Characterization of Lymphocyte-Dependent AngiogenesisUsing a SCID Mouse: Human Skin Model of Psoriasis

Brian J. NickoloffDepartment of Pathology, Skin Cancer Research Laboratories, Cardinal Bernardin Cancer Center, Loyola University Medical Center, Maywood,

Illinois, U.S.A.

From a clinical perspective, angiogenesis is animportant component of acute and chronic psoriaticskin lesions as they are erythematous and display atendency to bleed after super®cial removal of scale.By routine histology, numerous microscopic vascularabnormalities are also present. The structural expan-sion of capillaries and distinctive activated phenotypeof lesional endothelial cells are believed not only tobe clinical and pathologic hallmarks of the disease,but to play a central role in the pathogenesis of psor-iatic plaques. Despite over 20 years of research byleading angiogenesis experts and numerous studies,many details regarding the cellular and molecularbasis for angiogenesis in psoriasis remain unknown.In this review, 10 different sections are presented toupdate recent progress in this active ®eld of investi-gative skin biology. Highlights of this review includethe phenotypic characterization of endothelial cellsin acute and chronic psoriatic plaques, and a reviewof a novel animal model of psoriasis using human

skin engrafted onto severe combined immunode®-cient mice followed by injection of activated immu-nocytes. This new experimental model represents areproducible and pharmacologically validatedmethod to trigger neovascularization and bona ®depsoriatic plaque formation. In addition, the potentialcontribution of epidermal keratinocytes and dermalmacrophages to the angiogenic tissue reaction is pre-sented, and a series of questions are then posed thatcan be answered using the severe combined immu-node®cient mouse model of psoriasis. Finally, amodel is proposed integrating all available data intoa coherent multistep reaction schema that includesactive participation by multiple cell types includingnatural killer T cells, keratinocytes, macrophages,and microvascular endothelial cells. Key words:endothelial cells/immunocytes/macrophages/T cells. Journalof Investigative Dermatology Symposium Proceedings5:67±73, 2000

Psoriasis is a chronic in¯ammatory disease characterizedby prominent epidermal proliferation (Wrone-Smithand Nickoloff, 1996). Using a novel severe combinedimmunode®cient (SCID) mouse:human skin chimericanimal model, it has been established that activated

immunocytes are responsible for triggering the keratinocytehyperplasia producing a thick epidermis with altered differentiationand scale production (Nickoloff et al, 1995; Boehncke et al, 1996;Gilhar et al, 1997; Sugai et al, 1998; Yamamoto et al, 1998). Inaddition to activated T cells and natural killer T cells being involvedin provoking rapid and substantial keratinocyte hyperplasia, theengrafted skin samples also consistently display rather remarkableand rapid new blood vessel formation (i.e., angiogenesis) followinginjection of activated immunocytes (Boehncke et al, 1996;Nickoloff et al, 1999). When engrafted prepsoriatic skin is injectedwith saline, the dermal vascularization in the human dermiscontains only relatively inconspicuous blood vessels (Fig 1A), butwithin 2 weeks of injection of activated immunocytes, the humandermis contains numerous blood vessels representing an angiogenic

tissue reaction (Fig 1B). In dozens of injected grafts, psoriaticplaques created in this model system are always accompanied by aprominent angiogenic tissue reaction.

There are many changes in the blood vessels within a chronic,well-established psoriatic plaque including: dilation of capillarybeds, increased capillary permeability, increased endothelial cellproliferation, and tortuoisity of capillary loops with increase inblood ¯ow through the skin (Braverman and Yen, 1977; Ryan,1980; Braverman and Sibley, 1982; Klemp and Staberg, 1983;Heng et al, 1991). The study of the vascular changes in psoriaticplaques has been of interest to many leading scientists interested inangiogenesis for several decades (Folkman, 1972, 1995). By usingthe SCID mouse model, it is possible to determine the earliestmolecular and cellular changes in the epidermis and dermisfollowing injection of activated immunocytes that lead to thecharacteristic angiogenic tissue response observed in psoriasis.

