Gene expression profile of the fibrotic response in the peritoneal cavity

Long-Term Controlled Growth Factor Release Using Layer-by-Layer Assembly for the Development of In Vivo Tissue-Engineered Blood Vessels

The development of a well-designed tissue-engineered blood vessel (TEBV) still remains a challenge. In recent years, approaches in which the host response to implanted biomaterials is used to generate vascular constructs within the patient's body have gained increasing interest. The delivery of growth factors to these in situ-engineered vascular grafts might enhance myofibroblast recruitment and the secretion of essential extracellular matrix proteins, thereby optimizing their functional properties. Layer-by-layer (LbL) coating has emerged as an innovative technology for the controlled delivery of growth factors in tissue engineering applications. In this study, we combined the use of surface-etched polymeric rods with LbL coatings to control the delivery of TGF-β1, PDGF-BB, and IGF-1 and steer the foreign body response toward the formation of a functional vascular graft. Results showed that the regenerated tissue is composed of elastin, glycosaminoglycans, and circumferentially oriented collagen fibers, without calcification or systemic spill of the released growth factors. Functional controlled delivery was observed, whereas myofibroblast-rich tissue capsules were formed with enhanced collagen and elastin syntheses using TGF-β1 and TGF-β1/PDGF-BB releasing rods, when compared to control rods that were solely surface-engineered by chloroform etching. By combining our optimized LbL method and surface-engineered rods in an in vivo bioreactor approach, we could regulate the fate and ECM composition of in situ-engineered vascular grafts to create a successful in vivo vascular tissue-engineered replacement.

Characterization of Inflammatory and Fibrotic Aspects of Tissue Remodeling of Acellular Dermal Matrix in a Nonhuman Primate Model

Human acellular dermal matrices (hADMs) are applied in various soft tissue reconstructive surgeries as scaffolds to support tissue remodeling and regeneration. To evaluate the clinical efficacy of hADM implants, it is integral that the hADM does not induce a host chronic inflammatory response leading to fibrotic encapsulation of the implant. In this study, we characterized the inflammatory and fibrosis-related tissue remodeling response of 2 commercial hADM products (SimpliDerm and AlloDerm RTU) in a nonhuman primate model using histology and gene expression profiling.

METHODS

Eighteen African green monkeys with abdominal wall defects were applied to evaluate the performance of SimpliDerm and AlloDerm RTU implants (N = 3) at 2, 4, and 12-weeks post-implantation. Using histology and gene expression profiling, tissue responses such as implant integration, degradation, cell infiltration, immune response, neovascularization, and pro-fibrotic responses over time were evaluated.

RESULTS

SimpliDerm showed a lower initial inflammatory response and slower implant degradation rate than AlloDerm RTU evidenced by histomorphological analysis. These factors led to a more anti-inflammatory and pro-remodeling microenvironment within SimpliDerm, demonstrated by lower TNFα levels and lower expression levels of pro-fibrotic markers, and promoted tissue repair and regeneration by 3-months post-implantation.

CONCLUSIONS

Overall, histology and gene expression profiling analyses shown in this study demonstrated an effective model for analyzing hADM performance in terms of host inflammatory and fibrotic response. Further studies are warranted to fully evaluate the utility of this novel hADM in the clinical setting and verify the prognosis of our pre-clinical analysis model.

