B cells, progress or regulate disease manifestations of SSc. Collectively, B cell abnormalities in SSc is most likely participating in fibrosis and tissue damage of SSc. If the relationship between SSc-specific tissue damage and B cell abnormalities is revealed, these findings lead to novel effective therapy for SSc.

Identification of novel splicing variants of protein tyrosine phosphatase receptor type Z

Protein tyrosine phosphatase receptor type Z (PTPRZ, also known as PTPζ or RPTPβ) is preferentially expressed in the central nervous system (CNS). PTPRZ plays important roles during development and adulthood in CNS myelination, learning and memory. Three splicing isoforms for PTPRZ have been identified to date: two receptor type isoforms, PTPRZ-A and PTPRZ-B, and one secretory isoform, PTPRZ-S. We herein identified novel PTPRZ receptor sub-isoforms without a seven-amino acid sequence encoded by exon 16. This sequence forms a part of the helix-turn-helix segment called the 'wedge' structure, which is located at the N-terminal region in the membrane-proximal protein tyrosine phosphatase domain. In contrast to conventional receptor isoforms with uniform expression, the deleted isoforms were expressed in the brain, but not in the retina, indicating the tissue-specific splicing of exon 16. Biochemical analyses of PTPRZ intracellular regions revealed differences in the characteristics of the deleted form, namely, stronger binding activity to postsynaptic density protein 95 (PSD95) and greater enrichment in the postsynaptic density fraction than the full-length form. Furthermore, the exon 16-deleted form exhibited higher catalytic efficiency in vitro. These results suggest that sub-isoforms of PTPRZ have different functions because of variations in the wedge structure.

Regulation of receptor-type protein tyrosine phosphatases by their C-terminal tail domains

Protein tyrosine phosphatases (PTPs) perform specific functions in vivo, despite being vastly outnumbered by their substrates. Because of this and due to the central roles PTPs play in regulating cellular function, PTP activity is regulated by a large variety of molecular mechanisms. We review evidence that indicates that the divergent C-terminal tail sequences (C-terminal domains, CTDs) of receptor-type PTPs (RPTPs) help regulate RPTP function by controlling intermolecular associations in a way that is itself subject to physiological regulation. We propose that the CTD of each RPTP defines an 'interaction code' that helps determine molecules it will interact with under various physiological conditions, thus helping to regulate and diversify PTP function.

Identification of function-regulating antibodies targeting the receptor protein tyrosine phosphatase sigma ectodomain

Receptor tyrosine phosphatase sigma (RPTPσ) plays an important role in the regulation of axonal outgrowth and neural regeneration. Recent studies have identified two RPTPσ ligands, chondroitin sulfate proteoglycans (CSPGs) and heparan sulfate proteoglycans (HSPG), which can modulate RPTPσ activity by affecting its dimerization status. Here, we developed a split luciferase assay to monitor RPTPσ dimerization in living cells. Using this system, we demonstrate that heparin, an analog of heparan sulfate, induced the dimerization of RPTPσ, whereas chondroitin sulfate increased RPTPσ activity by inhibiting RPTPσ dimerization. Also, we generated several novel RPTPσ IgG monoclonal antibodies, to identify one that modulates its activity by inducing/stabilizing dimerization in living cells. Lastly, we demonstrate that this antibody promotes neurite outgrowth in SH-SY5Y cells. In summary, we demonstrated that the split luciferase RPTPσ activity assay is a novel high-throughput approach for discovering novel RPTPσ modulators that can promote axonal outgrowth and neural regeneration.

Dysregulated homeostasis of target tissues or autoantigens - A novel principle in autoimmunity

Monogenic autoimmune disorders provide a powerful tool for our understanding of the principles of autoimmunity due to the obvious impact of a single gene on the disease. So far, approximately 100 single gene defects causing murine monogenic autoimmune disorders have been reported and the functional characterization of these genes will provide significant progress in understanding the nature of autoimmunity. According to their function, genes leading to monogenic autoimmune disorders can be categorized into two groups. An expectable first group contains genes involved in the homeostasis of the immune system, including homeostasis of immune organs and immune cells. Intriguingly, the second group consists of genes functionally involved in the homeostasis of target tissues or autoantigens. According to our novel hypothesis, we propose that autoimmunity represents a consequence of a dysregulated homeostasis of the immune system and/or its targets including autoantigens and target tissues. In this review we refer to both aspects of homeostasis in autoimmunity with a highlight on the role of the homeostasis of target tissues and autoantigens.

