Designing biomaterials with immunomodulatory properties for tissue engineering and regenerative medicine

Abstract

Recent research in the vaccine and immunotherapy fields has revealed that biomaterials have the ability to activate immune pathways, even in the absence of other immune-stimulating signals. Intriguingly, new studies reveal these responses are influenced by the physicochemical properties of the material. Nearly all of this work has been done in the vaccine and immunotherapy fields, but there is tremendous opportunity to apply this same knowledge to tissue engineering and regenerative medicine. This review discusses recent findings that reveal how material properties-size, shape, chemical functionality-impact immune response, and links these changes to emerging opportunities in tissue engineering and regenerative medicine. We begin by discussing what has been learned from studies conducted in the contexts of vaccines and immunotherapies. Next, research is highlighted that elucidates the properties of materials that polarize innate immune cells, including macrophages and dendritic cells, toward either inflammatory or wound healing phenotypes. We also discuss recent studies demonstrating that scaffolds used in tissue engineering applications can influence cells of the adaptive immune system-B and T cell lymphocytes-to promote regenerative tissue microenvironments. Through greater study of the intrinsic immunogenic features of implantable materials and scaffolds, new translational opportunities will arise to better control tissue engineering and regenerative medicine applications.

Keywords: biomaterial; immunology; intrinsic immunogenicity; nanoparticle and microparticle; regenerative medicine; tissue engineering; vaccine and immunotherapy.

Figure 1

Intrinsic properties of materials influence immune responses. Biomaterials commonly used in vaccine, immunotherapy, and tissue engineering approaches exhibit features including size, shape, surface charge, hydrophobicity, and molecular weight that alter interactions with the immune system. Encountering components of the innate and adaptive immune system with these materials results in formation of fibrotic capsule to isolate the material, differential activation of dendritic cells and macrophages, recognition and removal by antibody and complement proteins, and even manipulating adaptive immune response

Figure 2

Particle shape dictates immune cell uptake and activation. (a) Spherical polymeric particles fabricated from polystyrene‐polyethylene oxide that exhibit rough surfaces (left) were preferentially taken up by macrophages and induced a pro‐inflammatory response compared to smooth particles (right) (Scale bar, 10 µm; inset scale bar, 5 µm). (b) Electron micrographs of gold nanoconstructs with spherical (left), cube (center), or rod‐like (right) shapes. When incubated with DCs, rod‐like particles induced inflammatory IL‐1β and activated the inflammasome, while sphere and cubes caused secretion of TNF‐α (Scale bar, 40 nm). (c) Spherical PLGA particles that are mechanically stretched to form ellipsoidal particles increase surface interactions with immune cells, leading to increased T cell proliferation. (Scale bar, 10 µm). (a) Reprinted with permission from Vaine CA, Patel MK, Zhu J, et al. Tuning innate immune activation by surface texturing of polymer microparticles: the role of shape in inflammasome activation. J Immunol. 2013;190(7):3525‐3532. (b) Reprinted with permission from Niikura K, Matsunaga T, Suzuki T, et al. Gold nanoparticles as a vaccine platform: influence of size and shape on immunological responses in vitro and in vivo. ACS Nano. 2013;7(5):3926‐3938. (c) Reprinted from Sunshine JC, Perica K, Schneck JP, Green JJ. Particle shape dependence of CD8+ T cell activation by artificial antigen presenting cells. Biomaterials. 2014;35(1):269‐277 with permission from Elsevier

Figure 3

Surface chemistry of particulate systems impacts immunogenicity. (a) Gold nanoparticles are functionalized with different chemical (“R”) groups to exhibit varied hydrophobicity, denoted as “Log P.” (b) Immune cells isolated from mouse spleens were treated with these particles and revealed a correlation between increased hydrophobicity and elevated gene expression of pro‐inflammatory cytokines (TNF‐α). (c) Silica particles functionalized with different polypeptides with defined charges and levels of hydrophobicity increase the IL‐1β response to treatment with an immune stimulant (LPS). Increasingly hydrophobic surface chemistries promote increased IL‐1β secretion. (d) When these silica particles were treated in conjunction with a model antigen (OVA), particle immunogenicity increased T cell production of pro‐inflammatory IFN‐γ, with cationic particles inducing the highest levels. (a and b) Adapted with permission from Moyano DF, Goldsmith M, Solfiell DJ, et al. Nanoparticle hydrophobicity dictates immune response. J Am Chem Soc. 2012;134(9):3965‐3967. Copyright 2012 American Chemical Society. (c–d) Reprinted from Kakizawa Y, Seok Lee J, Bell B, Fahmy TM. Precise manipulation of biophysical particle parameters enables control of proinflammatory cytokine production in presence of TLR 3 and 4 ligands. Acta Biomater. 2017 with permission from Elsevier

