Enhancement of antigen-induced T-cell proliferation by soluble CD26/dipeptidyl peptidase IV

Far-Red Light Triggered Production of Bispecific T Cell Engagers (BiTEs) from Engineered Cells for Antitumor Application

Bispecific T-cell engagers (BiTEs), which have shown potent antitumor activity in humans, are emerging as one of the most promising immunotherapeutic strategies for cancer treatment in recent years. However, the clinical application of BiTEs nowadays has been hampered by their short half-life in the circulatory system due to their low molecular weight and rapid renal clearance. Inevitable continuous infusion of BiTEs has become a routine operation in order to achieve effective treatment, although it is costly, inconvenient, time-consuming, and even painful for patients in some cases. To develop an on-demand, tunable, and reversible approach to overcome these limitations, we assembled a transcription-control device into mammalian cells based on a bacterial far-red light (FRL) responsive signaling pathway to drive the expression of a BiTE against Glypican 3 (GPC3), which is a highly tumor-specific antigen expressed in most hepatocellular carcinomas (HCC). As demonstrated in in vitro experiments, we proved that the FRL sensitive device spatiotemporally responded to the control of FRL illumination and produced a therapeutic level of BiTEs that recruited and activated human T cells to eliminate GPC3 positive tumor cells. By functionally harnessing the power of optogenetics to remotely regulate the production of BiTEs from bioengineered cells and demonstrating its effectiveness in treating tumor cells, this study provides a novel approach to achieve an in vivo supply of BiTEs, which could be potentially applied to other formats of bispecific antibodies and facilitate their clinical applications.

Delineating the Association Between Soluble CD26 and Autoantibodies Against G-Protein Coupled Receptors, Immunological and Cardiovascular Parameters Identifies Distinct Patterns in Post-Infectious vs. Non-Infection-Triggered Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Soluble cluster of differentiation 26 (sCD26) has a wide range of enzymatic functions affecting immunological, metabolic and vascular regulation. Diminished sCD26 concentrations have been reported in various autoimmune diseases and also in Myalgic Encephalomyelitis/Chronic fatigue syndrome (ME/CFS). Here we re-evaluate sCD26 as a diagnostic marker and perform a comprehensive correlation analysis of sCD26 concentrations with clinical and paraclinical parameters in ME/CFS patients. Though this study did find significantly lower concentrations of sCD26 only in the female cohort and could not confirm diagnostic suitability, results from correlation analyses provide striking pathomechanistic insights. In patients with infection-triggered onset, the associations of low sCD26 with elevated autoantibodies (AAB) against alpha1 adrenergic (AR) and M3 muscarinic acetylcholine receptors (mAChR) point to a pathomechanism of infection-triggered autoimmune-mediated vascular and immunological dysregulation. sCD26 concentrations in infection-triggered ME/CFS were found to be associated with activated T cells, liver enzymes, creatin kinase (CK) and lactate dehydrogenase (LDH) and inversely with Interleukin-1 beta (IL-1b). Most associations are in line with the known effects of sCD26/DPP-4 inhibition. Remarkably, in non-infection-triggered ME/CFS lower sCD26 in patients with higher heart rate after orthostatic challenge and postural orthostatic tachycardia syndrome (POTS) suggest an association with orthostatic regulation. These findings provide evidence that the key enzyme sCD26 is linked to immunological alterations in infection-triggered ME/CFS and delineate a different pathomechanism in the non-infectious ME/CFS subset.

A Novel Purification Procedure for Active Recombinant Human DPP4 and the Inability of DPP4 to Bind SARS-CoV-2

Proteases catalyse irreversible posttranslational modifications that often alter a biological function of the substrate. The protease dipeptidyl peptidase 4 (DPP4) is a pharmacological target in type 2 diabetes therapy primarily because it inactivates glucagon-like protein-1. DPP4 also has roles in steatosis, insulin resistance, cancers and inflammatory and fibrotic diseases. In addition, DPP4 binds to the spike protein of the MERS virus, causing it to be the human cell surface receptor for that virus. DPP4 has been identified as a potential binding target of SARS-CoV-2 spike protein, so this question requires experimental investigation. Understanding protein structure and function requires reliable protocols for production and purification. We developed such strategies for baculovirus generated soluble recombinant human DPP4 (residues 29–766) produced in insect cells. Purification used differential ammonium sulphate precipitation, hydrophobic interaction chromatography, dye affinity chromatography in series with immobilised metal affinity chromatography, and ion-exchange chromatography. The binding affinities of DPP4 to the SARS-CoV-2 full-length spike protein and its receptor-binding domain (RBD) were measured using surface plasmon resonance and ELISA. This optimised DPP4 purification procedure yielded 1 to 1.8 mg of pure fully active soluble DPP4 protein per litre of insect cell culture with specific activity >30 U/mg, indicative of high purity. No specific binding between DPP4 and CoV-2 spike protein was detected by surface plasmon resonance or ELISA. In summary, a procedure for high purity high yield soluble human DPP4 was achieved and used to show that, unlike MERS, SARS-CoV-2 does not bind human DPP4.

CD26 and Asthma: a Comprehensive Review

Asthma is a heterogeneous and chronic inflammatory family of disorders of the airways with increasing prevalence that results in recurrent and reversible bronchial obstruction and expiratory airflow limitation. These diseases arise from the interaction between environmental and genetic factors, which collaborate to cause increased susceptibility and severity. Many asthma susceptibility genes are linked to the immune system or encode enzymes like metalloproteases (e.g., ADAM-33) or serine proteases. The S9 family of serine proteases (prolyl oligopeptidases) is capable to process peptide bonds adjacent to proline, a kind of cleavage-resistant peptide bonds present in many growth factors, chemokines or cytokines that are important for asthma. Curiously, two serine proteases within the S9 family encoded by genes located on chromosome 2 appear to have a role in asthma: CD26/dipeptidyl peptidase 4 (DPP4) and DPP10. The aim of this review is to summarize the current knowledge about CD26 and to provide a structured overview of the numerous functions and implications that this versatile enzyme could have in this disease, especially after the detection of some secondary effects (e.g., viral nasopharyngitis) in type II diabetes mellitus patients (a subset with a certain risk of developing obesity-related asthma) upon CD26 inhibitory therapy.

