The electrospinning of a thermo-responsive polymer with peptide conjugates for phenotype support and extracellular matrix production of therapeutically relevant mammalian cells.

Current cell expansion methods for tissue engineering and regenerative medicine applications rely on the use of enzymatic digestion passaging and 2D platforms. However, this enzymatic treatment significantly reduces cell quality, due to the destruction of important cell-surface proteins. In addition, culture in 2D results in undesired de-differentiation of the cells caused by the lack of 3D similarity to the natural extracellular matrix (ECM) environment. Research has led to the development of thermo-responsive surfaces for the continuous culture of cells. These thermo-responsive materials properties can be used to passage cells from the surface when the cell culture temperature is reduced. Here we report the development of a PLA/thermo-responsive (PDEGMA) blend 3D electrospun fibre-based scaffold to create an enzymatic-free 3D cell culture platform for the expansion of mammalian cells with the desired phenotype for clinical use. Human corneal stromal cells (hCSCs) were used as an exemplar as they have been observed to de-differentiate to an undesirable myo-fibroblastic phenotype when cultured by conventional 2D cell culture methods. Scaffolds were functionalised with a cell adherence peptide sequence GGG-YIGSR by thiol-ene chemistry to improve cell adherence and phenotype support. This was obtained by functionalising the thermo-responsive polymer with a thiol (PDEGMA/PDEGSH) by co-polymerisation. These incorporated thiols react with the norbornene acid functionalised peptide (Nor-GGG-YIGSR) under UV exposure. Presence of the thiol in the scaffold and subsequent peptide attachment on the scaffolds were confirmed by fluorescence labelling, ToF-SIMS and XPS analysis. The biocompatibility of the peptide containing scaffolds was assessed by the adhesion, proliferation and immuno-staining of hCSCs. Significant increase in hCSC adherence and proliferation was observed on the peptide containing scaffolds. Immuno-staining showed maintained expression of the desired phenotypic markers ALDH, CD34 and CD105, while showing no or low expression of the undesired phenotype marker α-SMA. This desired expression was observed to be maintained after thermo-responsive passaging and higher when cells were cultured on PLA scaffolds with 10 wt% PDEGMA/4 mol% PDEGS-Nor-GGG-YIGSR. This paper describes the fabrication and application of a first generation, biocompatible peptide conjugated thermo-responsive fibrous scaffold. The ease of fabrication, successful adherence and expansion of a therapeutically relevant cell type makes these scaffolds a promising new class of materials for the application of cell culture expansion platforms in the biomaterials and tissue engineering field.

A design of experiments approach to identify the influencing parameters that determine poly-D,L-lactic acid (PDLLA) electrospun scaffold morphologies.

Electrospun fibrous materials have increasing applications in regenerative medicine due to the similarity of fibre constructs to the morphology of certain extracellular matrices. Although experimentally the electrospinning method is relatively simple, at the theoretical level the interactions between process parameters and their influence on the fibre morphology is not yet fully understood. Here, we hypothesised that a design of experiments (DoE) model could determine combinations of process parameters that result in significant effects on poly-D,L-lactic acid (PDLLA) fibre morphology. The process parameters used in this study were applied voltage, needle-to-collector distance, flow rate and polymer concentration. Data obtained for mean fibre diameter, standard deviation (SD) of the fibre diameter (measure of fibre morphology) and presence of 'beading' on the fibres (beads per µm2) were evaluated as a measure of PDLLA fibre morphology. Uniform fibres occurred at SDs of ≤500 nm, 'beads-on-string' morphologies were apparent between ±500 and 1300 nm and large beads were observed at ±1300-1800 nm respectively. Mean fibre diameter was significantly influenced by the applied voltage and interaction between flow rate and polymer concentration. Fibre morphology was mainly influenced by the polymer concentration, while bead distribution was significantly influenced by the polymer concentration as well as the flow rate. The resultant DoE model regression equations were tested and considered suitable for the prediction of parameters combinations needed for desired PDLLA fibre diameter and additionally provided information regarding the expected fibre morphology.

A novel electrospun biphasic scaffold provides optimal three-dimensional topography for in vitro co-culture of airway epithelial and fibroblast cells.