Figure 4

Polymer degradation and molecular weight influence DC activation in cell culture and mice. (a) PBAEs formulated with 4 (PBAE‐4, blue), 6 (PBAE‐6, red), or 10 (PBAE‐10, green) carbons in the diacrylate monomer backbone were synthesized with (b) different starting molecular weights, but similar degradation profiles. (c) PBAEs were degraded to distinct molecular weight ranges, formulated into particles, and used to treat DCs. Maximum activation, as indicated by CD40 expression on live (DAPI⁻) DCs, correlated with a molecular weight range of 1,500–3,000 Da. (d) After introduction into mice, immunogenic PBAE particles (red) induced statistically significant activation of lymph node resident immune cells compared to treatment with soluble (blue, “Free”) or buffer control (gray, “Vehicle”) treatments. (a–c) adapted with permission from Andorko JI, Pineault KG, Jewell CM. Impact of molecular weight on the intrinsic immunogenic activity of poly(beta amino esters). J Biomed Mater Res A. 2017;105(4):1219‐1229. (d) Reprinted with permission from Andorko JI, Hess KL, Pineault KG, Jewell CM. Intrinsic immunogenicity of rapidly‐degradable polymers evolves during degradation. Acta Biomater. 2016;32:24‐34

Figure 5

Scaffolds derived from extracellular matrix components of specific tissues polarize macrophage function. (a) Macrophages (stained with F4/80) and treated for 18 hr with control cytokines (MCSF, IFN‐γ + LPS, IL‐4) or solubilized extracellular matrix scaffolds cause differential expression of markers for the pro‐inflammatory M1 macrophage (iNOS) and the wound‐healing M2 macrophage (Fizz1) phenotypes. Solubilized scaffolds were produced via pepsin incubation of small intestine submucosa (SIS), urinary bladder (UBM), skeletal muscle (mECM), brain (bECM), esophagus (eECM), skin (dECM), liver (LECM), or colon (coECM) extracellular matrix components. MCSF was used as a negative control for macrophages while pepsin was a control for ECM solubilization, IFN‐γ + LPS was a positive control for M1 macrophages, and IL‐4 was a positive control for M2 macrophages. (b) Treatment with matrices also induces varied levels of M2 associated (CD206) and M1 associated (iNOS) proteins in macrophage lysates. Adapted with permission from Dziki JL, Wang DS, Pineda C, Sicari BM, Rausch T, Badylak SF. Solubilized extracellular matrix bioscaffolds derived from diverse source tissues differentially influence macrophage phenotype. J Biomed Mater Res. 2017;105(1):138‐147

Figure 6

Implant surface morphology and chemical composition induces innate cell activation and cytokine secretion. (a) Schematic depiction of glass cover slips coated with gold nanoparticles, then functionalized with allylamine (AA), octadiene (OD), or acrylic acid (AC) to form biomimetic surfaces abundant in amino, alkyl, or carboxylic acid groups, respectively. (b) Atomic force micrographs showing 2‐D (top) and 3‐D (bottom) surfaces with different roughness due to the different particles diameters. (Top scale bar, 1 µm; lower scale 5 µm × 5 µm). (c) Macrophages cultured on these surfaces exhibited different secretion levels of pro‐inflammatory cytokines TNF‐α (left), IL‐6 (center), and IL‐1β (right); cells cultures on the roughest surfaces (prepared with 68 nm particles) reduced inflammatory cytokine secretion. Reprinted with permission from Christo SN, Bachhuka A, Diener KR, Mierczynska A, Hayball JD, Vasilev K. The role of surface nanotopography and chemistry on primary neutrophil and macrophage cellular responses. Adv Healthcare Mater. 2016;5(8):956‐965

Figure 7

Implanted materials with distinct sizes and compositions alter fibrotic capsule formation. (a) Following implantation of alginate spheres of different sizes into the peritoneum of mice, gene expression profiles of the pro‐fibrotic markers α‐SMA (left), collagen 1α1 (center), and collagen 1α2 (right) revealed larger particles reduced fibrotic build‐up. (b) Images revealing the level of fibrosis for particles with diameters of 0.5 mm (medium) or 1.5–2 mm (large) prepared from alginate, stainless steel, glass, polycaprolactone, or polystyrene. For all materials, large particles were associated with reduced fibrosis 14 days after implantation. (Scale bar, 2 mm). Adapted with permission from Macmillan Publisher Ltd: Nature Materials from Veiseh O, Doloff JC, Ma M, et al. Size‐ and shape‐dependent foreign body immune response to materials implanted in rodents and non‐human primates. Nat Mater. 2015;14(6):643‐651, copyright 2015

Figure 8

Adaptive immune cells play a role in the response to implanted scaffolds. After inducing a critical muscle injury in mice, scaffolds derived from collagen, bone (B‐ECM), or cardiac muscle extracellular matrix (C‐ECM) were implanted. (a) CD3⁺ T cell transcriptome analysis was used to evaluate the gene expression of markers for distinct immune cell populations and phenotypes compared to a sham saline surgery control, denoted as “RQ to Saline.” Treatment with B‐ECM and C‐ECM scaffolds revealed an increase in T_H2‐associated genes (e.g., Jag2, IL‐4). (b) Treatment with scaffolds in wild‐type mice (blue, “WT”) increased IL‐4 gene expression in local (left) and distal lymph nodes (right) compared to saline treatments. When CD4⁺ T cell‐deficient mice (green, “Cd4^−/−”), or B and T cell‐deficient mice (red, “Rag1^−/−”) were treated with ECM scaffolds, IL‐4 gene expression decreased. (a and b) from Sadtler K, Estrellas K, Allen BW, et al. Developing a pro‐regenerative biomaterial scaffold microenvironment requires T helper 2 cells. Science. 2016;352(6283):366‐370. Reprinted with permission from AAAS