Previous investigators have observed that peripheral blood-derived mononuclear cells from psoriatic patients, or serum frompsoriatic patients, could induce angiogenesis (Majewski et al, 1985,1987); however, the exact cell-type (i.e., T cell subset), or themolecules responsible for this angiogenic response, were not furthercharacterized. More recently, attempts have been made to de®nethe cytokine network in psoriasis, and to identify angiogenic as wellas angiostatic cytokines that may contribute to the vascular changespresent in psoriatic plaques (Nickoloff, 1991). In this review, asummary of the literature concerning the molecular and cellular

1087-0024/00/$15.00 ´ Copyright # 2000 by The Society for Investigative Dermatology, Inc.

67

Manuscript received January 28, 2000; revised April 18, 2000; acceptedfor publication May 25, 2000.

Reprint requests to: Dr. Brian J. Nickoloff, Director, Skin CancerResearch Program, Cardinal Bernardin Cancer Center, Building #112,Room 301, 2160 South First Avenue, Maywood, IL 60153. Email:bnickol@lumc.edu

basis for angiogenesis in psoriasis will be followed by a briefdiscussion of currently unresolved questions, and a model for thepathogenesis of psoriasis integrating all available data will concludethe presentation.

CHARACTERIZATION OF PHENOTYPE OFACTIVATED ENDOTHELIAL CELLS IN CHRONIC

PSORIATIC PLAQUES

From a clinical and morphologic perspective, the prominentvasculature in the super®cial plexus of a chronic psoriatic plaquepoint to endothelial cell proliferation and activation (Wolfe, 1989).

To characterize the phenotype of the activated endothelial cell,investigators have used immunohistochemical staining and com-pared various antigens expressed by endothelial cells of normal skinobtained from healthy individuals (NN skin), with clinicallysymptomless skin removed from patients with psoriasis elsewhere(PN skin), as well as chronic psoriatic plaques (PP skin). Becausethe angiogenic pathway by which early endothelial cells sprout andextend to form tubes and elongate capillary beds involvesinteractions with extracellular matrix proteins, various integrinshave been examined (Ingber, 1992). Integrins are cell surfacemolecules that play key roles in the angiogenic process because theymediate cell±matrix interaction and trigger intracellular signaling

Figure 1.Angiogenic tissue reaction in en-grafted prepsoriatic human skin transplantedonto SCID mice. When orthotopically trans-planted skin is injected with saline, no angiogen-esis is present (A), but 2 weeks after injection ofactivated immunocytes the human dermis containsnumerous small and large blood vessels re¯ecting alocal angiogenic tissue response that accompaniesthe other clinical and microscopic changes charac-teristic of a bona ®de plaque of psoriasis (B).

Figure 2.Differential expression of avb3 versus avb5 integrins in NN, PN, and chronic psoriatic plaques (PP skin), as well as acute psoriaticlesions created using the SCID mouse model. Immunoperoxidase staining; Vectastain (anti-avb3 integrin MoAb; MoAb1976, and anti-aVb5 integrinMoAb; MoAb1961; anti-ELAM-1; CD62E). 3-amino-4-ethylcarbazole as chromagen producing positive red reaction product. (A) avb3 expression isabsent in NN skin, but is focally present in PN skin and diffusely positive in endothelial cells in chronic PP skin. An acute psoriatic lesion created by injectingactivated autologous immunocytes into engrafted PN skin reveals endothelial cell positivity for avb3 integrin (panel labelled SCID). (B) avb5 expression isfocally present on upper dermal dendritic cells, and on basal keratinocytes and dermal dendritic cells in PN skin. In a chronic psoriatic plaque, whileperivascular and interstitial dendritic cells are avb5 positive, the endothelial cells are negative. An acute psoriatic lesion created on a SCID mouse (far rightpanel) has strong and diffuse dendritic cell avb5 expression, but no endothelial cell positivity. Inset reveals ELAM-1 expression on microvascular endothelialcells.

68 NICKOLOFF JID SYMPOSIUM PROCEEDINGS

producing endothelial cell activation and proliferation (Hynes,1992).

