Pattern-recognition receptors in pulp defense.

Initial sensing of infection is mediated by germline-encoded pattern-recognition receptors (PRRs), the activation of which leads to the expression of inflammatory mediators responsible for the elimination of pathogens and infected cells. PRRs act as immune sensors that provide immediate cell responses to pathogen invasion or tissue injury. Here, we review the expression of PRRs in human dental pulp cells, namely, receptors from the Toll-like (TLR) and Nod-like NLR families, by which cells recognize bacteria. Particular attention is given to odontoblasts, which are the first cells encountered by pathogens and represent, in the tooth, the first line of defense for the host. Understanding cellular and molecular mechanisms associated with the recognition of bacterial pathogens by odontoblasts is critical for the development of therapeutic strategies that aim at preventing excessive pulp inflammation and related deleterious effects.

Different roles of odontoblasts and fibroblasts in immunity.

Odontoblasts and fibroblasts are suspected to influence the innate immune response triggered in the dental pulp by micro-organisms that progressively invade the human tooth during the caries process. To determine whether they differ in their responses to oral pathogens, we performed a systematic comparative analysis of odontoblast-like cell and pulp fibroblast responses to TLR2-, TLR3-, and TLR4-specific agonists (lipoteichoic acid [LTA], double-stranded RNA, and lipopolysaccharide [LPS], respectively). Cells responded to these agonists by differential up-regulation of chemokine gene expression. CXCL2 and CXCL10 were thus increased by LTA only in odontoblast-like cells, while LPS increased CCL7, CCL26, and CXCL11 only in fibroblasts. Supernatants of stimulated cultures increased migration of immature dendritic cells compared with controls, odontoblast-like cells being more potent attractants than fibroblasts. Analysis of these data suggests that odontoblasts and pulp fibroblasts differ in their innate immune responses to oral micro-organisms that invade the pulp tissue.

HUGO (FNDC3A): a new gene overexpressed in human odontoblasts.

Previously, we established a subtractive cDNA library enriched in odontoblast-specific genes and hypothesized that new, previously unidentified, markers would be present, associated with the odontoblast phenotype. In this paper, we report the first characterization of a new gene we have named HUGO, and its associated deduced protein sequence. This gene expression is under the control of two alternative promoters, resulting in the synthesis of two proteins, one of which, HUGO2, is included in the other, HUGO1. HUGO proteins are mainly composed of a proline-rich region at the N-terminus, 8 type III-fibronectin modules, and a transmembranous helix at the C-terminus. In odontoblasts, the proteins are located in Golgi vesicles. However, they display a broader expression pattern, since they are also expressed by nerve fibers in the dental pulp and other tissues (e.g., trachea, brain, kidney), as demonstrated by immunohistochemistry and qPCR, respectively. Their location in odontoblasts suggests a role in collagen and glycosaminoglycan synthesis.

In vitro adhesion of human epidermal Langerhans cells to laminin and fibronectin occurs through beta 1 integrin receptors.

Human epidermal Langerhans cells are dendritic cells that can capture, process, and present antigens to T cells. It was previously shown that these Langerhans cells express the very late activation antigen (VLA) protein family of beta 1 integrins. beta 1 integrins mainly mediate the adhesion of cells to a number of extracellular components, such as laminin, fibronectin, and collagen, which are present in the skin. In this report, we demonstrate that a large percentage of the Langerhans cell population was able in vitro to attach to laminin and fibronectin but not to collagen. An ultrastructural study of adherent Langerhans cells showed that they were spread largely on laminin, with a loss of their round shape, and partially on fibronectin. Langerhans cell binding to laminin or fibronectin induced a decrease of the Birbeck granule number. Specific inhibitions of cell adhesion were performed, and it was demonstrated that VLA-6 was the major receptor involved in Langerhans cell adhesion to laminin. This adhesion was not RGD dependent and was slightly enhanced by Mn2+. VLA-3 and especially VLA-5 mediated Langerhans cell binding to fibronectin through the RGDS sequence of the protein. Mn2+ sharply increased the Langerhans cell adhesion to fibronectin. VLA-6 mediated in vitro Langerhans cell adhesion to laminin, which suggests that in vivo VLA-6 could permit Langerhans cells to attach and migrate through the basement membrane. Moreover, VLA-5 and VLA-3 take part in the in vitro Langerhans cell binding to fibronectin, suggesting that in vivo these fibronectin receptors could facilitate Langerhans cell passage throughout the fibronectin network of the dermis before migration to lymph nodes.

