Arch Dermatol Res (2008) 300:561–567
DOI 10.1007/s00403-008-0862-1
ORIGINAL PAPER
Distribution of dipeptidyl-peptidase IV on keratinocytes in the margin zone of a psoriatic lesion: a comparison with hyperproliferation and aberrant diVerentiation markers
R. G. van Lingen · M. K. P. Poll · M. M. B. Seyger · E. M. G. J. de Jong · P. C. M. van de Kerkhof · P. E. J. van Erp
Received: 15 January 2008 / Revised: 3 April 2008 / Accepted: 28 April 2008 / Published online: 22 May 2008© The Author(s) 2008
Abstract The inXammation process in psoriatic skin ischaracterized by inXux of leukocytes, hyperproliferationand aberrant diVerentiation of keratinocytes regulated viacytokines. Dipeptidyl-peptidase IV (DPPIV) is known to beupregulated on keratinocytes in the psoriatic lesion. Theobjective was to gain insight into dynamics of DPPIVexpression and enzyme activity together with keratinocyteproliferation and diVerentiation markers during develop-ment of a psoriatic lesion, in order to investigate coherencein mechanisms behind the upregulation of DPPIV in psori-atic skin. The expression of DPPIV, Ki-67 antigen and ker-atin-16 (K16) was studied in the dynamic margin zone ofthe psoriatic lesion, examining skin sections of the clini-cally uninvolved skin, the early lesion and the chroniclesion of psoriatic patients compared to healthy volunteersusing immunohistochemical and enzymehistochemicalstaining methods. DPPIV-expression and enzyme activity,Ki-67 antigen and K16 are signiWcantly upregulated in thecentre and inner margin of the lesion compared to clinicallyuninvolved skin and the healthy volunteers skin. Mutuallybetween the centre and inner margin, this upregulation didnot diVer signiWcantly. The clinical symptomless skinproved to have signiWcantly elevated DPPIV enzyme activ-ity compared to the skin of healthy volunteers. We demon-strate that DPPIV is expressed and enzymatically activewell before the development of an overt psoriatic lesion.The abnormal DPPIV distribution in psoriatic skin does notcoincide with known markers of aberrant growth and diVer-
entiation of keratinocytes, which makes DPPIV (expressionand enzyme activity) a marker standing on its own.
Keywords Psoriasis · Dipeptidyl peptidase IV · Keratin 16 · Ki-67 · CD26
Introduction
Psoriasis is considered to be an organ-speciWc autoimmunedisease, caused by a combination of genetic and environ-mental factors and is triggered and maintained by an acti-vated cellular immune system. Its pathology is mainlycharacterized by cutaneous inXammatory inXux of leuko-cytes and hyperproliferation and aberrant diVerentiation ofkeratinocytes. Within the psoriatic lesion, there is a mutualinteraction between these keratinocytes and mononuclearleukocytes by means of a third involved party, the cyto-kines [5, 11]. The chemical modiWcation of cytokines andhence the modiWcation of their physiological function is ofmajor importance to understand their actual role in thepathophysiology of the psoriatic process.
Dipeptidyl peptidase IV (CD26) is a 110-kDa mem-brane-anchored glycoprotein with a variety of functions [1,4]. In general, it is widely distributed throughout the humanbody and varies in density and form between variousorgans and cell types. Its proteolytic properties enable it todirectly modify substrates such as cytokines in their naturalin vivo form, (e.g., CXCL12, CCL5, CXCL11, CXCL10,GLP-1/2) [1, 4, 7, 14].
In human skin, DPPIV is known to be expressed onkeratinocytes and known to be upregulated in psoriasis [4,13, 19, 22]. However, little data are available concerningthe potential functional implications of such an upregula-tion. Selective blockers of DPPIV activity are currently
R. G. van Lingen (&) · M. K. P. Poll · M. M. B. Seyger · E. M. G. J. de Jong · P. C. M. van de Kerkhof · P. E. J. van ErpDepartment of Dermatology, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlandse-mail: [emailprotected]
123
562 Arch Dermatol Res (2008) 300:561–567
being developed as modulators of immunological responses[15]. Recently, it was shown that such an inhibition couldsuppress keratinocyte proliferation in vitro and partiallyrestore keratinocyte diVerentiation in vivo [19]. Mappingthe DPPIV glycoprotein in psoriasis may therefore add toour current knowledge. Considering the common groundsof the three main pathological components of psoriasis (Tcells, keratinocytes and cytokines) and the various proper-ties of DPPIV, it is attractive to speculate that DPPIV mightbe involved in the modulation of the inXammatory processand the keratinocyte abnormalities in psoriasis.
