Enzymatic activity of CD26 (dipeptidylpeptidase IV) is not required for its signalling function in T cells

Targeting Chronic Myeloid Leukemia Stem/Progenitor Cells Using Venetoclax-Loaded Immunoliposome

CML is a hematopoietic stem-cell disorder emanating from breakpoint cluster region/Abelson murine leukemia 1 (BCR/ABL) translocation. Introduction of different TKIs revolutionized treatment outcome in CML patients, but CML LSCs seem insensitive to TKIs and are detectable in newly diagnosed and resistant CML patients and in patients who discontinued therapy. It has been reported that CML LSCs aberrantly express some CD markers such as CD26 that can be used for the diagnosis and for targeting. In this study, we confirmed the presence of CD26+ CML LSCs in newly diagnosed and resistant CML patients. To selectively target CML LSCs/progenitor cells that express CD26 and to spare normal HSCs/progenitor cells, we designed a venetoclax-loaded immunoliposome (IL-VX). Our results showed that by using this system we could selectively target CD26+ cells while sparing CD26- cells. The efficiency of venetoclax in targeting CML LSCs has been reported and our system demonstrated a higher potency in cell death induction in comparison to free venetoclax. Meanwhile, treatment of patient samples with IL-VX significantly reduced CD26+ cells in both stem cells and progenitor cells population. In conclusion, this approach showed that selective elimination of CD26+ CML LSCs/progenitor cells can be obtained in vitro, which might allow in vivo reduction of side effects and attainment of treatment-free, long-lasting remission in CML patients.

The Dipeptidyl Peptidase Family, Prolyl Oligopeptidase, and Prolyl Carboxypeptidase in the Immune System and Inflammatory Disease, Including Atherosclerosis

Research from over the past 20 years has implicated dipeptidyl peptidase (DPP) IV and its family members in many processes and different pathologies of the immune system. Most research has been focused on either DPPIV or just a few of its family members. It is, however, essential to consider the entire DPP family when discussing any one of its members. There is a substantial overlap between family members in their substrate specificity, inhibitors, and functions. In this review, we provide a comprehensive discussion on the role of prolyl-specific peptidases DPPIV, FAP, DPP8, DPP9, dipeptidyl peptidase II, prolyl carboxypeptidase, and prolyl oligopeptidase in the immune system and its diseases. We highlight possible therapeutic targets for the prevention and treatment of atherosclerosis, a condition that lies at the frontier between inflammation and cardiovascular disease.

Anti-inflammatory role of DPP-4 inhibitors in a nondiabetic model of glomerular injury

Dipeptidyl peptidase (DPP)-4 is an enzyme that cleaves and inactivates incretin hormones capable of stimulating insulin secretion from pancreatic β-cells. DPP-4 inhibitors are now widely used for the treatment of type 2 diabetes. Experimental studies have suggested a renoprotective role of DPP-4 inhibitors in various models of diabetic kidney disease, which may be independent of lowering blood glucose levels. In the present study, we examined the effect of DPP-4 inhibitors in the rat Thy-1 glomerulonephritis model, a nondiabetic glomerular injury model. Rats were injected with OX-7 (1.2 mg/kg iv) and treated with the DPP-4 inhibitor alogliptin (20 mg·kg(-1)·day(-1)) or vehicle for 7 days orally by gavage. Alogliptin significantly reduced the number of CD68-positive inflammatory macrophages in the kidney, which was associated with a nonsignificant tendency to ameliorate glomerular injury and reduce proteinuria. Another DPP-4 inhibitor, anagliptin (300 mg·kg(-1)·day(-1) mixed with food) and a glucagon-like peptide-1 receptor agonist, exendin-4 (10 mg/kg sc), similarly reduced CD68-positive macrophage infiltration to the kidney. Furthermore, ex vivo transmigration assays using peritoneal macrophages revealed that exendin-4, but not alogliptin, dose dependently reduced monocyte chemotactic protein-1-stimulated macrophage infiltration. These data suggest that DPP-4 inhibitors reduced macrophage infiltration directly via glucagon-like peptide-1-dependent signaling in the rat Thy-1 nephritis model and indicate that the control of inflammation by DPP-4 inhibitors is useful for the treatment of nondiabetic kidney disease models.