Role of specific successive contacts between extracellular matrix proteins and epidermal Langerhans cells in the control of their directed migration.

To migrate from epidermis to regional lymph nodes, antigen-bearing epidermal Langerhans cells (LC) must move through extracellular matrix (ECM) of various composition. The present study was designed to contrast the ability of basement membrane (BM)- and dermis-ECM components to successively stimulate the adhesion of normal human epidermal LC, in vitro. For this purpose, we used highly enriched LC suspensions (70%-80%), allowed them to attach to one ECM substrate, and then studied the readhesion properties of these recovered ECM-attached LC to the same and different ECM substrates. Each of four ECM molecules (laminin (LM), fibronectin (FN), type I and type IV collagen) was tested in pairs. Readhesion of recovered LM and type IV collagen-attached cells did not affect readhesion to FN and type I collagen, indicating that the interaction of LC with the BM components can be normally followed by interaction with the dermis-ECM molecules. In contrast, readhesion of recovered FN-, type I collagen-, and type IV collagen-attached cells to LM was significantly reduced. The findings indicate that following contact to BM components, epidermal LC are able to attach to ECM proteins present in the dermis, whereas once they have made contact with ECM present in the dermis, they reduce their binding capacity to the BM laminin, suggesting the contact with the dermal components could prevent LC from reentering the epidermal compartment. Binding to LM and FN was also shown to induce a decline in the expression of CD1a, known as a specific marker restricted to epidermal LC.(ABSTRACT TRUNCATED AT 250 WORDS)

A combination of MIP-3alpha and TGF-beta1 is required for the attraction of human Langerhans precursor cells through a dermal-epidermal barrier.

Signals regulating the traffic of Langerhans cell precursors from blood to the epidermis are not yet fully understood. The observations that TGF-beta1 is of unique importance in Langerhans cells (LC) ontogeny and that macrophage inflammatory protein-3alpha (MIP-3alpha) is able to attract LC within the epidermis, prompted us to study the effect of MIP-3alpha and TGF-beta1 on the migration of LC precursors. The migratory capacity of immature dendritic cells (DC) was assessed using a reconstituted basement membrane assay (Matrigel), mimicking the prerequisite passage through the dermal-epidermal basement membrane on the way into the epidermis. DC differentiated from cord blood CD34 cells in the presence of GM-CSF plus TNF-alpha were subjected to migration using modified Boyden chambers. Day-6 DC progenitors migrated in a dose-dependent fashion in response to MIP-3alpha, and CD1alpha+ LC precursors responded preferentially to the chemokine. Immature DC did not respond strongly to TGF-beta1 alone in migration assays, but up to 68% of the cells migrated in response to MIP-3alpha plus TGF-beta1. Among them, at least 50% expressed CD1a and E-cadherin and can be considered LC precursors. The allostimulatory function of these cells was significantly more potent than that which migrated in response to MIP-3alpha alone. Our results show that a significant proportion of immature DC is able to migrate through a dermal-epidermal basement membrane equivalent. In the presence of TGF-beta1, the DC which respond to MIP-3alpha have the phenotype and the functional capacity of epidermal LC. Our findings underline the role of MIP-3alpha and TGF-beta1 in attraction and localization of immature LC within the epidermis under normal conditions.

Reactivity pattern of a monoclonal antikeratin antibody (KL1).

A monoclonal antibody against keratins (KL1) from normal human stratum corneum was obtained using hybridoma techniques. Spleen cells from immunized BALB/c mice were fused with NS1, a mouse myeloma cell line, to produce hybrids. Antibody activity to epidermal keratins was tested using an indirect immunofluorescence test on cryostat sections of human epidermis and rabbit lip. A stable clone producing antikeratin antibody was isolated and an ascitic fluid was produced and used as a source of antibody (IgG1 kappa). KL1 was characterized by its immunohistochemical staining of various epithelia and by its recognition of 55-57 kilodalton (kd) keratin polypeptide from normal epidermis using the immunoblot technique. Frozen and deparaffinized sections of normal human epidermis, mucosa, and esophagus were stained by this antibody only in the upper cell layers as demonstrated by both indirect immunofluorescence and immunoperoxidase techniques. Approximatively 80% of normal keratinocytes isolated after trypsinization were labeled by KL1 whereas most negative cells showed basement membrane zone antigens. This confirmed differences in the expression of medium-sized polypeptides between basal and supra-basal cells during the course of human epidermal differentiation. All epithelial cells from other human epithelia (thymus, thyroid, bronchial mucosa, stomach, intestines) were positive with KL1 whereas nonepithelial cells and tissues did not show any staining. In view of these results KL1 promises to be a useful tool in the exploration of human epithelial differentiation.