Ensued from this, our goal was to map the epidermalexpression and enzyme activity of DPPIV in relation toknown markers of proliferation (Ki-67) and abnormal mat-uration (Keratin 16) of psoriatic keratinocytes in the epider-mis [10]. Coherence in dynamics of expression of thesethree markers could result in expanding knowledge on theexisting epidermal disbalance in psoriasis. We studied theexpression and enzyme activity of DPPIV in the dynamicmargin zone of the psoriatic lesion in order to investigatewhether DPPIV distribution diVers with the distribution ofK16 and Ki-67 in clinically uninvolved psoriatic skin, inthe early lesion and subsequently in the chronic psoriaticlesion. Accordingly, coherence in dynamics of DPPIV(expression and enzyme activity) with keratinocyte prolif-eration and diVerentiation markers during the developmentof a psoriatic plaque could reveal more insight into themechanisms behind the known upregulation of DPPIV inpsoriatic skin.
Patients and methods
Patients
A total of seven psoriatic patients (mean age § SEM,63 § 3 years) with mild-to-moderate plaque psoriasis [Pso-riasis Area and Severity Index (PASI) ¸2 and ·12] and acontrol group, composed of three healthy volunteers (meanage 52 § 6 years), participated in this study. Patients hadstable plaque-type psoriasis. None of the participants hadused any systemic anti-psoriatic or immune-modulatingtherapy for at least 4 weeks and no topical therapy for atleast 2 weeks prior to the biopsy. The healthy volunteershad no history of dermatological or immunological disease.All participants had given written informed consent beforeenrollment.
Biopsies
From the psoriatic patients three 4-mm punch biopsies wereobtained, respectively, from the centre of the plaque, theinner margin of the plaque comprising the outmost clini-
cally involved area and the clinically uninvolved skin (atleast 2 cm from the lesion). From the healthy volunteersone 4-mm punch biopsy was obtained. All participantsreceived local anesthesia (1% xylocaine/adrenaline) beforethe biopsies were taken. Skin defects were optionallyclosed with one suture. The obtained specimens wereembedded in Tissue-Tek OCT compound (Sakura, Zoeter-woude, Netherlands), snap-frozen in liquid nitrogen andstored at ¡80°C until use.
Histochemical methods
Immunohistochemical procedure
The immunohistochemical staining was performed asdescribed previously [2, 23]. BrieXy, cryostat sections of7 �m were sliced onto (3-amino propyl)triethoxysilane(AAS)-coated slides, air-dried for 10 min and Wxed in coldacetone for 10 min. After blocking for endogenous peroxi-dase for 20 min, sections were incubated with either CD26monoclonal antibody (1:50) (clone BA-5, Santa Cruz Bio-technology, Santa Cruz, FL), Ki-67 (1:25) (clone MIB-1,DAKO, Copenhagen, Denmark) or anti-Keratin-16 (1:20)(clone LL025, Monosan, Uden, Netherlands), diluted in1%BSA/PBS for 1 h. Subsequently, sections underwentsecondary incubation with IgG-labelled polymer, horserad-ish peroxidase antimouse-antibody EnVision+ (DAKO,Copenhagen, Denmark) for 30 min. As a coloring agentaminoethylcarbazole + high-sensitivity substrate chromo-gen (DAKO, Copenhagen, Denmark) was used for 10 minat 37°C. Before and after each incubation, sections werewashed with PBS for 15 min. All incubations were per-formed in a dark, humid environment. Counterstaining wasperformed with Mayer’s hematoxylin. Subsequently, thesections were mounted in glycerol gelatin.