In NN skin and PN skin there was low to absent expression of theavb3 integrin (Brooks et al, 1994), but in psoriatic plaques there wasstrong and diffuse endothelial cell expression of avb3 integrin(Creamer et al, 1995). In our own series of six different skinbiopsies, we con®rmed these observations with respect to avb3integrin expression (Fig 2). In contrast to this pattern foravb3 integrin expression, there is actually a decrease in expression ofb4 integrin in PP skin compared with PN skin, and no change in b1integrin expression between these different skin samples (Creameret al, 1995). Because cytokines can induce angiogenesis, Cheresh et alhave de®ned at least two different angiogenic pathways by differentialendothelial cell expression of avb3 versus avb5 integrins (Friedlanderet al, 1995). This group reported that avb3-dependent angiogenesiswas induced by basic ®broblast growth factor (bFGF) or tumornecrosis factor alpha (TNF-a), whereas avb5-dependent angio-genesis was triggered by either vascular endothelial growth factor(VEGF), or transforming growth factor-a (TGF-a), or phorbol ester(Friedlander et al, 1995). To examine the different expressions ofavb3 versus avb5, the same six NN, PN, and PP skin biopsies wereimmunostained. Figure 2 reveals that while there was focal basalkeratinocytes and dermal dendritic cell expression of avb5 in NN andPN skin, there was nodetectable endothelial cell positivity. Moreoverin all PP skin biopsies there was more extensive dermal dendritic cellavb5 expression, but no endothelial cell positivity. The lack ofendothelial cell avb5 was rather surprising because the cytokinenetwork in psoriatic plaques includes the presence of VEGF and

TGF-a (inducers of avb5 in endothelial cells). We will return to thistopic later in the paper.

Before concluding this section, it should also be noted that thereare several other phenotypic characteristics that distinguishendothelial cells in PP skin from endothelial cells in NN and PNskin. Other surface antigens prominently expressed by endothelialcells in PP skin include: endothelial cells adhesion molecule-1(ELAM-1; also called E-selectin) (Petzelbauer et al, 1994),intercellular adhesion molecule-1 (ICAM-1) (Groves et al, 1993),vascular cell adhesion molecule-1 (VCAM-1) (Grif®ths et al, 1989),and the MS-1 antigen (Goerdt et al, 1991). Thus, it is clear that theendothelial cells express a wide variety of markers of activation thatdistinguish them in the angiogenic tissue reaction characteristic ofchronic psoriatic plaques.

SWITCHING ON ANGIOGENESIS DURING WOUNDHEALING

Whereas the previous section focused on chronic psoriatic plaques,in this section the focus will be on describing the dynamicsequential events as recorded during cutaneous wound healing.These types of experiments are relevant to the biology of psoriasisas the ®nal psoriatic plaque may actually represent an abnormalwound-healing response (Lingen and Nickoloff, 1999). There aremany different types of wound-healing models in which to studythe molecular and cellular basis of the regulation of angiogenesis,but only three models will be discussed; one dealing with porcineskin and the other two using human skin. In the porcine model, theinitiation of angiogenesis triggered by full-thickness wounding

Figure 3. Psoriatic plaque contains a rich admixture of macrophages and dendritic cells. The macrophage population includes a pan-macrophagemarker CD14, macrophages with classical activation markers (i.e., CD16, CD32) as well as alternatively activated macrophages (CD163, mannose receptor).In addition, there are epidermal and dermal dendritic cells expressing CD1a, CD80, and CD83, indicating various degrees of maturation of these professionalantigen-presenting cells.