Flow cytometric sorting of human epidermal pure basal cell suspensions using a specific antikeratin monoclonal antibody.

A method is described for flow cytometric sorting of epidermal basal cells, based on the cellular keratin content. KL1, a monoclonal antikeratin antibody specific for the 56.5 kD polypeptide present in suprabasal cells was used to distinguish suprabasal from basal cells. After the action of Triton X-100, epidermal cells were stained in suspensions with KL1 antibody, and KL1-positive cells were separated by flow cytometry from KL1-negative cells (basal cells). This makes it possible to sort and analyze pure fractions of human epidermal basal cells since no less than 99% of the KL1 sorted cells were bearing bullous pemphigoid antigen. Therefore, the use of specific antibodies to cytoplasmic antigens can be of help in sorting pure fractions of cells of a particular tissue for further studies.

Fibronectin upregulates in vitro generation of dendritic Langerhans cells from human cord blood CD34+ progenitors.

Several studies have demonstrated that dendritic cells can be generated in vitro from CD34+ hematopoietic progenitor cells. In vivo, dendritic cells are found in many tissues and reside in direct proximity to extracellular matrix proteins. Because extracellular matrix proteins affect differentiation and location of cells in tissues, this study was designed to investigate potential effects of extracellular matrix proteins on differentiation of dendritic cells. Dendritic cells were generated from CD34+ human cord blood cells in the presence of granulocyte-macrophage colony-stimulating factor and tumor necrosis factor-alpha for 6 d and subsequently cultured for an additional 6-d period on tissue culture plates coated with various extracellular matrix proteins. Among the extracellular matrix proteins tested, exposure to fibronectin stimulated dendritic cell/Langerhans cell differentiation as indicated by the 50% increase of the number of cells expressing the Birbeck granule-associated marker Lag and displaying numerous Birbeck granules. Adhesion on fibronectin was shown to be specifically mediated by the integrin alpha5beta1. Because laminin and collagen were unable to cause similar changes in Langerhans cell development, these results suggest that fibronectin may cause changes affecting cellular differentiation of progenitors. Hematopoietic progenitors may exhibit maturational regulated differences in response to both matrix molecules and cytokines. The influence of combined signals emanating from a supportive microenvironment, specific integrins, and particular cytokines in the differentiation of Langerhans cells is discussed.

Human epidermal Langerhans cells express beta 1 integrins that mediate their adhesion to laminin and fibronectin.

Members of the beta 1 or very late antigen (VLA) integrin family represent the predominant class of integrin extracellular matrix receptors. Adhesion assays were developed for the identification of the beta 1 integrins involved in the adhesive interactions between Langerhans cells (which mainly express alpha 4 beta 1, alpha 5 beta 1, and alpha 6 beta 1) and extracellular matrix proteins. For this purpose, binding assays were performed on fibronectin-, laminin-, collagen type IV-, and collagen type I-coated plates. 59% +/- 21% of Langerhans cells (LC) specifically attached to fibronectin. Using as inhibitory probes monoclonal antibodies against the beta 1, alpha 5, and alpha 3 chains and the synthetic peptide GRGDSP resulted in a decrease of 43%, 41%, 15%, and 42% respectively of LC binding to fibronectin. 76% +/- 20% of LC specifically adhered to laminin. Anti-alpha 6 monoclonal antibody potently inhibited this adhesion, which dropped to 36%, whereas the synthetic peptide GRGDSP was ineffective. A low number of LC adhered to type I and type IV collagen (13-15%). These results indicate that alpha 5 beta 1 and alpha 6 beta 1 were the major beta 1 integrins involved in LC adhesion to fibronectin and laminin. Ultrastructural cell morphology of adherent cells was examined and showed that LC were largely spread on laminin and became tightly bound to the substrate on a large portion of membrane. On fibronectin surface, the contact between LC and substrate was smaller, thus cells could conserve their general round aspect. Moreover, LC binding to fibronectin and laminin induced a significative decrease of the Birbeck granule number. The finding that LC attach to LM and FN in vitro suggests they exist similarly in vivo. By mediating a passage through basement membrane and migration throughout the fibronectin network of the dermis, alpha 5 beta 1 and alpha 6 beta 1 could contribute to the ability of LC to migrate into and out of the epidermis.