Site-Specific DC Surface Signatures Influence CD4+ T Cell Co-stimulation and Lung-Homing

Dendritic cells (DCs) that drain the gut and skin are known to favor the establishment of T cell populations that home to the original site of DC-antigen (Ag) encounter by providing soluble “imprinting” signals to T cells in the lymph node (LN). To study the induction of lung T cell-trafficking, we used a protein-adjuvant murine intranasal and intramuscular immunization model to compare in vivo-activated Ag⁺ DCs in the lung and muscle-draining LNs. Higher frequencies of Ag⁺ CD11b⁺ DCs were observed in lung-draining mediastinal LNs (MedLN) compared to muscle-draining inguinal LNs (ILN). Ag⁺ CD11b⁺ MedLN DCs were qualitatively superior at priming CD4⁺ T cells, which then expressed CD49a and CXCR3, and preferentially trafficked into the lung parenchyma. CD11b⁺ DCs from the MedLN expressed higher levels of surface podoplanin, Trem4, GL7, and the known co-stimulatory molecules CD80, CD86, and CD24. Blockade of specific MedLN DC molecules or the use of sorted DC and T cell co-cultures demonstrated that DC surface phenotype influences the ability to prime T cells that then home to the lung. Thus, the density of dLN Ag⁺ DCs, and DC surface molecule signatures are factors that can influence the output and differentiation of lung-homing CD4⁺ T cells.

Development of novel monoclonal antibodies with specific binding affinity for denatured human CD26 in formalin-fixed paraffin-embedded and decalcified specimens

A 110-kDa type II transmembrane glycoprotein with dipeptidyl peptidase IV (DPPIV) activity in its extracellular region, CD26 has a multitude of biological functions and plays an important role in the regulation of inflammatory responses and tumor biology. Our work has focused on CD26 as a novel therapeutic target for various tumors and immune disorders, and we have recently developed a humanized anti-CD26 monoclonal antibody (mAb), YS110, which has promising safety profile and clinical activity in patients with malignant pleural mesothelioma. The development of an anti-human CD26 mAb that can clearly and reliably detect the denatured CD26 molecule in formalin-fixed paraffin-embedded (FFPE) tissues in the clinical setting is therefore of the utmost importance. To develop novel anti-CD26 mAbs capable of binding to denatured CD26, we immunized mice with urea-treated CD26 protein. Hybridoma supernatants were screened for specific reactivity with human CD26 by immunostaining through the use of a set of FFPE human CD26-positive or negative tumor cell lines. This screening method enables us to develop novel anti-human CD26 mAbs suitable for immunohistochemical staining of CD26 in FFPE non-tumor and tumor tissue sections with reliable clarity and intensity. Specifically, these mAbs display strong binding affinity to denatured human CD26 rather than undenatured human CD26, and are capable of detecting denatured human CD26 in decalcified specimens. These novel anti-CD26 mAbs are potentially useful for the analysis of CD26 expression in cancer patients with bony metastasis, and may help decide the appropriateness of YS110 therapy for future cancer patients.

Lymphocytes are a major source of circulating soluble dipeptidyl peptidase 4

Dipeptidyl peptidase 4 (DPP4, CD26) is a serine protease that is expressed constitutively by many haematopoietic and non-haematopoietic tissues. It exists as a membrane-associated protein, as well as in an active, soluble form (herein called sDPP4), present at high concentrations in bodily fluids. Despite the proposed use of sDPP4 as a biomarker for multiple diseases, its cellular sources are not well defined. Here, we report that individuals with congenital lymphocyte immunodeficiency had markedly lower serum concentrations of sDPP4, which were restored upon successful treatment and restoration of lymphocyte haematopoiesis. Using irradiated lymphopenic mice and wild-type to Dpp4-/- reciprocal bone marrow chimeric animals, we found that haematopoietic cells were a major source of circulating sDPP4. Furthermore, activation of human and mouse T lymphocytes resulted in increased sDPP4, providing a mechanistic link between immune system activation and sDPP4 concentration. Finally, we observed that acute viral infection induced a transient increase in sDPP4, which correlated with the expansion of antigen-specific CD8+ T cell responses. Our study demonstrates that sDPP4 concentrations are determined by the frequency and activation state of lymphocyte populations. Insights from these studies will support the use of sDPP4 concentration as a biomarker for inflammatory and infectious diseases.

The effects of DPP-4 inhibitor on hypoxia-induced apoptosis in human umbilical vein endothelial cells

Dipeptidyl peptidase-4 (DPP-4) inhibitors are a new class of oral hypoglycemic agents for patients with type 2 diabetes mellitus and have potential antiatherosclerotic properties. Meanwhile, it is unclear how DPP-4 inhibitors have protective effects on atherosclerosis. Our aim was to determine the effects and its mechanisms of DPP-4 inhibitors on cultured endothelial cells. Human umbilical vein endothelial cells (HUVECs) were cultured in hypoxic condition. To evaluate the protective effects of DPP-4 inhibitor on HUVECs, DPP-4 inhibitor was added in the cell culture medium and the cell viability was assessed by TUNEL assay. And we examined the intracellular signaling pathways in relation to the effects of DPP-4 inhibitor. DPP-4 inhibition had beneficial effects by inhibiting the apoptosis under hypoxic conditions in HUVECs. The antiapoptotic effects of DPP-4 inhibitor were abolished by the pretreatment with a CXCR4 antagonist or a Stat3 inhibitor. DPP-4 inhibition has beneficial effects on HUVECs by inhibiting the apoptosis under hypoxic conditions. SDF-1α/CXCR4/Stat3 pathways might be involved in the mechanisms of the cytoprotective effects of DPP-4 inhibitor. These results suggested that DPP-4 inhibitor has a potential for protecting vessels.

Plasma DPP4 activity is associated with no-reflow and major bleeding events in Chinese PCI-treated STEMI patients

Dipeptidyl peptidase-4 (DPP4) is an important regulator of incretins and inflammation, and it is involved in the pathophysiological process of myocardial infarction (MI). This study investigated the role of plasma DPP4 activity (DPP4a) in patients with ST-segment elevation myocardial infarction (STEMI) who had undergone percutaneous coronary intervention (PCI). We recruited 747 consecutive PCI-treated STEMI patients from a tertiary referral center from January 2014 to October 2015. The outcomes of interest were the rates of no-reflow, in-hospital major adverse cardiac or cerebrovascular events (iMACCE), in-hospital complications (IHC) and in-hospital major bleeding. The DPP4a was lower in STEMI patients compared with the controls (p < 0.0001). Multivariate logistic-regression analyses (adjusted for confounding variables) showed that a 1 U/L increase in DPP4a was associated with an increased rate of no-reflow events (odds ratio [OR]: 1.07; 95% CI: 1.02–1.11; p < 0.01) and a decreased rate of major bleeding events (OR: 0.90; 95% CI: 0.82–0.98; p = 0.02). There were no associations between DPP4a and either iMACCE or IHC. In conclusions, high levels of DPP4a are independently associated with an increased rate of no-reflow events and a decreased rate of major bleeding events in PCT-treated STEMI patients.