B lymphocytes in systemic sclerosis: Abnormalities and therapeutic targets

Systemic sclerosis (SSc) is a connective tissue disease characterized by excessive extracellular matrix deposition in the skin and visceral organs with an autoimmune background. Although the pathogenic relationship between systemic autoimmunity and the clinical manifestations remains unknown, SSc patients have immunological abnormalities including the production of disease-specific autoantibodies. Recent studies have demonstrated that B cells play a crucial role in systemic autoimmunity and disease expression via various functions in addition to autoantibody production. Recent studies show that B cells from SSc patients demonstrate an upregulated CD19 signaling pathway, which is a crucial regulator of B-cell activation, that induces SSc-specific autoantibody production in SSc. In addition, B cells from SSc patients exhibit an overexpression of CD19. Consistently, in CD19 transgenic mice, CD19 overexpression induces SSc-specific autoantibody production. SSc patients have also intrinsic B-cell abnormalities characterized by chronic hyperreactivity of memory B cells, possibly due to CD19 overexpression. Similarly, B cells from a tight-skin mouse, a genetic model of SSc, show augmented CD19 signaling and chronic hyperreactivity. Furthermore, in bleomycin-induced SSc model mice, endogenous ligands for Toll-like receptors, induced by bleomycin treatment, stimulate B cells to produce various fibrogenic cytokines and autoantibodies. Remarkably, CD19 loss results in inhibition of B-cell hyperreactivity and elimination of autoantibody production, which is associated with improvement of fibrosis. Taken together, altered B-cell function may result in tissue fibrosis, as well as autoimmunity, in SSc. Although further studies and greater understanding are needed, B cells are potential therapeutic target in SSc.

CD45-mediated control of TCR tuning in naïve and memory CD8+ T cells

Continuous contact with self-major histocompatibility complex (MHC) ligands is essential for survival of naïve T cells but not memory cells. This surprising finding implies that T cell subsets may vary in their relative T-cell receptor (TCR) sensitivity. Here we show that in CD8⁺T cells TCR sensitivity correlates inversely with levels of CD5, a marker for strong self-MHC reactivity. We also show that TCR sensitivity is lower in memory CD8⁺ T cells than naïve cells. In both situations, TCR hypo-responsiveness applies only to short-term TCR signalling events and not to proliferation, and correlates directly with increased expression of a phosphatase, CD45 and reciprocal decreased expression of activated LCK. Inhibition by high CD45 on CD8⁺ T cells may protect against overt TCR auto-MHC reactivity, while enhanced sensitivity to cytokines ensures strong responses to foreign antigens.

Naïve T cells establish self-tolerance via negative selection of cells with strong reactivity for self-peptide/MHC complexes, but undergo T-cell receptor (TCR) desensitisation when leaving the thymus. Here, Cho et al. show that TCR desensitisation correlates with cell-surface density of the phosphatase CD45.

B lymphocytes in systemic lupus erythematosus: lessons from therapy targeting B cells

Systemic lupus erythematosus (SLE) is a complex disease characterizedby numerous autoantibodies and clinical involvement in multiple organ systems. Autoantibodies are usually present in serum for years before the onset of clinical disease. Autoimmunity begins with a limited number of autoantibodiesand evolves to become progressivelymore diverse. Eventually clinical disease ensues. The immunological events triggering the onset of clinical manifestations have not yet been defined. While undoubtedly T cells and dendritic cells appear to play major roles in SLE, a central role for B cells in the pathogenesis of this disease has been brought to the fore in the last few years by work performed both in mice and humans by multiple laboratories.As a result, there is little doubt about the importance of B cells in the development of SLE. Yet much remains to be learned about their role in the ongoing disease process and the merit of targeting B cells for the treatment of SLE. This article will review the role of B cells in human SLE as well as the currently available data on the treatment of SLE by depleting B cells with anti-CD20 (rituximab).

An extracatalytic function of CD45 in B cells is mediated by CD22

The receptor-like tyrosine phosphatase CD45 regulates antigen receptor signaling by dephosphorylating the C-terminal inhibitory tyrosine of the src family kinases. However, despite its abundance, the function of the large, alternatively spliced extracellular domain of CD45 has remained elusive. We used normally spliced CD45 transgenes either incorporating a phosphatase-inactivating point mutation or lacking the cytoplasmic domain to uncouple the enzymatic and noncatalytic functions of CD45 in lymphocytes. Although these transgenes did not alter T-cell signaling or development irrespective of endogenous CD45 expression, both partially rescued the phenotype of CD45-deficient B cells. We identify a noncatalytic role for CD45 in regulating tonic, but not antigen-mediated, B-cell antigen receptor (BCR) signaling through modulation of the function of the inhibitory coreceptor CD22. This finding has important implications for understanding how naïve B cells maintain tonic BCR signaling while restraining inappropriate antigen-dependent activation to preserve clonal "ignorance."

