Identification and characterization of human DPP9, a novel homologue of dipeptidyl peptidase IV

We used an in silico approach to identify new cDNAs with homology to dipeptidyl peptidase IV (DPP IV). DPP IV (EC 3.4.14.5) is a serine protease with a rare enzyme activity having an important role in the regulation of various processes, such as blood glucose control and immune responses. Here, we report the identification and characterization of a novel DPP IV-like molecule, termed dipeptidyl peptidase-like protein 9 (DPP9). The deduced amino acid sequence of DPP9 has a serine protease motif, GWSYG, identical to that found in DPP IV. The presence of this motif, together with a conserved order and spacing of the Ser, Asp, and His residues that form the catalytic triad in DPP IV, places DPP9 in the "DPP IV gene family". Northern blots showed that DPP9 is ubiquitously expressed, with the highest expression levels in skeletal muscle, heart, and liver, and the lowest in brain. In vitro translation of the cloned full-length DPP9 sequence resulted in a DPP9 protein product that migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis at a position similar to the predicted protein size of 98 kDa. Consistent with the lack of predicted transmembrane domains and a signal sequence, DPP9 was found in a soluble, putative cytosolic form. A DPP9 orthologue in mice was identified by expressed sequence tag database searches and verified by cDNA cloning.     

Characterization and cDNA cloning of dipeptidyl peptidase IV from the venom of Gloydius blomhoffi brevicaudus

Dipeptidyl peptidase activity was investigated in snake venoms from Gloydius blomhoffi brevicaudus, Gloydius halys blomhoffii, Trimeresurus flavoviridis and Crotalus atrox. The strongest dipeptidyl peptidase IV (DPP IV) activity was found in venom from G. blomhoffi brevicaudus. The substrate specificity, susceptibility to inhibitors, and pH optimum of the partially purified enzyme were similar to those of known DPP IVs from bacteria and eukaryotes. The G. blomhoffi brevicaudus venom gland cDNA library was screened to isolate cDNA clones using probes based on amino acid sequences highly conserved in known DPP IVs. Two cDNA species encoding DPP IV were obtained, and designated as DPP IVa and DPP IVb. This is the first study to report the primary structure of DPP IV from a reptile. The deduced amino acid sequences for DPP IVa and DPP IVb both consist of 751amino acid residues and are highly homologous to each other. A putative catalytic triad for serine proteases, Ser-616, Asp-694, and His-726, is present. It is of particular interest that the deduced NH(2)-terminal sequence associated with the characteristic signal peptide is identical to that determined from the purified DPP IV. This indicates that the signal peptide of snake venom DPP IV is not cleaved off during biosynthesis, unlike those of other snake venom proteins.     

Development of monoclonal antibodies against different protein and carbohydrate epitopes of dipeptidyl peptidase IV from rat liver plasma membranes.

Dipeptidyl peptidase IV (DPP IV) is a serine exopeptidase expressed at high levels in rat kidney, liver and lung. We established eight monoclonal antibodies against partially purified DPP IV from rat liver plasma membranes. By means of a competitive dot blot assay with purified DPP IV, these antibodies were shown to recognize four different epitopes of the glycoprotein, designated A - D. The epitopes are located on the extracellular domain of DPP IV, as shown by papain digestion of liver plasma membranes. Treatment of DPP IV with neuraminidase and glycopeptide N-glycosidase F, as well as incubation of hepatocytes with the alpha-mannosidase I inhibitor deoxymannojirimycin, revealed that epitope A may be formed by a mannose-rich sugar chain and epitope D might represent a complex carbohydrate structure in the mature glycoprotein, while the epitopes B and C are formed by the protein moiety. Concanavalin A reduced the binding of monoclonal antibody to epitope A by 78%. Binding to epitope D was blocked by 73% with wheat germ lectin, and by more than 99% with sialic acid; epitopes B and C were unaffected by any of the lectins or sugars tested. The immunological cross-reactivity with DPP IV from Morris hepatoma 7777 was demonstrated with monoclonal antibodies against epitopes A-C. Epitope D was not recognized on hepatoma DPP IV. However, in addition to DPP IV, four hepatoma plasma membrane glycoproteins were precipitated by the monoclonal antibody against the epitope D, indicating that this epitope is not uniquely restricted to DPP IV.     

