Mast cells and tissue reaction to intraperitoneally implanted polymer capsules

Antifibrotic strategies for medical devices

A broad range of medical devices initiate an immune reaction known as the foreign body response (FBR) upon implantation. Here, collagen deposition at the surface of the implant occurs as a result of the FBR, ultimately leading to fibrous encapsulation and, in many cases, reduced function or failure of the device. Despite significant efforts, the prevention of fibrotic encapsulation has not been realized at this point in time. However, many next-generation medical technologies including cellular therapies, sensors and devices depend on the ability to modulate and control the FBR. For these technologies to become viable, significant advances must be made in understanding the underlying mechanism of this response as well as in the methods modulating this response. In this review, we highlight recent advances in the development of materials and coatings providing a reduced FBR and emphasize key characteristics of high-performing approaches. We also provide a detailed overview of the state-of-the-art in strategies relying on controlled drug release, the surface display of bioactive signals, materials-based approaches, and combinations of these approaches. Finally, we offer perspectives on future directions in this field.

Biomaterials: Foreign Bodies or Tuners for the Immune Response?

The perspectives of regenerative medicine are still severely hampered by the host response to biomaterial implantation, despite the robustness of technologies that hold the promise to recover the functionality of damaged organs and tissues. In this scenario, the cellular and molecular events that decide on implant success and tissue regeneration are played at the interface between the foreign body and the host inflammation, determined by innate and adaptive immune responses. To avoid adverse events, rather than the use of inert scaffolds, current state of the art points to the use of immunomodulatory biomaterials and their knowledge-based use to reduce neutrophil activation, and optimize M1 to M2 macrophage polarization, Th1 to Th2 lymphocyte switch, and Treg induction. Despite the fact that the field is still evolving and much remains to be accomplished, recent research breakthroughs have provided a broader insight on the correct choice of biomaterial physicochemical modifications to tune the reaction of the host immune system to implanted biomaterial and to favor integration and healing.

Cerebrospinal fluid from Alzheimer patients affects cell-mediated nerve growth factor production and cell survival in vitro

Alzheimer's disease (AD) is characterized by early degeneration of cholinergic neurons and decreased levels of nerve growth factor (NGF). Thus, increasing the NGF levels by for instance encapsulated cell bio-delivery (ECB) is a potential treatment strategy. The results from our previous first-in-human studies on ECB of NGF to the basal forebrain cholinergic neurons were promising, but indicated some variability of long-term viability of the encapsulated cells and associated reduced NGF-release. Here we studied the effect of amyloid beta-peptides (Aβ), interleukin 1-beta (IL-1β), and CSF from AD, Lewy body dementia (LBD) or subjective cognitive impairment (SCI) patients on the NGF overproducing cell line NGC-0295. At physiological concentrations, neither Aβ₄₀ nor Aβ₄₂ had any major impact on cell viability or NGF-production. In contrast, IL-1β dose-dependently affected NGF-production over time. Exposure of NGF-producing cells to CSF from AD patients showed significantly reduced NGF-release as compared to CSF from LBD or SCI patients. By mass spectrometry we found 3 proteins involved in inflammatory pathways to have an altered expression in AD CSF compared to LBD and SCI. Cell survival and NGF-release were not affected by Aβ. NGF-release was affected by IL-1β, suggesting that inflammation has a negative effect on ECB cells.

Embedded Adrenal Cells Graft Reduced Local and Early Nonspecific Inflammatory Phenomena Which Follow Agarose Beads Implantation

Microencapsulation of adrenal cells is proposed for reducing the nonspecific inflammatory reaction observed around polymer implants. This hypothesis was tested by comparing both host cellular reaction and the surrounding graft cell populations which appeared either when agarose embedded cells or when empty agarose beads were implanted. Our results showed that the fibrotic material that surrounded the implanted empty agarose microbeads was not as severe and important when adrenal cells were present. Similarly, T lymphocyte population surrounding the graft was considerably reduced together with the percentage of CD4 and CD8 positive cell subpopulations. The activation macrophage marker IaD disappeared. Our results support the hypothesis that embedded adrenal cells may be a suitable solution for reducing early inflammatory events due to microcapsules implantation.

