Disruption of lymphocyte function and signaling in CD45-associated protein-null mice

Lymphocyte Phosphatase-Associated Phosphoprotein (LPAP) as a CD45 Protein Stability Regulator

Lymphocyte phosphatase-associated phosphoprotein (LPAP) is a binding partner of the phosphatase CD45, but its function remains poorly understood. Its close interaction with CD45 suggests that LPAP may potentially regulate CD45, but direct biochemical evidence for this has not yet been obtained. We found that in the Jurkat lymphoid cells the levels of LPAP and CD45 proteins are interrelated and well correlated with each other. Knockout of LPAP leads to the decrease in the surface expression of CD45, while its overexpression, on the contrary, caused its increase. No such correlation was found in the non-lymphoid K562 cells. We hypothesize that LPAP regulates expression level of CD45 and thus can affect lymphocyte activation.

Toxin metalloproteinases exert a dominant influence on pro-inflammatory response and anti-inflammatory regulation in jellyfish sting dermatitis

Toxin metalloproteinases are the primary components responsible for various toxicities in jellyfish venom, and there is still no effective specific therapy for jellyfish stings. The comprehension of the pathogenic mechanisms underlying toxin metalloproteinases necessitates further refinement. In this study, we conducted a differential analysis of a dermatitis mouse model induced by jellyfish Nemopilema nomurai venom (NnNV) samples with varying levels of metalloproteinase activity. Through skin tissue proteomics and serum metabolomics, the predominant influence of toxin metalloproteinase activity on inflammatory response was revealed, and the signal pathway involved in its regulation was identified. In skin tissues, many membrane proteins were significantly down-regulated, which might cause tissue damage. The expression of pro-inflammatory factors was mainly regulated by PI3K-Akt signaling pathway. In serum, many fatty acid metabolites were significantly down-regulated, which might be the anti-inflammation feedback regulated by NF-κB p65 signaling pathway. These results reveal the dermatitis mechanism of toxin metalloproteinases and provide new therapeutic targets for further studies. SIGNIFICANCE: Omics is an important method to analyze the pathological mechanism and discover the key markers, which can reveal the pathological characteristics of jellyfish stings. Our research first analyzed the impact of toxin metalloproteinases on jellyfish sting dermatitis by skin proteomics and serum metabolomics. The present results suggest that inhibition of toxin metalloproteinases may be an effective treatment strategy, and provide new references for further jellyfish sting studies.

IGF1R as a Potential Pharmacological Target in Allergic Asthma

Background: Asthma is a chronic lung disease characterized by reversible airflow obstruction, airway hyperresponsiveness (AHR), mucus overproduction and inflammation. Although Insulin-like growth factor 1 receptor (IGF1R) was found to be involved in asthma, its pharmacological inhibition has not previously been investigated in this pathology. We aimed to determine if therapeutic targeting of IGF1R ameliorates allergic airway inflammation in a murine model of asthma. Methods: C57BL/6J mice were challenged by house dust mite (HDM) extract or PBS for four weeks and therapeutically treated with the IGF1R tyrosine kinase inhibitor (TKI) NVP-ADW742 (NVP) once allergic phenotype was established. Results: Lungs of HDM-challenged mice exhibited a significant increase in phospho-IGF1R levels, incremented AHR, airway remodeling, eosinophilia and allergic inflammation, as well as altered pulmonary surfactant expression, all of being these parameters counteracted by NVP treatment. HDM-challenged lungs also displayed augmented expression of the IGF1R signaling mediator p-ERK1/2, which was greatly reduced upon treatment with NVP. Conclusions: Our results demonstrate that IGF1R could be considered a potential pharmacological target in murine HDM-induced asthma and a candidate biomarker in allergic asthma.