Unbiased modifier screen reveals that signal strength determines the regulatory role murine TLR9 plays in autoantibody production

The autoimmune disease systemic lupus erythematosus has a complex environmental and multifactorial genetic basis. Genome-wide association studies have recently identified numerous disease-associated polymorphisms, but it remains unclear in which cells and during which step of pathogenesis specific polymorphisms interact to cause disease. Using a mouse model in which the same activating mutation (CD45E613R) causes distinct genetic background-dependent disease phenotypes, we performed a screen for genetic modifiers of autoreactivity between anti-nuclear Ab (ANA)-resistant CD45E613R.B6 and ANA-permissive CD45E613R.BALB/c mice. Within a novel autoreactivity-associated locus on chromosome 9, we identify a putative modifier, TLR9. Validating a role for TLR9 in modifying autoreactivity in the context of the CD45E613R mutation, manipulation of TLR9 gene dosage eliminates ANA in CD45E613R.BALB/c mice, but confoundingly permits ANA in CD45E613R.B6 mice. We demonstrate that sensitivity to ANA is modulated by strength of TLR9 signal, because stronger TLR9(B6) signals, but not weaker TLR9(BALB/c) signals, negatively regulate CD45E613R B cell development during competitive reconstitution at the central tolerance checkpoint. Our results identify a novel autoreactivity-associated locus and validate Tlr9 as a candidate gene within the locus. We further demonstrate a novel role for TLR9 signal strength in central tolerance, providing insight into the interplay of disease-associated polymorphisms at a discrete step of systemic lupus erythematosus pathogenesis.

Abnormal B lymphocyte activation and function in systemic sclerosis

Systemic sclerosis (SSc) is characterized by tissue fibrosis and autoimmunity. Although the pathogenic relationship between autoimmunity and clinical manifestations of SSc remains unknown, SSc patients display abnormal immune responses including the production of disease-specific autoantibodies. Previous studies have demonstrated that B cells play a critical role in systemic autoimmunity and disease expression through various functions such as induction of the activation of other immune cells in addition to autoantibody production. CD19 is a crucial regulator of B cell activation. Recent studies demonstrated that B cells from SSc patients showed an up-regulated CD19 signaling pathway that induced SSc-specific autoantibody production in SSc mouse models. CD19 transgenic mice lost tolerance for autoantigen and generated autoantibodies spontaneously. B cells from SSc patients exhibited an overexpression of CD19 that induced SSc-specific autoantibody production in transgenic mice. Moreover, SSc patients displayed intrinsic B cell abnormalities characterized by chronic hyper-reactivity of memory B cells, which was possibly due to CD19 overexpression. Similarly, B cells from a tight-skin mouse, a genetic model of SSc, showed augmented CD19 signaling. In bleomycin-induced SSc mouse models, endogenous ligands for toll-like receptor 4 induced by bleomycin stimulated B cells to produce various fibrogenic cytokines and autoantibodies. Remarkably, the loss of CD19 resulted in the inhibition of B cell hyper-reactivity and autoantibody production, which are associated with improvements in fibrosis and a parallel decrease in fibrogenic cytokine production by B cells. Taken together, the findings suggest that altered B cell function may result in tissue fibrosis as well as autoimmunity in SSc.

Mutation in the CD45 inhibitory wedge modulates integrin activation and leukocyte recruitment during inflammation

Neutrophil recruitment to the site of inflammation plays a pivotal role in host defense. Src family kinases (SFKs) activation is required for integrin and chemokine signaling as well as immune cell function. The receptor-like protein tyrosine phosphatase CD45 positively regulates chemoattractant signaling acting on SFK activity. To further investigate the role of CD45 in neutrophil recruitment and function, we analyzed transgenic mice carrying a single point mutation (CD45E613R), which constitutively activates CD45. By using intravital microscopy experiments, we demonstrated that different steps of the leukocyte recruitment cascade were affected in CD45E613R mutant mice. The rolling velocity of CD45E613R mutant neutrophils was decreased compared with wild-type neutrophils that subsequently resulted in an increased number of adherent cells. The analysis of β2 integrins LFA-1 and macrophage-1 Ag (Mac-1) showed that in CD45E613R mutant neutrophils LFA-1 adhesiveness was impaired, and avidity was enhanced, whereas Mac-1 adhesiveness was increased. Because of the increased Mac-1 adhesiveness, neutrophil crawling was impaired in CD45E613R mutant compared with wild-type neutrophils. In an Escherichia coli lung infection model, CD45E613R mice displayed a decreased neutrophil recruitment into the alveolar compartment, which resulted in an increased number of CFUs in the lung. Our data demonstrate that the CD45E613R mutation modulates integrin activation and leukocyte recruitment during inflammation.

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.

Cellular biochemistry methods for investigating protein tyrosine phosphatases

SIGNIFICANCE

The protein tyrosine phosphatases (PTPs) are a family of proteins that play critical roles in cellular signaling and influence many aspects of human health and disease. Although a wealth of information has been collected about PTPs since their discovery, many questions regarding their regulation and function still remain.

CRITICAL ISSUES

Of particular importance are the elucidation of the biological substrates of individual PTPs and understanding of the chemical and biological basis for temporal and spatial resolution of PTP activity within a cell.