Enzymehistochemical procedure
The demonstration of DPPIV activity was achieved usingthe method, described by Khalaf et al. [9] with some minormodiWcations [22]. The frozen biopsies were sliced into 10-�m sections on AAS-coated slides. These were air-dried for20 min and Wxed in cold acetone for 10 min. For the incu-bation mixture, 3 mg of glycyl-propyl-4-methoxy-2-naph-thylamide (GPMN) (Bachem, Bubendorf, Switzerland) wasdissolved in 0.2 ml dimethylformamide. This substrate wasadded to 4.6 ml of PBS and subsequently mixed with 5 mgof fast blue B salt (FBB) (Sigma–Alderich, St. Louis, MO)dissolved in 0.2 ml dimethylformamide and Wltered. Thesamples were incubated in a humid, dark chamber at roomtemperature for 45 min with 200 �l of the incubation mix-ture added to each section. After incubation the slides werewashed with distilled water, brieXy counterstained with
123
Arch Dermatol Res (2008) 300:561–567 563
Mayer’s hematoxylin and immersed for 30 s in alkaline tapwater. After rinsing with distilled water, they were mountedin glycerol gelatin.
Negative and positive controls
In the immunohistochemical procedure, as a negative con-trol the primary antibody was omitted. As a negative con-trol in the enzymehistochemical procedure, inhibition ofDPPIV activity was accomplished by pretreating the slideswith 200 �l of 5 mM Diprotin A (Peptides International,Louisville, KY) for 15 min at 37°C [3]. As positive control,murine kidney tissue was used in the enzymehistochemicalprocedure [18, 21].
Image analysis
QuantiWcation
The immunohistochemical and enzymehistochemicalDPPIV-stained sections were analyzed using transmissionlight microscopy with an objective magniWcation of 200£.Per patient, three of the diVerent biopsy sections (center,inner margin, clinically uninvolved psoriatic skin) wereanalyzed for the full width of the section. For quantiWcationof the epidermal DPPIV expression and enzyme activity,the following semi-quantitative Wve-point scale was used:0 = no staining, 1 = slight staining, 2 = moderate staining,3 = moderately pronounced staining, 4 = pronounced stain-ing.
In order to analyze K16 and Ki-67 positive cells, digitalphotographs were made at 100£ magniWcation. Each pho-tograph was analyzed using IP-lab software (Scanalytics,USA). For quantiWcation of the number of cells positive forKi-67 and the percentage epidermal area positive for K16, adeWned window was set in the epidermis as the region ofinterest (ROI) which was representative for the whole sec-tion. The total ROI and the K16-positive area within theROI were measured. The K16 value was expressed as per-centage K16-positive area of the ROI (%). The number ofKi-67 positive cells was determined by counting the Ki-67positive nuclei in a representative epidermal ROI. A line,following the stratum basale of the ROI, was set out andmeasured. The number of Ki-67 positive nuclei wasexpressed as positive cells per mm length of basem*ntmembrane.
Statistical analysis
Results are presented as means §SD of the mean. All anal-yses were carried out using Statistica® statistical software,version 7.0. Comparison of the non-parametric immunohis-tochemical and enzymehistochemical DPPIV values for the
plaque centre, inner margin and clinically uninvolved skinwas performed using the Kruskal–Wallis analysis of vari-ance. For analysis of the markers K16 and Ki-67 concern-ing the plaque centre, inner margin and clinicallyuninvolved skin we performed analysis of variance(ANOVA) and if signiWcant, Duncan’s post hoc compari-son was performed. The Student’s t test was carried out tostudy the diVerence in expression of markers between theuninvolved psoriatic skin and the healthy volunteers skin.Correlation was tested with the Spearman’s correlation test.Statistical signiWcance was set at P < 0.05.
Results
Patients
Histologically, the skin of the healthy volunteers was nor-mal (hematoxylin–eosin staining). Seven patients (5 males,2 females) diagnosed with psoriasis participated. In order toassess the severity of the psoriatic lesion used for biopsy,SUM scores (0–12) were determined. SUM score is a psori-atic lesion severity score, comprising erythema (0–4), indu-ration (0–4) and desquamation (0–4) wherein a score of 0represents no psoriasis and 12 the highest possible severityof the plaque. The mean SUM score in this study was5.57 § 0.34.