Short and dysfunctional telomeres protect from allergen-induced airway inflammation

Asthma is a chronic inflammatory disease affecting 300 million people worldwide. As telomere shortening is a well-established hallmark of aging and that asthma incidence decreases with age, here we aimed to study the role of short telomeres in asthma pathobiology. To this end, wild-type and telomerase-deficient mice with short telomeres (third-generation (G3 Tert^-/- mice)) were challenged with intranasal house dust mite (HDM) extract. We also challenged with HDM wild-type mice in which we induced a telomere dysfunction by the administration of 6-thio-2´-deoxyguanosine (6-thio-dG). Following HDM exposure, G3 Tert^-/- and 6-thio-dG treated mice exhibited attenuated eosinophil counts and presence of hematopoietic stem cells in the bone marrow, as well as lower levels of IgE and circulating eosinophils. Accordingly, both G3 Tert^-/- and 6-thio-dG treated wild-type mice displayed reduced airway hyperresponsiveness (AHR), as indicated by decreased airway remodeling and allergic airway inflammation markers in the lung. Furthermore, G3 Tert^-/- and 6-thio-dG treated mice showed lower differentiation of Club cells, attenuating goblet cell hyperplasia. Club cells of G3 Tert^-/- and 6-thio-dG treated mice displayed increased DNA damage and senescence and reduced proliferation. Thus, short/dysfunctional telomeres play a protective role in murine asthma by impeding both AHR and mucus secretion after HDM exposure. Therefore, our findings imply that telomeres play a relevant role in allergen-induced airway inflammation.

Proteogenomic Characterization Reveals Therapeutic Vulnerabilities in Lung Adenocarcinoma

To explore the biology of lung adenocarcinoma (LUAD) and identify new therapeutic opportunities, we performed comprehensive proteogenomic characterization of 110 tumors and 101 matched normal adjacent tissues (NATs) incorporating genomics, epigenomics, deep-scale proteomics, phosphoproteomics, and acetylproteomics. Multi-omics clustering revealed four subgroups defined by key driver mutations, country, and gender. Proteomic and phosphoproteomic data illuminated biology downstream of copy number aberrations, somatic mutations, and fusions and identified therapeutic vulnerabilities associated with driver events involving KRAS, EGFR, and ALK. Immune subtyping revealed a complex landscape, reinforced the association of STK11 with immune-cold behavior, and underscored a potential immunosuppressive role of neutrophil degranulation. Smoking-associated LUADs showed correlation with other environmental exposure signatures and a field effect in NATs. Matched NATs allowed identification of differentially expressed proteins with potential diagnostic and therapeutic utility. This proteogenomics dataset represents a unique public resource for researchers and clinicians seeking to better understand and treat lung adenocarcinomas.

T cell receptor signaling results in ERK-dependent Ser163 phosphorylation of lymphocyte phosphatase-associated phosphoprotein

Lymphocyte phosphatase-associated phosphoprotein (LPAP) is a transmembrane protein tightly associated with the phosphatase CD45, which regulates antigen specific lymphocyte activation. Although LPAP is positioned in close proximity to key signaling molecules, its function remains unknown. In this study, we investigated signaling pathways involved in LPAP phosphorylation. Using phosphospecific antibodies generated in our laboratory, we analyzed changes in LPAP phosphorylation in response to various stimuli. Cross-linking with antibodies against TCR or BCR, as well as ionophores and Thapsigargin, resulted in rapid dephosphorylation at Ser172 and Ser99 followed by phosphorylation at Ser163. A panel of inhibitors allowed us to show that PMA and TCR cross-linkage engages the MEK-ERK pathway to drive phosphorylation of LPAP at Ser163. The ERK1/2 kinase was the most distal element in the cascade, which when inhibited prevented changes in LPAP phosphorylation. Supporting this, we found that ERK1 is capable of phosphorylating LPAP at Ser163 in vitro. Although the functional role of this event is yet to be revealed, we provide evidence for a new ERK1/2 target in lymphocytes, namely LPAP, representing a potential regulatory mechanism in the signaling cascade.