Inflammasome components ASC and AIM2 modulate the acute phase of biomaterial implant-induced foreign body responses

Detailing the inflammatory mechanisms of biomaterial-implant induced foreign body responses (FBR) has implications for revealing targetable pathways that may reduce leukocyte activation and fibrotic encapsulation of the implant. We have adapted a model of poly(methylmethacrylate) (PMMA) bead injection to perform an assessment of the mechanistic role of the ASC-dependent inflammasome in this process. We first demonstrate that ASC^−/− mice subjected to PMMA bead injections had reduced cell infiltration and altered collagen deposition, suggesting a role for the inflammasome in the FBR. We next investigated the NLRP3 and AIM2 sensors because of their known contributions in recognising damaged and apoptotic cells. We found that NLRP3 was dispensable for the fibrotic encapsulation; however AIM2 expression influenced leukocyte infiltration and controlled collagen deposition, suggesting a previously unexplored link between AIM2 and biomaterial-induced FBR.

Innate Immunity and Biomaterials at the Nexus: Friends or Foes

Biomaterial implants are an established part of medical practice, encompassing a broad range of devices that widely differ in function and structural composition. However, one common property amongst biomaterials is the induction of the foreign body response: an acute sterile inflammatory reaction which overlaps with tissue vascularisation and remodelling and ultimately fibrotic encapsulation of the biomaterial to prevent further interaction with host tissue. Severity and clinical manifestation of the biomaterial-induced foreign body response are different for each biomaterial, with cases of incompatibility often associated with loss of function. However, unravelling the mechanisms that progress to the formation of the fibrotic capsule highlights the tightly intertwined nature of immunological responses to a seemingly noncanonical “antigen.” In this review, we detail the pathways associated with the foreign body response and describe possible mechanisms of immune involvement that can be targeted. We also discuss methods of modulating the immune response by altering the physiochemical surface properties of the biomaterial prior to implantation. Developments in these areas are reliant on reproducible and effective animal models and may allow a “combined” immunomodulatory approach of adapting surface properties of biomaterials, as well as treating key immune pathways to ultimately reduce the negative consequences of biomaterial implantation.

The localisation of inflammatory cells and expression of associated proteoglycans in response to implanted chitosan

Implantation of a foreign material almost certainly results in the formation of a fibrous capsule around the implant however, mechanistic events leading to its formation are largely unexplored. Mast cells are an inflammatory cell type known to play a role in the response to material implants, through the release of pro-inflammatory proteases and cytokines from their α-granules following activation. This study examined the in vivo and in vitro response of mast cells to chitosan, through detection of markers known to be produced by mast cells or involved with the inflammatory response. Mast cells, identified as Leder stained positive cells, were shown to be present in response to material implants. Additionally, the mast cell receptor, c-kit, along with collagen, serglycin, perlecan and chondroitin sulphate were detected within the fibrous capsules, where distribution varied between material implants. In conjunction, rat mast cells (RBL-2H3) were shown to be activated following exposure to chitosan as indicated by the release of β-hexosaminidase. Proteoglycan and glycosaminoglycans produced by the cells showed similar expression and localisation when in contact with chitosan to when chemically activated. These data support the role that mast cells play in the inflammatory host response to chitosan implants, where mediators released from their α-granules impact on the formation of a fibrous capsule by supporting the production and organisation of collagen fibres.

Molecular determinants of biocompatibility

The biocompatibility of medical implants dictates the fate of almost all medical devices. It is well established that medical devices trigger a variety of adverse tissue responses, such as inflammation, fibrosis, infection and thrombosis. However, the mechanisms involved in biomaterial-mediated tissue responses remain largely unknown. The lack of such knowledge hinders the development of biomaterials with better biocompatibility and safety. The aim of this review is to summarize our current understanding of the processes governing foreign body reactions to tissue-contact devices. Obviously, this information is urgently needed for assisting the rational design of materials or medical devices to minimize undesirable tissue reactions upon implantation and, in addition, to promote the wound healing process.