WNT signaling is required for peritoneal membrane angiogenesis

The wingless-type mouse mammary tumor virus integration site family (WNT) signaling pathway is involved in wound healing and fibrosis. We evaluated the WNT signaling pathway in peritoneal membrane injury. We assessed WNT1 protein expression in the peritoneal effluents of 54 stable peritoneal dialysis (PD) patients and WNT-related gene expression in ex vivo mesothelial cell cultures from 21 PD patients. In a transforming growth factor-β (TGF-β)-mediated animal model of peritoneal fibrosis, we evaluated regulation of the WNT pathway and the effect of WNT inhibition on peritoneal fibrosis and angiogenesis. WNT1 and WNT2 gene expression were positively correlated with peritoneal membrane solute transport in PD patients. In the mouse peritoneum, TGF-β-induced peritoneal fibrosis was associated with increased expression of WNT2 and WNT4. Peritoneal β-catenin protein was significantly upregulated after infection with adenovirus expressing TGF-β (AdTGF-β) along with elements of the WNT signaling pathway. Treatment with a β-catenin inhibitor (ICG-001) in mice with AdTGF-β-induced peritoneal fibrosis resulted in attenuation of peritoneal angiogenesis and reduced vascular endothelial growth factor. Similar results were also observed with the WNT antagonist Dickkopf-related protein (DKK)-1. In addition to this, DKK-1 blocked epithelial-mesenchymal transition and increased levels of the cell adhesion protein E-cadherin. We provide evidence that WNT signaling is active in the setting of experimental peritoneal fibrosis and WNT1 correlates with patient peritoneal membrane solute transport in PD patients. Intervention in this pathway is a possible therapy for peritoneal membrane injury.

Temporal changes in peritoneal cell phenotype and neoelastic matrix induction with hyaluronan oligomers and TGF-β1 after implantation of engineered conduits

The neoassembly and maturation of elastic matrix is an important challenge for engineering small-diameter grafts for patients with peripheral artery disease. We have previously shown that hyaluronan oligomers and transforming growth factor-β (elastogenic factors or EFs) promote elastogenesis in smooth muscle cell (SMC) culture. However, their combined effects on macrophages and inflammatory cells in vivo are unknown. This information is needed to use the body (e.g., peritoneal cavity) as an "in vivo bioreactor" to recruit autologous cells to implanted EF-functionalized scaffolds. In this study, we determined if peritoneal fluid cells respond to EFs like smooth muscle cells and if these responses differ between cells sourced during different stages of inflammation triggered by scaffold implantation. Electrospun poly(ε-caprolactone)/collagen conduits were implanted in the peritoneal cavity prior to peritoneal fluid collection at 3-42 days postimplantation. Cells from the fluid were cultured in vitro with and without EFs to determine their response. Their phenotype/behaviour was assessed with a DNA assay, quantitative real-time PCR, and immunofluorescence. The EFs reduced peritoneal cell proliferation, maintained cell contractility, and unexpectedly did not exhibit proelastic effects, which we attributed to differences in cell density. We found the greatest elastin deposition in regions containing a high cell density. Further, we found that cells isolated from the peritoneal cavity at longer times after conduit implantation responded better to the EFs and exhibited more CD31 expression than cells at an earlier time point. Overall, this study provides information about the potential use of EFs in vivo and can guide the design of future tissue-engineered vascular grafts.

Peritoneal pre-conditioning reduces macrophage marker expression in collagen-containing engineered vascular grafts

Engineered vascular grafts have shown promise as arteriovenous shunts, but they have not yet progressed to clinical trials for coronary arteries <4 mm in diameter such as the coronary arteries. Control over initial biomaterial properties and remodeling are necessary to generate viable grafts. In this study, we blended collagen with a synthetic material, poly(ε-caprolactone), to modulate the post-grafting inflammatory response while avoiding aneurysmal-like dilation and failure that can occur with pure collagen grafts. We also used pre-implantation in an "in vivo bioreactor" to recruit autologous cells and improve patency after grafting. Electrospun conduits were pre-implanted within rat peritoneal cavities and then grafted autologously into abdominal aortae. Conduit collagen percentages and pre-implantation were tested for their impact on graft remodeling and patency. Burst pressures >2000 mmHg, reproducible expansion with systole/diastole, and maintenance of mechanical integrity were observed. More importantly, peritoneal pre-implantation reduced overall lipid oxidation, intimal layer thickness, and expression of an M1 macrophage marker. The condition with the most collagen, 25%, exhibited the lowest expression of macrophage markers but also resulted in a thicker intimal layer. Overall, the 10% collagen/PCL with peritoneal pre-implantation condition appeared to exhibit the best combination of responses, and may result in improved clinical graft viability.