Lymphocyte Phosphatase-Associated Phosphoprotein Is a Substrate of Protein Kinase CK2

Lymphocyte phosphatase-associated phosphoprotein (LPAP) is a molecular partner of CD45 phosphatase that plays a key role in the regulation of antigen-specific activation of lymphocytes. The functions of LPAP still remain unknown. We believe that studying LPAP phosphorylation pathways could shed light on its functions. In this work, we studied the phosphorylation of LPAP ectopically expressed in non-lymphoid cells in order to determine the effect of LPAP interaction partners on its phosphorylation. We found that phosphorylation at Ser153 and Ser163 in non-hematopoietic HEK293 cells was conserved, while phosphorylation at Ser99 and Ser172 was almost absent. The pattern of LPAP phosphorylation in K562 erythroid and U937 myeloid cells expressing endogenous CD45 protein was similar to that observed in T and B lymphocytes. We demonstrated for the first time that LPAP is a substrate for protein kinase CK2 that phosphorylates it at Ser153, presumably ensuring LPAP resistance to degradation.

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.

Constitutive and activation-dependent phosphorylation of lymphocyte phosphatase-associated phosphoprotein (LPAP)

Lymphocyte phosphatase-associated phosphoprotein (LPAP) is a small transmembrane protein expressed exclusively in lymphocytes. LPAP is a component of a supramolecular complex composed of the phosphatase CD45, the co-receptor CD4, and the kinase Lck. In contrast to its immunologically important partners, the function of LPAP is unknown. We hypothesized that the biological role of LPAP may be determined by analyzing LPAP phosphorylation. In the present study, we identified LPAP phosphorylation sites by site-directed mutagenesis, phospho-specific antibodies, and protein immunoprecipitation coupled to mass spectrometry analysis. Our results confirmed previous reports that Ser-99, Ser-153, and Ser-163 are phosphorylated, as well as provided evidence for the phosphorylation of Ser-172. Using various SDS-PAGE techniques, we detected and quantified a set of LPAP phosphoforms that were assigned to a combination of particular phosphorylation events. The phosphorylation of LPAP appears to be a tightly regulated process. Our results support the model: following phorbol 12-myristate 13-acetate (PMA) or TCR/CD3 activation of T cells, LPAP is rapidly dephosphorylated at Ser-99 and Ser-172 and slowly phosphorylated at Ser-163. Ser-153 exhibited a high basal level of phosphorylation in both resting and activated cells. Therefore, we suggest that LPAP may function as a co-regulator of T-cell signaling.

Genetic determinants of fibro-osseous lesions in aged inbred mice

Fibro-osseous lesions in mice are progressive aging changes in which the bone marrow is replaced to various degrees by fibrovascular stroma and bony trabeculae in a wide variety of bones. The frequency and severity varied greatly among 28 different inbred mouse stains, predominantly affecting females, ranging from 0% for 10 strains to 100% for KK/HlJ and NZW/LacJ female mice. Few lesions were observed in male mice and for 23 of the strains, no lesions were observed in males for any of the cohorts. There were no significant correlations between strain-specific severities of fibro-osseous lesions and ovarian (r=0.11; P=0.57) or endometrial (r=0.03; P=0.89) cyst formation frequency or abnormalities in parathyroid glands. Frequency of fibro-osseous lesions was most strongly associated (P<10(-6)) with genome variations on chromosome (Chr) 8 at 90.6 and 90.8Mb (rs33108071, rs33500669; P=5.0·10(-10), 1.3·10(-6)), Chr 15 at 23.6 and 23.8Mb (rs32087871, rs45770368; P=7.3·10(-7), 2.7·10(-6)), and Chr 19 at 33.2, 33.4, and 33.6Mb (rs311004232, rs30524929, rs30448815; P=2.8·10(-6), 2.8·10(-6), 2.8·10(-6)) in genome-wide association studies (GWAS). The relatively large number of candidate genes identified in the GWAS analyses suggests that this may be an extremely complex polygenic disease. These results indicate that fibro-osseous lesions are surprisingly common in many inbred strains of laboratory mice as they age. While this presents little problem in most studies that utilize young animals, it may complicate aging studies, particularly those focused on bone.