Inflammatory response to peritoneal implantation of alginate–poly-l-lysine microcapsules

A thorough understanding of the mechanisms involved in the host reaction to alginate-poly-L-lysine microcapsules (HRM) is important to design methods for the evaluation, selection, and development of biocompatible biomaterials and microcapsules or treatments to control this reaction. The objective of this study was to identify those immune cells and cytokines involved in the pathogenesis of the HRM. The total and differential cell counts were evaluated, and the mRNA expression of TNF-alpha, IL-1beta, IL-6 and TGF-beta1 was measured in peritoneal washings at 3, 17, 48, 96 and 168 h after saline or microcapsule injections. Neutrophil number and IL-1beta and IL-6 m-RNA expression presented an early transient increase, with no differences between saline and microcapsule injections, suggesting a reaction to the procedure. Macrophages, lymphocytes and TNF-alpha were significantly more activated over a longer period of time, after microcapsule implantation than saline injection. They are likely involved in transforming the reaction into a chronic inflammatory process. TGF-beta1 and IL-1beta presented a late (day 7) significant increase after microcapsule but not saline injections. They are likely involved in transforming the reaction into a fibrogenic process. These results suggest that macrophages, lymphocytes, TNF-alpha, IL-1beta and TGF-beta1 play a role in the pathogenesis of the HRM.

In vivo degradation and biocompatibility of a new class of alternate poly(ester-anhydrides) based on aliphatic and aromatic diacids

The degradation, tissue compatibility, and toxicology of a novel class of alternate poly(ester-anhydrides) were assessed in rats. It was observed that the degradation rate of the polymers in vivo was slower than that in vitro. In addition, erosion and intact zone were observed for all the polymers. IR and SEM analysis of the outer erosion and inner intact zone revealed that the outer zone degraded more rapidly than the inner zone. Such results were similar to that in vitro. All the studied poly(ester-anhydrides) produced mild inflammatory reactions and tissue encapsulation by layers of fibroblastic cells in vivo. Observation of liver and kidney tissue by light microscopy suggested the hydrolytic products of the studied poly(ester-anhydrides) had no harmful effects on the normal tissue/organs. In addition, the polymer and the breakdown products were found to be non-mutagenic by examination of micronucleus in bone marrow.

The participation of P- and E-selectins on biomaterial-mediated tissue responses

Biomaterial-mediated inflammatory responses often compromise the functions of implantable devices. The mechanism(s) involved in the inflammatory responses, which can be arbitrarily divided into phagocyte transmigration, chemotaxis, and adhesion to implant surfaces, are not totally understood. Because adhesion molecules have been shown to involved in phagocyte transmigration, this study was designed to investigate the participation of endothelial adhesion molecules in the pathogenesis of biomaterial-mediated inflammatory responses and fibrotic tissue formation. Using transgenic adhesion molecule knockout mice, we found that (1) deficiency of P-selectin reduced polymorphonuclear neutrophils (PMN) but not macrophages/monocytes (Mphi) transmigration and adhesion. (2) Furthermore, absence of both P- and E-selectin (P/E-deficient) dramatically diminished both PMN and Mphi recruitment to the peritoneal cavity and accumulation on implanted biomaterials. (3) Finally, the impairment of inflammatory responses in P/E-deficient mice significantly reduced the extent of subsequent biomaterial-mediated fibrotic responses. We conclude that P- and E-selectins are important for both biomaterial-mediated inflammatory and fibrotic reactions. Our results also indicate that the reduction of phagocyte accumulation might be responsible to the decrease of fibrotic tissue formation surrounding material implants. Better understanding of such sequence of events may help the rational design of biomaterials with desired tissue reactivity.

Technology of mammalian cell encapsulation

Entrapment of mammalian cells in physical membranes has been practiced since the early 1950s when it was originally introduced as a basic research tool. The method has since been developed based on the promise of its therapeutic usefulness in tissue transplantation. Encapsulation physically isolates a cell mass from an outside environment and aims to maintain normal cellular physiology within a desired permeability barrier. Numerous encapsulation techniques have been developed over the years. These techniques are generally classified as microencapsulation (involving small spherical vehicles and conformally coated tissues) and macroencapsulation (involving larger flat-sheet and hollow-fiber membranes). This review is intended to summarize techniques of cell encapsulation as well as methods for evaluating the performance of encapsulated cells. The techniques reviewed include microencapsulation with polyelectrolyte complexation emphasizing alginate-polylysine capsules, thermoreversible gelation with agarose as a prototype system, interfacial precipitation and interfacial polymerization, as well as the technology of flat sheet and hollow fiber-based macroencapsulation. Four aspects of encapsulated cells that are critical for the success of the technology, namely the capsule permeability, mechanical properties, immune protection and biocompatibility, have been singled out and methods to evaluate these properties were summarized. Finally, speculations regarding future directions of cell encapsulation research and device development are included from the authors' perspective.