Characterisation of the <Emphasis Type="Italic">Aspergillus niger dapB</Emphasis> gene,which encodes a novel fungal type IV dipeptidyl aminopeptidase

We have cloned the Aspergillus niger dapB gene. Analysis of its nucleotide sequence and the corresponding protein sequence indicates that the gene encodes a type IV dipeptidyl aminopeptidase (DPP IV). Based upon its deduced sequence we predict the presence of a transmembrane domain in the protein. Furthermore, dapB-overexpressing transformants display an increase in intracellular DPP IV activity. This is the first reported characterisation of a dipeptidyl aminopeptidase with a transmembrane domain from a filamentous fungus. Using the dapB sequence as a query, we were able to identify 14 DPP IV-encoding genes, and 12 additional DPPIV proteases in public genomic databases. Phylogenetic analysis reveals that in yeasts there are two clades of genes that encode DPP IV proteases with a transmembrane domain. In this study we demonstrate that, as in yeasts, two classes of DPP IV-encoding genes exist in filamentous fungi. However, only one of these codes for DPP IV proteases with a transmembrane domain. The second type present in filamentous fungi encodes extracellular DPP IV proteases. The dapB gene belongs to the first cluster. We propose that DapB plays a role in the proteolytic maturation of enzymes produced by A. niger.Electronic Supplementary Material Supplementary material is available for this article at     

The cytoplasmic/transmembrane domain of dipeptidyl peptidase IV, a type II glycoprotein, contains an apical targeting signal that does not specifically interact with lipid rafts

We investigated the signals involved in the apical targeting of dipeptidyl peptidase IV (DPP IV/CD26), an archetypal type II transmembrane glycoprotein. A secretory construct, corresponding to the DPP IV ectodomain, was first stably expressed in both the enterocytic-like cell line Caco-2 and the epithelial kidney MDCK cells. Most of the secretory form of the protein was delivered apically in MDCK cells, whereas secretion was 60% basolateral in Caco-2 cells, indicating that DPP IV ectodomain targeting is cell-type-dependent. A chimera (CTM-GFP) containing only the cytoplasmic and transmembrane domains of mouse DPP IV plus the green fluorescent protein was then studied. In both cell lines, this chimera was preferentially expressed at the apical membrane. By contrast, a secretory form of GFP was randomly secreted, indicating that GFP by itself does not contain cryptic targeting information. Comparison of the sequence of the transmembrane domain of DPP IV and several other apically targeted proteins does not show any consensus, suggesting that the apical targeting signal may be conformational. Neither the DPP IV nor the CTM-GFP chimera was enriched in lipid rafts. Together these results indicate that, besides the well-known raft-dependent apical targeting pathway, the fate of the CTM domain of DPP IV may reveal a new raft-independent apical pathway.     

A collagen-binding glycoprotein on the surface of mouse fibroblasts is identified as dipeptidyl peptidase IV.

A dipeptidyl aminopeptidase (DPP) was detected in plasma membranes from normal (3T3) and transformed (3T12) mouse fibroblasts. This enzyme was active in cleaving the prolyl bond in the synthetic dipeptide nitroanilide Gly-Pro-NH-Np, which is a specific substrate for DPP IV (Km 0.63 mM and Vmax 6.1 nmol/min per mg at pH 6.0 and 37 degrees C). However, it did not degrade Pro-NH-Np or other dipeptide nitroanilides such as Gly-Arg-NH-Np or Val-Ala-NH-Np. The enzyme was totally inhibited by di-isopropyl phosphorofluoridate (Pri2-P-F) and by phenylmethanesulphonyl fluoride, indicating a serine catalytic site for the proteinase. DPP IV is a glycoprotein that specifically recognized immobilized gelatin and type I collagen. Upon molecular exclusion chromatography, the proteinase exhibited an apparent Mr of 100,000. SDS/polyacrylamide-gel electrophoresis under non-reducing and reducing conditions revealed that the [3H]Pri2-P-protein was exclusively represented by a polypeptide of Mr 55,000. This suggested that DPP IV consists of two non-covalently linked 55,000-Mr subunits. Fibroblast adhesion to native or denatured collagen was significantly inhibited by the two dipeptide inhibitors of DPP IV, Gly-Pro-Ala and Ala-Pro-Gly, but not by the peptides Gly-Pro and Gly-Gly-Gly, which are not inhibitors of the proteinase. Moreover, preliminary fractionation of DPP IV by molecular exclusion chromatography and affinity chromatography indicated that this material was active in disrupting cell adhesion to collagens. Taken together, the above data suggest that a fibroblast membrane-associated collagen-binding glycoprotein, DPP IV, may play a role in cell attachment to collagen.     