RECENT ADVANCES

Drawing from recent advances in both biology and chemistry, innovative approaches have been developed to study the intracellular biochemistry and physiology of PTPs. We provide a summary of PTP-tailored techniques and approaches, emphasizing methodologies to study PTP activity within a cellular context. We first provide a discussion of methods for identifying PTP substrates, including substrate-trapping mutants and synthetic peptide libraries for substrate selectivity profiling. We next provide an overview of approaches for monitoring intracellular PTP activity, including a discussion of mechanistic-based probes, gel-based assays, substrates that can be used intracellularly, and assays tied to cell growth. Finally, we review approaches used for monitoring PTP oxidation, a key regulatory pathway for these enzymes, discussing the biotin switch method and variants of this approach, along with affinity trapping techniques and probes designed to detect PTP oxidation.

FUTURE DIRECTIONS

Further development of approaches to investigate the intracellular PTP activity and functions will provide specific insight into their mechanisms of action and control of diverse signaling pathways.

Bi-domain protein tyrosine phosphatases reveal an evolutionary adaptation to optimize signal transduction

SIGNIFICANCE

The bi-domain protein tyrosine phosphatases (PTPs) exemplify functional evolution in signaling proteins for optimal spatiotemporal signal transduction. Bi-domain PTPs are products of gene duplication. The catalytic activity, however, is often localized to one PTP domain. The inactive PTP domain adopts multiple functional roles. These include modulation of catalytic activity, substrate specificity, and stability of the bi-domain enzyme. In some cases, the inactive PTP domain is a receptor for redox stimuli. Since multiple bi-domain PTPs are concurrently active in related cellular pathways, a stringent regulatory mechanism and selective cross-talk is essential to ensure fidelity in signal transduction.

RECENT ADVANCES

The inactive PTP domain is an activator for the catalytic PTP domain in some cases, whereas it reduces catalytic activity in other bi-domain PTPs. The relative orientation of the two domains provides a conformational rationale for this regulatory mechanism. Recent structural and biochemical data reveal that these PTP domains participate in substrate recruitment. The inactive PTP domain has also been demonstrated to undergo substantial conformational rearrangement and oligomerization under oxidative stress.

CRITICAL ISSUES AND FUTURE DIRECTIONS

The role of the inactive PTP domain in coupling environmental stimuli with catalytic activity needs to be further examined. Another aspect that merits attention is the role of this domain in substrate recruitment. These aspects have been poorly characterized in vivo. These lacunae currently restrict our understanding of neo-functionalization of the inactive PTP domain in the bi-domain enzyme. It appears likely that more data from these research themes could form the basis for understanding the fidelity in intracellular signal transduction.

Diverse levels of sequence selectivity and catalytic efficiency of protein-tyrosine phosphatases

The sequence selectivity of 14 classical protein-tyrosine phosphatases (PTPs) (PTPRA, PTPRB, PTPRC, PTPRD, PTPRO, PTP1B, SHP-1, SHP-2, HePTP, PTP-PEST, TCPTP, PTPH1, PTPD1, and PTPD2) was systematically profiled by screening their catalytic domains against combinatorial peptide libraries. All of the PTPs exhibit similar preference for pY peptides rich in acidic amino acids and disfavor positively charged sequences but differ vastly in their degrees of preference/disfavor. Some PTPs (PTP-PEST, SHP-1, and SHP-2) are highly selective for acidic over basic (or neutral) peptides (by >10(5)-fold), whereas others (PTPRA and PTPRD) show no to little sequence selectivity. PTPs also have diverse intrinsic catalytic efficiencies (kcat/KM values against optimal substrates), which differ by >10(5)-fold due to different kcat and/or KM values. Moreover, PTPs show little positional preference for the acidic residues relative to the pY residue. Mutation of Arg47 of PTP1B, which is located near the pY-1 and pY-2 residues of a bound substrate, decreased the enzymatic activity by 3-18-fold toward all pY substrates containing acidic residues anywhere within the pY-6 to pY+5 region. Similarly, mutation of Arg24, which is situated near the C-terminus of a bound substrate, adversely affected the kinetic activity of all acidic substrates. A cocrystal structure of PTP1B bound with a nephrin pY(1193) peptide suggests that Arg24 engages in electrostatic interactions with acidic residues at the pY+1, pY+2, and likely other positions. These results suggest that long-range electrostatic interactions between positively charged residues near the PTP active site and acidic residues on pY substrates allow a PTP to bind acidic substrates with similar affinities, and the varying levels of preference for acidic sequences by different PTPs are likely caused by the different electrostatic potentials near their active sites. The implications of the varying sequence selectivity and intrinsic catalytic activities with respect to PTP in vivo substrate specificity and biological functions are discussed.