Sitagliptin, a DPP-4 inhibitor, alters the subsets of circulating CD4+ T cells in patients with type 2 diabetes

OBJECTIVE

CD26/DPP-4 is highly expressed by T cells, especially CD4+ T cells (T helper cells; Th) and may regulate the differentiation, maturation, or proliferation of these cells. We investigated the effects of sitagliptin, a DPP-4 inhibitor, on the absolute number and percentage of various subsets of circulating CD4+ T cells in patients with type 2 diabetes.

METHODS

We enrolled 30 consecutive patients (16 women and 14 men) with type 2 diabetes in a prospective, randomized, open-label, blinded endpoint study. Eligible participants were randomly assigned at a 2:1 ratio to either a sitagliptin group (sitagliptin at 50mg/day) or an active control group (glimepiride at 1mg/day). Patients were followed for 12 weeks with monthly review. Peripheral blood mononuclear cells were examined by flow cytometry for intracellular expression of cytokines (IFN-γ as a marker of Th1cells, IL-4 for Th2 cells, and IL-17 for Th17 cells) and for expression of CD4, CD25, and Foxp3 (regulatory T cells [Treg]).

RESULTS

Both groups showed similar improvement of glycemic control. The total number of CD4+ T cells was decreased by treatment with sitagliptin, while it did not change in the control group. The number and percentage of Th17 cells and Treg cells both decreased significantly in the sitagliptin group, but not in the control group. There was a significant positive correlation between changes in the percentage of Th17 cells and Treg cells after treatment with sitagliptin.

CONCLUSIONS

Treatment with sitagliptin for 12 weeks reduced the number of circulating CD4+ T cells, especially Th17 and Treg cells, in patients with type 2 diabetes.

Dpp4 inhibition as a therapeutic strategy in cardiometabolic disease: Incretin-dependent and -independent function

Cardiometabolic disorders including obesity, diabetes and cardiovascular disease are among the most severe health problems worldwide. DPP4 enzymatic inhibitors were first developed as anti-diabetic reagents which preserve incretin hormones and promote post-prandial insulin secretion. It's been shown in animal studies that incretin-based therapy has a beneficial effect on cardiovascular disease. Recent studies demonstrated novel non-catalytic functions of DPP4 that may play a role in cardiometabolic disease. Although the role of DPP4 inhibition-mediated incretin effects has been well-reviewed, little information of its incretin-independent actions was introduced in cardiometabolic disease. In the current review, we will summarize the catalytic dependent and independent effects of DPP4 inhibition on cardiometabolic disease.

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Discuss the findings from recent large scale clinical trials (EXAMINE and SAVOR-TIMI 53)

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Summarize the catalytic dependent and independent effects of DPP4 inhibition on cardiometabolic disease

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Focus on recent evidence linking DPP4 inhibition therapy with cardiovascular disease

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Provide mechanistic insights into the cardiovascular effect of DPP4

Recent Advances in Dipeptidyl-Peptidase-4 Inhibition Therapy: Lessons from the Bench and Clinical Trials

DPP4 inhibitors (DPP4i) are a class of newly developed antidiabetic drugs which preserve incretin hormones and promote postprandial insulin secretion. Although the cardiovascular effect of DPP4 inhibition has been substantially studied, the exact role of DPP4 in cardiovascular disease especially in humans remains elusive. Previous small studies and meta-analyses have suggested a benefit in both surrogate outcomes and cardiovascular events for these agents. However, there was growing evidence in recent years questioning the cardioprotective effect of DPP4i. Further, a signal of heart failure hospitalization in a recent large scale clinical trial SAVOR-TIMI 53 has called into question the safety of these agents and their utility in the treatment of cardiovascular disease. In this review, we will revisit the physiologic function of DPP4 and discuss its role in cardiometabolic disease based on recent experimental and clinical studies.

Potential role of dipeptidyl peptidase IV in the pathophysiology of heart failure

Dipeptidyl peptidase IV (DPPIV) is a widely expressed multifunctional serine peptidase that exists as a membrane-anchored cell surface protein or in a soluble form in the plasma and other body fluids. Numerous substrates are cleaved at the penultimate amino acid by DPPIV, including glucagon-like peptide-1 (GLP-1), brain natriuretic peptide (BNP) and stromal cell-derived factor-1 (SDF-α), all of which play important roles in the cardiovascular system. In this regard, recent reports have documented that circulating DPPIV activity correlates with poorer cardiovascular outcomes in human and experimental heart failure (HF). Moreover, emerging evidence indicates that DPPIV inhibitors exert cardioprotective and renoprotective actions in a variety of experimental models of cardiac dysfunction. On the other hand, conflicting results have been found when translating these promising findings from preclinical animal models to clinical therapy. In this review, we discuss how DPPIV might be involved in the cardio-renal axis in HF. In addition, the potential role for DPPIV inhibitors in ameliorating heart disease is revised, focusing on the effects of the main DPPIV substrates on cardiac remodeling and renal handling of salt and water.

Establishment of monoclonal anti-human CD26 antibodies suitable for immunostaining of formalin-fixed tissue

Background

A T cell costimulatory molecule with dipeptidyl peptidase IV (DPPIV) activity in its extracellular region, CD26 is a multifunctional molecule associated with various proteins such as adenosine deaminase, caveolin-1, CXCR4, collagen, and fibronectin, while playing an important role in the regulation of inflammatory responses and tumor biology. We have focused on CD26 as a novel therapeutic target for various tumors and immune disorders, and have developed a humanized anti-CD26 monoclonal antibody (mAb), YS110, which is currently being evaluated in a phase I clinical trial for patients with CD26-expressing tumors, including malignant mesothelioma. Since detection of tumor CD26 expression is required for determining potential eligibility for YS110 therapy, the development of anti-human CD26 mAb that can clearly and reliably detect the denatured CD26 molecule in the formalin-fixed paraffin-embedded tissues is critical.

Methods

To develop novel anti-CD26 mAbs capable of binding to the denatured CD26, we immunized mice with CD26 protein denatured in urea buffer. After the fusion of splenocytes and myeloma cells, the mAbs were screened for specific reactivity with human CD26 by flow cytometry, enzyme-linked immunosorbent assay, and immunohistochemistry. The binding competitiveness of novel anti-CD26 mAbs with the humanized anti-CD26 mAb YS110 was also examined.