Diprolyl nitriles as potent dipeptidyl peptidase IV inhibitors

Dipeptidyl peptidase IV (DPP4) is a multifunctional type II transmembrane serine peptidase which regulates various physiological processes, most notably plasma glucose homeostasis by cleaving peptide hormones glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide. Inhibition of DPP4 is a potentially valuable therapy for type 2 diabetes. Synthesis and structure-activity relationships of a series of substituted diprolyl nitriles are described, leading to the identification of compound 1 with a measured DPP4 K(i) of 3.6 nM.     

Characterization of specific proteases associated with the surface of human skin fibroblasts, and their modulation in pathology.

Human skin fibroblasts were probed for cell surface protease activity. One activity removing dipeptides from the NH2-terminal end of Gly-Pro-pNA was specifically inhibited by di-isopropyl-fluorophosphate (DFP), phenylmethanesulphony fluoride (PMSF), and diprotin A, and thus was identified as dipeptidyl peptidase IV (DPP IV). A group of bestatin-sensitive N-exoaminopeptidase activities was also characterized when Ala-, Leu-, and Arg-pNA were used as chromogenic substrates. Using human monoclonal antibodies anti-CD 13 and anti-CD 26 that recognized, respectively, an N-Ala-aminopeptidase and DPP IV, it was found that human dermal fibroblasts expressed the CD 13 and CD 26 antigen on their surface. In addition, both peptidases were specifically immunoprecipitated by monoclonal antibodies anti-CD 13 and anti-CD 26 from plasma membranes. Cell surface proteolytic activities were also investigated in human fibroblasts derived from dermatological and rheumatic diseases (i.e., psoriasis, rheumatoid arthritis, and lichen planus). It was found that these fibroblasts also expressed both types of proteinases initially identified on normal skin fibroblasts and that the levels of Ala-aminopeptidase activities were similar in all cases. In contrast, the levels of Arg-, Leu-exoaminopeptidase, and DPP IV activities were significantly higher (up to 6.6-fold) in the three pathological fibroblast populations than in their normal counterparts. These proteolytic enzymes, therefore, can potentially serve as markers in dermatological diseases. Taken together, our results suggest that skin fibroblast-derived proteinases associated with both serine and N-aminopeptidase activities may play an important role by participating in the extracellular events associated with fibroblast behaviour.     

Two highly conserved glutamic acid residues in the predicted beta propeller domain of dipeptidyl peptidase IV are required for its enzyme activity

Dipeptidyl peptidase IV (DPP IV) is a member of the prolyl oligopeptidase family and modifies the biological activities of certain chemokines and neuropeptides by cleaving their N-terminal dipeptides. This paper reports the identification and possible significance of a novel conserved sequence motif Asp-Trp-(Val/Ile/Leu)-Tyr-Glu-Glu-Glu (DW(V/I/L)YEEE) in the predicted beta propeller domain of the DPP IV-like gene family. Single amino acid point mutations in this motif identified two glutamates, at positions 205 and 206, as essential for the enzyme activity of human DPP IV. This observation suggests a novel role in proteolysis for residues of DPP IV distant from the Ser-Asp-His catalytic triad.     

Evidence that thymocyte-activating molecule is mouse CD26 (dipeptidyl peptidase IV).

