Julia E. Babensee

Host response to tissue engineered devices

The two main components of a tissue engineered device are the transplanted cells and the biomaterial, creating a device for the restoration or modification of tissue or organ function. The implantation of polymer/cell constructs combines concepts of biomaterials and cell transplantation. The interconnections between the host responses to the biomaterial and transplanted cells determines the biocompatibility of the device. This review describes the inflammatory response to the biomaterial component and immune response towards transplanted cells. Emphasis is on how the presence of the transplanted cell construct affects the host response. The inflammatory response towards a biomaterial can impact the immune response towards transplanted cells and vice versa. Immune rejection is the most important host response towards the cellular component of tissue engineered devices containing allogeneic, xenogeneic or immunogenic ex vivo manipulated autologous cells. The immune mechanisms towards allografts and xenografts are outlined to provide a basis for the mechanistic hypotheses of the immune response towards encapsulated cells, with antigen shedding and the indirect pathway of antigen presentation predominating. A review of experimental evidence illustrates examples of the inflammatory response towards biodegradable polymer scaffold materials, examples of devices appropriately integrated as assessed morphologically with the host for various applications including bone, nerve, and skin regeneration, and of the immune response towards encapsulated allogeneic and xenogeneic cells.

Growth Factor Delivery for Tissue Engineering

A tissue-engineered implant is a biologic-biomaterial combination in which some component of tissuehas been combined with a biomaterial to create a device for the restoration or modification of tissue ororgan function. Specific growth factors, released from a delivery device or from co-transplanted cells,would aid in the induction of host paraenchymal cell infiltration and improve engraftment of co-deliveredcells for more efficient tissue regeneration or ameliorate disease states. The characteristic properties ofgrowth factors are described to provide a biological basis for their use in tissue engineered devices. Theprinciples of polymeric device development for therapeutic growth factor delivery in the context of tissueengineering are outlined. A review of experimental evidence illustrates examples of growth factor deliveryfrom devices such as micropaticles, scaffolds, and encapsulated cells, for their use in the applicationareas of musculoskeletal tissue, neural tissue, and hepatic tissue.

Macrophage and dendritic cell phenotypic diversity in the context of biomaterials

Macrophages (Mϕ) and dendritic cells (DCs) are critical antigen presenting cells that play pivotal roles in host responses to biomaterial implants. Although Mϕs have been widely studied for their roles in the inflammatory responses against biomaterials, the roles that DCs play in the host responses toward implanted materials have only recently been explored. DCs are of significant research interest because of the emergence of a large number of combination products that cross-traditional medical device boundaries. These products combine biomaterials with biologics, including cells, nucleic acids, and/or proteins. The biomaterial component may evoke an inflammatory response, primarily mediated by neutrophils and Mϕs, whereas the biologic component may elicit an immunogenic immune response, initiated by DCs involving lymphocyte activation. Control of Mϕ phenotypic balance from proinflammatory M1 to reparative M2 is a goal of investigators to optimize the host response to biomaterials. Similarly, control of DC phenotype from proinflammatory to toleragenic is of interest in vaccine delivery and tissue engineering/transplantation situations, respectively. This review discusses the interconnection between innate and adaptive immunity, the comparative and contrasting phenotypes and roles of Mϕs and DCs in immunity, their responses to biomaterials and the strategies to modulate their phenotype for applications in tissue engineering and vaccine delivery. Furthermore, the collaboration between and unique roles of DCs and Mϕs needs to be addressed in future studies to gain a more complete picture of host responses toward combination products.

Reduced Acute Inflammatory Responses to Microgel Conformal Coatings

Implantation of synthetic materials into the body elicits inflammatory host responses that limit medical device integration and biological performance. This inflammatory cascade involves protein adsorption, leukocyte recruitment and activation, cytokine release, and fibrous encapsulation of the implant. We present a coating strategy based on thin films of poly(N-isopropylacrylamide) hydrogel microparticles (i.e. microgels) cross-linked with poly(ethylene glycol) diacrylate. These particles were grafted onto a clinically relevant polymeric material to generate conformal coatings that significantly reduced in vitro fibrinogen adsorption and primary human monocyte/macrophage adhesion and spreading. These microgel coatings also reduced leukocyte adhesion and expression of pro-inflammatory cytokines (TNF-alpha, IL-1beta, MCP-1) in response to materials implanted acutely in the murine intraperitoneal space. These microgel coatings can be applied to biomedical implants as a protective coating to attenuate biofouling, leukocyte adhesion and activation, and adverse host responses for biomedical and biotechnological applications.