VOL. 5, NO. 1 DECEMBER 2000 IMMUNOCYTE-MEDIATED ANGIOGENESIS IN HUMAN SKIN 69

revealed the transient functional involvement of the avb3 integrinby endothelial cells, but not avb5 or b1 integrins (Clark et al,1996). When NN skin was engrafted onto SCID mice, alongitudinal dermo±epidermal excisional wound was created andthe subsequent wound healing and angiogenesis process wasinvestigated (Christo®dou-Solonidou et al, 1997). As early as 2 daysafter wounding, there was upregulation of avb3, avb5, and avb6integrins by the proliferating endothelial cells, but only inhibitors ofavb3 integrin blocked new vessel formation during human woundhealing (Christo®dou-Solonidou et al, 1997). To de®ne theprimary mediators of angiogenesis in acute wound repair, surgicalwound ¯uid was collected from surgical patients undergoingmastectomy or neck dissection from postproductive days 1±7(Nissen et al, 1998). It was determined that the initial angiogenicstimulus was provided by bFGF-2, which was followed by a moresustained pro-angiogenic stimulus elicited by VEGF (Nissen et al,1998). The early appearance of bFGF-2 that peaked almostimmediately after surgery was presumably released from cells orsequestered by extracellular matrix, and could account for thesubsequent upregulation of avb3 integrin expression as notedearlier by the investigators.

PHENOTYPE OF DERMAL TISSUE REACTION DURINGGENESIS OF PSORIATIC LESIONS

Up until recently, it was dif®cult to reproducibly induce and studythe onset of psoriasis because no suitable animal model wasavailable; however, with the advent of transplantation technologyand availability of SCID mice, an animal model using entirelyhuman skin components was developed and validated (Nickoloffet al, 1995). It is now possible to inject either engrafted PN or NNskin and create psoriatic plaques that include a prominentangiogenic tissue response (Boehncke et al, 1996; Nickoloff et al,1999). This neovascularization of human dermis takes place within2±4 weeks following intradermal injection of activated immuno-cytes (Nickoloff et al, 1999). By using this experimental modelsystem we have begun to characterize the phenotype of the acutelyactivated and proliferating human dermal microvascular endothelialcells. Figure 2 reveals that acute lesion of psoriasis includesendothelial cells that express avb3, but not avb5 integrins. Inaddition, note the expression of ELAM-1 by the endothelial cells(Fig 2, inset). Thus, as regards the differential expression ofintegrins, the acutely generated angiogenic tissue response is similarto chronic psoriatic plaques with respect to the phenotype of thevascular endothelial cells.

Now that we can use this experimental approach in whichprominent angiogenesis is rapidly and predictably induced by the

injection of activated immunocytes including T cells and naturalkiller T cells, it will now be possible to more fully explore themolecular mechanisms governing this remarkable change in theclinical and histologic appearance of the skin (Boehncke et al, 1996;Nickoloff and Wrone-Smith, 1999; Nickoloff et al, 1999).

POTENTIAL DIRECT MEDIATORS PRODUCED BYACTIVATED T CELLS THAT CAN MODULATE

ANGIOGENESIS

Perhaps the simplest view of how the injection of activatedimmunocyte triggers psoriasis is to consider a direct interaction bywhich T cells themselves can serve as the source of pro-angiogenicstimuli. This is a reasonable and straightforward possibility as it iswell documented that activated T cells can produce pro-angiogenicresponses by endothelial cells. Examples of such T cell-derivedfactors include various cytokines (Lingen and Nickoloff, 1999), aswell as scatter factor (hepatocyte growth factor) (Naidu et al, 1994).Scatter factor is a potent angiogenic factor that is present in psoriaticplaques (Grant et al, 1993). Besides pro-angiogenic cytokines, itshould also be remembered that T cells in psoriatic plaques alsoproduce cytokines that have direct antiangiogenic effects such asgamma interferon (IFN-g) and interferon inducible protein 10 (IP-10) (Reaman and Tosato, 1995). IFN-g has pleiotropic effects(Boehm et al, 1997), as it also is a potent inducer of nitric oxide thatcan have profound effects on the microvasculature includingprominent vasodilation (Xie et al, 1993; Kolb-Bachofen et al, 1994).Thus, it should be clear that there are both positive and negativeregulators of microvasculature growth, and that the overall balanceamongst these factors determines the ®nal net result with each tissuesite of interest (Folkman, 1995).