Results

We have succeeded in developing novel anti-human CD26 mAbs suitable for immunohistochemical staining of CD26 in formalin-fixed tissue sections with reliable clarity and intensity. Importantly, some of these mAbs exhibit no cross-reactivity with the humanized anti-CD26 mAb.

Conclusions

These novel mAbs are potentially useful as companion diagnostic agents to analyze CD26 expression in the clinical setting while advancing future CD26-related research.

Virtual slides

The virtual slides for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/5987140221097729

Identification of altered dipeptidyl-peptidase activities as potential biomarkers for unipolar depression

BACKGROUND

Changes in circulatory aminopeptidases [dipeptidyl-peptidase-IV (DPP-IV), Prolyl-oligopeptidase (POP) and Leucine aminopeptidase (LAP)] activities have been found to be associated with psychiatric illnesses and inflammatory diseases.

METHODS

The discriminatory indices of aminopeptidases activities were assessed by enzymatic assays in plasma samples from 240 unipolar depression (UD) patients and 264 matched controls. In addition the relationship between soluble and cellular DPP-IV activity was determined in plasma and blood cells from healthy subjects.

RESULTS

Greater than 95% of the plasma DPP-IV activity could be blocked by inhibitors, demonstrating the specificity of the assay. Also, DPP-IV protein and activity levels were strongly correlated. In contrast, only 50% of the membrane-bound activity in blood cells was inhibited, which suggested that other similar peptidases may be present in these cells. UD patients had decreased plasma levels of DPP-IV and POP activities compared to healthy controls with a concomitant increase in LAP activity. Finally, testing of the LAP/DPP-IV ratio resulted in good discrimination of UD patients from controls with an area under the curve-receiver operating characteristic of 0.70.

LIMITATIONS

Further biological validation studies using different cohorts are warranted.

CONCLUSIONS

The finding that plasma DPP-IV activity was decreased and LAP activity was increased in UD patients suggests the potential value for testing the levels of these enzymes for improved classification of patients. In addition, the changes in these enzymes, suggests that the proteolytic maturation of their proneuropeptide and prohormone subtrates may also be affected in UD, resulting in altered production of the associated bioactive peptides.

Soluble CD26/Dipeptidyl Peptidase IV Enhances the Transcription of IL-6 and TNF-α in THP-1 Cells and Monocytes

CD26 is a 110-kDa multifunctional molecule having dipeptidyl peptidase IV (DPPIV) enzyme activity and is present on the surface of human T cells. Soluble CD26 (sCD26) exists in human blood and enhances the proliferation of peripheral T lymphocytes induced by tetanus toxoid (TT). The mechanisms by which CD26 enhances the activation of T cells and monocytes remain to be fully elucidated. In this study, we compared the stimulation of THP-1 cells and isolated human monocytes with a combination of recombinant sCD26 and lipopolysaccharide (LPS) and the stimulation of these cells with LPS alone. We found that addition of sCD26 increased TNF-α and IL-6 mRNA and protein expression and enhanced ERK1/2 levels in the cytosol as well as c-Fos, NF-κB p50, NF-κB p65, and CUX1 levels in the nuclei of these cells. On the other hand, the selective DPPIV inhibitor sitagliptin inhibited the increase in TNF-α mRNA and protein expression as well as the increase in ERK, c-Fos, NF-κB p50, NF-κB p65, and CUX1 levels. However, it did not inhibit the increase in IL-6 mRNA and protein expression. We then demonstrated that sCD26 enhanced binding of transcription factors to the TNF- and IL-6 promoters and used reporter assays to demonstrate that transcription factor binding enhanced promoter activity. Once again, we observed differential activities at the TNF- and IL-6 promoters. Finally, we demonstrated that CUX-1 overexpression enhanced TNF- production on sCD26/LPS stimulation, while CUX-1 depletion had no effect. Neither CUX-1 overexpression nor CUX-1 depletion had an effect on IL-6 stimulation. These results are discussed in the context of a model that describes the mechanisms by which stimulation of monocytic cells by sCD26 and LPS leads to elevation of TNF- and IL-6 expression. CUX-1 is identified as a new transcription factor that differently regulates the activities of the TNF- and IL-6 promoters.

Proteomic analysis of two types of exosomes in human whole saliva

Saliva contains a large number of proteins that participate in the protection of oral tissue. Exosomes are small vesicles (30-100 nm in diameter) with an endosome-derived limiting membrane that are secreted by a diverse range of cell types. We have recently demonstrated that exosomes are present in human whole saliva. In this study, we found that whole saliva contained at least two types of exosomes (exosome I and exosome II) that are different in size and protein composition. Proteomic analysis revealed that both types of exosomes contained Alix, Tsg101 and Hsp70, all exosomal markers, immunoglobulin A and polymeric immunoglobulin receptor, whereas they had different protein compositions. Most of dipeptidyl peptidase IV known as CD26 in whole saliva, was present on the exosome II and metabolically active in cleaving chemokines (CXCL11 and CXCL12). Human whole saliva exosomes might participate in the catabolism of bioactive peptides and play a regulatory role in local immune defense in the oral cavity.

Dipeptidyl peptidase in autoimmune pathophysiology

CD26 is a 110-kDa surface glycoprotein with intrinsic dipeptidyl peptidase IV (DPPIV) activity that is expressed on various cell types and has many biological functions. An important aspect of CD26 biology is its peptidase activity and its functional and physical association with molecules with key roles in human immunological programs. CD26 role in immune regulation has been extensively characterized, with recent findings elucidating its link age with signaling pathways and structures involved in T cell activation a well as antigen-presenting cell-T cell interaction, being a marker of diseas behavior clinically as well as playing an important role in autoimmune pathogenesis and development. Through the use of various experimental approaches and agents to influence CD26/DPPIV expression and activity, such as anti-CD26 antibodies, CD26/DPPIV chemical inhibitors, siRNAs to inhibit CD26 expression, overexpressing CD26 transfectants, soluble CD26 molecules and proteomic approach, we have shown that CD26 interacts with structures with essential cellular functions in T cell responses. We will review emerging data that suggest CD26 may be an appropriate therapeutic target for the treatment of selected immune disorders.