Epitope mapping of lymphocyte phosphatase-associated phosphoprotein

Lymphocyte phosphatase-associated phosphoprotein (LPAP) is a transmembrane protein with unknown function. The available data on its close association with phosphatase CD45 and its phosphorylation depending on cell activation suggest that LPAP can play a significant role in the antigenic stimulation of lymphocytes. We have localized three antigenic epitopes of the LPAP molecule that can be detected using monoclonal antibodies prepared earlier. Experiments on reactions of antibodies with point mutants and shortened forms of the LPAP protein revealed regions of the amino acid sequence that correspond to the epitopes recognized by the antibodies.

Lymphocyte phosphatase-associated phosphoprotein proteoforms analyzed using monoclonal antibodies

Phosphatase CD45 regulates the activation of lymphocytes by controlling the level of receptor and signal molecule phosphorylation. However, it remains unknown which molecules mediate the phosphatase activity of CD45. A candidate for such a molecule is a small transmembrane adapter protein called lymphocyte phosphatase-associated phosphoprotein (LPAP). LPAP forms a supramolecular complex that consists of not only CD45 molecule but also CD4 and Lck kinase. The function of LPAP has not been defined clearly. In our study, we determined the pattern of LPAP expression in various cell types and characterized its proteoforms using new monoclonal antibodies generated against the intracellular portion of the protein. We show that LPAP is a pan-lymphocyte marker, and its expression in cells correlates with the expression of CD45. The majority of T, B and NK cells express high levels of LPAP, whereas monocytes, granulocytes, monocyte-derived dendritic cells, platelets and red blood cells are negative for LPAP. Using one- and two-dimensional protein gel electrophoresis, we demonstrate that LPAP has at least four sites of phosphorylation. The resting cells express at least six different LPAP phosphoforms representing mono-, di- and tri-phosphorylated LPAP. T and B cells differ in the distribution of the protein between phosphoforms. The activation of lymphocytes with PMA reduces the diversity of phosphorylated forms. Our experiments on Lck-deficient Jurkat cells show that Lck kinase is not involved in LPAP phosphorylation. Thus, LPAP is a dynamically phosphorylated protein, the function of which can be understood, when all phosphosites and kinases involved in its phosphorylation will be identified.

Distinct Transcriptomic Features are Associated with Transitional and Mature B-Cell Populations in the Mouse Spleen

Splenic transitional B-cells (T1 and T2) are selected to avoid self-reactivity and to safeguard against autoimmunity, then differentiate into mature follicular (FO-I and FO-II) and marginal zone (MZ) subsets. Transcriptomic analysis by RNA-seq of the five B-cell subsets revealed T1 cell signature genes included RAG suggesting a potential for receptor revision. T1 to T2 B-cell differentiation was marked by a switch from Myb to Myc, increased expression of the PI3K adapter DAP10 and MHC class II. FO-II may be an intermediate in FO-I differentiation and may also become MZ B-cells as suggested by principle component analysis. MZ B-cells possessed the most distinct transcriptome including down-regulation of CD45 phosphatase-associated protein (CD45-AP/PTPRC-AP), as well as upregulation of IL-9R and innate molecules TLR3, TLR7, and bactericidal Perforin-2 (MPEG1). Among the endosomal TLRs, stimulation via TLR3 further enhanced Perforin-2 expression exclusively in MZ B-cells. Using gene-deleted and overexpressing transgenic mice we show that IL-9/IL-9R interaction resulted in rapid activation of STAT1, 3, and 5, primarily in MZ B-cells. Importantly, CD45-AP mutant mice had reduced transitional and increased mature MZ and FO B-cells, suggesting that it prevents premature entry of transitional B-cells to the mature B-cell pool or their survival and proliferation. Together, these findings suggest, developmental plasticity among splenic B-cell subsets, potential for receptor revision in peripheral tolerance whereas enhanced metabolism coincides with T2 to mature B-cell differentiation. Further, unique core transcriptional signatures in MZ B-cells may control their innate features.

