Hydrogels and scaffolds for immunomodulation

Abstract

For over two decades, immunologists and biomaterials scientists have co-existed in parallel world with the rationale of understanding the molecular profile of immune responses to vaccination, implantation, and treating incurable diseases. Much of the field of biomaterial-based immunotherapy has relied on evaluating model antigens such as chicken egg ovalbumin in mouse models but their relevance to humans has been point of much discussion. Nevertheless, such model antigens have provided important insights into the mechanisms of immune regulation and served as a proof-of-concept for plethora of biomaterial-based vaccines. After years of extensive development of numerous biomaterials for immunomodulation, it is only recently that an experimental scaffold vaccine implanted beneath the skin has begun to use the human model to study the immune responses to cancer vaccination by co-delivering patient-derived tumor lysates and immunomodulatory proteins. If successful, this scaffold vaccine will change the way we approached untreatable cancers, but more importantly, will allow a faster and more rational translation of therapeutic regimes to other cancers, chronic infections, and autoimmune diseases. Most materials reviews have focused on immunomodulatory adjuvants and micro-nano-particles. Here we provide an insight into emerging hydrogel and scaffold based immunomodulatory approaches that continue to demonstrate efficacy against immune associated diseases.

Keywords: biomaterials; hydrogels; immunology; immunomodulation; polymers; scaffolds.

Figure 1. Dendritic cell trafficking and modulation using scaffolds

A) Cell-loaded vaccination nodes: Ex vivo primed DC (purple) are delivered through injections or implantation of pre-fabricated scaffolds, subcutaneously into mice. A few of the programmed DCs migrate to the draining lymph nodes and present antigen to the T-cells in T-cell zone. Cytokines and chemokines secreted by programmed DCs at the implantation site recruits host’s own naïve DCs (green) and programmed T-cells to induce a robust immune response. B) Pathogen-mimicking immune centers with no cells: In situ crosslinked hydrogels or implanted scaffolds release chemokines and growth factors in the tissue to attract naïve DCs. Recruited DCs engulf antigenic vaccines and return to draining lymph node to present antigen to naïve T helper cells (CD4+ T-cells). Programmed T-cells can then migrate to the implant site or nearby tumor regions to destroy malignant cells.

Figure 2. Lymphoid tissue engineering using scaffolds

A) Naturally occurring collagen-based lymphoid organoids. When transplanted in mouse, tissue-engineered organoids are structure similar to secondary lymphoid organs such as mesenteric lymph node (LN). (i) CD4+ T-cells in stained red and CD8+ T-cells are stained green. (ii) Mesenteric LN and transplant were stained for Thy1.2+ T-cells in red, B220+ B-cells in green and CD11c+ DCs in blue. B) Composite PEG-based inverse opal scaffolds: (i) Image represents an oblique view of fluorescently-labeled inverse opal PEG scaffolds with macroscale pores. The interior of these scaffolds could be decorated with cell supportive ECM proteins such as fibronectin or laminin. (ii) Plan view of inverse opal scaffolds with naive CD4+ T-cells (red) interacting with DCs (green) that are spread on the interior side of the ECM protein-conjugated PEG scaffold. Reproduced with permission ^[73],[74].

Figure 3. Injectable and implantable strategies for immunomodulation

A) In situ crosslinkable alginate gel co-delivering immunomodulatory factors. (i) 3D injectable alginate gel with calcium-crosslinked alginate microspheres. Alginate microspheres (non-fluorescent voids) were distributed throughout the fluorescent alginte matrix. (ii) GFP⁺ DCs in alginate hydrogels, explanted 22 h after s.c. injection in C57Bl/6 mice. White arrows indicate spread DCs. (iii) Infiltrating GFP+ cells infiltrate and occupy void spaces in porous alginate matrix (purple) explanted from mice 48 h after injection Scale bars: 50 µm. B) Implantable EVA rods with chemokines and antigens. (i) Accumulation of Langerhans cells around BSA rod (left) and MIP-3β rod (right) implanted in mice and examined after 24 h. Original magnification, 200; scale bars, 100 µm. (ii) Co-implantation of MIP-3β rods and OVA rods initiates protective immunity in mice challenged with E.G7-OVA tumor cells in the scapular region five days after rod implantation (C) Engineered PLGA scaffold vaccine against tumor (i) SEM of PLGA scaffold. (ii) Photograph of lymph nodes from control mice and infection mimic mice after 10 days of implantation of matrices incorporating 10 µg CpG-ODN+3,000 ng GM-CSF. (iii) Survival times of mice vaccinated with PLGA vaccines 14 days before B16-F10 melanoma tumor challenge. (D) Drug-responsive hydrogel vaccine depot. (i) Gyrase B (Gyr B) functionalized 8-arm PEG hydrogel providing a molecular switch based on the interaction of the GyrB to the aminocoumarin antibiotics coumermycin and novobiocin. (ii) Mice were sacrificed at day 98 after treatment with or without Novobiocin. Non-dissolved hydrogel of the group 2 mouse is indicated by an arrow. (E) An injectable synthetic immune-priming center made of Dextran Vinyl Sulfone and 4-arm PEG-SH mediates efficient T-cell class switching and T-helper 1 response against B-cell lymphoma (i) Formation of in situ crosslinkable hydrogels in mouse quad muscle, retrieved after 8 h post injection. (ii) Primary DC infiltration through 3D hydrogels (iii) Kaplan–Meier survival curve indicating protection against A20 B-lymphoma in Balb/C immunized mice with various indicated formulations. Reproduced with permission ^{[64],[46],[47],[48],[23],[24]}

Figure 4. Scaffold based immunomodulation of T_Reg cells in type 1 diabetes mellitus (T1DM)

Schematic represents immunological mechanism associated with T1DM leading to apoptosis of pancreatic β-cell islets. Islets can be encapsulated in scaffolds made of polymeric materials such as PLGA. Protection of scaffold transplanted islets by T_Reg cells can be associated with insulin production and Foxp3+ T_Reg co-localization around islets.

Figure 5. Immunomodulation in arthritis

A) Schematic of immune regulation in arthritis; B) hydrogel nanoparticle-based for enhanced knee joint retention to reduce inflammation in osteoarthritis. Schematic of nanoparticle self-assembly based on protein/polymer complexation. PHEMA–pyridine was synthesized by reacting PHEMA with nicotinoyl chloride hydrochloride in tetrahydrofuran and pyridine. SEM images of Fibronectin nanoparticles (FN-NP, Scale: 200 nm); C) IL-1Ra-tethered particles are distributed throughout the intra-articular joint space. IL-1Ra was tagged with a Dylight-IR-650 dye prior to tethering IL-1Ra to particles. Tagged IL-1Ra-tethered particles or soluble IL-1Ra was injected into the right stifle joint of 8–10 wk old rats while the left stifle joints received saline. Cryosectioned samples were counterstained with DAPI to localize dye tagged protein. Scale bar = 50 µm. Reproduced with permission ^[97],[96].