Conventional airway in vitro models focus upon the function of individual structural cells cultured in a two-dimensional monolayer, with limited three-dimensional (3D) models of the bronchial mucosa. Electrospinning offers an attractive method to produce defined, porous 3D matrices for cell culture. To investigate the effects of fibre diameter on airway epithelial and fibroblast cell growth and functionality, we manipulated the concentration and deposition rate of the non-degradable polymer polyethylene terephthalate to create fibres with diameters ranging from nanometre to micrometre. The nanofibre scaffold closely resembles the basement membrane of the bronchiole mucosal layer, and epithelial cells cultured at the air-liquid interface on this scaffold showed polarized differentiation. The microfibre scaffold mimics the porous sub-mucosal layer of the airway into which lung fibroblast cells showed good penetration. Using these defined electrospinning parameters we created a biphasic scaffold with 3D topography tailored for optimal growth of both cell types. Epithelial and fibroblast cells were co-cultured onto the apical nanofibre phase and the basal microfibre phase respectively, with enhanced epithelial barrier formation observed upon co-culture. This biphasic scaffold provides a novel 3D in vitro platform optimized to mimic the different microenvironments the cells encounter in vivo on which to investigate key airway structural cell interactions in airway diseases such as asthma.

Evaluation of skeletal tissue repair, part 1: assessment of novel growth-factor-releasing hydrogels in an ex vivo chick femur defect model.

Current clinical treatments for skeletal conditions resulting in large-scale bone loss include autograft or allograft, both of which have limited effectiveness. In seeking to address bone regeneration, several tissue engineering strategies have come to the fore, including the development of growth factor releasing technologies and appropriate animal models to evaluate repair. Ex vivo models represent a promising alternative to simple in vitro systems or complex, ethically challenging in vivo models. We have developed an ex vivo culture system of whole embryonic chick femora, adapted in this study as a critical size defect model to investigate the effects of novel bone extracellular matrix (bECM) hydrogel scaffolds containing spatio-temporal growth factor-releasing microparticles and skeletal stem cells on bone regeneration, to develop a viable alternative treatment for skeletal degeneration. Alginate/bECM hydrogels combined with poly (d,l-lactic-co-glycolic acid) (PDLLGA)/triblock copolymer (10-30% PDLLGA-PEG-PDLLGA) microparticles releasing VEGF, TGF-ß3 or BMP-2 were placed, with human adult Stro-1+ bone marrow stromal cells, into 2mm central segmental defects in embryonic chick femurs. Alginate/bECM hydrogels loaded with HSA/VEGF or HSA/TGF-ß3 demonstrated a cartilage-like phenotype, with minimal collagen I deposition, comparable to HSA-only control hydrogels. The addition of BMP-2 releasing microparticles resulted in enhanced structured bone matrix formation, evidenced by increased Sirius red-stained matrix and collagen expression within hydrogels. This study demonstrates delivery of bioactive growth factors from a novel alginate/bECM hydrogel to augment skeletal tissue formation and the use of an organotypic chick femur defect culture system as a high-throughput test model for scaffold/cell/growth factor therapies for regenerative medicine.

Evaluation of skeletal tissue repair, part 2: enhancement of skeletal tissue repair through dual-growth-factor-releasing hydrogels within an ex vivo chick femur defect model.

There is an unmet need for improved, effective tissue engineering strategies to replace or repair bone damaged through disease or injury. Recent research has focused on developing biomaterial scaffolds capable of spatially and temporally releasing combinations of bioactive growth factors, rather than individual molecules, to recapitulate repair pathways present in vivo. We have developed an ex vivo embryonic chick femur critical size defect model and applied the model in the study of novel extracellular matrix (ECM) hydrogel scaffolds containing spatio-temporal combinatorial growth factor-releasing microparticles and skeletal stem cells for bone regeneration. Alginate/bovine bone ECM (bECM) hydrogels combined with poly(d,l-lactic-co-glycolic acid) (PDLLGA)/triblock copolymer (10-30% PDLLGA-PEG-PLDLGA) microparticles releasing dual combinations of vascular endothelial growth factor (VEGF), chondrogenic transforming growth factor beta 3 (TGF-ß3) and the bone morphogenetic protein BMP2, with human adult Stro-1+bone marrow stromal cells (HBMSCs), were placed into 2mm central segmental defects in embryonic day 11 chick femurs and organotypically cultured. Hydrogels loaded with VEGF combinations induced host cell migration and type I collagen deposition. Combinations of TGF-ß3/BMP2, particularly with Stro-1+HBMSCs, induced significant formation of structured bone matrix, evidenced by increased Sirius red-stained matrix together with collagen expression demonstrating birefringent alignment within hydrogels. This study demonstrates the successful use of the chick femur organotypic culture system as a high-throughput test model for scaffold/cell/growth factor therapies in regenerative medicine. Temporal release of dual growth factors, combined with enriched Stro-1+HBMSCs, improved the formation of a highly structured bone matrix compared to single release modalities. These studies highlight the potential of a unique alginate/bECM hydrogel dual growth factor release platform for bone repair.