Different culture conditions modulate the immunological properties of adipose stem cells

The potential of human adipose stem cells (ASCs) for regenerative medicine has received recognition owing to their ease of isolation and their multilineage differentiation capacity. Additionally, low immunogenicity and immunosuppressive properties make them a relevant cell source when considering immunomodulation therapies and allogeneic stem cell treatments. In the current study, immunogenicity and immunosuppression of ASCs were determined through mixed lymphocyte reactions. The immunogenic response was analyzed after cell isolation and expansion in fetal bovine serum (FBS), human serum (HS)-supplemented medium, and xeno-free and serum-free (XF/SF) conditions. Additionally, the immunophenotype and the secretion of CXC chemokine ligand 8 (CXCL8), CXCL9, CXCL10, C-C chemokine ligand 2 (CCL2), CCL5, interleukin 2 (IL-2), IL-4, IL-6, IL-10, IL-17A, tumor necrosis factor-α, interferon-γ, transforming growth factor-β1, indoleamine 2,3-deoxygenase, Galectin-1, and Galectin-3 were analyzed. The results showed that ASCs were weakly immunogenic when expanded in any of the three conditions. The significantly strongest suppression was observed with cells expanded in FBS conditions, whereas higher ASC numbers were required to display suppression in HS or XF/SF conditions. In addition, statistically significant differences in protein secretion were observed between direct versus indirect cocultures and between different culture conditions. The characteristic immunophenotype of ASCs was maintained in all conditions. However, in XF/SF conditions, a significantly lower expression of CD54 (intercellular adhesion molecule 1) and a higher expression of CD45 (lymphocyte common antigen) was observed at a low passage number. Although culture conditions have an effect on the immunogenicity, immunosuppression, and protein secretion profile of ASCs, our findings demonstrated that ASCs have low immunogenicity and promising immunosuppressive potential whether cultured in FBS, HS, or XF/SF conditions.

A regulatory polymorphism at position -309 in PTPRCAP is associated with susceptibility to diffuse-type gastric cancer and gene expression

PTPRCAP (CD45-AP) is a positive regulator of protein tyrosine phosphatase PTPRC (CD45), which activates Src family kinases implicated in tumorigenesis. Single-nucleotide polymorphism (SNP) rs869736 located at position -309 of the PTPRCAP promoter was associated with susceptibility to diffuse-type gastric cancer in the current case-control study. The minor-allele homozygote was significantly associated with a 2.5-fold increased susceptibility to diffuse-type gastric cancer (P = .0021, n = 252), but not to intestinal-type (P = .30, n = 178), versus the major-allele homozygote, when comparing unrelated Korean patients with healthy controls (n = 406). Nine other SNPs were in nearly perfect linkage disequilibrium (r(2) >or= 0.97) with this SNP, exhibiting the same association, and spread out for 26 kb on chromosome 11q13.1 covering RPS6KB2, PTPRCAP, CORO1B, and GPR152. Among the four genes, however, only PTPRCAP expression was affected by haplotypes of the 10 SNPs. Endogenous transcript levels of PTPRCAP were linearly correlated with copy numbers (0, 1, and 2) of the risk-haplotype (P = .0060) in 12 lymphoblastoid cells derived from blood samples, but those of the other three genes were not. Furthermore, the cancer-risk, minor-allele T of rs869736 increased both promoter activity and specific nuclear protein-binding affinity than the nonrisk, major-allele G in luciferase reporter and electrophoretic mobility shift assays, respectively. Accordingly, the minor allele of rs869736 in the PTPRCAP promoter is associated with increased susceptibility to diffuse-type gastric cancer by increasing PTPRCAP expression, possibly leading to activation of the oncogenic Src family kinases.

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.

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-associated protein promotes the response of primary CD4 T cells to low-potency T-cell receptor (TCR) stimulation and facilitates CD45 association with CD3/TCR and lck