Epidermal distribution of dipeptidyl-peptidase IV, Ki-67 and Keratin-16 (Fig. 1)
In correspondence with earlier Wndings [22], upregulationof DPPIV on keratinocytes was seen in all of the patients,although to a diVerent extent. In line with these earlier Wnd-ings, the immunohistochemical and the enzymehistochemi-cal staining of DPPIV followed a similar epidermal pattern;focally pronounced patches of staining, mainly displaying ahoneycomb like structure, in psoriatic sections. In those ofthe healthy volunteers this pattern was practically absent. Inbrief, the psoriatic pattern was generally more extensiveand predominantly located para-papillary with an optimumin the epidermal area around the top of the papillae. Usuallythe staining was conWned to the lower two third of the totalepidermis, or more speciWcally to the lower three quartersof the stratum spinosum (Fig. 1). In both healthy and psori-atic skin, no staining in the stratum lucidum was observed.In the psoriatic coupes, additional column-like structurescould regularly be observed.
The DPPIV immunohistochemical expression andenzyme activity in the centre of the plaque, the inner mar-gin and the clinically uninvolved skin showed a comparablepattern with correlations, respectively, of 0.79, 0.85 and0.87 (P<0.02). Considering this comparable pattern, the
123
564 Arch Dermatol Res (2008) 300:561–567
enzyme activity of DPPIV was chosen in this study as a ref-erence for drawing comparisons with the Ki-67 and K16markers as it encompasses more potential to study dynam-ics than the protein expression does. Namely, proteinexpression only conWrms the presence of DPPIV whereasenzyme activity denotes presence and activity of DPPIV.
As far as the proliferation marker Ki-67 is concerned, itpredominantly appeared in the stratum basale in both psori-atic and healthy skin although to a diVerent extent (Fig. 1).Solitary Ki-67 positive cells could be observed sporadicallyin the stratum spinosum of the clinically uninvolved skin ofpsoriatics and the skin of healthy volunteers. Obviously, inthe centre and inner margin of the psoriatic lesion, gener-ally a greater number of Ki-67-positive cells was observedin all patients.
Keratin-16, a diVerentiation marker, only stained posi-tive in the epidermis of the plaque centre and inner marginconWrming the aberrant diVerentiation in these parts asopposed to the clinically uninvolved psoriatic and healthy
skin (Fig. 1). The staining was seen throughout the wholestratum spinosum, with an optimum towards the stratumbasale.
Dynamics in epidermal distribution of dipeptidyl-peptidase IV, Ki-67 and Keratin-16 (Fig. 2)
Throughout the dynamics of the margin zone of the psoriaticlesion, a stronger signiWcant DPPIV enzymatic activity couldbe noticed in both the inner margin and centre of the lesioncompared to the clinically uninvolved psoriatic skin(1.38 § 0.24 and 1.57 § 0.27 vs. 0.62 § 0.15; P < 0.01 andP < 0.05) (Fig. 2). Clinically uninvolved psoriatic skin alsoshowed a signiWcant increase in enzymatic activity comparedto skin of healthy volunteers (P < 0.05). However, whenobserving the expression in the centre and the inner marginmutually, no signiWcant diVerence in expression was detected.
Equal to the DPPIV enzyme activity, Ki-67 antigen wassigniWcantly strongly expressed in both the plaque centre
Fig. 1 Expression of DPPIV protein expression, enzyme activity, Keratin 16 and Ki-67 throughout the margin zone of a psoriatic lesion compared to nor-mal skin. Plaque centre, inner margin, clinically uninvolved skin (UIS): £200 magniWcation. Healthy volunteers skin (HV): £100 magniWcation. Notice-ably, in the skin of healthy vol-unteers the DPPIV enzyme-activity staining creates a brownish background staining
Fig. 2 Mean scores of DPPIV enzyme activity, Ki67 and K16 expression throughout the mar-gin zone. * = signiWcant. (Mean § SEM)
123
Arch Dermatol Res (2008) 300:561–567 565
and the inner margin compared to the clinically uninvolvedpsoriatic skin (172.65 § 16.06 and 167.60 § 13.84 vs.70.44 § 7.97; both P < 0.01) (Fig. 2). The clinically unin-volved psoriatic skin showed a stronger proliferation ten-dency of keratinocytes than the healthy volunteers skin,albeit borderline signiWcant (P = 0.057). However, whenobserving the Ki-67 expression in the centre and the innermargin mutually again, no signiWcant diVerence in expres-sion was detected.