Immunocytochemical and ultrastructural localization of keratin polypeptides in normal epidermal and mucosal cells and tissues.

Previous studies revealed that the filaments of cytokeratin express different antigenic sites in the basal and suprabasal cell compartment of the skin and oral mucosa. It has been demonstrated that the keratin polypeptide subunit of molecular weight 67,000 dalton (67K) is a valuable marker of the suprabasal cells, since the normal basal cells remain unlabeled by antibodies against this large polypeptide. In the present study, the distribution of the 67K polypeptide has been investigated on human and guinea pig keratinocyte suspensions, purified basal cells of guinea pigs, normal human abdominal skin and normal human buccal mucosa, using the indirect immunoperoxidase method. The results of this investigation confirmed ultrastructurally that the 67K-antigen expression is lacking in the epidermal basal cell compartment. However, it could be shown that 67K polypeptide is a component of the tonofilaments in the upper Malpighian cell layers of the skin and oral mucosa. These observations suggest that keratinocyte differentiation is accompanied by modifications of antigenic determinants of the keratin filaments.

Integration of Langerhans cells into a pigmented reconstructed human epidermis.

The majority of in vitro reconstructed human epidermis is composed of keratinocytes only. Recently, the introduction of melanocytes into epidermal reconstructs has enlarged their field of application. The completion of reconstructed epidermis by introducing Langerhans cells remained an important challenge because Langerhans cells, unlike the other epidermal cell types, cannot be subcultured and expanded. To solve this problem, we used cord blood-derived CD34+ hematopoietic progenitors. Seeding these cells, after induction of their differentiation by granulocyte macrophage-colony stimulating factor and tumor necrosis factor-alpha, onto a reconstructing epidermis, composed of keratinocytes and melanocytes, gives rise to a pigmented epidermis with melanocytes in the basal layer and resident epidermal Langerhans cells located suprabasally. Interestingly, the same result was obtained by co-seeding a mixture of keratinocytes, melanocytes, and nondifferentiated CD34+ hematopoietic progenitors on the dermal equivalent, indicating that keratinocytes provide the environmental conditions for hematopoietic progenitors to differentiate into resident epidermal Langerhans cells, expressing major histocompatibility complex class II molecules, CD1a antigen, and Birbeck granules.

Human epidermal Langerhans cells express integrins of the beta 1 subfamily.

The expression of VLA integrins on human epidermal Langerhans cells (LC) was investigated by indirect immunogold labeling on transmission electron microscopy, followed by a quantitative analysis. Labelings on suspensions enriched in freshly isolated LC were carried out with an antibody recognizing the beta 1 subunit, common to all members of the VLA family, and with antibodies specific for the six different alpha subunits, alpha 1 to alpha 6. Normal human epidermal LC were all beta 1 positive, of which 60% were highly positive. By labelings with different VLA-alpha-chain MoAb, there appeared two subpopulations of LC: one positive and one negative. On average 40% of LC bore small amounts of VLA-1 and VLA-3, 53% and 77% of LC expressed moderate amounts of VLA-2 and VLA-5, respectively, and VLA-4 and VLA-6 were expressed by 67% and 90% of LC, respectively. VLA proteins are mainly extracellular matrix protein receptors. VLA-6 (laminin receptor) and VLA-5 (fibronectin receptor) are expressed mainly by LC, and in this way could subserve LC to leave the epidermal compartment through the basement membrane, to migrate throughout the fibronectin network of the dermis before migrating via the afferent lymphatics to the regional lymph nodes, where they present antigen to T cells. VLA proteins such as VLA-4, VLA-3, or VLA-2, which are found involved in cell-cell contacts, could contribute to the promotion of T-cell activation by facilitating adherence between LC and T cells.

Normal and psoriatic human skin grafts on "nude" mice: morphological and immunochemical studies.