STATEMENT OF SIGNIFICANCE

This manuscript describes a rodent study to systematically determine the benefits of both pre-implantation in the peritoneal cavity and specific ratios of collagen on engineered vascular graft viability. We show that pre-implantation had a significant benefit, including decreasing the expression of macrophage markers and reducing lipid oxidation after grafting. This study is the first time that the benefits of peritoneal pre-implantation have been compared to an "off the shelf," directly grafted control condition. We also demonstrated the importance of specific collagen ratio on the response after grafting. Overall, we feel that this article will be of interest to the field and it has the potential to address a significant clinical need: a graft for coronary arteries <4 mm in diameter.

Time course study of long-term biocompatibility and foreign body reaction to intraneural polyimide-based implants

The foreign body reaction (FBR) against an implanted device is characterized by the formation of a fibrotic tissue around the implant. In the case of interfaces for peripheral nerves, used to stimulate specific group of axons and to record different nerve signals, the FBR induces a matrix deposition around the implant creating a physical separation between nerve fibers and the interface that may reduce its functionality over time. In order to understand how the FBR to intraneural interfaces evolves, polyimide non-functional devices were implanted in rat peripheral nerve. Functional tests (electrophysiological, pain and locomotion) and histological evaluation demonstrated that implanted devices did not cause any alteration in nerve function, in myelinated axons or in nerve architecture. The inflammatory response due to the surgical implantation decreased after 2 weeks. In contrast, inflammation was higher and more prolonged in the device implanted nerves with a peak after 2 weeks. With regard to tissue deposition, a tissue capsule appeared soon around the devices, acquiring maximal thickness at 2 weeks and being remodeled subsequently. Immunohistochemical analysis revealed two different cell types implicated in the FBR in the nerve: macrophages as the first cells in contact with the interface and fibroblasts that appear later at the edge of the capsule. Our results describe how the FBR against a polyimide implant in the peripheral nerve occurs and which are the main cellular players. Increasing knowledge of these responses will help to improve strategies to decrease the FBR against intraneural implants and to extend their usability. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 746-757, 2018.

A Biodesigned Nanocomposite Biomaterial for Auricular Cartilage Reconstruction

Current biomaterials for auricular replacement are associated with high rates of infection and extrusion. The development of new auricular biomaterials that mimic the mechanical properties of native tissue and promote desirable cellular interactions may prevent implant failure. A porous 3D nanocomposite scaffold (NS) based on POSS-PCU (polyhedral oligomeric silsesquioxane nanocage into polycarbonate based urea-urethane) is developed with an elastic modulus similar to native ear. In vitro biological interactions on this NS reveal greater protein adsorption, increased fibroblast adhesion, proliferation, and collagen production compared with Medpor (the current synthetic auricular implant). In vivo, the POSS-PCU with larger pores (NS2; 150-250 μm) have greater tissue ingrowth (≈5.8× and ≈1.4 × increase) than the POSS-PCU with smaller pores (NS1; 100-50 μm) and when compared to Medpor (>100 μm). The NS2 with the larger pores demonstrates a reduced fibrotic encapsulation compared with NS1 and Medpor (≈4.1× and ≈1.6×, respectively; P < 0.05). Porosity also influences the amount of neovascularization within the implants, with no blood vessel observed in NS1 (12 weeks postimplantation). The lack of chronic inflammatory response for all materials may indicate that the elastic modulus and pore size of the implant scaffold could be important design considerations for influencing fibrotic responses to auricular and other soft tissue implants.