With respect to the expression of K16 in the variousparts of the margin zone, the plaque centre and the innermargin showed signiWcantly more expression of K16 incomparison to the clinically uninvolved psoriatic skin(67.0 § 3.13 and 65.43 § 2.1 vs. 0 § 0) (Fig. 2). This wasin line with the expected expression pattern because inuninvolved psoriatic skin and healthy volunteers skin thereexists a natural absence of K16 expression. Mutually how-ever, the centre and margin did, once more, not diVer sig-niWcantly in K16 expression.
Correlation of epidermal distribution of dipeptidyl-peptidase IV, Ki-67antigen and K16
In general, there were no signiWcant correlations observedbetween enzymatic cytochemical activity of DPPIV andexpression of aberrant proliferation or diVerentiation ofkeratinocytes throughout the dynamics of the margin zone.
Remarkably, the Ki-67 positive cell count and percent-age of K16 positively stained epidermis showed a strongoverall correlation in the psoriatic patients, taking togetherboth the plaque centre and the inner margin (� = 0.88;P < 0.05). Though, when investigating the correlation ofthese markers per locus separately, no signiWcant correla-tion could be observed (plaque centre � = 0.54; inner mar-gin � = 0.3).
Discussion
Dipeptidyl peptidase IV has diverse properties as an ecto-peptidase, enabling it to play a key role in the chronic pso-riatic immune response which is mainly orchestrated by Tcells, cytokines and keratinocytes in psoriasis. Direct evi-dence that DPPIV inhibitors are relevant to keratinocytebiology has been provided by the observation that suchinhibitors can suppress keratinocyte proliferation in vitroand can partially restore keratinocyte diVerentiation invivo [19]. This would imply that DPPIV may aVect andinteract with the aberrant keratinocyte abnormalities inpsoriasis. In line with previous Wndings [22], the presentstudy conWrms a strong upregulation of DPPIV on kerati-nocytes in psoriatic epidermis but additionally reveals itto be continuously present throughout the dynamics of the
margin zone. Moreover, it aimed to shed light on the func-tional implication of this upregulation combining it withthe expression of markers characteristic for proliferation(Ki-67) and diVerentiation (K16) in psoriatic keratino-cytes as coherence in dynamics of expression of thesemarkers could result in creating an epidermal biomarkerin psoriasis.
Topographically, the distribution of the enzyme activityof DPPIV did not coincide entirely with the areas of hyper-proliferation and aberrant diVerentiation of keratinocytes ina psoriatic section which ultimately makes DPPIV expres-sion (enzyme activity and protein expression) an entity initself in psoriasis. The margin zone was previously provento be a suitable model for studying the dynamics of a psori-atic lesion by representing the chronic and the early processand the uninvolved skin [6, 20, 23]. The inner margin waspresumed to represent an early active lesion and thereforereXecting a diVerent level of expression. DPPIV enzymeactivity, Ki-67 and K16, all three show, throughout themargin zone, signiWcant upregulation in the centre andinner margin of the plaque compared to the clinically unin-volved skin and the healthy volunteers skin. However,mutually the upregulation in expression in the centre andinner margin did not diVer signiWcantly for both the Ki-67,K16 marker as well as for DPPIV enzyme activity. Onecould suggest that in this study within a psoriatic lesionthere exists a hom*ogeneous level of keratinocyte prolifera-tion, diVerentiation and DPPIV enzyme activity as partsrepresenting an early (inner margin) or more chronic (cen-ter) lesion do not seem to make a diVerence in expressionfor the markers under study. Moreover, correlation testsapplied to the margin zone in this investigation showed nocoherence in dynamics of epidermal DPPIV enzyme activ-ity, hyperproliferation and aberrant diVerentiation of kerati-nocytes.
The symptomless skin at 2-cm distance from a psoriaticlesion proved to have signiWcantly elevated DPPIV enzymeactivity as compared to skin of healthy volunteers, whereasKi-67 and K16 were essentially slightly present or absent inthe symptomless skin. Although the symptomless skin mayharbor some “preclinical lesions” or can be subclinicallyinvolved, we can conclude that in an early phase of the pso-riatic process well before an overt lesion has developed, anincreased DPPIV enzyme activity is present clearly show-ing that DPPIV is involved in an early phase of the psoriaticcascade [8, 12].