ACTIVATED T CELLS CAN INFLUENCE ENDOTHELIALCELLS INDIRECTLY VIA THE EPIDERMAL

KERATINOCYTE

Prior to the discovery of cytokines, epidermal keratinocytes wereviewed as rather immunologically inert cells, primarily relegated toa brick and mortar function as regards the barrier function of skin.When T cells were present in the skin, keratinocytes wereportrayed as simple passive targets not capable of either initiating orperpetuating in¯ammatory or immune-mediated reactions (Barkeret al, 1991). It is now clear, however, that keratinocytes areimportant coconspirators with T cells and dendritic cells, and caneven function as nonprofessional antigen presenting cells (Nickoloffand Turka, 1993). The ability of keratinocytes to produce variouscytokines, chemotactic polypeptides, and adhesion molecules has

Figure 4.Model for understanding the cellularand molecular basis for the angiogenic tissuereaction in psoriasis highlighting the role ofactivated immunocytes producing cytokinesthat trigger neovascularization of the dermis.The upper panels depict the microscopic and clin-ical appearance of symptomless skin engrafted ontoa SCID mouse, and the subsequent psoriatic pla-que that is created following injection of activatedimmunocytes (inset includes prominent neovascu-larization of dermis underlying newly createdpsoriatic plaque). The lower panels represent amodel in which the intradermal injected activatedimmunocytes (i.e., natural killer T cells) producecytokines such as IFN-g that are accompanied byseveral keratinocyte-derived, macrophage-derived,and dermal dendrocyte-derived pro-angiogenicfactors. The net result of this cytokine network isactivation and proliferation of aVb3 integrin posi-tive endothelial cells.

70 NICKOLOFF JID SYMPOSIUM PROCEEDINGS

led to the recognition that they can in¯uence events in dermisincluding the activation of endothelial cells (Grif®ths andNickoloff, 1991; Detmar et al, 1995; Reaman and Tosato, 1995).In this section we will discuss the series of reciprocal interactions bywhich activated T cells and natural killer T cells move from thedermis into the epidermis and release cytokines that activatekeratinocytes, which in turn release additional cytokines that canthen activate dermal microvascular endothelial cells in psoriasis.

The two most important primary cytokines in psoriatic plaquesare IFN-g and TNF-a (Nickoloff, 1991). These cytokines canin¯uence keratinocytes to produce a wide array of other cytokinesthat have angiogenic activity. Examples of pro-angiogenicregulators produced by cytokine activated keratinocytes include:transforming growth factor-alpha (TGF-a) (Gottlieb et al, 1988;Elder et al, 1989), VEGF (Detmar et al, 1994), IL-8 (Schroder andChristophers, 1986; Nickoloff et al, 1991), scatter factor (Grant DSet al, 1993), and TNF-a (Ettchad et al, 1994). In addition to thesepro-angiogenic cytokines, psoriatic keratinocytes underproduce theangioinhibitory molecule thrombospondin (Nickoloff et al, 1994).Given the mitogenic activity of several of these cytokines to alsopromote further keratinocyte hyperplasia, which can contribute toelongation of psoriatic capillaries (Bacharach-Buchies et al, 1994),and the net pro-angiogenic pro®le of this cytokine network, it isclear that keratinocytes activated by T cells can promote thevascular proliferation that is a hallmark of the psoriatic plaque.

ACTIVATED T CELLS AND NATURAL KILLER T CELLINTERACTION WITH MACROPHAGES THAT

TRIGGERS ANGIOGENESIS

One of the earliest cellular interactions to occur in the skin duringthe onset of psoriatic lesions involves lymphocytes and macro-phages (Schubert and Christophers, 1985). There are also importantreactions between T cells and dermal dendritic cells that contributeto the release of cytokines that impact endothelial cells (Nestle et al,1994). Numerous investigators have documented the presence ofvarious types of monocyte/macrophage and dendritic cells in andaround the blood vessels of psoriatic plaques (Paukkonen et al,1992; Goerdt et al, 1993; Schopf et al, 1993; Sterry and Boehncke,1993; Oord Van Den and Wolf-Peeters, 1994; Djemadji-Oudjelet al, 1996). Rather than reviewing how the local immune reactionis generated involving antigen presenting cells and T cells, in thissection a brief review of the importance of how T cells can impactmacrophages, followed by how these activated macrophages canimpact endothelial cells will be highlighted.