BANK1 and BLK act through phospholipase C gamma 2 in B-cell signaling

The B cell adaptor protein with ankyrin repeats (BANK1) and the B lymphoid tyrosine kinase (BLK) have been genetically associated with autoimmunity. The proteins of these genes interact physically and work in concert during B-cell signaling. Little is know about their interactions with other B-cell signaling molecules or their role in the process. Using yeast two hybrid (Y2H) we sought for factors that interact with BANK1. We found that the molecular switch PLCg2 interacts with BANK1 and that the interaction is promoted by B-cell receptor (BCR) stimulation. We found further that the kinase activity of BLK enhanced BANK1- PLCg2 binding and that the interaction was suppressed upon BLK depletion. Immunoprecipitation and mutational analysis demonstrated that the interaction between BANK1 and PLCg2 was dependent on specific tyrosine and proline residues on the adaptor protein. Our results provide new information important to understand the role of these two genes in basic B-cell physiology and immune-related diseases.

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.

The microbial capsular polysaccharide galactoxylomannan inhibits IL-17A production in circulating T cells from rheumatoid arthritis patients

The persistence of activated T cells in rheumatoid arthritis (RA) synovium may be attributable to increased homing, increased retention or a possible imbalance between cell proliferation and programmed cell death. Induction of apoptosis may represent a potential therapeutic approach. Galactoxylomannan (GalXM) from the opportunistic fungus Cryptococcus neoformans can interact with T cells and induce T-cell apoptosis through the inhibition of CD45 phosphatase activity. The aim of this study was to determine the effect of GalXM on circulating T cells from patients with RA and the underlying mechanisms. GalXM immunomodulating effect on apoptosis and signal transduction pathway involved in IL-17A production was evaluated on T cells. RA T-cell apoptosis, higher than that of control T cells, was further increased by GalXM through induction of caspase-3 activation. Activated T cells expressing the CD45RO molecule and producing IL-17A were the main target of GalXM-induced apoptosis. GalXM induced consistent impairment of IL-17A production and inhibition of STAT3, which was hyperactivated in RA. In conclusion, GalXM triggered apoptosis of activated memory T cells and interfered with IL-17A production in RA. These data suggest therapeutic targeting of deleterious Th17 cells in RA and other autoimmune diseases.

A phospholipase C-γ1-independent, RasGRP1-ERK-dependent pathway drives lymphoproliferative disease in linker for activation of T cells-Y136F mutant mice

Mice expressing a germline mutation in the phospholipase C-γ1-binding site of linker for activation of T cells (LAT) show progressive lymphoproliferation and ultimately die at 4-6 mo age. The hyperactivated T cells in these mice show defective TCR-induced calcium flux but enhanced Ras/ERK activation, which is critical for disease progression. Despite the loss of LAT-dependent phospholipase C-γ1 binding and activation, genetic analysis revealed RasGRP1, and not Sos1 or Sos2, to be the major Ras guanine exchange factor responsible for ERK activation and the lymphoproliferative phenotype in these mice. Analysis of isolated CD4(+) T cells from LAT-Y136F mice showed altered proximal TCR-dependent kinase signaling, which activated a Zap70- and LAT-independent pathway. Moreover, LAT-Y136F T cells showed ERK activation that was dependent on Lck and/or Fyn, protein kinase C-θ, and RasGRP1. These data demonstrate a novel route to Ras activation in vivo in a pathological setting.

Regulation of TCR signalling by tyrosine phosphatases: from immune homeostasis to autoimmunity

More than half of the known protein tyrosine phosphatases (PTPs) in the human genome are expressed in T cells, and significant progress has been made in elucidating the biology of these enzymes in T-cell development and function. Here we provide a systematic review of the current understanding of the roles of PTPs in T-cell activation, providing insight into their mechanisms of action and regulation in T-cell receptor signalling, the phenotypes of their genetically modified mice, and their possible involvement in T-cell-mediated autoimmune disease. Our projection is that the interest in PTPs as mediators of T-cell homeostasis will continue to rise with further functional analysis of these proteins, and PTPs will be increasingly considered as targets of immunomodulatory therapies.

Autoimmunity in primary antibody deficiency is associated with protein tyrosine phosphatase nonreceptor type 22 (PTPN22)

BACKGROUND

The 1858T allele of protein tyrosine phosphatase nonreceptor type 22 (PTPN22; R620W) exhibits one of the strongest and most consistent associations with sporadic autoimmune disease. Although autoimmunity is common in patients with primary antibody deficiency (PAD), it remains unknown whether its pathogenesis is similar when it arises in this context compared with in immunocompetent patients.

OBJECTIVE

We set out to determine whether the 1858T allele of PTPN22 was associated with PAD or with autoimmunity in the context of PAD.

METHODS

We genotyped rs2476601 (g.1858C>T), a single nucleotide polymorphism encoding substitution of arginine for tryptophan in PTPN22 (R620W), in 193 patients with PAD and 148 control subjects from an Australian cohort. We also performed a subgroup analysis according to the presence of autoimmunity and B-cell phenotypes.