A number of other molecules exhibiting DPPIV-likeenzyme activity have been introduced recently, and termed“DPP-IV activity and/or structure-hom*ologues” (DASH)[17]. This group comprises several proteases including attr-actin, DPP 8 and 9 [16, 17]. DASH have potential abilitiesto complement and/or functionally substitute DPPIV on thelevel of its enzymatic activity. In this light, a marginal note
123
566 Arch Dermatol Res (2008) 300:561–567
has to be made as the DASH might be involved in DPPIV-like processes (as could be the case in the current investiga-tion). Due to the ubiquitous expression pattern and multi-functional nature of DPPIV speciWcally and the majority ofthe DASH in general, it may be diYcult to deduce theselectivity of DPPIV from the DASH group. However, ourresults did not show discrepancies between the immunohis-tochemical and enzymehistochemical staining, whichimplies that the possible presence of DPPIV hom*ologuesdoes not interfere with the interpretation of the data in thecurrent study.
Our goal was to determine whether the distribution ofDPPIV in the epidermis throughout the dynamics of themargin zone, coheres with markers for proliferation anddiVerentiation of keratinocytes in psoriasis in order toexpand knowledge on the existing epidermal dysbalance inpsoriasis. Considering the localization of DPPIV expres-sion and enzyme activity in the epidermis, being primarilyconcentrated towards the (supra) basal layers, it is possibleto presume an active role for DPPIV in the proliferation ofkeratinocytes in the pathogenesis of psoriasis. On the otherhand, considering the functional enzymatic characteristicsof DPPIV, it may well be feasible that the peptidase activityactually restrains the inXammatory disbalance in the psori-atic skin through down-regulation of cytokines secondarilyaVecting the keratinocyte malfunctioning. Therefore, thepsoriatic skin represents an interesting eVector organ forDPPIV inhibitors which could tackle not only keratinocytesand cytokines but also (DPPIV+) T cells, key players in thepsoriatic pathogenesis all sharing common grounds withDPPIV. Future studies should address the eVect of suchinhibitors in psoriasis.
In conclusion, we have demonstrated diVerential expres-sion and activity of DPPIV in normal and diseased skinthroughout the dynamics of the margin zone. In addition,we have shown that DPPIV (protein expression andenzyme activity) is expressed well before the developmentof the overt psoriatic lesion before the accumulation of Tcells and other inXammatory cells. Moreover, the DPPIVenzyme activity does not increase signiWcantly in the morechronic phase of the psoriatic process. Therefore, DPPIV isa consistent feature of the psoriatic lesion. The abnormalenzyme activity in psoriatic diseased skin does not seem tocorrelate with known markers of aberrant growth anddiVerentiation of keratinocytes, which makes DPPIV amarker standing on its own. Future studies should focus onwhether DPPIV can be regarded a useful biomarker fortherapy evaluation.
Open Access This article is distributed under the terms of theCreative Commons Attribution Noncommercial License whichpermits any noncommercial use, distribution, and reproduction in anymedium, provided the original author(s) and source are credited.
References
1. Boonacker E, Van Noorden CJ (2003) The multifunctional ormoonlighting protein CD26/DPPIV. Eur J Cell Biol 82:53–73
2. Bovenschen HJ, Seyger MM, van de Kerkhof PC (2005) Plaquepsoriasis versus atopic dermatitis and lichen planus: a comparisonfor lesional T cell subsets, epidermal proliferation and diVerentia-tion. Br J Dermatol 153:72–78
3. Christopherson KW, Cooper S, Broxmeyer HE (2003) Cell sur-face peptidase CD26/DPPIV mediates G-CSF mobilization ofmouse progenitor cells. Blood 101:4680–4686
4. De Meester I, Korom S, Van Damme J, Scharpe S (1999) CD26,let it cut or cut it down. Immunol Today 20:367–375