Human airway smooth muscle maintain in situ cell orientation and phenotype when cultured on aligned electrospun scaffolds.

Human airway smooth muscle (HASM) contraction plays a central role in regulating airway resistance in both healthy and asthmatic bronchioles. In vitro studies that investigate the intricate mechanisms that regulate this contractile process are predominantly conducted on tissue culture plastic, a rigid, 2D geometry, unlike the 3D microenvironment smooth muscle cells are exposed to in situ. It is increasingly apparent that cellular characteristics and responses are altered between cells cultured on 2D substrates compared with 3D topographies. Electrospinning is an attractive method to produce 3D topographies for cell culturing as the fibers produced have dimensions within the nanometer range, similar to cells' natural environment. We have developed an electrospun scaffold using the nondegradable, nontoxic, polymer polyethylene terephthalate (PET) composed of uniaxially orientated nanofibers and have evaluated this topography's effect on HASM cell adhesion, alignment, and morphology. The fibers orientation provided contact guidance enabling the formation of fully aligned sheets of smooth muscle. Moreover, smooth muscle cells cultured on the scaffold present an elongated cell phenotype with altered contractile protein levels and distribution. HASM cells cultured on this scaffold responded to the bronchoconstrictor bradykinin. The platform presented provides a novel in vitro model that promotes airway smooth muscle cell development toward a more in vivo-like phenotype while providing topological cues to ensure full cell alignment.

Post-processing of polymer foam tissue scaffolds with high power ultrasound: a route to increased pore interconnectivity, pore size and fluid transport.

The aim of this work is to demonstrate that the structural and fluidic properties of polymer foam tissue scaffolds, post-fabrication but prior to the introduction of cells, can be engineered via exposure to high power ultrasound. Our analysis is supported by measurements of fluid uptake during insonification and imaging of the scaffold microstructure via X-ray computed tomography, scanning electron microscopy and acoustic microscopy. The ultrasonic treatment is performed with a frequency of 30 kHz, average intensities up to 80,000 Wm(-2) and exposure times up to 20 h. The treatment is found to increase the mean pore size by over 10%. More striking is the improvement in fluid uptake: for scaffolds with only 40% water uptake via standard immersion techniques, we can routinely achieve full saturation of the scaffold over approximately one hour of exposure. These desirable modifications occur with negligible loss of scaffold integrity and mass, and are optimized when the ultrasound treatment is coupled to a pre-wetting stage with ethanol. Our findings suggest that high power ultrasound is highly targeted towards flow obstructions in the scaffold architecture, thereby providing an efficient means to promote pore interconnectivity and fluid transport in thick foam tissue scaffolds.

Hydrogels derived from demineralized and decellularized bone extracellular matrix.

The extracellular matrix (ECM) of mammalian tissues has been isolated, decellularized and utilized as a scaffold to facilitate the repair and reconstruction of numerous tissues. Recent studies have suggested that superior function and complex tissue formation occurred when ECM scaffolds were derived from site-specific homologous tissues compared with heterologous tissues. The objectives of the present study were to apply a stringent decellularization process to demineralized bone matrix (DBM), prepared from bovine bone, and to characterize the structure and composition of the resulting ECM materials and DBM itself. Additionally, we sought to produce a soluble form of DBM and ECM which could be induced to form a hydrogel. Current clinical delivery of DBM particles for treatment of bone defects requires incorporation of the particles within a carrier liquid. Differences in osteogenic activity, inflammation and nephrotoxicity have been reported with various carrier liquids. The use of hydrogel forms of DBM or ECM may reduce the need for carrier liquids. DBM and ECM hydrogels exhibited sigmoidal gelation kinetics consistent with a nucleation and growth mechanism, with ECM hydrogels characterized by lower storage moduli than the DBM hydrogels. Enhanced proliferation of mouse primary calvarial cells was achieved on ECM hydrogels, compared with collagen type I and DBM hydrogels. These results show that DBM and ECM hydrogels have distinct structural, mechanical and biological properties and have the potential for clinical delivery without the need for carrier liquids.