Up until recently, macrophages and dendritic cells were believedto be of distinct origins with nonoverlapping phenotypes andfunctions (Banchereau and Steinman, 1998); however, it is nowclear that monocytes can give rise to dendritic cells and thatdendritic cells can undergo phagocytosis (the term dendrophagewas introduced to signify this point) (Nickoloff and Nestle, 1995).Currently, it is also possible to delineate at least two differentpathways for the activation of macrophages as well as dendritic cells(Goerdt and Orfanos, 1999). These pathways have been identi®edas either involving classical activation versus alternative activation(s).It has been suggested that acute in¯ammatory processes may beperpetuated into chronic conditions because of these antigenpresenting cells (i.e., macrophage and dendritic cells) (Goerdt andOrfanos, 1999). The classical activation pathway involves cytokinessuch as IL-1, IL-6, IL-12, IFN-g, and TNF-a whereas thealternative activation pathway involves the TH2 type cytokines IL-4 and IL-10. The classical macrophage pathways mediatesimmunologic reactions that produce in¯ammation, whereas thealternative macrophage pathway is envisioned as a counter-regulatory process predominantly manifesting itself with down-regulation of in¯ammation, angiogenesis, and elimination of tissuedebris via phagocytosis as occurs during wound healing.

A classical activation-type macrophage has a distinctive im-munophenotypic pro®le of cell surface markers including CD16,CD32, and CD64; whereas the alternatively activated macrophage

does not express these markers, but does express CD163 and themacrophage mannose receptor (MMR) (Goerdt and Orfanos,1999). One previous report demonstrated the presence ofalternatively activated macrophages in psoriasis, but did notexamine the same biopsies for evidence of classical activatedmacrophages or dendritic cells (Djemadji-Oudjel et al, 1996). Todetermine if alternatively activated macrophages are the dominantmacrophage population in psoriasis, we examined a series ofpsoriatic plaques with a broad panel of monoclonal antibodies.Because alternatively activated macrophages have been linked toangiogenic responses (Goerdt and Orfanos, 1999), we wereintrigued by the possible role for such cells in psoriasis.

As shown in Fig 3, a chronic psoriatic plaque contains a richadmixture of dendritic cells and macrophage-appearing cells in theupper dermis immunoreactive for the classical activation markersCD16 and CD32. In addition, there are also numerous positivelystained mononuclear cells with alternative activation markersCD163 and MMR. Thus it appears that both types of activatedcell phenotypes are present in psoriatic plaques. Once activated,macrophages can induce prominent and rapid angiogenesis bysecreting various mediators including TNF-a, TGF-a, IGF-1,PDGF-B, TGF-b, and scatter factor (Leibovich et al, 1987; Kodeljaet al, 1997). Further studies are indicated to determine their relativeimportance and functional contribution to the in¯ammatoryprocess and angiogenic tissue reaction.