Osteoarthritis-like disorder in rats with vascular deprivation-induced necrosis of the femoral head

The reparative processes following vascular deprivation-induced necrosis of the femoral head were studied histologically in rats sacrificed 2, 7, 14, 21, 42 and 92 days postoperatively. The blood supply was severed by incision of the periosteum at the neck of the femoral head and transection of the ligamentum teres. Granulation tissue and a well-vascularized fibrous tissue originating from the joint capsule invaded the necrotic marrow spaces. With progressive resorption of the necrotic tissues and osteoneogenesis, both appositional and intramembranous, within the fibrotic intertrabecular spaces, the remodeling process led to a shift of the normal spongy architecture of the femoral head to a compacta-like one. In a few cases, osseous bridges bisected a necrotic physeal cartilage at the latest time intervals. The remodeling was associated with flattening of the femoral heads as well as with degenerative, regenerative and reparative alterations of the articular cartilage. In one of the two femoral heads obtained three months postoperatively, cystic spaces developed in the fibrous subchondral zone. Our findings are consistent with the view that ineffective attempts at restoring the prenecrotic state of the femoral head by replacing the necrotic with viable tissue triggers the collapse of the femoral head. Thickening and condensation of the subchondral bone, leading to increased stiffness of the subchondral zone, result in the osteoarthritis-like disorder. Mimicking the well-known phases of human osteonecrosis, the model readily allows for preclinical studies of therapeutic regimens.

Mechanisms of fibrinogen domains: biomaterial interactions

Spontaneous adsorption of fibrinogen is critical to the pathogenesis of biomaterial-mediated inflammatory responses. However, the mechanism by which adsorbed fibrinogen affects phagocyte responses is still not clear. To investigate the molecular interaction between fibrinogen and biomaterials, fibrinogen fragments (D100 and E50) were generated and used in the present study. The results indicate that biomaterial: D100 interaction is essential to fibrinogen-mediated inflammatory responses, because biomaterials precoated with D100, but not E50, prompt strong inflammatory responses. Furthermore, the results from in vitro studies show that whole molecule fibrinogen and D100 exhibit very similar protein:surface interactions. Specifically: (1) both D100 and fibrinogen have high affinity for biomaterial surfaces; and (2) the retention rates of adsorbed D100 in both in vivo and in vitro environments are as high as that for adsorbed fibrinogen. On the other hand, E50 does bind to biomaterials but with low affinity because, once bound, it is not tightly adherent to the biomaterial surfaces. Taken together, the results suggest that the mechanism of fibrinogen-mediated inflammatory responses may involve the following three consecutive events: (1) after contact with blood or tissue fluid, the D domain tends to interact with biomaterial surfaces and is important in the tight binding of fibrinogen to implant surfaces; (2) the biomaterial surface then promotes conformational changes within the D domain, exposing P1 epitope (gamma 190-202, which interacts with phagocyte Mac-1 integrin); and (3) the engagement of Mac-1 integrin with P1 epitope then triggers subsequent phagocyte adherence and reactions.

Mast cells mediate acute inflammatory responses to implanted biomaterials

Implanted biomaterials trigger acute and chronic inflammatory responses. The mechanisms involved in such acute inflammatory responses can be arbitrarily divided into phagocyte transmigration, chemotaxis, and adhesion to implant surfaces. We earlier observed that two chemokines-macrophage inflammatory protein 1alpha/monocyte chemoattractant protein 1-and the phagocyte integrin Mac-1 (CD11b/CD18)/surface fibrinogen interaction are, respectively, required for phagocyte chemotaxis and adherence to biomaterial surfaces. However, it is still not clear how the initial transmigration of phagocytes through the endothelial barrier into the area of the implant is triggered. Because implanted biomaterials elicit histaminic responses in the surrounding tissue, and histamine release is known to promote rapid diapedesis of inflammatory cells, we evaluated the possible role of histamine and mast cells in the recruitment of phagocytes to biomaterial implants. Using i.p. and s. c. implantation of polyethylene terephthalate disks in mice we find: (i) Extensive degranulation of mast cells, accompanied by histamine release, occurs adjacent to short-term i.p. implants. (ii) Simultaneous administration of H1 and H2 histamine receptor antagonists (pyrilamine and famotidine, respectively) greatly diminishes recruitment and adhesion of both neutrophils (<20% of control) and monocytes/macrophages (<30% of control) to implants. (iii) Congenitally mast cell-deficient mice also exhibit markedly reduced accumulation of phagocytes on both i.p. and s.c implants. (iv) Finally, mast cell reconstitution of mast cell-deficient mice restores "normal" inflammatory responses to biomaterial implants. We conclude that mast cells and their granular products, especially histamine, are important in recruitment of inflammatory cells to biomaterial implants. Improved knowledge of such responses may permit purposeful modulation of both acute and chronic inflammation affecting implanted biomaterials.