5. Gaspari AA (2006) Innate and adaptive immunity and the patho-physiology of psoriasis. J Am Acad Dermatol 54:S67–S80
6. Gerritsen MJ, Elbers ME, de Jong EM, Van de Kerkhof PC (1997)Recruitment of cycling epidermal cells and expression of Wlaggrin,involucrin and tenascin in the margin of the active psoriaticplaque, in the uninvolved skin of psoriatic patients and in the nor-mal healthy skin. J Dermatol Sci 14:179–188
7. Gorrell MD, Gysbers V, McCaughan GW (2001) CD26: a multi-functional integral membrane and secreted protein of activatedlymphocytes. Scand J Immunol 54:249–264
8. Hammar H, Hellerstrom C (1970) The oxygen consumption of thegerminal epithelium in normal and psoriatic skin. Br J Dermatol83:371–375
9. Khalaf MR, Aqel NM, Hayhoe FG (1987) Histochemistry ofdipeptidyl aminopeptidase (DAP) II and IV in reactive lymphoidtissues and malignant lymphoma. J Clin Pathol 40:480–485
10. Kuijpers AL, Van Pelt JP, Bergers M, Boegheim PJ, Den BakkerJE, Siegenthaler G, Van de Kerkhof PC, Schalkwijk J (1998) TheeVects of oral liarozole on epidermal proliferation and diVerentia-tion in severe plaque psoriasis are comparable with those of acitre-tin. Br J Dermatol 139:380–389
11. Lowes MA, Bowco*ck AM, Krueger JG (2007) Pathogenesis andtherapy of psoriasis. Nature 445:866–873
12. Mier PD Cotton DWK (1976) The molecular biology of skin.Blackwell, Oxford
13. Novelli M, Savoia P, Fierro MT, Verrone A, Quaglino P, Bern-engo MG (1996) Keratinocytes express dipeptidyl-peptidase IV(CD26) in benign and malignant skin diseases. Br J Dermatol134:1052–1056
14. Oravecz T, Pall M, Roderiquez G, Gorrell MD, Ditto M, NguyenNY, Boykins R, Unsworth E, Norcross MA (1997) Regulation ofthe receptor speciWcity and function of the chemokine RANTES(regulated on activation, normal T cell expressed and secreted) bydipeptidyl peptidase IV (CD26)-mediated cleavage. J Exp Med186:1865–1872
15. Pratley RE, Salsali A (2007) Inhibition of DPP-4: a new therapeu-tic approach for the treatment of type 2 diabetes. Curr Med ResOpin 23:919–931
16. Schade J, Stephan M, Schmiedl A, Wagner L, Niestroj AJ, De-muth HU, Frerker N, Klemann C, Raber KA, Pabst R, von HörstenS (2008) Regulation of expression and function of dipeptidyl pep-tidase 4 (DP4), DP8/9, and DP10 in allergic responses of the lungin rats. J Histochem Cytochem 56:147–155
17. Sedo A, Malik R (2001) Dipeptidyl peptidase IV-like molecules:hom*ologous proteins or hom*ologous activities? Biochim BiophysActa 1550:107–116
18. Smith RE, Reynolds CJ, Elder EA (1992) The evolution of pro-teinase substrates with special reference to dipeptidylpeptidase IV.Histochem J 24:637–647
19. Thielitz A, Reinhold D, Vetter R, Bank U, Helmuth M, Hartig R,Wrenger S, Wiswedel I, Lendeckel U, Kahne T, Neubert K, FaustJ, Zouboulis CC, Ansorge S, Gollnick H (2007) Inhibitors of
123
Arch Dermatol Res (2008) 300:561–567 567
dipeptidyl peptidase IV and aminopeptidase N target major patho-genetic steps in acne initiation. J Invest Dermatol 127:1042–1051
20. Van de Kerkhof PC, van Rennes H, de Grood R, Bauer FW, MierPD (1983) Metabolic changes at the margin of the spreading pso-riatic lesion. Br J Dermatol 108:647–652
21. van der Velden V, Wierenga-Wolf AF, Adriaansen-Soeting PW,Overbeek SE, Moller GM, Hoogsteden HC, Versnel MA (1998)Expression of aminopeptidase N and dipeptidyl peptidase IV in thehealthy and asthmatic bronchus. Clin Exp Allergy 28:110–120
22. Van Lingen RG, Van de Kerkhof PCM, Seyger MMB, De JongEMGJ, van Rens DWA, Poll MKP, Zeeuwen PLJM, Van Erp PEJ(2008) CD26 / DPPIV in psoriatic skin: Upregulation and topo-graphical changes. Br J Dermatol (In Press, online early)
23. Vissers WH, Arndtz CH, Muys L, Van Erp PE, de Jong EM, Vande Kerkhof PC (2004) Memory eVector (CD45RO+) and cytotoxic(CD8+) T cells appear early in the margin zone of spreading pso-riatic lesions in contrast to cells expressing natural killer receptors,which appear late. Br J Dermatol 150:852–859
123