124 normal and psoriatic human skin grafts were performed on congenitally athymic nude mice. 5, 10, 15, 20, and 30 days after grafting, histological and immunochemical studies (using bullous pemphigoid, pemphigus vulgaris and anti 67 K keratin polypeptide immune sera) were performed. The preservation of certain immunological identities of grafted skin, at least with regard to intercellular substance and basement membrane, was observed and the keratinization pattern of human epidermis also remained unaltered throughout the post-grafting period. The typical clubbing of rate ridges/acanthosis pattern in psoriatic skin was preserved in 40% of involved psoriatic skin grafts. The possible significance of these findings is discussed.

Comparative studies on naturally occurring antikeratin antibodies in human sera.

Comparative studies on the specificity of the so-called antiepidermal antibodies (Abs) found in human sera were performed by immunoblotting, enzyme-linked immunosorbent assay (ELISA), and immunoelectron microscopy (IEM). After a screening test by indirect immunofluorescence (IF), sera obtained from patients with various diseases and controls could be classified in 5 different groups according to the IF patterns on the epidermis: sera reactive with: (1) the stratum corneum (SC); (2) the upper layer (U-Cyt); (3) the whole epidermis (G-Cyt); (4) basal cells (B-Cyt); and (5) negative ones. By immunoblotting, all the 23 IF-positive sera were found to bind to one or more keratin bands, and did not show any reactivity with epidermal Nonidet P-40 soluble proteins. SC-Abs were mainly directed against a 67 kD Keratin band, whereas U-Cyt- and G-Cyt-Abs bound to both 58-56 kD and 67-63 kD keratins. B-Cyt-Abs reacted strongly with 63 kD Keratins and slightly with a 50 kD band. Antikeratin Abs were detected by immunoblotting even in the IF-negative sera. The ELISA study showed that sera with high IF titers contained high levels of antikeratin Abs. In the IEM study using sera containing U-Cyt- or B-Cyt-Abs, 2 distinct reaction patterns were demonstrated: U-Cyt-Abs stained tonofilaments of suprabasal keratinocytes, while B-Cyt-Abs characteristically reacted with those of basal cells. Moreover, SC-, U-Cyt-, and G-Cyt-Abs were absorbed out by insoluble epidermal proteins, and B-Cyt-Abs were decreased in titer after the absorption test. The present study provides strong evidence that most, though not all, human antiepidermal Abs are directed against different keratin polypeptides, and that antikeratin Abs commonly occur in almost all human sera.

Distribution pattern of psoriatic keratoblasts: computer-assisted image-analysis for combined evaluation of DNA synthesis and expression of 67 kD keratin polypeptides in the epidermis of stable plaques of psoriasis.

Psoriatic epidermis is characterized by increased DNA synthesis and disturbed differentiation. Even though these processes are closely associated, most investigations do not give insight into temporal/spatial relationships between both events. We previously developed a double labeling method for the simultaneous demonstration of the germinative and differentiated epidermal compartments in normal human skin by using tritium-labeled thymidine ([3H] Thd) incorporation and immunoperoxidase staining of 67 kD keratin polypeptides. In this paper we report the results of combined evaluation of these compartments in stable plaques of psoriasis. Scanning of skin sections with an automatic image analyzer allows objective quantification of areas of total epidermis, 67 kD+ differentiated epidermis and numbers of [3H] Thdr+ nuclei. Our data indicate that the 67 kD- undifferentiated psoriatic epidermis is expanded. Increased numbers of [3H] Thd+ basal and suprabasal psoriatic keratinocytes are present and most of them (97.9%) pertain to the 67 kD- compartment. Keratin identification in scales taken from the same sites showed a variable but distinct decrease of 67 kD keratin polypeptides. Hence, the hyperplastic epidermis of stable plaques of psoriasis is characterized by the presence of increased numbers of [3H] Thd+ cells, which primarily belong to the undifferentiated (67 kD-) basal and suprabasal compartments, especially in the lowermost parts of the elongated interpapillary rete ridges. These changes are associated with a relative decrease of synthesis of 67 kD polypeptides and the presence in the scales of keratins that confer a characteristic hyperproliferative epidermal keratin pattern to the psoriatic plaque.

Langerhans cells in S-phase in normal skin detected by simultaneous analysis of cell surface antigen and BrdU incorporation.

We report on a double immunofluorescence staining for the detection of the Langerhans Cell (LC) population in S-phase. After trypsinization, the epidermal cell suspensions were enriched for LC and exposed to 10 microM BrdU for 2 h. Studies of BrdU labeled cells included the determination of their cell surface phenotype. Both membrane labeling and incorporated BrdU as revealed using anti-BrdU demonstrated the BrdU as revealed using anti-BrdU demonstrated the presence of LC in S-phase. We observed 6% of the epidermal LC in S-phase. This is a proof that LC are able to proliferate in the epidermal microenvironment. This technique, besides being rapid and free of radioactivity, allows the cytokinetic study of phenotypically defined LC in heterogeneous epidermal cell populations.