Humoral immune responses to model antigen co-delivered with biomaterials used in tissue engineering

A model shed antigen, ovalbumin (OVA), was co-delivered with polymeric biomaterial carrier vehicles in C57BL6 mice to test whether the presence of the biomaterial acted as an adjuvant in the immune response towards the associated antigen. The biomaterials tested were non-biodegradable polystyrene microparticles and biodegradable 50:50 or 75:25 poly(lactic-co-glycolic acid) (PLGA) microparticles or scaffolds. For each biomaterial carrier vehicle, to assess the resulting time-dependent systemic humoral immune response towards the co-delivered OVA, the OVA-specific IgG concentration and isotypes (IgG2a or IgG1, indicating a predominant Th1 or Th2 response, respectively) were determined using ELISA. OVA co-delivered with biomaterial carrier vehicles supported a moderate humoral immune response that was maintained for the 18-week duration of the experiment. This humoral immune response was primarily Th2 helper T cell-dependent as indicated by the predominant IgG1 isotype. Furthermore, this humoral immune response was not material chemistry-dependent within the material set tested here. With the presence of the biomaterial resulting in an enhancement of the humoral immune response to co-delivered antigen, it appears that the biomaterial acts as an adjuvant in the development of an adaptive immune response to co-delivered antigen.

Differential functional effects of biomaterials on dendritic cell maturation.

The immunological outcome of dendritic cell (DC) treatment with different biomaterials was assessed to demonstrate the range of DC phenotypes induced by biomaterials commonly used in combination products. Immature DCs (iDCs) were derived from human peripheral blood monocytes, and treated with different biomaterial films of alginate, agarose, chitosan, hyaluronic acid (HA), or 75:25 poly(lactic-co-glycolic acid) (PLGA) and a comprehensive battery of phenotypic functional outcomes was assessed. Different levels of functional changes in DC phenotype were observed depending on the type of biomaterial films used to treat the DCs. Treatment of DCs with PLGA or chitosan films supported DC maturation, with higher levels of DC allostimulatory capacity, pro-inflammatory cytokine release, and expression of CD80, CD86, CD83, HLA-DQ and CD44 compared with iDCs, and lower endocytic ability compared with iDCs. Alginate film induced pro-inflammatory cytokine release from DCs at levels higher than from iDCs. Dendritic cells treated with HA film expressed lower levels of CD40, CD80, CD86 and HLA-DR compared with iDCs. They also exhibited lower endocytic ability and CD44 expression than iDCs, possibly due to an insolubilized (cross-linked) form of high molecular weight HA. Interestingly, treatment of DCs with agarose film maintained the DC functional phenotype at levels similar to iDCs except for CD44 expression, which was lower than that of iDCs. Taken together, these results can provide selection criteria for biomaterials to be used in immunomodulating applications and can inform potential outcomes of biomaterials within combination products on associated immune responses as desired by the application.

Biomaterial Strategies for Immunomodulation

Strategies to enhance, suppress, or qualitatively shape the immune response are of importance for diverse biomedical applications, such as the development of new vaccines, treatments for autoimmune diseases and allergies, strategies for regenerative medicine, and immunotherapies for cancer. However, the intricate cellular and molecular signals regulating the immune system are major hurdles to predictably manipulating the immune response and developing safe and effective therapies. To meet this challenge, biomaterials are being developed that control how, where, and when immune cells are stimulated in vivo, and that can finely control their differentiation in vitro. We review recent advances in the field of biomaterials for immunomodulation, focusing particularly on designing biomaterials to provide controlled immunostimulation, targeting drugs and vaccines to lymphoid organs, and serving as scaffolds to organize immune cells and emulate lymphoid tissues. These ongoing efforts highlight the many ways in which biomaterials can be brought to bear to engineer the immune system.