Bladder Acellular Matrix Grafts Seeded with Adipose-Derived Stem Cells and Incubated Intraperitoneally Promote the Regeneration of Bladder Smooth Muscle and Nerve in a Rat Model of Bladder Augmentation

The objective of this study was to investigate the feasibility of bladder acellular matrix grafts (BAMGs) seeded with adipose-derived stem cells (ASCs) followed by intraperitoneal incubation for bladder reconstruction in a rat model of bladder augmentation, and to explore the underlying mechanism. Autologous CM-DiI-labeled ASC-seeded (experimental group) and unseeded (control group) BAMGs were incubated in the peritoneum of male rats for 2 weeks and then harvested for bladder augmentation. Histological analysis of the incubated BAMGs revealed numerous cells growing in homogeneous collagen bundles in both groups. In the control BAMGs, these cells were mesenchyme derived, while in the ASC-seeded BAMGs, myofibroblasts and mesothelial cells were found inside and on the surface of the scaffold, respectively. Immunofluorescence analysis demonstrated that some of the myofibroblasts were transdifferentiated from the ASCs after 2 weeks of intraperitoneal incubation. The greater bladder capacity was found in the experimental group than the control group both 4 and 14 weeks postoperatively. Histological analysis revealed that the entire urothelium regenerated well both in the experimental group and the control group without significant difference 4 weeks and 14 weeks postoperatively. From the quantitative data of immunohistochemical and immunofluorescence staining, the smooth muscle cells (SMCs) regenerated significantly better in the experimental group than the control group both 4 weeks and 14 weeks postoperatively. Also significantly more nerve cells were found in the experimental group 14 weeks postoperatively. At 4 weeks postoperatively, the immunofluorescence double staining revealed that some SMCs in the BAMG were transdifferentiated from the implanted ASCs, but no CM-DiI labeling of ASCs was detected 14 weeks postoperatively. Taken together, our results demonstrate that ASC-seeded and peritoneally incubated BAMGs promote not only the morphological regeneration of the bladder smooth muscle and nerve, but also the bladder capacity, which indicates their potential for bladder regeneration.

Neutrophil Responses to Sterile Implant Materials

In vivo implantation of sterile materials and devices results in a foreign body immune response leading to fibrosis of implanted material. Neutrophils, one of the first immune cells to be recruited to implantation sites, have been suggested to contribute to the establishment of the inflammatory microenvironment that initiates the fibrotic response. However, the precise numbers and roles of neutrophils in response to implanted devices remains unclear. Using a mouse model of peritoneal microcapsule implantation, we show 30–500 fold increased neutrophil presence in the peritoneal exudates in response to implants. We demonstrate that these neutrophils secrete increased amounts of a variety of inflammatory cytokines and chemokines. Further, we observe that they participate in the foreign body response through the formation of neutrophil extracellular traps (NETs) on implant surfaces. Our results provide new insight into neutrophil function during a foreign body response to peritoneal implants which has implications for the development of biologically compatible medical devices.

Preparation of Biotubes with vascular cells component by in vivo incubation using adipose-derived stromal cell-exuding multi-microporous molds

Biotubes, prepared using in-body tissue architecture (IBTA) technology, have adequate mechanical properties and excellent biocompatibility for vascular grafts. However, they have thin walls, lack vascular constructing cells, and are composed of subcutaneous connective tissues consisting mainly of collagen and fibroblasts. This study aimed to prepare Biotubes with a vascular-like structure including an endothelial cell lining and a smooth muscle cell by IBTA using adipose-derived vascular stromal cell (ADSCs)-exuding specially designed multiporous tubes (outer diameter 5 mm, length 24 mm, pore size 500 μm, pore number 180, cell number/tube >3.0 × 10(6)). ADSCs were separated from rat subcutaneous fat, suspended in a Matrigel™ solution at 4 °C, and then filled into the tubes. After the tubes were embedded into dorsal subcutaneous pouches of the same rats for 2 weeks, robust Biotubes with a wall thickness of >600 μm were formed surrounding the tubes. The luminal layer of the obtained Biotubes was dominated by the cells positive for an endothelial marker. Almost the entire intima, with a thickness of about 400 μm, was occupied with cells positive for a smooth muscle marker. Both cells were derived from ADSCs. Biotube walls were constructed by fusing ADSC-derived vascular constructing cells exuded from the tubes and fibroblasts and collagen from the surrounding connective tissue. A robust Biotubes with vascular cells component, were formed after only 2 weeks of subcutaneous incubation of ADSCs-exuding multiporous tubes.