MODEL LINKING THE GENETIC AND CELLULARFACTORS RESPONSIBLE FOR TRIGGERING THEANGIOGENIC TISSUE REACTION IN PSORIASIS

A line of inquiry that is currently being followed includes thepossibility that when one of our natural killer T cell lines is injectedinto the dermis, because these natural killer T cells can produce bothIFN-g (classical activation pathways) as well as IL-4 and IL-13(alternative activating pathway), neither opposing pathways canbecome dominantly established within the ®rst 2 weeks (Nickoloffet al 1999). We postulate this potentially unbalanced, and hencedysfunctional interaction, ultimately results in a chronic in¯ammatoryreaction with neovascularization that persists for a long periodof time.Our current working model includes several sequential cellular andmolecular events including the following (Fig 4). First, natural killerT cells in®ltrate the dermis and, upon contact with relevant CD1dbearing target cells such as epidermal keratinocytes, become activatedvia their natural killer receptors such as CD161 (Bonish et al, 2000).Second, these natural killer T cells begin producing a mixture ofcytokines including IFN-g and IL-13 (Nickoloff et al, 2000). Third,the avascular epidermal-based keratinocytes also become activated torelease several pro-angiogenic cytokines that in¯uence the under-lying vessels in a paracrine fashion. Fourth, the local production ofIFN-g and IL-13 activate tissue monocytes/macrophages/dendriticcells and drive both classical as well as alternative activation pathways.Finally, the tissue ``dendrophage'' release a variety of cytokines andangiogenic molecules that provoke a local angiogenic tissue reaction.The proliferating endothelial cells are characterized by a distinctivephenotype including avb3 integrin expression as well as variousadhesion molecules including ELAM-1, VCAM-1, and ICAM-1.Thus, the initial in¯ux of pathogenic immunocytes triggers a chainreaction of cellular and molecular events involving both epidermalkeratinocytes as well as angiocentric and interstitial dermal macro-phages and dendritic cells. It should be noted that the experimentalevidence for this new concept in which natural killer T cells cantrigger angiogenesis is still preliminary, and will require additionalstudy for comparison with more conventional T cell subsets.

REMAINING QUESTIONS THAT CAN BE ADDRESSEDUSING THE SCID MOUSE ANIMAL MODEL OF

PSORIASIS

Perhaps the most important currently unsolved issues as regardsangiogenesis and psoriasis is whether the neovascularization isrequired for the creation and/or maintenance of the plaque. Using

VOL. 5, NO. 1 DECEMBER 2000 IMMUNOCYTE-MEDIATED ANGIOGENESIS IN HUMAN SKIN 71

the SCID mouse:human skin chimeric model it is now possible toaddress this question by preinjecting the engrafted skin withblocking antibodies against the most important integrins such asavb3, followed by intradermal inoculation of activated immuno-cytes. Alternatively, it is possible to add neutralizing antibodyagainst VEGF to determine the relative importance of angiogenesisto psoriasis mediated by VEGF. If the angiogenic tissue responsecan be inhibited, it will be interesting to observe the degree (if any)of epidermal hyperplasia in such a setting. Similarly by injectingengrafted psoriatic plaques with inhibitory antibodies against avb3integrin or VEGF it could be determined whether the angiogenesiscould be reversed; and if it could, whether it would have any effecton the extent of keratinocyte hyperplasia in the overlyingepidermis. The reagents available that antagonize or disruptangiogenic blood vessels do so by inducing apoptosis of theendothelial cells (Brooks et al, 1994). An alternative approach tousing monoclonal antibodies would be to use cytokines such as IL-12 and IL-18 to attack the hyperplastic endothelial cells, and thenobserve the net effect on the psoriatic tissue (Coughlin et al, 1998).Thus, by taking this approach, investigators will ®nally be able todetermine if the angiogenic response in the dermis is of primary orsecondary importance to the genesis and propagation of psoriaticplaques.

Recent discoveries related to the positive and negative regulationof blood vessel growth have propelled the ®eld of angiogenesis tothe forefront of biomedical research. While it is clear that themajority of investigators are seeking to apply this new knowledgeto cancer therapy and to understanding tumorigenesis, it is likelythat these advances will impact our perspectives towards otherdisease processes such as psoriasis, rheumatoid arthritis, and manymore medical conditions (Folkman, 1995). Even though a numberof exciting naturally occurring and synthetic negative regulatorshave been discovered such as angiostatin, it will be important toidentify their target receptor(s) that are responsible for mediatingthe antiangiogenic tissue response. While many cellular andmolecular mediators of angiogenesis in psoriasis have beenelucidated, many questions remain as discussed in the previoussection. Nonetheless, with the development of the SCID mouseanimal model as a validated and useful experimental tool, as well asthe rapid advances made by investigators studying other diseaseprocesses (Eliceiri and Cheresh, 1999), it is likely that progress inunderstanding and treating psoriasis by attacking the vasculaturewill be part of our therapeutic strategy in the not so distant future.

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