Molecular aspects of microparticle phagocytosis by dendritic cells

The ability of immature dendritic cells (iDCs) derived from human peripheral blood mononuclear cells to phagocytose poly(lactic-co-glycolic acid) (PLGA) microparticles (MPs) as compared to polystyrene MPs and the molecular aspects of this phagocytosis were investigated. Treating iDCs with PLGA or polystyrene fluorospheres of approximately 3 μm in diameter resulted in the internalization of the particles as evidenced by confocal laser scanning micrographs. This uptake of fluorospheres by DCs was decreased by pretreatment of cells with cytochalasin D or by incubation with the fluorospheres at 4°C, and was sensitive to EDTA and trypsin pretreatments in a dose-dependent manner. In agreement with our previous studies, treatment of iDCs with PLGA MPs, but not with polystyrene MPs, led to DC maturation, as measured by increase in release of the autocrine maturation cytokine, tumor necrosis factor-α, which was dependent on ratio of PLGA MPs to DCs. Taken together, this work begins to address the role of phagocytosis on PLGA MP-induced DC maturation and the molecular mechanisms involved.

Dendritic cell responses to surface properties of clinical titanium surfaces

Dendritic cells (DCs) play pivotal roles in responding to foreign entities during the innate immune response and in initiating effective adaptive immunity as well as maintaining immune tolerance. The sensitivity of DCs to foreign stimuli also makes them useful cells to assess the inflammatory response to biomaterials. Elucidating material property-DC phenotype relationships using a well-defined biomaterial system is expected to provide criteria for immunomodulatory biomaterial design. Clinical titanium (Ti) substrates, including pretreatment (PT), sand blasted and acid etched (SLA), and modified SLA (modSLA), with different roughnesses and surface energies were used to treat DCs and resulted in differential DC responses. PT and SLA induced a mature DC (mDC) phenotype, while modSLA promoted a non-inflammatory environment by supporting an immature DC (iDC) phenotype, based on surface marker expression, cytokine production profiles and cell morphology. Principal component analysis (PCA) confirmed these experimental results, and also indicated that the non-stimulating property of modSLA covaried with certain surface properties, such as high surface hydrophilicity, percent oxygen and percent Ti of the substrates. In addition to previous research that demonstrated superior osteogenic properties of modSLA compared with PT and SLA, the results reported herein indicates that modSLA may further benefit implant osteointegration by reducing local inflammation and its associated osteoclastogenesis.

Biomaterial adjuvant effect is attenuated by anti-inflammatory drug delivery or material selection

Biomaterials have been shown to differentially support dendritic cell (DC) maturation, a prerequisite for an adjuvant effect. Treatment of DCs with poly(D,L-lactic-co-glycolic acid) (PLGA) films resulted in DC maturation but agarose films did not. In these studies, the biomaterial adjuvant effect was attenuated by material selection (PLGA or agarose scaffolds) or local delivery of an anti-inflammatory/immunosuppressive glucocorticoid, dexamethasone (DX), from PLGA scaffolds. Porous scaffolds (SCs) of PLGA or agarose were produced to deliver equivalent amounts of model antigen, ovalbumin (OVA). Alternatively, PLGA SCs with incorporated OVA were produced with or without DX. These SCs were implanted individually, subcutaneously, and dorsally in C57BL/6 mice. Blood was collected from mice at specific times over a 12-week duration for measurement of antibody production against OVA. Scaffolds were explanted at 12 weeks for histological examination of foreign body response. Scaffolds of PLGA, but not of agarose, were found to elicit higher antibody production against co-delivered OVA, than negative controls. Short-term delivery of DX from PLGA SCs delivering OVA temporarily delayed onset of anti-OVA antibody production. More sustained release of DX at an effective dose and with an appropriate time course is expected to extend the effect of DX on the biomaterial adjuvant effect. The immunomodulatory ability of biomaterials to affect the immune response to co-delivered antigen is demonstrated wherein this immunomodulatory ability correlates with the observed in vitro differential effects of biomaterials on DC maturation.