Altering in vivo macrophage responses with modified polymer properties

Macrophage reprogramming has long been the focus of research in disease therapeutics and biomaterial implantation. With different chemical and physical properties of materials playing a role in macrophage polarization, it is important to investigate and categorize the activation effects of material parameters both in vitro and in vivo. In this study, we have investigated the effects of material surface chemistry on in vivo polarization of macrophages. The library of materials used here include poly(N-isopropylacrylamide-co-acrylic acid) (p(NIPAm-co-AAc)) nanoparticles (∼600 nm) modified with various functional groups. This study also focuses on the development of a quantitative structure-activity relationship method (QSAR) as a predictive tool for determining the macrophage polarization in response to particular biomaterial surface chemistries. Here, we successfully use in vivo imaging and histological analysis to identify the macrophage response and activation. We demonstrate the ability to induce a spectrum of macrophage phenotypes with a change in material functionality as well as identify certain material parameters that seem to correlate with each phenotype. This suggests the potential to develop materials for a variety of applications and predict the outcome of macrophage activation in response to new surface chemistries.

The significance of macrophage phenotype in cancer and biomaterials

Macrophages have long been known to exhibit heterogeneous and plastic phenotypes. They show functional diversity with roles in homeostasis, tissue repair, immunity and disease. There exists a spectrum of macrophage phenotypes with varied effector functions, molecular determinants, cytokine and chemokine profiles, as well as receptor expression. In tumor microenvironments, the subset of macrophages known as tumor-associated macrophages generates byproducts that enhance tumor growth and angiogenesis, making them attractive targets for anti-cancer therapeutics. With respect to wound healing and the foreign body response, there is a necessity for balance between pro-inflammatory, wound healing, and regulatory macrophages in order to achieve successful implantation of a scaffold for tissue engineering. In this review, we discuss the multitude of ways macrophages are known to be important in cancer therapies and implanted biomaterials.

Tissue engineering bone using autologous progenitor cells in the peritoneum

Despite intensive research efforts, there remains a need for novel methods to improve the ossification of scaffolds for bone tissue engineering. Based on a common phenomenon and known pathological conditions of peritoneal membrane ossification following peritoneal dialysis, we have explored the possibility of regenerating ossified tissue in the peritoneum. Interestingly, in addition to inflammatory cells, we discovered a large number of multipotent mesenchymal stem cells (MSCs) in the peritoneal lavage fluid from mice with peritoneal catheter implants. The osteogenic potential of these peritoneal progenitor cells was demonstrated by their ability to easily infiltrate decalcified bone implants, produce osteocalcin and form mineralized bone in 8 weeks. Additionally, when poly(l-lactic acid) scaffolds loaded with bone morphogenetic protein-2 (a known osteogenic differentiation agent) were implanted into the peritoneum, signs of osteogenesis were seen within 8 weeks of implantation. The results of this investigation support the concept that scaffolds containing BMP-2 can stimulate the formation of bone in the peritoneum via directed autologous stem and progenitor cell responses.