Analysis of sintered polymer scaffolds using concomitant synchrotron computed tomography and in situ mechanical testing.

The mechanical behaviour of polymer scaffolds plays a vital role in their successful use in bone tissue engineering. The present study utilised novel sintered polymer scaffolds prepared using temperature-sensitive poly(DL-lactic acid-co-glycolic acid)/poly(ethylene glycol) particles. The microstructure of these scaffolds was monitored under compressive strain by image-guided failure assessment (IGFA), which combined synchrotron radiation computed tomography (SR CT) and in situ micro-compression. Three-dimensional CT data sets of scaffolds subjected to a strain rate of 0.01%/s illustrated particle movement within the scaffolds with no deformation or cracking. When compressed using a higher strain rate of 0.02%/s particle movement was more pronounced and cracks between sintered particles were observed. The results from this study demonstrate that IGFA based on simultaneous SR CT imaging and micro-compression testing is a useful tool for assessing structural and mechanical scaffold properties, leading to further insight into structure-function relationships in scaffolds for bone tissue engineering applications.

Growth-induced buckling of an epithelial layer.

We use a proof-of-concept experiment and two mathematical models to explore growth-induced tissue buckling, as may occur in colorectal crypt formation. Our experiment reveals how growth of a cultured epithelial monolayer on a thin flexible substrate can cause out-of-plane substrate deflections. We describe this system theoretically using a 'bilayer' model in which a growing cell layer adheres to a thin compressible elastic beam. We compare this with the 'supported-monolayer' model due to Edwards and Chapman (Bull Math Biol 69:1927-1942, 2007) for an incompressible expanding beam (representing crypt epithelium), which incorporates viscoelastic tethering to underlying stroma. We show that the bilayer model can exhibit buckling via parametric growth (in which the system passes through a sequence of equilibrium states, parameterised by the total beam length); in this case, non-uniformities in cell growth and variations in cell-substrate adhesion are predicted to have minimal effect on the shape of resulting buckled states. The supported-monolayer model reveals how competition between lateral supports and stromal adhesion influences the wavelength of buckled states (in parametric growth), and how non-equilibrium relaxation of tethering forces influences post-buckled shapes. This model also predicts that non-uniformities in growth patterns have a much weaker influence on buckled shapes than non-uniformities in material properties. Together, the experiment and models support the concept of patterning by growth-induced buckling and suggest that targeted softening of a growing cell layer provides greater control in shaping tissues than non-uniform growth.

Tracking large solid constructs suspended in a rotating bioreactor: A combined experimental and theoretical study.

We present a combined experimental and theoretical study of the trajectory of a large solid cylindrical disc suspended within a fluid-filled rotating cylindrical vessel. The experimental set-up is relevant to tissue-engineering applications where a disc-shaped porous scaffold is seeded with cells to be cultured, placed within a bioreactor filled with nutrient-rich culture medium, which is then rotated in a vertical plane to keep the growing tissue construct suspended in a state of "free fall." The experimental results are compared with theoretical predictions based on the model of Cummings and Waters (2007), who showed that the suspended disc executes a periodic motion. For anticlockwise vessel rotation three regimes were identified: (i) disc remains suspended at a fixed position on the right-hand side of the bioreactor; (ii) disc executes a periodic oscillatory motion on the right-hand side of the bioreactor; and (iii) disc orbits the bioreactor. All three regimes are captured experimentally, and good agreement between theory and experiment is obtained. For the tissue engineering application, computation of the fluid dynamics allows the nutrient concentration field surrounding a tissue construct (a property that cannot be measured experimentally) to be determined (Cummings and Waters, 2007). The implications for experimental cell-culture protocols are discussed.

In situ monitoring of 3D in vitro cell aggregation using an optical imaging system.