RESULTS

C/T and T/T PTPN22 genotypes were more common in patients with PAD than in the matched control subjects (C/T, 18.1% vs 9.5%; T/T, 1.04% vs 0.6%). The T allele was associated with an increased risk of PAD relative to control subjects (odds ratio, 2.10; 95% CI, 1.11-4.00). The distribution of genotypes in control subjects was similar to those reported previously and did not deviate significantly from Hardy-Weinberg equilibrium. We found a strong association between the 1858T allele and PAD with coexistent autoimmune diseases. In patients with PAD and autoimmunity, 16 (43.2%) of 37 had at least one T allele of PTPN22 compared with 27 (17.3%) of 156 with the C/C genotype (P=.0014; odds ratio, 3.64; 95% CI, 1.68-7.88). We found no evidence that this effect was mediated by enrichment of CD21low B cells.

CONCLUSION

The 1858T PTPN22 allele is strongly associated with autoimmunity in patients with PAD.

TACE activation by MAPK-mediated regulation of cell surface dimerization and TIMP3 association

Ectodomain shedding mediated by tumor necrosis factor-α (TNF-α)-converting enzyme [TACE; also known as ADAM17 (a disintegrin and metalloproteinase 17)] provides an important switch in regulating cell proliferation, inflammation, and cancer progression. TACE-mediated ectodomain cleavage is activated by signaling of the mitogen-activated protein kinases (MAPKs) p38 and ERK (extracellular signal-regulated kinase). Here, we found that under basal conditions, TACE was predominantly present as dimers at the cell surface, which required its cytoplasmic domain and enabled efficient association with tissue inhibitor of metalloproteinase-3 (TIMP3) and silencing of TACE activity. Upon activation of the ERK or p38 MAPK pathway, the balance shifted from TACE dimers to monomers, and this shift was associated with increased cell surface presentation of TACE and decreased TIMP3 association, which relieved the inhibition of TACE by TIMP3 and increased TACE-mediated proteolysis of transforming growth factor-α. Thus, cell signaling altered the dimer-monomer equilibrium and inhibitor association to promote activation of TACE-mediated ectodomain shedding, a regulatory mechanism that may extend to other ADAM proteases.

Protein tyrosine phosphatases in lymphocyte activation and autoimmunity

Lymphocyte activation must be tightly regulated to ensure sufficient immunity to pathogens and prevent autoimmunity. Protein tyrosine phosphatases (PTPs) serve critical roles in this regulation by controlling the functions of key receptors and intracellular signaling molecules in lymphocytes. In some cases, PTPs inhibit lymphocyte activation, whereas in others they promote it. Here we discuss recent progress in elucidating the roles and mechanisms of action of PTPs in lymphocyte activation. We also review the accumulating evidence that genetic alterations in PTPs are involved in human autoimmunity.

Experimental use of mouse models of systemic lupus erythematosus

Mouse models of lupus have for many years provided accessible and reliable research systems for the pathogenesis and therapy of systemic autoimmune disease, spanning a spectrum of inbred strains that develop spontaneous disease to experimentally induced, sometimes genetically manipulated animals. Nearly all the models share in common the development of glomerulonephritis and autoantibodies, including antinuclear and DNA specificities, the most common endpoints examined in experimental studies, but exhibit specific differences in the incidence of other end-organ manifestations such as hemolytic anemia, arthritis, dermatitis, and vasculitis. This chapter contrasts the clinical characteristics of these various models, providing an outline for their use and analysis.

The human cytomegalovirus UL11 protein interacts with the receptor tyrosine phosphatase CD45, resulting in functional paralysis of T cells

Human cytomegalovirus (CMV) exerts diverse and complex effects on the immune system, not all of which have been attributed to viral genes. Acute CMV infection results in transient restrictions in T cell proliferative ability, which can impair the control of the virus and increase the risk of secondary infections in patients with weakened or immature immune systems. In a search for new immunomodulatory proteins, we investigated the UL11 protein, a member of the CMV RL11 family. This protein family is defined by the RL11 domain, which has homology to immunoglobulin domains and adenoviral immunomodulatory proteins. We show that pUL11 is expressed on the cell surface and induces intercellular interactions with leukocytes. This was demonstrated to be due to the interaction of pUL11 with the receptor tyrosine phosphatase CD45, identified by mass spectrometry analysis of pUL11-associated proteins. CD45 expression is sufficient to mediate the interaction with pUL11 and is required for pUL11 binding to T cells, indicating that pUL11 is a specific CD45 ligand. CD45 has a pivotal function regulating T cell signaling thresholds; in its absence, the Src family kinase Lck is inactive and signaling through the T cell receptor (TCR) is therefore shut off. In the presence of pUL11, several CD45-mediated functions were inhibited. The induction of tyrosine phosphorylation of multiple signaling proteins upon TCR stimulation was reduced and T cell proliferation was impaired. We therefore conclude that pUL11 has immunosuppressive properties, and that disruption of T cell function via inhibition of CD45 is a previously unknown immunomodulatory strategy of CMV.