We previously described a developmentally regulated, Mr 115,000 (reduced) and 110,000/128,000 (nonreduced) mouse T cell-activating molecule (THAM) also expressed on a variety of epithelial cell surfaces, and associated with neutral exoaminopeptidase activity. In the present study, we show that THAM is the mouse counterpart of the human T cell-activating ectoenzyme CD26 (dipeptidyl peptidase IV, DPP IV) and that highly purified THAM lacks neutral exoaminopeptidase activity. This conclusion is based on the following: 1) the N-terminal segments of the THAM Mr 110,000 and 128,000 components shared the same amino acid sequence with the rat DPP IV. These N-termini comprised a short intracytoplasmic tail of six residues followed by a downstream hydrophobic transmembrane segment. 2) THAM-specific mAb H194-112-Affi-Gel immunoadsorbent was capable of removing DPP IV enzymatic activity from mouse thymoma cell detergent extracts. 3) H194-112 reactivity pattern on developing thymocytes was found to parallel that previously reported for membrane-bound DPP IV enzymatic activity. The extent of THAM N-glycosylation, as measured by N-glycanase treatment of H194-112 immunoprecipitates, was found to be similar to that of human and rat DPP IV (i.e., approximately 20 kDa). Cross-linking experiments indicated that THAM was expressed at the cell surface as a dimer of approximately 220 kDa. Its two subunits were found to be structurally related but not identical as shown by their different Mr under nonreducing conditions and by their slightly distinct peptide profiles after proteolytic cleavage. We conclude from these data that DPP IV, in addition to its extracellular matrix receptor and ectoenzymatic functions, is a T cell-activating structure in both human and mouse species.     

The expression of T-cell surface antigens CTLA-4, CD26, and CD28 is modulated by inhibition of dipeptidylpeptidase IV (DPP IV,CD26) activity in murine stress-induced abortions

Inhibition of DPP IV has been shown to abrogate the stress-related increase in murine abortions and a concomitant increase in gamma-interferon. The aim of the present study was to investigate a potential impact of the DPP IV inhibitor Isoleucine Cyanopyrrolidide on the expression of surface antigens involved in T-cell responses. DBA/2-mated CBA mice were stressed on day 5.5 of pregnancy and received injections of a DPP IV inhibitor. On day 13 of gestation, the animals were sacrificed and the percentage of abortions was determined. As shown before, stress failed to boost the abortion rate in mice receiving the DPP IV inhibitor. In stressed animals, a lower surface density of CTLA-4 on decidual CD26-positive lymphocytes was observed than in non-stressed animals. Inhibition of DPP IV restored CTLA-4 surface density to normal and decreased surface expression of CD26 and CD28 on decidual lymphocytes irrespective of stress exposure. These observations suggest that a modulation of T-cell surface antigens expression due to inhibition of DPP IV activity may contribute to the potent anti-abortogenic effect observed here.     

Heparan sulfate/heparin oligosaccharides protect stromal cell-derived factor-1 (SDF-1)/CXCL12 against proteolysis induced by CD26/dipeptidyl peptidase IV

Stromal cell-derived factor-1 (SDF-1) is a CXC chemokine that is constitutively expressed in most tissues and displayed on the cell surface in association with heparan sulfate (HS). Its numerous biological effects are mediated by a specific G protein-coupled receptor, CXCR4. A number of cells inactivate SDF-1 by specific processing of the N-terminal domain of the chemokine. In particular, CD26/dipeptidyl peptidase IV (DPP IV), a serine protease that co-distributes with CXCR4 at the cell surface, mediates the selective removal of the N-terminal dipeptide of SDF-1. We report here that heparin and HS specifically prevent the processing of SDF-1 by DPP IV expressed by Caco-2 cells. The level of processing increases with the level of differentiation of these cells, which correlates with an increase of DPP IV activity. A mutant SDF-1 that does not interact with HS is readily cleaved by DPP IV, a process that is not inhibited by HS, demonstrating that a productive interaction between HS and SDF-1 is required for the protection to take place. Moreover, we found that protection depends on the degree of polymerization of the HS sulfated S-domains. Finally a structural model of SDF-1, in complex with HS oligosaccharides of defined length, rationalizes the experimental data. The mechanisms by which HS regulates SDF-1 may thus include, in addition to its ability to locally concentrate the chemokine at the cell surface, a control of selective protease cleavage events that directly affect the chemokine activity.     

Dipeptidyl peptidase III from rat liver cytosol: purification, molecular cloning and immunohistochemical localization