Bioreactor systems that maintain cells and tissues in suspension are increasingly popular for culturing 3D constructs to avoid the loss of in vivo cell function associated with traditional 2D culture methods. There is a need for the online monitoring of such systems to provide better understanding and control of the processes involved and to prevent the disruption of these processes caused by offline sampling and endpoint analysis. We describe a system for the imaging and analysis of cell aggregation, over long periods, within a high aspect rotating vessel (HARV). The system exploits side illumination, using an adjustable beam pattern, to restrict the detected light to that scattered by the cell aggregates, thus eliminating the need for the fluorescent labeling of the cells. The in situ aggregation of mammalian cells (MCF-7 breast carcinoma cells) was monitored over an 8 h period and image sequences showing the growth and motion of the aggregates within the bioreactor were obtained. Detailed size and population data have been derived characterizing the development of the aggregates during this time. We show how the number of resolvable aggregates increases to reach a peak and then declines as these aggregates merge. Once formed, remaining aggregates are found to consolidate to form more tightly packed bodies, typically reducing in cross-sectional area by one third. These results provide the basis for the development of an automated feedback system to control the growth of 3D cell cultures for repeatable, reliable, and quality controlled experimentation.

Cell adhesion and mechanical properties of a flexible scaffold for cardiac tissue engineering.

Cardiac tissue engineering is focused on obtaining functional cardiomyocyte constructs to provide an alternative to cellular cardiomyoplasty. Mechanical stimuli have been shown to stimulate protein expression and the differentiation of mammalian cells from contractile tissues. Our aim was to obtain a flexible scaffold which could be used to apply mechanical forces during tissue regeneration. Poly(1,8-octanediol-co-citric acid) (POC) is an elastomer that can be processed into scaffolds for tissue engineering. We investigated the effect of modifying the porosity on the mechanical properties of the POC scaffolds. In addition, the effects of the storage method and strain rate on material integrity were assessed. The maximum elongation of POC porous films varied from 60% to 160% of their original length. A decrease in the porosity caused a rise in this elastic modulus. The attachment of HL-1 cardiomyocytes to POC was assessed on films coated with fibronectin, collagen and laminin. These extracellular matrix proteins promoted cell adhesion in a protein-type- and concentration-dependent manner. Therefore, POC scaffolds can be optimised to meet the mechanical and biological parameters needed for cardiac culture. This porous material has the potential to be used for cardiac tissue engineering as well as for other soft tissue applications.

The effect of anisotropic architecture on cell and tissue infiltration into tissue engineering scaffolds.

A common phenomenon in tissue engineering is rapid tissue formation on the outer edge of the scaffold which restricts cell penetration and nutrient exchange to the scaffold centre, resulting in a necrotic core. To address this problem, we generated scaffolds with both random and anisotropic open porous architectures to enhance cell and subsequent tissue infiltration throughout the scaffold for applications in bone and cartilage engineering. Hydroxyapatite (HA) and poly(D,L-lactic acid) (P(DL)LA) scaffolds with random open porosity were manufactured, using modified slip-casting and by supercritical fluid processing respectively, and subsequently characterised. An array of porous aligned channels (400 microm) was incorporated into both scaffold types and cell (human osteoblast sarcoma, for HA scaffolds; ovine meniscal fibrochondrocytes, for P(DL)LA scaffolds) and tissue infiltration into these modified scaffolds was assessed in vitro (cell penetration) and in vivo (tissue infiltration; HA scaffolds only). Scaffolds were shown to have an extensive random, open porous structure with an average porosity of 85%. Enhanced cell and tissue penetration was observed both in vitro and in vivo demonstrating that scaffold design alone can influence cell and tissue infiltration into the centre of tissue engineering scaffolds.

Tissue growth in a rotating bioreactor. Part I: mechanical stability.

We develop mathematical models to provide insights into the morphology of a tissue construct formed from a single-cell suspension in culture media, within a rotating bioreactor. The bioreactor consists of a cylindrical vessel of circular cross-section rotating about its longitudinal axis with constant angular speed. Experimental studies show that at rotation rates below a critical value, the cells 'self-assemble' to form smooth 'nodules' that are approximately cylindrical with elliptical cross-section; however, at rotation rates above a critical value, an amorphous construct forms with a highly irregular boundary. The construct is denser than the surrounding culture media and histological studies indicate that the interior of the construct, which is a mix of apoptotic cells and culture media, is surrounded by an outer rim of proliferating cells and collagen. The construct is modelled as a viscous fluid drop surrounded by an extensible membrane in a (less dense) immiscible viscous fluid within a rotating bioreactor. We consider both thin-disk and slender-pipe bioreactors for which the aspect ratio, L(*)/a(*) (where L(*) and a(*) are the bioreactor length and radius, respectively), is small and large, respectively, and obtain a series of spatially 2D problems (independent of the axial coordinate). We then examine the hypothesis that the construct morphology is a result of the mechanical forces that it experiences by considering the interfacial stability of an initially circular fluid-fluid interface to small-amplitude, oscillatory perturbations. The instability is driven by the density difference between the two fluids, and we investigate the effect of the rotation rate, the (time-dependent) gravitational field, and the material and geometrical properties of the system on the stability properties.