A surface glycoprotein complex related to the adhesive receptors of the VLA family, shared by epidermal Langerhans cells and basal keratinocytes.

A murine monoclonal antibody, designated K20, was raised by immunization with a human malignant T-cell line. It reacted specifically with membrane glycoprotein complexes on early haematopoietic cells, T cells, and monocytes. In epidermis, K20 specifically reacted with Langerhans cells and basal keratinocytes, as demonstrated by double labeling experiments. Membrane immunoprecipitation analysis demonstrated that the antigen identified by K20 on lymphoid cells and epidermal cells was different. While on lymphoid cells, K20 recognized glycoprotein complexes made of a constant 130-kD subunit associated with subunits of higher molecular weight ranging from 150 to 200 kD, a complex of 105-145 kD was precipitated from Langerhans and basal cells. Metabolic labeling studies demonstrated that these proteins were synthesized by the basal cells. The antigen identified by K20 was thought to belong to the integrins, a family of cell surface receptors that play a role in cell adhesion, cell interactions, wound healing, and immune defense mechanisms. K20 is the first monoclonal antibody that specifically recognizes a membrane antigen common to Langerhans and basal cells. Additionally, K20 is the first of five reported monoclonal antibodies to have been characterized on the epidermal cells that detect antigens shared by lymphoid subpopulations and normal basal keratinocytes.

Expression of the alpha 6 beta 4 integrin in lesional skin differentiates bullous pemphigoid (BP) from epidermolysis bullosa acquisita (EBA).

The integrin alpha 6 beta 4 complex is a protein of the membrane of basal keratinocytes, localized at the surface of cells in contact with the basement membrane zone in normal skin. The expression of alpha 6 beta 4 was investigated in several autoimmune blistering skin diseases including bullous pemphigoid (BP), epidermolysis bullosa acquisita (EBA), bullous systemic lupus erythematosus (BSLE), and pemphigus vulgaris (PV) by an indirect immunofluorescence technique. In lesional bullous skin of BP, alpha 6 beta 4 expression was either absent, or in some cases represented an unusual irregular patchy staining. In contrast, in lesional bullous skin from EBA, BSLE, and PV, alpha 6 beta 4 expression was comparable to that observed in normal skin, i.e., a linear staining of the BMZ. Thus, analysis of the alpha 6 beta 4 integrin reactivity on lesional skin, in conjunction with the typical localization of collagen IV, allows a rapid and accurate distinction between BP and EBA.

Comparative studies on the secretion and activation of MMPs in two reconstructed human skin models using HaCaT- and HaCaT-ras-transfected cell lines.

Matrix metalloproteinases play an important role in tissue regeneration, wound healing and tumor invasion. Our previous studies have shown a higher motility of HaCaT-ras-transfected cells compared with HaCaT or normal human keratinocytes (NHK) in correlation with a higher secretion of MMP-2 (72 kDa) or MMP-9 (92 kDa), according to the medium used for cell cultures. Presently, the expression and activity of MMPs were investigated in two reconstructed skin models, using a dead de-epidermized dermis (DED) or a dermal substitute including living fibroblasts. In all experiments, MMP-9 was essentially secreted by NHK and to a greater extent by HaCaT cells. Its active form (86 kDa) was only detected in both reconstructed skin models according to keratinocyte differentiation. MMP-2 was mainly secreted by living fibroblasts included in the dermal substitute skin model. In this case, its activation was up-regulated when HaCaT cell lines were seeded onto the dermal substitute according to their culture at air/liquid interface as shown for MMP-9. The collagenase MMP-1 and stromelysin-1 (MMP-3), susceptible to activate pro-MMP-2 and -9, respectively, were detected in their inactive form by ELISA. MMP-1 was expressed in both models but MMP-3 required the presence of living fibroblasts. Their activities were not detected using specific fluorogenic substrates. In the skin equivalent model using HaCaT, the extensive secretion and activation of MMP-2 and MMP-9 could explain the defect observed in basal membrane reconstruction, suggesting a direct interaction of HaCaT with fibroblasts.