Dipeptidyl peptidase III (DPP III) was purified to homogeneity from rat liver cytosol. The calculated molecular weight of the purified enzyme was 82845.6 according to TOF-MS and 82000 on non-denaturing PAGE, and 82000 on SDS-PAGE in the absence or presence of beta-mercaptoethanol. These findings suggest that the enzyme exists in a monomeric form in rat liver cytosol. The enzyme rapidly hydrolyzed the substrate Arg-Arg-MCA and moderately hydrolyzed Gly-Arg-MCA in the pH range of 7.5 to 9.5. The Km, k(cat) and k(cat)/Km values of DPP III at optimal pH (pH 8.5) were 290 microM, 18.0 s(-1) and 62.1 s(-1) x nM(-1) for Arg-Arg-MCA and 125 microM, 4.53 s(-1) and 36.2 s(-1) x nM(-1) for Ala-Arg-MCA, respectively. DPP III was potently inhibited by EDTA, 1,10-phenanthroline, DFP, PCMBS and NEM. These findings suggest that DPP III is an exo-type peptidase with characteristics of a metallo- and serine peptidase. For further information on the molecular structure, we screened a rat liver cDNA library using affinity-purified anti-rat DPP III rabbit IgG antibodies, determined the cDNA structure and deduced the amino acid sequence. The cDNA, designated as lambdaRDIII-11, is composed of 2640 bp and encodes 738 amino acids in the coding region. Although the enzyme has a novel zinc-binding motif, HEXXXH, DPP III is thought to belong to family 1 in clan MA in the metalloprotease kingdom. The DPP III antigen was detected in significant amounts in the cytosol of various rat tissues by immunohistochemical examination.     

Histochemistry of some proteases in the normal rabbit, pig and ox corneas

The distribution of activities of membrane aminopeptidases (aminopeptidase M (APM), aminopeptidase A (APA), dipeptidyl peptidase IV (DPP IV), gamma-glutamyltransferase (GGT) and lysosomal exopeptidases (dipeptidyl peptidase I (DPP I), dipeptidyl peptidase II (DPP II)) was investigated in rabbit, ox and pig corneas. Cryostat sections of snap-frozen corneas treated with chloroform-acetone (4 degrees C) were used for the demonstration of membrane-bound enzymes and sections of corneas fixed in 4% paraformaldehyde (4 degrees C) for the demonstration of lysosomal enzymes. In activities of proteases species differences were found. The rabbit cornea was most active, followed by ox and pig corneas. Individual corneal layers reacted differently. Of membrane proteases a high APM activity was found in keratocytes, whereas epithelium and endothelium were negative. On the other hand, APA and GGT were active in the epithelium and endothelium. Their activities in keratocytes were less pronounced. DPP IV activity was demonstrated in some keratocytes beneath the epithelium only. Lysosomal enzymes DPP I and DPP II were active in all corneal layers. The epithelium displayed the highest activity. Differences in activities in the centro-peripheral and epithelio-endothelial directions were found. DPP I, DPP II, and APM were most active in the limbal region in all corneal layers.     

Insulin-dependent phosphorylation of DPP IV in liver. Evidence for a role of compartmentalized c-Src

Dipeptidyl peptidase IV (DPP IV, CD26, EC 3.4.14.5) serves as a model aimed at elucidating protein sorting signals. We identify here, by MS, several tyrosine-phosphorylated proteins in a rat liver Golgi/endosome (G/E) fraction including DPP IV. We show that a pool of DPP IV is tyrosine-phosphorylated. Maximal phosphorylation was observed after 2 min following intravenous insulin injection. DPP IV coimmunoprecipitated with the cellular tyrosine kinase Src (c-Src) with maximal association also observed after 2 min following insulin injection. DPP IV was found phosphorylated after incubation of nonsolubilized G/E membranes with [gamma-32P]ATP. The c-Src inhibitor PP2 inhibited DPP IV phosphorylation. Oriented proteolysis experiments indicate that a large pool of c-Src is protected in G/E fractions. Following injection of the protein-tyrosine phosphatase inhibitor bpV(phen), DPP IV levels markedly decreased by 40% both in plasma membrane and G/E fractions. In the fraction designated Lh, DPP IV levels decreased by 50% 15 min following insulin injection. Therefore, a pool of DPP IV is tyrosine-phosphorylated in an insulin-dependent manner. The results suggest the presence of a yet to be characterized signalling mechanism whereby DPP IV has access to c-Src-containing signalling platforms.     

Biosynthesis and metabolism of dipeptidylpeptidase IV in primary cultured rat hepatocytes and Morris hepatoma 7777 cells.