The influence of dispersant concentration on the pore morphology of hydroxyapatite ceramics for bone tissue engineering.

There is a clinical need for synthetic scaffolds that will promote bone regeneration. Important factors include obtaining an optimal porosity and size of interconnecting windows whilst maintaining scaffold mechanical strength, enabling complete penetration of cells and nutrients throughout the scaffold, preventing the formation of necrotic tissue in the centre of the scaffold. To address this we investigated varying slip deflocculation in order to control the resulting porosity, pore size and interconnecting window size whilst maintaining mechanical strength. Hydroxyapatite (HA) porous ceramics were prepared using a modified slip casting process. Rheological measurements of the HA slips were used to identify deflocculation conditions which resulted in changes in the cell and window sizes of the resulting ceramics. Sintered ceramics were characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Pore and window size distribution was determined by SEM. XRD analysis confirmed that the crystal structure remained HA after the sintering process. SEM showed that HA porous ceramics presented a highly interconnected porous network with average pore sizes ranging from 391+/-39 to 495+/-25 microm. The average window size varied from 73+/-5 to 135+/-7 microm. Pore diameters obtained were controllable in the range 200-500 microm. Window sizes were in the range 30-250 microm. The use of dispersant concentration allows pore and window size to be modified whilst maintaining control over porosity demonstrated by a porosity of 85% for seven different dispersant concentrations. The advantage of this approach allows the correlation between the rheological conditions of the slip and the resultant sintered ceramic properties. In particular, optimising the ceramic strength by controlling the agglomeration during the casting process.

Seeding cells into needled felt scaffolds for tissue engineering applications.

Tissue engineering methods are under development that will enable the repair or replacement of a variety of tissues, including articular cartilage and bone. To engineer functional tissue it is necessary that scaffolds initially be seeded with a large number of cells distributed evenly throughout the scaffold structure. It previously has been shown that, compared to static seeding conditions, seeding scaffolds under dynamic conditions facilitates high seeding densities and even distributions of cells (Li et al., Biotechnology Progress 2001;17:935-944). The efficiency of seeding HOSTE85 cells and bovine chondrocytes into needled felt scaffolds following agitation at different speeds was determined. Seeding efficiency was determined using the Hoechst 33258 assay, and cell viability was assessed using the Alamar Blue trade mark assay. The distribution of cells within the scaffolds was imaged using scanning electron microscopy. It was found that the optimum seeding conditions varied for HOSTE85 cells and bovine chondrocytes, with different agitation speeds leading to different seeding efficiencies, cell viabilities, and distributions of cells within scaffolds. The optimum agitation speeds for seeding a high number of viable cells into scaffolds so that they were arranged evenly were 300 rpm for HOSTE85 cells and 200 rpm for bovine chondrocytes.

Expression of antimicrobial neutrophil defensins in epithelial cells of active inflammatory bowel disease mucosa.

BACKGROUND/AIMS: The normal intestinal epithelium is increasingly being recognised as an important component of the mucosal innate protection against microorganisms. Human neutrophil defensins 1-3 (HNP 1-3) and lysozyme are components of the systemic innate immunity. The aim of this study was to investigate the expression of HNP 1-3 and lysozyme in normal and active inflammatory bowel disease (IBD) mucosa. METHODS: Mucosal tissue sections were studied by immunohistochemistry using antibodies to neutrophil defensins 1-3 and lysozyme. Extracts of purified intestinal epithelial cells were used for immunoblotting studies and antimicrobial activity against the phoP negative strain of Salmonella typhimurium. RESULTS: Surface epithelial cells strongly immunoreactive for neutrophil defensins and lysozyme were seen in active ulcerative colitis and Crohn's disease (but not normal or inactive IBD) mucosal samples. Many of these cells coexpressed both of the antimicrobial proteins. Immunoblotting studies confirmed the expression of neutrophil defensins in extracts of purified ulcerative colitis epithelial cells, which also demonstrated antimicrobial activity. CONCLUSION: HNP 1-3 and lysozyme are expressed in surface enterocytes of mucosa with active IBD and they may play an important role in intestinal host defence against luminal microorganisms.