Soluble CD26 / dipeptidyl peptidase IV enhances human lymphocyte proliferation in vitro independent of dipeptidyl peptidase enzyme activity and adenosine deaminase binding

Human CD26 has dipeptidyl peptidase-4 (DPP IV) enzyme activity and binds to adenosine deaminase (ADA). CD26 is costimulatory for lymphocytes and has a circulating soluble form (sCD26). DPP IV enzyme inhibition is a new successful type 2 diabetes therapy. We examined whether the ADA binding and catalytic functions of sCD26 contribute to its effects on T-cell proliferation. Wildtype soluble recombinant human CD26 (srhCD26), an enzyme inactive mutant (srhCD26E-) and an ADA non-binding mutant (srhCD26A-) were co-incubated in in vitro T-cell proliferation assays with peripheral blood mononuclear cells (PBMC) stimulated with phytohaemagglutinin (PHA), muromonab-CD3 or Herpes simplex virus antigen (HSV Ag). Both srhCD26 and srhCD26E- enhanced PHA-induced T-cell proliferation dose-dependently in all six subjects tested. srhCD26 and srhCD26A- had no overall effect on anti-CD3-stimulated PBMC proliferation in four of five subjects. srhCD26, srhCD26E- and srhCD26A- enhanced HSV Ag induced PBMC proliferation in low responders to HSV Ag, but had no effect or inhibited proliferation in HSV-high responders. Thus, effects of soluble human CD26 on human T-cell proliferation are mechanistically independent of both the enzyme activity and the ADA-binding capability of sCD26.

Identification and characterization of dipeptidyl peptidase IV enzyme activity in the American crocodile (Crocodylus acutus)

Serum from the American crocodile was assayed for dipeptidyl peptidase IV (DPP4) activity. We measured the DPP4-mediated hydrolysis of Ala-Pro-AFC. The generation of AFC was dependent on the titer of serum, with significant DPP4 activity (0.20 + or - 0.03 nmol product formed) measured using only 2 microL of crocodile serum, with maximum activity measured using 500 microL of serum. The hydrolysis of substrate was inhibited in a concentration-dependent manner by diprotin A, a specific inhibitor of DPP4 activity, indicating that this activity was due to the presence of DPP4. The crocodile serum DPP4 exhibited classical Michaelis-Menten kinetics, with K(m) and V(max) extrapolated, by double-reciprocal plot, to be 14.7 + or - 1.3 microM and 75.5 + or - 4.3 nmol/min, respectively. Crocodile DPP4 catalyzed the hydrolysis of Ala-Pro-AFC rapidly, with substantial activity measured within 5 min of the addition of substrate. After an initial rapid increase in activity, near maximal activity (7.43 + or - 0.24 nmol product formed) measured at 180 min. Crocodile serum DPP4 activity was temperature-dependent, with steadily increased activity from 5 to 40 degrees C.

The dipeptidyl peptidase IV family in cancer and cell biology

Of the 600+ known proteases identified to date in mammals, a significant percentage is involved or implicated in pathogenic and cancer processes. The dipeptidyl peptidase IV (DPIV) gene family, comprising four enzyme members [DPIV (EC 3.4.14.5), fibroblast activation protein, DP8 and DP9] and two nonenzyme members [DP6 (DPL1) and DP10 (DPL2)], are interesting in this regard because of their multiple diverse functions, varying patterns of distribution/localization and subtle, but significant, differences in structure/substrate recognition. In addition, their engagement in cell biological processes involves both enzymatic and nonenzymatic capabilities. This article examines, in detail, our current understanding of the biological involvement of this unique enzyme family and their overall potential as therapeutic targets.

Decreased expression of T lymphocyte co-stimulatory molecule CD26 on invariant natural killer T cells in systemic lupus erythematosus

CD26, a T cell co-stimulatory molecule and dipeptidyl peptidase IV for the degradation of interferon-gamma-induced chemokine, participates in multiple immunopathological roles in leukocyte homing and inflammation. Decreased circulating concentration of soluble (s)CD26 in patients with systemic lupus erythematosus (SLE), rheumatoid arthritis and murine model of arthritis and encephalomyelitis have been reported. In the present study, the plasma concentration of sCD26 and chemokines, and cell surface expression of CD26 on monocytes, CD4+T lymphocytes, CD8+T lymphocytes, CD19+B lymphocytes and invariant natural killer T (iNKT) lymphocytes were analyzed using ELISA and flow cytometry, respectively, in 23 SLE patients and 14 sex- and age-matched control subjects. Although there was no significant difference between plasma concentrations of soluble CD26 in SLE patients with controls (p > 0.05), there was significant elevated Th1 chemokines CXCL10 and CXCL9 but not Th2 chemokine CCL2, and down-regulation in iNKT lymphocytes number and cell surface expression of CD26 on CD4+T and iNKT lymphocytes of SLE patients compared with controls (all p < 0.05). Decreased circulating number of iNKT cells and CD26 on iNKT cells can be important for the immunopathogenesis by exacerbating Th1-related inflammation in SLE.

Exosome-like vesicles with dipeptidyl peptidase IV in human saliva

Saliva contains a large number of proteins that participate in the protection of oral tissue. We found, for the first time, small vesicles (30-130 nm in diameter) in human whole saliva. Vesicles from saliva were identified by electron microscopy after isolation by gel-filtration on Sepharose CL-4B. They resemble exosomes, which are vesicles with an endosome-derived limiting membrane that are secreted by a diverse range of cell types. We performed a biochemical characterization of these vesicles by amino acid sequence analysis and Western blot analysis. We found that they contain dipeptidyl peptidase IV (DPP IV), galectin-3 and immunoglobulin A, which have potential to influence immune response. The DPP IV in the vesicles was metabolically active in cleaving substance P and glucose-dependent insulinotropic polypeptide to release N-terminal dipeptides. Our results demonstrate that human whole saliva contains exosome-like vesicles; they might participate in the catabolism of bioactive peptides and play a regulatory role in local immune defense in the oral cavity.

Revisiting an old acquaintance: CD26 and its molecular mechanisms in T cell function

The role of CD26 in human T cell biology is puzzling. Despite being extensively characterized, it has been called 'a moonlighting protein' since it has multifunctional effects, but a definitive native ligand has not been identified. We summarize the current knowledge on the molecular mechanisms of CD26-mediated T cell costimulation and immune regulation. Work identifying a ligand for CD26 and elucidating the proximal signaling events associated with CD26-mediated T cell costimulation is also described. Finally, we discuss the involvement of CD26 in various pathophysiologic states as well as its suitability as a potential therapeutic target in selected immune diseases.