Pharmacology, physiology, and mechanisms of action of dipeptidyl peptidase-4 inhibitors

Dipeptidyl peptidase-4 (DPP4) is a widely expressed enzyme transducing actions through an anchored transmembrane molecule and a soluble circulating protein. Both membrane-associated and soluble DPP4 exert catalytic activity, cleaving proteins containing a position 2 alanine or proline. DPP4-mediated enzymatic cleavage alternatively inactivates peptides or generates new bioactive moieties that may exert competing or novel activities. The widespread use of selective DPP4 inhibitors for the treatment of type 2 diabetes has heightened interest in the molecular mechanisms through which DPP4 inhibitors exert their pleiotropic actions. Here we review the biology of DPP4 with a focus on: 1) identification of pharmacological vs physiological DPP4 substrates; and 2) elucidation of mechanisms of actions of DPP4 in studies employing genetic elimination or chemical reduction of DPP4 activity. We review data identifying the roles of key DPP4 substrates in transducing the glucoregulatory, anti-inflammatory, and cardiometabolic actions of DPP4 inhibitors in both preclinical and clinical studies. Finally, we highlight experimental pitfalls and technical challenges encountered in studies designed to understand the mechanisms of action and downstream targets activated by inhibition of DPP4.

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.

Genetic ablation or pharmacological blockade of dipeptidyl peptidase IV does not impact T cell-dependent immune responses

BACKGROUND

Current literature suggests that dipeptidyl peptidase IV (DPP-IV; CD26) plays an essential role in T-dependent immune responses, a role that could have important clinical consequences. To rigorously define the role of DPP-IV in the immune system, we evaluated genetic and pharmacological inhibition of the enzyme on T-dependent immune responses in vivo.

RESULTS

The DPP-IV null animals mounted robust primary and secondary antibody responses to the T dependent antigens, 4-hydroxy-3-nitrophenylacetyl-ovalbumin (NP-Ova) and 4-hydroxy-3-nitrophenylacetyl-chicken gamma globulin (NP-CGG), which were comparable to wild type mice. Serum levels of antigen specific IgM, IgG1, IgG2a, IgG2b and IgG3 were similar between the two groups of animals. DPP-IV null animals mounted an efficient germinal center reaction by day 10 after antigen stimulation that was comparable to wild type mice. Moreover, the antibodies produced by DPP-IV null animals after repeated antigenic challenge were affinity matured. Similar observations were made using wild type animals treated with a highly selective DPP-IV inhibitor during the entire course of the experiments. T cell recall responses to ovalbumin and MOG peptide, evaluated by measuring proliferation and IL-2 release from cells isolated from draining lymph nodes, were equivalent in DPP-IV null and wild type animals. Furthermore, mice treated with DPP-IV inhibitor had intact T-cell recall responses to MOG peptide. In addition, female DPP-IV null and wild type mice treated with DPP-IV inhibitor exhibited normal and robust in vivo cytotoxic T cell responses after challenge with cells expressing the male H-Y minor histocompatibility antigen.

CONCLUSION

These data indicate Selective inhibition of DPP-IV does not impair T dependent immune responses to antigenic challenge.

Fluorogenic substrate [Ala-Pro]2-cresyl violet but not Ala-Pro-rhodamine 110 is cleaved specifically by DPPIV activity: a study in living Jurkat cells and CD26/DPPIV-transfected Jurkat cells

Fluorogenic substrates [Ala-Pro](2)-cresyl violet and Ala-Pro-rhodamine 110 have been tested for microscopic detection of protease activity of dipeptidyl peptidase IV (DPPIV) in living cells. DPPIV activity is one of the many functions of the multifunctional or moonlighting protein CD26/DPPIV. As a model we used Jurkat cells, which are T-cells that lack CD26/DPPIV expression, and CD26/DPPIV-transfected Jurkat cells. Ala-Pro-rhodamine 110 is not fluorescent, but after proteolytic cleavage rhodamine 110 fluoresces. [Ala-Pro](2)-cresyl violet is fluorescent by itself but proteolytic cleavage into cresyl violet induces a shift to longer wavelengths. This phenomenon enables the simultaneous determination of local (intracellular) substrate and product concentrations, which is important for analysis of kinetics of the cleavage reaction. [Ala-Pro](2)-cresyl violet, but not Ala-Pro-rhodamine 110, appeared to be specific for DPPIV. When microscopic analysis is performed on living cells during the first minutes of the enzyme reaction, DPPIV activity can be precisely localized in cells with the use of [Ala-Pro](2)-cresyl violet. Fluorescent product is rapidly internalized into submembrane granules in transfected Jurkat cells and is redistributed intracellularly via internalization pathways that have been described for CD26/DPPIV. We conclude that [Ala-Pro](2)-cresyl violet is a good fluorogenic substrate to localize DPPIV activity in living cells when the correct wavelengths are used for excitation and emission and images are captured in the early stages of the enzyme reaction.

Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV

Dipeptidyl-peptidase IV/CD26 (DPP IV) is a cell-surface protease belonging to the prolyloligopeptidase family. It selectively removes the N-terminal dipeptide from peptides with proline or alanine in the second position. Apart from its catalytic activity, it interacts with several proteins, for instance, adenosine deaminase, the HIV gp120 protein, fibronectin, collagen, the chemokine receptor CXCR4, and the tyrosine phosphatase CD45. DPP IV is expressed on a specific set of T lymphocytes, where it is up-regulated after activation. It is also expressed in a variety of tissues, primarily on endothelial and epithelial cells. A soluble form is present in plasma and other body fluids. DPP IV has been proposed as a diagnostic or prognostic marker for various tumors, hematological malignancies, immunological, inflammatory, psychoneuroendocrine disorders, and viral infections. DPP IV truncates many bioactive peptides of medical importance. It plays a role in glucose homeostasis through proteolytic inactivation of the incretins. DPP IV inhibitors improve glucose tolerance and pancreatic islet cell function in animal models of type 2 diabetes and in diabetic patients. The role of DPP IV/ CD26 within the immune system is a combination of its exopeptidase activity and its interactions with different molecules. This enables DPP IV/CD26 to serve as a co-stimulatory molecule to influence T cell activity and to modulate chemotaxis. DPP IV is also implicated in HIV-1 entry, malignant transformation, and tumor invasion.

The multifunctional or moonlighting protein CD26/DPPIV

CD26/DPPIV can be considered a moonlighting protein because it is a multifunctional protein that exerts its different functions depending on cell type and intra- or extracellular conditions in which it is expressed. In the present review, we summarize all its known functions in relation to physiological and pathophysiological conditions. The protein is a proteolytic enzyme, receptor, costimulatory protein, and is involved in adhesion and apoptosis. The CD26/DPPIV protein plays a major role in immune response. Abnormal expression is found in the case of autoimmune diseases, HIV-related diseases and cancer. Natural substrates for CD26/DPPIV are involved in immunomodulation, psycho/neuronal modulation and physiological processes in general. Therefore, targeting of CD26/ DPPIV and especially its proteolytic activity has many therapeutic potentials. On the other hand, there are homologous proteins with overlapping proteolytic activity, which thus may prevent specific modulation of CD26/DPPIV. In conclusion, CD26/DPPIV is a protein present both in various cellular compartments and extracellularly where it exerts different functions and thus is a true moonlighting protein.

Cellular targets for angiotensin II fragments: pharmacological and molecular evidence

Although angiotensin II has long been considered to represent the end product of the renin-angiotensin system (RAS), there is accumulating evidence that it encompasses additional effector peptides with diverse functions. In this respect, angiotensin IV (Ang IV) formed by deletion of the two N terminal amino acids, has sparked great interest because of its wide range of physiological effects. Among those, its facilitatory role in memory acquisition and retrieval is of special therapeutic relevance. High affinity binding sites for this peptide have been denoted as AT(4)- receptors and, very recently, they have been proposed to correspond to the membrane-associated OTase/ IRAP aminopeptidase. This offers new opportunities for examining physiological roles of Ang IV in the fields of cognition, cardiovascular and renal metabolism and pathophysiological conditions like diabetes and hypertension. Still new recognition sites may be unveiled for this and other angiotensin fragments. Recognition sites for Ang-(1-7) (deletion of the C terminal amino acid) are still elusive and some of the actions of angiotensin III (deletion of the N terminal amino acid) in the CNS are hard to explain on the basis of their interaction with AT(1)-receptors only. A more thorough cross-talk between in vitro investigations on native and transfected cell lines and in vivo investigations on healthy, diseased and transgenic animals may prove to be essential to further unravel the molecular basis of the physiological actions of these small endogenous angiotensin fragments.

Dipeptidyl peptidase IV-like molecules: homologous proteins or homologous activities?