The human cytomegalovirus (CMV) belongs to a class of viruses that interferes with the immune response of its host. Accordingly, infection with CMV is a severe risk for immunologically immature newborns and immunocompromised patients such as transplant recipients. The mechanisms by which CMV affects the immune system are not completely understood. Here we show that a CMV protein, pUL11, which is expressed on the surface of cells, binds to leukocytes by interacting with the receptor tyrosine phosphatase CD45. In T cells, CD45 is essential for transmission of activating signals received via the T cell receptor (TCR) to downstream effector molecules that ultimately lead to activation and proliferation of these immune cells. Binding of the CMV pUL11 protein to CD45 on T cells prevents signal transduction via the TCR and restricts T cell proliferation. Interestingly, the mechanism by which the activity of CD45 is regulated is a matter of debate and no specific cellular ligand of CD45 has yet been described. The identification of a first viral ligand for CD45 may provide the means to investigate CD45 regulatory mechanisms and also allow the development of therapies to interfere with CMV-mediated immunomodulation.

Elevated BCR signaling and decreased survival of Lyn-deficient transitional and follicular B cells

The Src-family tyrosine kinase Lyn negatively regulates BCR signaling and also myeloid cell activity. Mice deficient in Lyn have substantially decreased numbers of peripheral B cells, despite spontaneously producing IgG anti-DNA antibodies. Here, we examine the mechanism underlying the B-cell depletion in these mice. Lyn-deficient B cells were out-competed by WT B cells in mixed BM chimeras at two steps, at the T1 to T2 transitional maturation stage in the spleen and again between the T2 or T3 stage and the mature follicular B-cell population. Lyn-deficient T2 and follicular B cells expressed elevated levels of the pro-apoptotic factor Bim and deletion of Bim restored splenic B cells of Lyn-deficient mice to close to WT numbers. Lyn-deficient T2 and later stage B cells also had changes in cell surface phenotype consistent with increased in vivo BCR signaling. Similarly, an increased proportion of T2 and follicular B cells had elevated basal intracellular free calcium levels. Overall, these observations suggest that increased BCR signaling is responsible for increased death of weakly self-reactive Lyn-deficient B cells both at the T2 stage and additionally as these cells mature to follicular B cells.

A disease-associated polymorphism alters splicing of the human CD45 phosphatase gene by disrupting combinatorial repression by heterogeneous nuclear ribonucleoproteins (hnRNPs)

Alternative splicing is typically controlled by complexes of regulatory proteins that bind to sequences within or flanking variable exons. The identification of regulatory sequence motifs and the characterization of sequence motifs bound by splicing regulatory proteins have been essential to predicting splicing regulation. The activation-responsive sequence (ARS) motif has previously been identified in several exons that undergo changes in splicing upon T cell activation. hnRNP L binds to this ARS motif and regulates ARS-containing exons; however, hnRNP L does not function alone. Interestingly, the proteins that bind together with hnRNP L differ for different exons that contain the ARS core motif. Here we undertake a systematic mutational analysis of the best characterized context of the ARS motif, namely the ESS1 sequence from CD45 exon 4, to understand the determinants of binding specificity among the components of the ESS1 regulatory complex and the relationship between protein binding and function. We demonstrate that different mutations within the ARS motif affect specific aspects of regulatory function and disrupt the binding of distinct proteins. Most notably, we demonstrate that the C77G polymorphism, which correlates with autoimmune disease susceptibility in humans, disrupts exon silencing by preventing the redundant activity of hnRNPs K and E2 to compensate for the weakened function of hnRNP L. Therefore, these studies provide an important example of the functional relevance of combinatorial function in splicing regulation and suggest that additional polymorphisms may similarly disrupt function of the ESS1 silencer.

Why is PTPN22 a good candidate susceptibility gene for autoimmune disease?

The PTPN22 locus is one of the strongest risk factors outside of the major histocompatability complex that associates with autoimmune diseases. PTPN22 encodes lymphoid protein tyrosine phosphatase (Lyp) which is expressed exclusively in immune cells. A single base change in the coding region of this gene resulting in an arginine to tryptophan amino acid substitution within a polyproline binding motif associates with type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosis, Hashimotos thyroiditis, Graves disease, Addison's disease, Myasthenia Gravis, vitiligo, systemic sclerosis juvenile idiopathic arthritis and psoriatic arthritis. Here, we review the current understanding of the PTPN22 locus from a genetic, geographical, biochemical and functional perspective.