Snake venom dipeptidyl peptidase IV: taxonomic distribution and quantitative variation

The present study examined the taxonomic distribution of dipeptidyl peptidase IV (DPP IV) activity in venoms of 59 ophidian taxa, representing seven subfamilies of the Families Elapidae and Viperidae. DPP IV activity is extremely variable at all taxonomic levels. It ranged from essentially none in laticaudine, hydrophiine, and some bungarine and elapine venoms, to 10.72 mumol 4-methoxy-beta-naphthylamine liberated per min per 200 mug venom, for Ophiophagus hannah. Intra- and interpopulational variation were examined among eight populations of prairie rattlesnakes (Crotalus viridis viridis), Great Basin rattlesnakes (Crotalus viridis lutosus) and southern Pacific rattlesnakes (Crotalus viridis helleri). Among these populations, the mean weighted range of variation was 4.9-fold, and even among litter mates of C. v. lutosus, DPP IV activity varied as much as 5.6-fold. The two most salient findings, the near ubiquity of DPP IV in snake venoms and its great quantitative variability, even among full siblings, are paradoxical. The widespread distribution of the enzyme suggests an important role in envenomation, while the variable activity levels suggest that DPP IV and by extension, other individual enzymatic constituents, may not be under much individual selective pressure.

In active chronic rheumatoid arthritis, dipeptidyl peptidase IV density is increased on monocytes and CD4(+) T lymphocytes

The effect of low-dose methotrexate (MTX) treatment on the CD26 density on circulating monocytes and CD4(+) T lymphocytes or levels of soluble CD26 (sCD26) has not yet been described in rheumatoid arthritis (RA). While CD26 in T lymphocytes is involved in the activation and proliferation of T lymphocytes, little is known of the role of CD26 in monocytes as it has only recently been localized to monocytes. We analysed the CD26 density by flow cytometry and levels of sCD26 in plasma before initiation of MTX treatment and 12 weeks later. This was done on 34 RA patients fulfilling the 1987 American College of Rheumatology (ACR) criteria followed for 16 weeks after starting MTX treatment. CD26 density on monocytes was increased in RA patients compared with healthy controls before MTX treatment (P < 0.01). After 12 weeks of MTX treatment, the CD26 density on monocytes decreased significantly in the ACR-50% group (P = 0.03), but not in the ACR-20% and the non-responder group (P = 0.15 and 0.87). The increased CD26 density on CD4(+) T lymphocytes (P < 0.01) was unaffected by the reduction in disease activity in relation to MTX treatment. The percentage of monocytes and CD4(+) T lymphocytes among peripheral blood circulating mononuclear cells did not change during MTX treatment. No effect of MTX treatment was observed on the plasma levels of sCD26. Active chronic RA is characterized by enhanced CD26 density on circulating monocytes and CD4(+) T lymphocytes. MTX treatment decreased CD26 density on monocytes in the ACR-50% responder group and was associated with decreased disease activity. The enhanced CD26 density on CD4(+) T lymphocytes was uninfluenced by MTX treatment.

Caveolin-1 triggers T-cell activation via CD26 in association with CARMA1

CD26 is a widely distributed 110-kDa cell surface glycoprotein with an important role in T-cell costimulation. We demonstrated previously that CD26 binds to caveolin-1 in antigen-presenting cells, and following exogenous CD26 stimulation, Tollip and IRAK-1 disengage from caveolin-1 in antigen-presenting cells. IRAK-1 is then subsequently phosphorylated to up-regulate CD86 expression, resulting in subsequent T-cell proliferation. However, it is unclear whether caveolin-1 is a costimulatory ligand for CD26 in T-cells. Using soluble caveolin-1-Fc fusion protein, we now show that caveolin-1 is the costimulatory ligand for CD26, and that ligation of CD26 by caveolin-1 induces T-cell proliferation and NF-kappaB activation in a T-cell receptor/CD3-dependent manner. We also demonstrated that the cytoplasmic tail of CD26 interacts with CARMA1 in T-cells, resulting in signaling events that lead to NF-kappaB activation. Ligation of CD26 by caveolin-1 recruits a complex consisting of CD26, CARMA1, Bcl10, and IkappaB kinase to lipid rafts. Taken together, our findings provide novel insights into the regulation of T-cell costimulation via the CD26 molecule.

T-cell activation via CD26 and caveolin-1 in rheumatoid synovium

CD26 is a T-cell costimulatory molecule with dipeptidyl peptidase IV (DPPIV) activity in its extracellular region. We previously reported that recombinant soluble CD26 enhances peripheral blood T-cell proliferation induced by the recall antigen tetanus toxoid (TT). Recently, we demonstrated that CD26 binds caveolin-1 on antigen-presenting cell (APC), and that residues 201–211 of CD26 along with the serine catalytic site at residue 630, which constitute a pocket structure of CD26/DPPIV, contribute to binding to caveolin-1 scaffolding domain. In addition, following CD26–caveolin-1 interaction on TT-loaded monocytes, caveolin-1 is phosphorylated, with linkage to NF-κB activation, followed by upregulation of CD86. Finally, reduced caveolin-1 expression on APC inhibits CD26-mediated CD86 upregulation and abrogates CD26 effect on TT-induced T-cell proliferation, and immunohistochemical studies revealed an infiltration of CD26+ T cells in the sublining region of rheumatoid synovium and high expression of caveolin-1 in the increased vasculature and synoviocytes of the rheumatoid synovium. Taken together, these results strongly suggest that CD26–cavolin-1 interaction plays a role in the upregulation of CD86 on TT-loaded APC and subsequent engagement with CD28 on T cells, leading to antigen-specific T-cell activation such as the T-cell-mediated antigen-specific response in rheumatoid arthritis.

Activation Antigens during the Proliferative and Secretory Phases of Endometrium and Early-Pregnancy Decidua

BACKGROUND

Clarifying the normal distribution of activation antigens will contribute to database construction studies of monoclonal-antibody-based therapies in endometrial disorders.

METHODS

In this study, endometrial tissue samples obtained during proliferative and secretory phases and decidual samples of early pregnancies were immunostained by the monoclonal antibodies anti-CD26, anti-CD30, anti-CD70, anti-CD71, and anti-CD98 using the indirect immunoperoxidase method.

RESULTS

CD26 is expressed on the glandular epithelium in the endometrium and decidua. Endothelial CD26 is expressed less in the decidua when compared to the endometrium. CD30 is strongly expressed by decidual cells. It is only weakly expressed on endometrial and decidual vessels. Glandular and endothelial CD70 expression is mainly seen in the proliferative phase of the menstrual cycle. Glandular CD71 expression is less in the decidua when compared to the endometrium. Its expression on stromal cells is more in the secretory phase of the menstrual cycle and in early pregnancy deciduae. It is expressed on endometrial vessels but not on decidual vessels. Glandular CD98 is expressed more in the decidua when compared to the endometrium. This antigen exists on endometrial lymphocytes. It is strongly expressed on the endothelium in the endometrium and decidua.