Local Fas/APO-1 (CD95) ligand-mediated tumor cell killing in vivo

Fas/APO-1 (CD95) is a cell surface receptor which mediates apoptosis when ligated by specific antibodies or by its recently cloned natural ligand, FasL. We have studied the cytotoxic potential of FasL in vivo using Fas/APO-1-expressing Yac-1 cells as targets. Supernatant harvested from Neuro-2a cells transfected with the murine FasL cDNA contains FasL and transduces a potent apoptotic signal to Yac-1 cells in vitro. Specificity of FasL-mediated cytotoxicity was confirmed by competition assays using soluble Fas or anti-Fas/APO-1 F(ab')2 fragments which specifically interfere with FasL-Fas/APO-1 interactions. Intraperitoneal injection of FasL-containing supernatant efficiently killed Yac-1 target cells which had been implanted in capsules into the peritoneal cavity of mice. Analysis of the target cells revealed DNA fragmentation and nuclear changes typical of apoptosis. As previously shown, intraperitoneal injection of anti-Fas/APO-1 antibodies caused liver failure (Ogasawara, J., Watanabe, F.R., Adachi, M., Matsuzawa, A., Kasugai, T., Kitamura, Y., Itoh, N., Suda, T. and Nagata, S., Nature 1993. 364:806) and was observed at doses which did not reduce Yac-1 cell viability. In contrast, FasL did not induce histopathology in the liver when applied intraperitoneally at doses cytotoxic for Yac-1 cells. However, intravenous administration of FasL induced lethal liver hemorrhages and hepatocyte apoptosis. Thus, locally applied FasL kills tumor cells very efficiently without systemic toxicity and may therefore represent a candidate for local tumor treatment.

Recent progress in immunoisolated cell therapy

Biohybrid implants represent a new class of medical device in which living cells, supported in a hydrogel matrix, and surrounded by a semipermiable membrane, produce and deliver therapeutic reagents to specific sites within a host. First proposed in the mid-1970s for diabetes, this treatment modality has progressed rapidly in the past four years and is now being investigated not just for endocrine disorders but also for alleviation of chronic pain, treatment of neurodegenerative disorders, and delivery of neurotrophic factors to sites within the blood brain barrier, and as a practical alternative to conventional ex vivo.

Dacron vascular biomaterial triggers macrophage ectoenzyme activity without change in cell membrane fluidity

Biomaterials induce an inflammatory reaction characterized by a rapid recruitment at the implantation site of polymorphonuclear cells and macrophages. In the course of the inflammatory response, the cellular activation triggers expression of a number of enzymes, such as 5'-nucleotidase, which is widely distributed in animal cell membranes as an ectoenzyme. It is now well established that 5'-nucleotidase activity decreases following the contact of inflammatory cells with foreign particles. In this paper we investigate a possible correlation between the enzymatic activities and the dynamic properties of the cell membrane bilayer. Dacron pieces were introduced into rats' peritoneal cavities for a period of 6 h, after which the peritoneal cells were harvested, and various enzyme assays performed, including those for cytoplasmic, lysosomal, and ectoenzymes. In parallel, we studied cell membrane fluidity, using fluorescence polarization of 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), and cellular ultrastructural alteration resulting from the cell-biomaterial interactions using scanning and transmission electron microscopy. Our results show that: 1) macrophages spread around the Dacron fibers with cytoplasmic finger-like projections, but no phagolysosomes, 2) 5'-nucleotidase levels decrease with surgical trauma in comparison with the resident cell exudate, 3) implantation of biomaterials slightly modify the 5'-nucleotidase levels observed in the sham animal, 4) no differences in the anisotropy values indicating that membrane lipid order within the cells could not account for the observed decrease of 5'-nucleotidase activity. Thus, we can suggest that 5'-nucleotidase expression may reflect a particular feature of cell activation without a phagocytic process.