Although it is clear that the CD45 tyrosine phosphatase is required for efficient T-cell activation and T-cell development, the factors that regulate CD45 function remain uncertain. Previous data have indicated that there is an association of CD45 with CD4 and the T-cell receptor (TCR) complex controlled by the variable ectodomain of CD45 and, following activation, by high- and low-potency peptides. This suggests that controlling substrate access to CD45 may be an important regulatory mechanism during T-cell activation. In the present study we have examined the role of the transmembrane adapter-like molecule CD45-associated protein (CD45-AP) in regulating the association of CD45 with CD3/TCR and lck, and in regulating primary CD4(+) T-lymphocyte activation. In CD4(+) T cells from CD45-AP-deficient mice, coimmunoprecipitation of CD45 with the CD3/TCR complex, in addition to lck, is significantly reduced compared with wild-type T cells. Functionally, this correlates with a decreased proliferative response, a decrease in interleukin (IL)-2 production, and a decrease in calcium flux upon stimulation with a low-potency altered peptide ligand. However, the response of CD45-AP-deficient T cells to stimulation with a high-avidity agonist peptide was largely intact, except for a modest decrease in IL-2 production. These data suggest that CD45-AP promotes or stabilizes the association of CD45 with substrates and regulates the threshold of T-cell activation.

CD45: all is not yet crystal clear

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

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)

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.

CD45: new jobs for an old acquaintance

Identified as the first and prototypic transmembrane protein tyrosine phosphatase (PTPase), CD45 has been extensively studied for over two decades and is thought to be important for positively regulating antigen-receptor signaling via the dephosphorylation of Src kinases. However, new evidence indicates that CD45 can function as a Janus kinase PTPase that negatively controls cytokine-receptor signaling. A point mutation in CD45, which appears to affect CD45 dimerization, and a genetic polymorphism that affects alternative CD45 splicing are implicated in autoimmunity in mice and multiple sclerosis in humans. CD45 is expressed in multiple isoforms and the modulation of specific CD45 splice variants with antibodies can prevent transplant rejections. In addition, loss of CD45 can affect microglia activation in a mouse model for Alzheimer's disease. Thus, CD45 is moving rapidly back into the spotlight as a drug target and central regulator involved in differentiation of multiple hematopoietic cell lineages, autoimmunity and antiviral immunity.

Receptor protein tyrosine phosphatases in hematopoietic cells

PTPs and PTKs control the level of tyrosine phosphorylation of cellular proteins. Although many substrates for PTKs have been identified, the specific targets of individual PTP family members, along with the consequences of protein dephosphorylation for cellular physiology, remain largely unknown. Fine regulation of tyrosine phosphorylation events is required for the proper progression of hematopoiesis. In this review, we have summarized the characterization of tyrosine phosphatases in hematopoietic cells and delineated their potential role in the process of hematopoiesis and the development of hematopoietic disorders.

The CD45 tyrosine phosphatase: a positive and negative regulator of immune cell function

The CD45 phosphotyrosine phosphatase is one of the most abundant glycoproteins expressed on immune cells. Previously, the serpentine twists and turns of the CD45 research field have tended to draw attention to CD45 either as a positive or negative regulator of immune cell function. This review draws heavily on CD45 knockout mouse data to emphasize that CD45 has both positive and negative actions in regulating receptor thresholds, and these roles vary according to cell lineage and developmental stage. Previously conflicting results are reconciled in a model suggesting how CD45 regulates the p56(lck)tyrosine kinase in T cell signalling and development.

Emerging issues in receptor protein tyrosine phosphatase function: lifting fog or simply shifting?

Transmembrane (receptor) tyrosine phosphatases are intimately involved in responses to cell-cell and cell-matrix contact. Several important issues regarding the targets and regulation of this protein family are now emerging. For example, these phosphatases exhibit complex interactions with signaling pathways involving SRC family kinases, which result from their ability to control phosphorylation of both activating and inhibitory sites in these kinases and possibly also their substrates. Similarly, integrin signaling illustrates how phosphorylation of a single protein, or the activity of a pathway, can be controlled by multiple tyrosine phosphatases, attesting to the intricate integration of these enzymes in cellular regulation. Lastly, we are starting to appreciate the roles of intracellular topology, tyrosine phosphorylation and oligomerization among the many mechanisms regulating tyrosine phosphatase activity.