Transcriptional switching in macrophages associated with the peritoneal foreign body response

We previously demonstrated that myeloid cells are the source of fibrotic tissue induced by foreign material implanted in the peritoneal cavity. This study utilised the MacGreen mouse, in which the Csf1r promoter directs myeloid-specific enhanced green fluorescent protein (EGFP) expression, to determine the temporal gene expression profile of myeloid subpopulations recruited to the peritoneal cavity to encapsulate implanted foreign material (cubes of boiled egg white). Cells with high EGFP expression (EGFP(hi)) were purified from exudate and encapsulating tissue at different times during the foreign body response, gene expression profiles determined using cDNA microarrays, and data clustered using the network analysis tool, Biolayout Express(3D). EGFP(hi) cells from all time points expressed high levels of Csf1r, Emr1 (encoding F4/80), Cd14 and Itgam (encoding Mac-1) providing internal validation of their myeloid nature. Exudate macrophages (days 4-7) expressed a large cluster of cell cycle genes; these were switched off in capsule cells. Early in capsule formation, Csf1r-EGFP(hi) cells expressed genes associated with tissue turnover, but later expressed both pro- and anti-inflammatory genes alongside a subset of mesenchyme-associated genes, a pattern of gene expression that adds weight to the concept of a continuum of macrophage phenotypes rather than distinct M1/M2 subsets. Moreover, rather than transdifferentiating to myofibroblasts, macrophages contributing to later stages of the peritoneal foreign body response warrant their own classification as 'fibroblastoid' macrophages.

Composition of intraperitoneally implanted electrospun conduits modulates cellular elastic matrix generation

Improving elastic matrix generation is critical to developing functional tissue engineered vascular grafts. Therefore, this study pursued a strategy to grow autologous tissue in vivo by recruiting potentially more elastogenic cells to conduits implanted within the peritoneal cavity. The goal was to determine the impacts of electrospun conduit composition and hyaluronan oligomer (HA-o) modification on the recruitment of peritoneal cells, and their phenotype and ability to synthesize elastic matrix. These responses were assessed as a function of conduit intra-peritoneal implantation time. This study showed that the blending of collagen with poly(ε-caprolactone) (PCL) promotes a faster wound healing response, as assessed by trends in expression of macrophage and smooth muscle cell (SMC) contractile markers and in matrix deposition, compared to the more chronic response for PCL alone. This result, along with the increase in elastic matrix production, demonstrates the benefits of incorporating as little as 25% w/w collagen into the conduit. In addition, PCR analysis demonstrated the challenges in differentiating between a myofibroblast and an SMC using traditional phenotypic markers. Finally, the impact of the tethered HA-o is limited within the inflammatory environment, unlike the significant response found previously in vitro. In conclusion, these results demonstrate the importance of both careful control of implanted scaffold composition and the development of appropriate delivery methods for HA-o.

Porous implants modulate healing and induce shifts in local macrophage polarization in the foreign body reaction

The foreign body reaction (FBR) to implanted materials is of critical importance when medical devices require biological integration and vascularization to support their proper function (e.g., transcutaneous devices, implanted drug delivery systems, tissue replacements, and sensors). One class of materials that improves FBR outcomes is made by sphere-templating, resulting in porous structures with uniform, interconnected 34 μm pores. With these materials we observe reduced fibrosis and increased vascularization. We hypothesized that improved healing is a result of a shift in macrophage polarization, often measured as the ratio of M1 pro-inflammatory cells to M2 pro-healing cells. In this study, macrophage polarity of 34 μm porous implants was compared to non-porous and 160 μm porous implants in subcutaneous mouse tissue. Immunohistochemistry revealed that macrophages in implant pores displayed a shift towards an M1 phenotype compared to externalized cells. Macrophages in 34 μm porous implants had up to 63% greater expression of M1 markers and up to 85% reduction in M2 marker expression (p < 0.05). Macrophages immediately outside the porous structure, in contrast, showed a significant enrichment in M2 phenotypic cells. This study supports a role for macrophage polarization in driving the FBR to implanted materials.