Membrane-bound proteases are widely distributed among various cell systems. Their expression in a particular cell type is finely regulated, reflecting the specific functional cell implications and engagement in defined physiological pathways. Protein turnover, ontogeny, inflammation, tissue remodeling, cell migration and tumor invasion are among the many physiological and pathological events in which membrane proteases play a crucial role, both as effector as well as regulatory molecules. The presence of proline residues gives unique structural features to peptide chains, substantially influencing the susceptibility of proximal peptide bond to protease cleavage. Among the rare group of proline-specific proteases, dipeptidyl peptidase IV (DPP-IV, EC 3.4.14.5) was originally believed to be the only membrane-bound enzyme specific for proline as the penultimate residue at the amino-terminus of the polypeptide chain. However, other molecules, even structurally non-homologous with the DPP-IV but bearing corresponding enzyme activity, have been identified recently. This review summarizes the present knowledge of "DPP-IV activity- and/or structure-homologues" (DASH) and provides some insight into their multifunctional roles.

CD26: a multifunctional integral membrane and secreted protein of activated lymphocytes

CD26 has proved interesting in the fields of immunology, endocrinology, cancer biology and nutrition owing to its ubiquitous and unusual enzyme activity. This dipeptidyl aminopeptidase (DPP IV) activity generally inactivates but sometimes alters or enhances the biological activities of its peptide substrates, which include several chemokines. CD26 costimulates both the CD3 and the CD2 dependent T-cell activation and tyrosine phosphorylation of TCR/CD3 signal transduction pathway proteins. CD26 in vivo has integral membrane protein and soluble forms. Soluble CD26 is at significant levels in serum, these levels alter in many diseases and soluble CD26 can modulate in vitro T-cell proliferation. CD26, being an adenosine deaminase binding protein (ADAbp), functions as a receptor for ADA on lymphocytes. The focus of this review is the structure and function of CD26 and the influence of its ligand binding activity on T-cell proliferation and the T cell costimulatory activity of CD26.

Signal transduction events induced or affected by inhibition of the catalytic activity of dipeptidyl peptidase IV (DP IV, CD26)

DP IV (CD26) represents an accessory surface molecule playing an important role in the process of activation and proliferation of human lymphocytes. The molecular events mediated by this ectoenzyme are only partly established and the necessity of DP IV enzymatic activity for its signalling capacity has been discussed controversial. Focusing on the putative role of the catalytic domain of this peptidase, it could be shown that inhibition of the catalytic activity can provoke many cellular effects, including induction of tyrosine phosphorylations and p38 MAP kinase activation as well as suppression of DNA synthesis and reduced production of various cytokines. TGF-beta 1, the production and secretion of which is increased after DP IV inhibition, supposedly mediates the observed suppressive effects by maintaining p27kip expression levels which leads to a cell cycle arrest in G1. Moreover, anti-CD3-induced signalling pathways, including Ca2+ mobilisation, MEK1-, Erk1/2- and PKB-activation, can be strongly affected by DP IV inhibition. Thus, the enzymatic activity or at least the interaction of effectors with the catalytic domain of CD26 seems to be important for crucial functions of this cell surface antigen.

Early phosphorylation events induced by DPIV/CD26-specific inhibitors

Dipeptidyl peptidase IV (DPIV, CD26) is known to be involved in the regulation of T lymphocyte and NK cell activation and proliferation in vitro. The molecular events of lymphocyte activation mediated by this ectopeptidase as well as their physiological ligands are only partly established. Particularly, the necessity of catalytic dipeptidase activity for the costimulatory function of this molecule has been controversial. Here we provide evidence for a direct involvement of DPIV/CD26 in early phosphorylation mechanisms which are known to be essential in the signal transduction cascade of human T lymphocytes. We have found that DPIV-specific inhibitors (Lys[Z(NO2)]-thiazolidide and -piperidide) are capable of inducing intracellular tyrosine phosphorylation in resting human T cells. On the other hand, both inhibitors decreased the PMA-induced tyrosine phosphorylation in human T cells in a dose-dependent manner. Furthermore, a linkage between CD26 and the tyrosine kinase p56(lck) was shown by inhibition of PMA-induced hyperphosphorylation of p56(lck) by means of DPIV-specific inhibitors. The data presented here suggest that the inhibition of DPIV enzymatic activity induces a inhibitory signal transmitted by tyrosine kinases which leads to a block in a PMA-induced downstream pathway. These results support the assumption that DPIV/CD26 is directly involved in early processes of T cell activation via its enzymatic activity.