CD45-associated protein is not essential for the regulation of antigen receptor-mediated signal transduction

CD45 is a transmembrane protein tyrosine phosphatase required for signaling through the T-and B-cell antigen receptors. In lymphocytes, CD45 interacts with CD45-associated protein (CD45AP), a 32 000 Mr phosphoprotein, through their respective transmembrane domains. To determine whether CD45AP affects the ability of CD45 to regulate antigen receptor signaling, CD45AP-deficient mice were generated. Thymocyte development was grossly normal. Moreover, the cellularity of the thymus and spleens were normal. CD45 expression on thymocytes and splenocytes, ascertained by flow cytometry, was comparable between CD45AP-deficient mice and littermate controls. In contrast to a previous report (Matsuda et al., J. Exp. Med. 1998 187: 1863 - 1870). CD45AP-deficient and normal thymocytes and splenocytes proliferated similarly in response to various mitogens or antigen receptor cross-linking. Furthermore, thymocyte CD45-associated p56(lck) kinase activity was similar between CD45AP-deficient and normal cells. We conclude that CD45AP is not essential for the regulation of Src-family kinase activity by CD45.

Biochemical and functional analysis of mice deficient in expression of the CD45-associated phosphoprotein LPAP

The role of the CD45-associated phosphoprotein (LPAP / CD45-AP) during an immune response remains unclear. To understand better the function of LPAP we generated LPAP-deficient mice by disrupting exon 2 of the LPAP gene. LPAP-null mice were healthy and did not show gross abnormalities compared to their wild-type littermates. However, immunofluorescence analysis of T and B lymphocytes revealed a reduced expression of CD45, which did not affect a particular subpopulation. In contrast to a recent report (Matsuda et al., J. Exp. Med. 1998. 187: 1863 - 1870) we neither observed significant alterations of the assembly of the CD45 / lck-complex nor of polyclonal T-cell responses. However, lymphnodes from LPAP-null mice showed increased cellularity, which could indicate that expression of LPAP might be required to prevent expansion of lymphocytes in particular lymphatic organs rather than potentiating immune responses.

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

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

Biochemical association of CD45 with the T cell receptor complex: regulation by CD45 isoform and during T cell activation

CD45 is the predominant transmembrane tyrosine phosphatase in lymphocytes and is required for the efficient induction of T cell receptor signaling and activation. However, the regulation of CD45 activity and substrate specificity are poorly understood. In the present study, we demonstrate a basal biochemical association of CD45 with the T cell receptor complex that is regulated in part by CD45 isoform expression. Further, maintenance of CD45/TCR association is differentially regulated following TCR ligation with peptide: a partial agonist peptide induces CD45/TCR dissociation while an agonist peptide promotes sustained association in a CD4-dependent manner. These data suggest that T cell receptor signaling pathways may be modulated by altering access of CD45 to TCR-associated substrates involved in T cell activation.

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

Interaction between CD45-AP and protein-tyrosine kinases involved in T cell receptor signaling

CD45-AP associates specifically with CD45, a protein-tyrosine phosphatase essential for antigen receptor-mediated signal transduction. CD45 modulates the activity of Src family protein-tyrosine kinases involved at the onset of antigen receptor-mediated signaling by dephosphorylating their regulatory tyrosyl residues. We have shown that lymphocyte responses to antigen receptor stimulation are impaired in CD45-AP-null mice. To examine the possibility that CD45-AP coordinates the interaction between CD45 and its substrates, we investigated the associations of CD45-AP with several protein-tyrosine kinases. Endogenous CD45-AP coimmunoprecipitated with Lck and ZAP-70 in both CD45-positive T cells and their CD45-negative variants after stimulation by antigen receptor ligation. Concomitantly, CD45 coimmunoprecipitated with Lck and ZAP-70 after T cell receptor-mediated stimulation of CD45-positive cells. Recombinant CD45-AP exhibited specific binding to Lck and ZAP-70 protein-tyrosine kinases, but not to Fyn or Csk, in lysates of both CD45-positive and -negative T cells. Specific interactions were demonstrated between the respective recombinant proteins as well. These results demonstrate that CD45-AP associates directly and selectively with Lck and ZAP-70 in response to T cell receptor-mediated stimulation. The associations of CD45-AP with Lck and ZAP-70 may mediate the functional interactions of these kinases with CD45 during antigen receptor stimulation.