Transcriptional patterns in peritoneal tissue of encapsulating peritoneal sclerosis, a complication of chronic peritoneal dialysis

Encapsulating peritoneal sclerosis (EPS) is a devastating complication of peritoneal dialysis (PD), characterized by marked inflammation and severe fibrosis of the peritoneum, and associated with high morbidity and mortality. EPS can occur years after termination of PD and, in severe cases, leads to intestinal obstruction and ileus requiring surgical intervention. Despite ongoing research, the pathogenesis of EPS remains unclear. We performed a global transcriptome analysis of peritoneal tissue specimens from EPS patients, PD patients without EPS, and uremic patients without history of PD or EPS (Uremic). Unsupervised and supervised bioinformatics analysis revealed distinct transcriptional patterns that discriminated these three clinical groups. The analysis identified a signature of 219 genes expressed differentially in EPS as compared to PD and Uremic groups. Canonical pathway analysis of differentially expressed genes showed enrichment in several pathways, including antigen presentation, dendritic cell maturation, B cell development, chemokine signaling and humoral and cellular immunity (P value<0.05). Further interactive network analysis depicted effects of EPS-associated genes on networks linked to inflammation, immunological response, and cell proliferation. Gene expression changes were confirmed by qRT-PCR for a subset of the differentially expressed genes. EPS patient tissues exhibited elevated expression of genes encoding sulfatase1, thrombospondin 1, fibronectin 1 and alpha smooth muscle actin, among many others, while in EPS and PD tissues mRNAs encoding leptin and retinol-binding protein 4 were markedly down-regulated, compared to Uremic group patients. Immunolocalization of Collagen 1 alpha 1 revealed that Col1a1 protein was predominantly expressed in the submesothelial compact zone of EPS patient peritoneal samples, whereas PD patient peritoneal samples exhibited homogenous Col1a1 staining throughout the tissue samples. The results are compatible with the hypothesis that encapsulating peritoneal sclerosis is a distinct pathological process from the simple peritoneal fibrosis that accompanies all PD treatment.

Expanded applications, shifting paradigms and an improved understanding of host-biomaterial interactions

The conventional approach to biomaterial design and development typically focuses upon the mechanical and material properties with long-term objectives that include an inert host immune response and long-lasting mechanical and structural support. The emergence of and interest in tissue engineering and regenerative medicine have driven the development of novel cell-friendly biomaterials, materials with tailored degradation rates, materials with highly specific architectures and surfaces, and vehicles for delivery of bioactive molecules, among numerous other advancements. Each of these biomaterial developments supports specific strategies for tissue repair and reconstruction. These advancements in biomaterial form and function, combined with new knowledge of innate and acquired immune system biology, provide an impetus for re-examination of host-biomaterial interactions, including host-biomaterial interface events, spatial and temporal patterns of in vivo biomaterial remodeling, and related downstream functional outcomes. An examination of such issues is provided herein with a particular focus on macrophage polarization and its implications in tissue engineering and regenerative medicine.

Impact of electrospun conduit fiber diameter and enclosing pouch pore size on vascular constructs grown within rat peritoneal cavities

The generation of vascular grafts by recruiting autologous cells within the peritoneal cavity has shown promise. However, the microenvironment affects cell differentiation and elastic matrix production. Therefore, this study determined the impact of systematic changes in the average fiber diameter of electrospun poly(ɛ-caprolactone) conduits, and the pore size of pouches used to enclose the conduits, on recruited cells. After 2 weeks in the peritoneal cavity, fibrous capsules formed containing macrophages, α-smooth muscle actin (α-SMA)(+) and SM22α(+) myofibroblastic or smooth muscle like-cells, and what appeared to be mesothelial cells on the outer surfaces. These cells infiltrated and deposited matrix (e.g., collagen, hyaluoronan, and limited elastin) within conduit walls. Constructs enclosed within the largest pore pouches exhibited significantly better tissue generation responses (e.g., better cell infiltration, elongation, and matrix deposition). Additionally, the healing response was impacted by the conduit average fiber diameter, and consequently, the effective pore diameter, with the largest diameter fibers promoting the most positive healing response (e.g., greater total cellularity, extracellular matrix deposition, and α-SMA(+) cells). Six weeks post-intra-aortal grafting, constructs were occluded, but significant remodeling also occurred in the arterial microenvironment. Overall, these results demonstrate the importance of microenvironmental cues on recruited peritoneal cells and the necessity of developing strategies to further improve elastic matrix synthesis.

