Synthetic Semiflexible and Bioactive Brushes.

Polymer brushes are extensively used for the preparation of bioactive surfaces. They form a platform to attach functional (bio)molecules and control the physicochemical properties of the surface. These brushes are nearly exclusively prepared from flexible polymers, even though much stiffer brushes from semiflexible polymers are frequently found in nature, which exert bioactive functions that are out of reach for flexible brushes. Synthetic semiflexible polymers, however, are very rare. Here, we use polyisocyanopeptides (PICs) to prepare high-density semiflexible brushes on different substrate geometries. For bioconjugation, we developed routes with two orthogonal click reactions, based on the strain-promoted azide-alkyne cycloaddition reaction and the (photoactivated) tetrazole-ene cycloaddition reaction. We found that for high brush densities, multiple bonds between the polymer and the substrate are necessary, which was achieved in a block copolymer strategy. Whether the desired biomolecules are conjugated to the PIC polymer before or after brush formation depends on the dimensions and required densities of the biomolecules and the curvature of the substrate. In either case, we provide mild, aqueous, and highly modular reaction strategies, which make PICs a versatile addition to the toolbox for generating semiflexible bioactive polymer brush surfaces.

Semiflexible Immunobrushes Induce Enhanced T Cell Activation and Expansion.

A variety of bioactive materials developed to expand T cells for adoptive transfer into cancer patients are currently evaluated in the clinic. In most cases, T cell activating biomolecules are attached to rigid surfaces or matrices and form a static interface between materials and the signaling receptors on the T cells. We hypothesized that a T cell activating polymer brush interface might better mimic the cell surface of a natural antigen-presenting cell, facilitating receptor movement and concomitant advantageous mechanical forces to provide enhanced T cell activating capacities. Here, as a proof of concept, we synthesized semiflexible polyisocyanopeptide (PIC) polymer-based immunobrushes equipped with T cell activating agonistic anti-CD3 (αCD3) and αCD28 antibodies placed on magnetic microbeads. We demonstrated enhanced efficiency of ex vivo expansion of activated primary human T cells even at very low numbers of stimulating antibodies compared to rigid beads. Importantly, the immunobrush architecture appeared crucial for this improved T cell activating capacity. Immunobrushes outperform current benchmarks by producing higher numbers of T cells exhibiting a combination of beneficial phenotypic characteristics, such as reduced exhaustion marker expression, high cytokine production, and robust expression of cytotoxic hallmarks. This study indicates that semiflexible immunobrushes have great potential in making T cell-based immunotherapies more effective.

Robust Antigen-Specific T Cell Activation within Injectable 3D Synthetic Nanovaccine Depots.

Synthetic cancer vaccines may boost anticancer immune responses by co-delivering tumor antigens and adjuvants to dendritic cells (DCs). The accessibility of cancer vaccines to DCs and thereby the delivery efficiency of antigenic material greatly depends on the vaccine platform that is used. Three-dimensional scaffolds have been developed to deliver antigens and adjuvants locally in an immunostimulatory environment to DCs to enable sustained availability. However, current systems have little control over the release profiles of the cargo that is incorporated and are often characterized by an initial high-burst release. Here, an alternative system is designed that co-delivers antigens and adjuvants to DCs through cargo-loaded nanoparticles (NPs) incorporated within biomaterial-based scaffolds. This creates a programmable system with the potential for controlled delivery of their cargo to DCs. Cargo-loaded poly(d,l-lactic-co-glycolic acid) NPs are entrapped within the polymer walls of alginate cryogels with high efficiency while retaining the favorable physical properties of cryogels, including syringe injection. DCs cultured within these NP-loaded scaffolds acquire strong antigen-specific T cell-activating capabilities. These findings demonstrate that introduction of NPs into the walls of macroporous alginate cryogels creates a fully synthetic immunostimulatory niche that stimulates DCs and evokes strong antigen-specific T cell responses.

Influence of Network Topology on the Viscoelastic Properties of Dynamically Crosslinked Hydrogels.