N-Glycosylation, biosynthesis and degradation of dipeptidylpeptidase IV (EC 3.4.14.5) (DPP IV) were comparatively studied in primary cultured rat hepatocytes and Morris hepatoma 7777 cells (MH 7777 cells). DPP IV had a molecular mass of 105 kDa in rat hepatocytes and of 103 kDa in MH 7777 cells as assessed by SDS/PAGE under reducing conditions. This difference in molecular mass was caused by differences in covalently attached N-glycans. DPP IV from hepatoma cells contained a higher proportion of N-glycans of the oligomannosidic or hybrid type and therefore migrated at a slightly lower molecular mass. In both cell types DPP IV was initially synthesized as a 97-kDa precursor which was completely susceptible to digestion with endo-beta-N-acetylglucosaminidase H converting the molecular mass to 84 kDa. The precursor was processed to the mature forms of DPP IV, glycosylated with N-glycans mainly of the complex type with a half-life of 20-25 min. The transit of newly synthesized DPP IV to the cell surface displayed identical or very similar kinetics in both cell types with the major portion of DPP IV appearing at the cell surface after 60 min. DPP IV molecules were very slowly degraded in hepatocytes as well as in hepatoma cells with half-lives of approximately 45 h. Inhibition of oligosaccharide processing with 1-deoxymannojirimycin led to the formation of DPP IV molecules containing N-glycans of the oligomannosidic type. This glycosylation variant was degraded with the same half-life as complex-type glycosylated DPP IV. By contrast, inhibition of N-glycosylation with tunicamycin resulted into rapid degradation of non-N-glycosylated DPP IV molecules in both cell types. Non-N-glycosylated DPP IV could not be detected at the cell surface indicating an intracellular proteolytic process soon after biosynthesis.     

Expression and spatial heterogeneity of dipeptidyl peptidases in endothelial cells of conduct vessels and capillaries

Dipeptidyl peptidase IV (DPPIV)/CD26 is by far the most extensively studied member of the prolyl oligopeptidase family of serine proteases. The discovery of the related enzymes DPP8 and DPP9 necessitates a (re-)evaluation of the DPPIV-like enzymatic activity in cells and organs. In this study, we aimed (1) to investigate the expression of the individual dipeptidyl peptidases in different types of endothelial cells (ECs) and (2) to reconsider published data in relation to our findings. Examination of DPP expression in rat primary ECs of aortic, endocardial and cardiac microvascular origin revealed the presence of DPPIV-like activity in all cell lysates. More than half of this activity could be attributed to DPP8/9. Western blot analysis revealed an abundance of the DPP8 protein as compared to DPP9. The expression of DPPIV and DPP8 was significantly higher in the cardiac microvascular endothelium than in the other ECs, suggesting a more pronounced role of these DPPs in the microvasculature. In situ, DPP activity in ventricular microvasculature was completely inhibited by sitagliptin, indicating that DPPIV is the predominant DPPIV-like enzyme in this organ. By contrast, immunohistochemical studies indicated DPP9 as the predominant DPP in human carotid artery ECs. In conclusion, our results support a highly regulated expression of individual DPPs in ECs, with a spatial heterogeneity in the cardiovascular tree.     

CD26 and DPP IV expression in T acute lymphoblastic leukemia cells: immunocytochemistry and enzyme cytochemistry

Recent studies have revealed the dipeptidyl peptidase IV (DPP IV) enzymatic activity of CD26 antigen. In this paper, the possible identity of DPP IV and CD26 expression in phenotypically defined T-ALL has been examined. The combination of enzyme cytochemistry and immunocytochemistry was used. The correlation between the CD26 antigen expression and DPP IV positivity in the vast majority of T lymphoblasts in T-ALL patients was observed. No CD26 was expressed on DPP IV negative T cells. The variable CD4 and/or CD8 antigen expression, frequent CD7 positivity and absence of membrane CD3 antigen expression were the characteristic immunophenotypic features of CD26/DPP IV positive T cells. CD26/DPP IV activity strongly paralleled the CD71 antigen (transferrin receptor, T cell activation/proliferation antigen) expression. The phenotypic features of CD26/DPP IV positive T cells are characteristic for the relative immature cell population. Noteworthy was the slight disassociation between the very high CD26 antigen J expression and moderate DPP IV activity in cells of some T-ALL patients. The possible existence of enzymatically inactive structures of CD26 antigen or inactive precursors of DPP IV detectable only by immunocytochemistry was discussed. Our study indicates that CD26 antigen expression is tended to identify cells with DPP IV enzymatic activity in T-ALL patients. The results provide information of CD26 antigen possible involvement in the pathology of leukemic cells via its DPP IV enzyme activity.