Macrophage phenotype as a predictor of constructive remodeling following the implantation of biologically derived surgical mesh materials

Macrophages have been classified as having plastic phenotypes which exist along a spectrum between M1 (classically activated; pro-inflammatory) and M2 (alternatively activated; regulatory, homeostatic). To date, the effects of polarization towards an M1 or M2 phenotype have been studied largely in the context of response to pathogen or cancer. Recently, M1 and M2 macrophages have been shown to play distinct roles in tissue remodeling following injury. In the present study, the M1/M2 paradigm was utilized to examine the role of macrophages in the remodeling process following implantation of 14 biologically derived surgical mesh materials in the rat abdominal wall. In situ polarization of macrophages responding to the materials was examined and correlated to a quantitative measure of the observed tissue remodeling response to determine whether macrophage polarization is an accurate predictor of the ability of a biologic scaffold to promote constructive tissue remodeling. Additionally the ability of M1 and M2 macrophages to differentially recruit progenitor-like cells in vitro, which are commonly observed to participate in the remodeling of those ECM scaffolds which have a positive clinical outcome, was examined as a possible mechanism underlying the differences in the observed remodeling responses. The results of the present study show that there is a strong correlation between the early macrophage response to implanted materials and the outcome of tissue remodeling. Increased numbers of M2 macrophages and higher ratios of M2:M1 macrophages within the site of remodeling at 14 days were associated with more positive remodeling outcomes (r(2)=0.525-0.686, p<0.05). Further, the results of the present study suggest that the constructive remodeling outcome may be due to the recruitment and survival of different cell populations to the sites of remodeling associated with materials that elicit an M1 vs. M2 response. Both M2 and M0 macrophage conditioned media were shown to have higher chemotactic activities than media conditioned by M1 macrophages (p<0.05). A more thorough understanding of these issues will logically influence the design of next generation biomaterials and the development of regenerative medicine strategies for the formation of functional host tissues.

Altered AIB1 or AIB1Δ3 expression impacts ERα effects on mammary gland stromal and epithelial content

Amplified in breast cancer 1 (AIB1) (also known as steroid receptor coactivator-3) is a nuclear receptor coactivator enhancing estrogen receptor (ER)α and progesterone receptor (PR)-dependent transcription in breast cancer. The splice variant AIB1Δ3 demonstrates increased ability to promote ERα and PR-dependent transcription. Both are implicated in breast cancer risk and antihormone resistance. Conditional transgenic mice tested the in vivo impact of AIB1Δ3 overexpression compared with AIB1 on histological features of increased breast cancer risk and growth response to estrogen and progesterone in the mammary gland. Combining expression of either AIB1 or AIB1Δ3 with ERα overexpression, we investigated in vivo cooperativity. AIB1 and AIB1Δ3 overexpression equivalently increased the prevalence of hyperplastic alveolar nodules but not ductal hyperplasia or collagen content. When AIB1 or AIB1Δ3 overexpression was combined with ERα, both stromal collagen content and ductal hyperplasia prevalence were significantly increased and adenocarcinomas appeared. Overexpression of AIB1Δ3, especially combined with overexpressed ERα, led to an abnormal response to estrogen and progesterone with significant increases in stromal collagen content and development of a multilayered mammary epithelium. AIB1Δ3 overexpression was associated with a significant increase in PR expression and PR downstream signaling genes. AIB1 overexpression produced less marked growth abnormalities and no significant change in PR expression. In summary, AIB1Δ3 overexpression was more potent than AIB1 overexpression in increasing stromal collagen content, inducing abnormal mammary epithelial growth, altering PR expression levels, and mediating the response to estrogen and progesterone. Combining ERα overexpression with either AIB1 or AIB1Δ3 overexpression augmented abnormal growth responses in both epithelial and stromal compartments.