Trp53 negatively regulates autoimmunity via the STAT3-Th17 axis

Emerging evidence suggests that the tumor suppressor p53 is also a crucial regulator for many physiological processes. Previous observations indicate that p53 suppresses inflammation by inhibiting inflammatory antigen-presenting cells. To investigate the potential role of p53 in autoimmune effector T cells, we generated p53(null)CD45.1 mice by crossing p53(null)CD45.2 and CD45.1 mice. We demonstrate that p53(null)CD45.1 mice spontaneously developed autoimmunity, with a significant increase in IL-17-producing Th17 effectors in their lymph nodes (4.7 ± 1.0%) compared to the age-matched counterparts (1.9 ± 0.8% for p53(null)CD45.2, 1.1 ± 0.2% for CD45.1, and 0.5 ± 0.1% for CD45.2 mice). Likewise, p53(null)CD45.1 mice possess highly elevated serum levels of inflammatory cytokines IL-17 and IL-6. This enhanced Th17 response results largely from an increased sensitivity of p53(null)CD45.1 T cells to IL-6-induced STAT3 phosphorylation. Administration of STAT3 inhibitor S31-201 (IC50 of 38.0 ± 7.2 μM for IL-6-induced STAT3 phosphorylation), but not PBS control, to p53(null)CD45.1 mice suppressed Th17 effectors and alleviated autoimmune pathology. This is the first report revealing that p53 activity in T cells suppresses autoimmunity by controlling Th17 effectors. This study suggests that p53 serves as a guardian of immunological functions and that the p53-STAT3-Th17 axis might be a therapeutic target for autoimmunity.

Role of CD45 signaling pathway in galactoxylomannan-induced T cell damage

Previously, we reported that Galactoxylomannan (GalXM) activates the extrinsic and intrinsic apoptotic pathways through an interaction with the glycoreceptors on T cells. In this study we establish the role of the glycoreceptor CD45 in GalXM-induced T cell apoptosis, using CD45(+/+) and CD45(-/-) cell lines, derived from BW5147 murine T cell lymphoma. Our results show that whereas CD45 expression is not required for GalXM association by the cells, it is essential for apoptosis induction. In CD45(+/+) cells, CD45 triggering by GalXM reduces the activation of Lck, ZAP70 and Erk1/2. Conversely, in CD45(-/-) cells, Lck was hyperphosphorylated and did not show any modulation after GalXM stimulation. On the whole, our findings provide evidence that the negative regulation of Lck activation occurs via CD45 engagement. This appears to be related to the capacity of GalXM to antagonize T cell activation and induce T cell death. Overall this mechanism may be responsible for the immune paralysis that follows GalXM administration and could explain the powerful immunosuppression that accompanies cryptococcosis.

Reversible phosphorylation in haematological malignancies: potential role for protein tyrosine phosphatases in treatment?

Most aspects of leukocyte physiology are under the control of reversible tyrosine phosphorylation. It is clear that excessive phosphorylation of signal transduction elements is a pivotal element of many different pathologies including haematological malignancies and accordingly, strategies that target such phosphorylation have clinically been proven highly successful for treatment of multiple types of leukemias and lymphomas. Cellular phosphorylation status is dependent on the resultant activity of kinases and phosphatases. The cell biology of the former is now well understood; for most cellular phosphoproteins we now know the kinases responsible for their phosphorylation and we understand the principles of their aberrant activity in disease. With respect to phosphatases, however, our knowledge is much patchier. Although the sequences of whole genomes allow us to identify phosphatases using in silico methodology, whereas transcription profiling allows us to understand how phosphatase expression is regulated during disease, most functional questions as to substrate specificity, dynamic regulation of phosphatase activity and potential for therapeutic intervention are still to a large degree open. Nevertheless, recent studies have allowed us to make meaningful statements on the role of tyrosine phosphatase activity in the three major signaling pathways that are commonly affected in leukemias, i.e. the Ras-Raf-ERK1/2, the Jak-STAT and the PI3K-PKB-mTOR pathways. Lessons learned from these pathways may well be applicable elsewhere in leukocyte biology as well.

Consequences of increased CD45RA and RC isoforms for TCR signaling and peripheral T cell deficiency resulting from heterogeneous nuclear ribonucleoprotein L-like mutation

CD45 is the most abundant protein tyrosine phosphatase in the plasma membrane of T cells and serves a critical role in TCR signaling. Different CD45 isoforms are made by alternative mRNA splicing depending on the stage of T cell development and activation, yet their role remains unclear. Expression of CD45RA and RC isoforms is increased 20- to 200-fold on T cells from thunder mice with a loss-of-function mutation in the RNA-binding protein, heterogeneous nuclear ribonucleoprotein L-like (hnRNPLL), although total CD45 expression is unaltered. In this study, we test the hypothesis that this shift in CD45 isoform expression alters TCR signaling, thymic selection, and accumulation of peripheral T cells. There was no discernable effect of the change in CD45 isoform expression upon Lck phosphorylation or T cell positive and negative selection, whereas these indices were strongly affected by a decrease in the overall amount of CD45 in Ptprc mutant animals. The one exception to this conclusion was in thymocytes from Ptprc(loc/loc) animals with 4% of normal CD45 protein levels, where Lck505 phosphorylation was increased 25% in Hnrpll mutant cells, suggesting that high m.w. CD45 isoforms had lower Lck505 phosphatase activity in this context. In T cells with no CD45 protein, hnRNPLL mutation still diminished peripheral T cell accumulation, demonstrating that hnRNPLL regulates T cell longevity independently from its effects on CD45 splicing.

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.