CONCLUSION

It seems that CD26 and CD70 are not involved in the functions of endometrial and decidual stromal cells. CD30 and CD71 are thought to be involved in decidualization. Absence of activation antigens other than CD98 on lymphocytes indicated an antigenic profile for large granular lymphocytes that is different from regular lymphocytes.

Expression of CD28 and cytotoxic T lymphocyte antigen 4 at the maternal-fetal interface in women with unexplained pregnancy loss

OBJECTIVE

To investigate the expression of CD28 and CTLA-4, two costimulatory molecules involved in T-cell activation at the maternal-fetal interface in women with unexplained pregnancy loss.

METHODS

A total of 57 women, 39 with unexplained spontaneous abortion (SA) and 18 with unexplained recurrent spontaneous abortion (RSA), were enrolled in the study. A high-resolution spectratyping analysis of fluorescent bands was performed to detect CD28 and CTLA-4 expression in decidual tissues.

RESULTS

The mean expression of CTLA-4 mRNA was significantly higher in women with SA than in controls (P<0.05). Moreover, the expression of CTLA-4 was higher in SA patients with genotype AA and phenotype A+ (AA+AG) than in controls (P<0.01). The expression of CTLA-4 was not significantly different in patients with RSA and in controls. The expression of CD28 was significantly decreased in patients with RSA (P<0.01) and SA (P<0.05) compared with controls. The mean ratios of CTLA-4/CD28 were significantly higher in patients with RSA (P<0.01) and SA (P<0.05) than in controls.

CONCLUSIONS

This study suggests that an imbalance in CTLA-4/CD28 expression or suppressed T-cell activity at the maternal-fetal interface may confer susceptibility to unexplained pregnancy loss.

Dipeptidyl peptidase IV activity and/or structure homologs: contributing factors in the pathogenesis of rheumatoid arthritis?

Several of the proinflammatory peptides involved in rheumatoid arthritis pathogenesis, including peptides induced downstream of tumor necrosis factor-α as well as the monocyte/T cell-attracting chemokines RANTES and stromal cell-derived factor (SDF)-1α and the neuropeptides vasoactive intestinal peptide (VIP) and substance P, have their biological half-lives controlled by dipeptidyl peptidase IV (DPPIV). Proteolysis by DPPIV regulates not only the half-life but also receptor preference and downstream signaling. In this article, we examine the role of DPPIV homologs, including CD26, the canonical DPPIV, and their substrates in the pathogenesis of rheumatoid arthritis. The differing specific activities of the DPPIV family members and their differential inhibitor response provide new insights into therapeutic design.

CD26 mediates dissociation of Tollip and IRAK-1 from caveolin-1 and induces upregulation of CD86 on antigen-presenting cells

CD26 is a T-cell costimulatory molecule with dipeptidyl peptidase IV enzyme activity in its extracellular region. We have previously reported that the addition of recombinant soluble CD26 resulted in enhanced proliferation of human T lymphocytes induced by the recall antigen tetanus toxoid (TT) via upregulation of CD86 on monocytes and that caveolin-1 was a binding protein of CD26, and the CD26-caveolin-1 interaction resulted in caveolin-1 phosphorylation (p-cav-1) as well as TT-mediated T-cell proliferation. However, the mechanism involved in this immune enhancement has not yet been elucidated. In the present work, we perform experiments to identify the molecular mechanisms by which p-cav-1 leads directly to the upregulation of CD86. Through proteomic analysis, we identify Tollip (Toll-interacting protein) and IRAK-1 (interleukin-1 receptor-associated serine/threonine kinase 1) as caveolin-1-interacting proteins in monocytes. We also demonstrate that following stimulation by exogenous CD26, Tollip and IRAK-1 dissociate from caveolin-1, and IRAK-1 is then phosphorylated in the cytosol, leading to the upregulation of CD86 via activation of NF-kappaB. Binding of CD26 to caveolin-1 therefore regulates signaling pathways in antigen-presenting cells to induce antigen-specific T-cell proliferation.

Ku80 autoantigen as a cellular coreceptor for human parvovirus B19 infection

Human parvovirus B19 (B19) infects human erythroid cells expressing P antigen. However, some cell lines that were positive for P antigen failed to bind B19, whereas some cell lines had an ability to bind B19 despite undetectable expression of P antigen. We here demonstrate that B19 specifically binds with Ku80 autoantigen on the cell surface. Furthermore, transfection of HeLa cells with the gene of Ku80 enabled the binding of B19 and allowed its entry into cells. Moreover, reduction of cell-surface expression of Ku80 in KU812Ep6 cells, which was a high-sensitive cell line for B19 infection, by short interfering RNA for Ku80 resulted in the marked inhibition of B19 binding in KU812Ep6 cells. Although Ku80 originally has been described as a nuclear protein, human bone marrow erythroid cells with glycophorin A or CD36, B cells with CD20, or T cells with CD3 were all positive for cell-surface expression of Ku80. B19 infection of KU812Ep6 cells and bone marrow cells was inhibited in the presence of anti-Ku80 antibody. Our data suggest that Ku80 functions as a novel coreceptor for B19 infection, and this finding may provide an explanation for the pathologic immunity associated with B19 infection.

Dipeptidyl peptidase IV and related enzymes in cell biology and liver disorders

DP (dipeptidyl peptidase) IV is the archetypal member of its six-member gene family. Four members of this family, DPIV, FAP (fibroblast activation protein), DP8 and DP9, have a rare substrate specificity, hydrolysis of a prolyl bond two residues from the N-terminus. The ubiquitous DPIV glycoprotein has proved interesting in the fields of immunology, endocrinology, haematology and endothelial cell and cancer biology and DPIV has become a novel target for Type II diabetes therapy. The crystal structure shows that the soluble form of DPIV comprises two domains, an alpha/beta-hydrolase domain and an eight-blade beta-propeller domain. The propeller domain contains the ADA (adenosine deaminase) binding site, a dimerization site, antibody epitopes and two openings for substrate access to the internal active site. FAP is structurally very similar to DPIV, but FAP protein expression is largely confined to diseased and damaged tissue, notably the tissue remodelling interface in chronically injured liver. DPIV has a variety of peptide substrates, the best studied being GLP-1 (glucagon-like peptide-1), NPY (neuropeptide Y) and CXCL12. The DPIV family has roles in bone marrow mobilization. The functional interactions of DPIV and FAP with extracellular matrix confer roles for these proteins in cancer biology. DP8 and DP9 are widely distributed and indirectly implicated in immune function. The DPL (DP-like) glycoproteins that lack peptidase activity, DPL1 and DPL2, are brain-expressed potassium channel modulators. Thus the six members of the DPIV gene family exhibit diverse biological roles.