Dipeptidyl-peptidase IV/CD26 on T cells: analysis of an alternative T-cell activation pathway

CD26 is a proteolytic enzyme (dipeptidyl-peptidase IV) with a wide tissue distribution and a unique specificity that was already described 27 years ago. CD26 is expressed on a fraction of resting T cells at low density but is strongly upregulated following T-cell activation. Recent results indicate that CD26 is a multifunctional molecule that may have important functions on T cells and in the immune system. It is associated with molecules of immunological importance such as the protein tyrosine phosphatase CD45 and adenosine deaminase (ADA) on the cell surface. Synthetic inhibitors of the enzymatic activity of CD26 have been shown to suppress certain immune reactions in vitro and in vivo. An interesting feature of CD26 is its ability to transmit a transmembrane signal to trigger functional programs in T cells. This triggering requires crosslinking of CD26 on a cell membrane. The enzymatic activity of CD26 is not obligatory for the activation of T cells via CD26. Since CD26 is a type II membrane protein with only six intracellular amino acids, it must deliver its signal via a signal-transducing molecule. Signaling is dependent on the expression of the T-cell receptor (TCR) complex with a special need for a functional zeta-chain. In this context the zeta-chain of the TCR complex is required for CD26-mediated signaling but, in contrast to other co-stimulatory molecules such as the CD2 molecule, is not sufficient for triggering the T cell.

In vivo inhibition of dipeptidyl peptidase IV activity by pro-pro-diphenyl-phosphonate (Prodipine)

Dipeptidyl peptidase IV (DPP IV, EC 3.4.14.5), also known as CD26, is a membrane-bound serine protease that cleaves off aminoterminal dipeptides from peptides with a penultimate proline (or, at a much slower rate, a penultimate alanine). Recently, we synthesized and characterized a number of dipeptide-derived diphenylphosphonates. Out of the resulting series of slow-binding irreversible inhibitors of DPP IV, diphenyl 1-(S)-prolylpyrrolidine-2(R,S)-phosphonate hydrochloride (Pro-Pro-diphenylphosphonate or Prodipine) was selected for further study. We investigated the in vivo applicability of Prodipine. Male rabbits weighing 3-4 kg received a single intravenous injection with 10 mg Prodipine or saline. After 1 hr, plasma DPP IV activity had decreased to less than 20% of the preinjection value and remained unchanged during a 24-hr observation period. In a next step, we aimed to study (i) the dose dependency and (ii) the duration of the effect after a single intravenous dose of Prodipine. A profound and long-lasting inhibition of plasma DPP IV activity was observed in the treated animals (1, 5 or 10 mg). It took 5 to 8 days to reach half of the pretreatment DPP IV activity and generally more than 20 days for a complete recovery. Systemic treatment with Prodipine not only led to inhibition of plasma DPP IV activity but also decreased tissue DPP IV activity in circulating mononuclear cells, kidney cortex, thymus, spleen, lung, and liver. No differences in activities of the related peptidases aminopeptidase P (APP, EC 3.4.11.9), prolyl oligopeptidase (PO, EC 3.4.21.26), or aminopeptidase M (mAAP, EC 3.4.11.2) were detected between Prodipine-treated and control rabbits. The in vivo applicability of this chemically stable, irreversible inhibitor of DPP IV opens new possibilities, not only to further unravel the biological functions of this intriguing ectopeptidase, but also to explore this enzyme as a new target in various fields of pharmacological research.

T cell signalling through CD73

CD73 (ecto-5'-nucleotidase), a glycosyl phosphatidylinositol (GPI) anchored purine salvage enzyme expressed on the surface of human T and B lymphocytes, catalyzes the conversion of purine and pyrimidine ribo- and deoxyribonucleoside monophosphates to the corresponding nucleosides. The cellular distribution, cDNA sequence, and structure of CD73 are reviewed. CD73 serves as a costimulatory molecule in activating T cells. A Jurkat.T cell line transfected with the CD73 cDNA revealed that neither enzymatic activity nor the GPI anchor is necessary for T cell activation in vitro via CD73, while expression of p56kk, CD45 and the T cell receptor are required. Models for the transmission of signals via CD73 and other GPI-anchored proteins are discussed. CD73 generated adenosine functions in cell signalling in many physiologic systems, including intestinal epithelium, ischemic myocardium, and cholinergic synapses. The hypothesis that CD73 produces adenosine that is important for T cell development is presented.