CD26 up-regulates expression of CD86 on antigen-presenting cells by means of caveolin-1

CD26 is a T cell costimulatory molecule with dipeptidyl peptidase IV activity in its extracellular region. We previously reported that recombinant soluble CD26 enhanced T cell proliferation induced by the recall antigen tetanus toxoid (TT). However, the mechanism involved in this enhancement is not yet elucidated. We now demonstrate that CD26 binds Caveolin-1 on antigen-presenting cells, and that residues 201-211 of CD26 along with the serine catalytic site at residue 630 contribute to binding to caveolin-1 scaffolding domain. In addition, after CD26-caveolin-1 interaction on TT-loaded monocytes, caveolin-1 is phosphorylated, which links to activate NF-kappaB, followed by up-regulation of CD86. Finally, reduced caveolin-1 expression on monocytes inhibits CD26-mediated CD86 up-regulation and abrogates CD26 effect on TT-induced T cell proliferation. Taken together, these results strongly suggest that CD26-caveolin-1 interaction plays a role in the up-regulation of CD86 on TT-loaded monocytes and subsequent engagement with CD28 on T cells, leading to antigen-specific T cell activation.

Human Miscarriage is Associated with Increased Number of CD26+ Decidual Lymphocytes

Dipeptidyl peptidase-IV (DPP-IV, CD26), a serine protease with broad distribution in mammalian tissues and known activity in serum, participates in T-cell activation and promotes a Th1-like cytokine response. Previous data on murine abortion indicate that DPP-IV may play a critical role in pregnancy failure by inducing a Th1 local response. Here, we investigated the possible participation of DPP-IV in the onset of human spontaneous abortion (SA). The systemic (peripheral blood) and local (decidua) percentages of CD4(+), CD8(+), CD26(+) and CD56(+) cells as well as the number of Th1 lymphocytes (CCR5(+) cells) were assessed in samples from women after SAs (n = 20) and from women with normally progressing pregnancies (NPs) (n = 27) using flow cytometry and immunohistochemistry. We further measured the DPP-IV activity and concentrations of Th1 (interferon-gamma and tumour necrosis factor-alpha), Th2 [interleukin-4 (IL-4), IL-10] and Th3 (transforming growth factor-beta2) cytokines in serum samples. We could not find any difference in the number of CD4(+), CD8(+), CD26(+), CD26(+)/CD4(+) or CD8(+)/CD26(+) blood cells between NP and SA patients. No differences in the Th1, Th2 or Th3 cytokine levels could be observed between both groups. However, the percentages of decidual CD26(+) lymphocytes as well as the number of decidual Th1 cells were significantly higher in SA samples compared to samples from patients with NP. Our data support the hypothesis that CD26(+) decidual lymphocytes with DPP-IV activity may play a critical role in SAs, as previously suggested in an abortion mice model. This abortive effect may be mediated by enhancing the levels of Th1 abortogenic cytokines only locally.

Dipeptidyl peptidase IV activity and/or structure homologues (DASH) and their substrates in cancer

Post-translational modification of proteins is an important regulatory event. Numerous biologically active peptides that play an essential role in cancerogenesis contain an evolutionary conserved proline residue as a proteolytic-processing regulatory element. Proline-specific proteases could therefore be viewed as important "check-points". Limited proteolysis of such peptides may lead to quantitative but, importantly, due to the change of receptor preference, also qualitative changes of their signaling potential. Dipeptidyl peptidase-IV (DPP-IV, EC 3.4.14.5, identical with CD26) was for many years believed to be a unique cell membrane protease cleaving X-Pro dipeptides from the N-terminal end of peptides and proteins. Subsequently, a number of other molecules were discovered, exhibiting various degree of structural homology and DPP-IV-like enzyme activity, capable of cleaving similar set of substrates. These comprise for example, seprase, fibroblast activation protein alpha, DPP6, DPP8, DPP9, attractin, N-acetylated-alpha-linked-acidic dipeptidases I, II and L, quiescent cell proline dipeptidase, thymus-specific serine protease and DPP IV-beta. It is tempting to speculate their potential participation on DPP-IV biological function(s). Disrupted expression and enzymatic activity of "DPP-IV activity and/or structure homologues" (DASH) might corrupt the message carried by their substrates, promoting abnormal cell behavior. Consequently, modulation of particular enzyme activity using e.g. DASH inhibitors, specific antibodies or DASH expression modification may be an attractive therapeutic concept in cancer treatment. This review summarizes recent information on the interactions between DASH members and their substrates with respect to their possible role in cancer biology.

3D structure of the CD26-ADA complex obtained by cryo-EM and single particle analysis

The specific binding of adenosine deaminase to the multifunctional membrane glycoprotein dipeptidyl peptidase IV is thought to be immunologically relevant for certain regulatory and co-stimulatory processes. In this study we present the 3D structure of the complete CD26-ADA complex obtained by single particle cryo-EM at 22A resolution. ADA binding occurs at the outer edges of the beta-propeller of CD26. Docking calculations of available CD26 and ADA crystal data into the obtained EM density map revealed that the ADA-binding site is stretched across CD26 beta-propeller blades 4 and 5 involving the outermost distal hydrophobic amino acids L294 and V341 but not T440 and K441 as suggested by antibody binding. Though the docking of the ADA orientation appears less significant due to the lack of distinct surface features, non-ambiguous conclusions can be drawn in the combination with earlier indirect non-imaging methods affirming the crucial role of the ADA alpha2-helix for binding.

CD26/dipeptidyl peptidase IV enhances expression of topoisomerase II alpha and sensitivity to apoptosis induced by topoisomerase II inhibitors

CD26/dipeptidyl peptidase IV (DPPIV) is a cell surface-bound ectopeptidase with important roles in T-cell activation and tumour biology. We now report that CD26/DPPIV enhances sensitivity to apoptosis induced by the antineoplastic agents doxorubicin and etoposide. In particular, CD26/DPPIV presence is associated with increased susceptibility to the mitochondrial pathway of apoptosis, documented by enhanced cleavage of poly (ADP ribose) polymerase (PARP), caspase-3 and caspase-9, Bcl-xl, and Apaf-1, as well as increased expression of death receptor 5 (DR5). We also show that the caspase-9-specific inhibitor z-LEHD-fmk inhibits drug-mediated apoptosis, leading to decreased PARP and caspase-3 cleavage, and reduced DR5 expression. Importantly, through detailed studies that demonstrate the association between topoisomerase II alpha expression and DPPIV activity, our data provide further evidence of the key role played by CD26 in biological processes.