Inhibition of dipeptidyl peptidase IV (CD26) by peptide boronic acid dipeptides

Peptide boronic acid dipeptide compounds were analyzed for their ability to inhibit recombinant human dipeptidylpeptidase IV (CD26, DPPIV). Rate constants for the peptide boronates are difficult to obtain because the active boronic acid dipeptide exists in equilibrium with a cyclic inactive species in aqueous solution. Rate constants were determined for the inhibition of DPPIV using several peptide boronates at different pH values. Val-boroPro forms the most tightly bound complex with DPPIV; the first order half life for dissociation of the inactive enzyme-inhibitor complex at 23 degrees C is approximately 27 days.

Substrates containing phosphorylated residues adjacent to proline decrease the cleavage by proline-specific peptidases

Thirteen dipeptide rho-nitroanilides of the common structure H-Xaa-Pro-4-NA (Xaa = serine, threonine and tyrosine) and seven tripeptide rho-nitroanilides of the common structure H-Gly-Xaa-Pro-4-NA (Xaa = serine or threonine) were prepared and analyzed as substrates of the proline-specific peptidases dipeptidyl peptidase IV and prolyl endopeptidase, respectively. The side chains of the hydroxy amino acids were synthetically modified by various acyl-, benzyl- and phosphate residues. The presence of aliphatic or aromatic residues attached to the side chain of the P2-hydroxy amino acids resulted in no significant change of the specificity constants of the enzyme-catalyzed substrate hydrolysis. In some cases, however, substrate inhibition was observed. In contrast, the reactivity of dipeptidyl peptidase IV and prolyl endopeptidase decreases more than two orders of magnitude towards the phosphorylated di- and tripeptide substrates compared to the hydrolysis of unmodified substrates. The kinetic data obtained with the model compounds suggest that side-chain modification of proline-containing peptide substrates may influence their resistance towards the hydrolytic activity of proline-specific hydrolases. Additionally, the results support that structural changes of the substrate during enzyme-hydrolysis may be involved in the mechanism of action of proline-specific serine peptidases. From this result we speculate that posttranslational phosphorylation of peptide sequences found in protein kinase recognition motifs such as -Xaa-Ser/Thr-Pro-Yaa- and -Xaa-Pro-Ser/Thr-Yaa- may serve as structural determinants that modulate their proteolytic stability.

Alcohol modulation of human normal T-cell activation, maturation, and migration

We are interested in the characterization of the effects of alcohol on human T-cell activation, maturation, and migration, because this cell population is crucial in the initiation, regulation, and propagation of cellular immunity. We and others have described the effects of both acute and chronic exposure of human immune cells to ethanol (EtOH) in vitro. Herein, we briefly, review these reports and expand this body of literature with the inclusion of new data recently obtained in our laboratory. We confirm the blunting effects of EtOH on the production of interleukin-2 and mitogen proliferative response following T-cell mitogen stimulation, and on the expression of membrane markers of activation. We show that EtOH significantly alters the expression of the CD4 cell-associated marker of activation, CD26. We report the effect of EtOH on the expression of the homing receptor CD62L by CD4+ cells, and on their ability to adhere by a CD18-mediated process to a defined cellular substratum. Furthermore, we demonstrate the effects of EtOH and EtOH and beta-endorphin pretreatment on the activation of CD4+ lymphocytes endowed with the homing receptor CD62L.

The cytoplasmic tail of the T cell receptor zeta chain is required for signaling via CD26

The protease dipeptidylpeptidase IV (CD26) provides an alternative activation pathway for T lymphocytes and is involved in several aspects of T cell function. Activation via CD26 requires the expression of the T cell receptor (TcR)/CD3 complex. Here we have investigated the role of the TcR zeta chain for T cell activation via CD26. T cell hybridomas expressing TcR with various deletions in the CD3 zeta chain were transfected with a CD26 cDNA and the response of the transfected cells to anti-CD26 monoclonal antibodies was tested. Our data show that the zeta chain is essential and that at least one YXXL motif in the cytoplasmic tail of the zeta chain is required for CD26-mediated signaling. Other TcR components do not replace the zeta chain.

CD26: a surface protease involved in T-cell activation

CD26 is a proteolytic enzyme (dipeptidylpeptidase IV) with a wide tissue distribution and a unique specificity. Recent developments indicate that CD26 is a multifunctional molecule that may have important functions in the immune system. Here, Bernhard Fleischer reviews the current knowledge of CD26 and discusses the possible functions of this molecule in T lymphocytes.