Biological materials combine stress relaxation and self-healing with non-linear stress-strain responses. These characteristic features are a direct result of hierarchical self-assembly, which often results in fiber-like architectures. Even though structural knowledge is rapidly increasing, it has remained a challenge to establish relationships between microscopic and macroscopic structure and function. Here, we focus on understanding how network topology determines the viscoelastic properties, i.e., stress relaxation, of biomimetic hydrogels. We have dynamically crosslinked two different synthetic polymers with one and the same crosslink. The first polymer, a polyisocyanopeptide (PIC), self-assembles into semi-flexible, fiber-like bundles, and thus displays stress-stiffening, similar to many biopolymer networks. The second polymer, 4-arm poly(ethylene glycol) (starPEG), serves as a reference network with well-characterized structural and viscoelastic properties. Using one and the same coiled coil crosslink allows us to decouple the effects of crosslink kinetics and network topology on the stress relaxation behavior of the resulting hydrogel networks. We show that the fiber-containing PIC network displays a relaxation time approximately two orders of magnitude slower than the starPEG network. This reveals that crosslink kinetics is not the only determinant for stress relaxation. Instead, we propose that the different network topologies determine the ability of elastically active network chains to relax stress. In the starPEG network, each elastically active chain contains exactly one crosslink. In the absence of entanglements, crosslink dissociation thus relaxes the entire chain. In contrast, each polymer is crosslinked to the fiber bundle in multiple positions in the PIC hydrogel. The dissociation of a single crosslink is thus not sufficient for chain relaxation. This suggests that tuning the number of crosslinks per elastically active chain in combination with crosslink kinetics is a powerful design principle for tuning stress relaxation in polymeric materials. The presence of a higher number of crosslinks per elastically active chain thus yields materials with a slow macroscopic relaxation time but fast dynamics at the microscopic level. Using this principle for the design of synthetic cell culture matrices will yield materials with excellent long-term stability combined with the ability to locally reorganize, thus facilitating cell motility, spreading, and growth.

Injectable Biomimetic Hydrogels as Tools for Efficient T Cell Expansion and Delivery.

Biomaterial-based scaffolds are promising tools for controlled immunomodulation. They can be applied as three dimensional (3D) culture systems in vitro, whereas in vivo they may be used to dictate cellular localization and exert spatiotemporal control over cues presented to the immune system. As such, scaffolds can be exploited to enhance the efficacy of cancer immunotherapies such as adoptive T cell transfer, in which localization and persistence of tumor-specific T cells dictates treatment outcome. Biomimetic polyisocyanopeptide (PIC) hydrogels are polymeric scaffolds with beneficial characteristics as they display reversible thermally-induced gelation at temperatures above 16°C, which allows for their minimally invasive delivery via injection. Moreover, incorporation of azide-terminated monomers introduces functional handles that can be exploited to include immune cell-modulating cues. Here, we explore the potential of synthetic PIC hydrogels to promote the in vitro expansion and in vivo local delivery of pre-activated T cells. We found that PIC hydrogels support the survival and vigorous expansion of pre-stimulated T cells in vitro even at high cell densities, highlighting their potential as 3D culture systems for efficient expansion of T cells for their adoptive transfer. In particular, the reversible thermo-sensitive behavior of the PIC scaffolds favors straightforward recovery of cells. PIC hydrogels that were injected subcutaneously gelated instantly in vivo, after which a confined 3D structure was formed that remained localized for at least 4 weeks. Importantly, we noticed no signs of inflammation, indicating that PIC hydrogels are non-immunogenic. Cells co-delivered with PIC polymers were encapsulated within the scaffold in vivo. Cells egressed gradually from the PIC gel and migrated into distant organs. This confirms that PIC hydrogels can be used to locally deliver cells within a supportive environment. These results demonstrate that PIC hydrogels are highly promising for both the in vitro expansion and in vivo delivery of pre-activated T cells. Covalent attachment of biomolecules onto azide-functionalized PIC polymers provides the opportunity to steer the phenotype, survival or functional response of the adoptively transferred cells. As such, PIC hydrogels can be used as valuable tools to improve current adoptive T cell therapy strategies.

Nonlinear mechanics of hybrid polymer networks that mimic the complex mechanical environment of cells.

The mechanical properties of cells and the extracellular environment they reside in are governed by a complex interplay of biopolymers. These biopolymers, which possess a wide range of stiffnesses, self-assemble into fibrous composite networks such as the cytoskeleton and extracellular matrix. They interact with each other both physically and chemically to create a highly responsive and adaptive mechanical environment that stiffens when stressed or strained. Here we show that hybrid networks of a synthetic mimic of biological networks and either stiff, flexible and semi-flexible components, even very low concentrations of these added components, strongly affect the network stiffness and/or its strain-responsive character. The stiffness (persistence length) of the second network, its concentration and the interaction between the components are all parameters that can be used to tune the mechanics of the hybrids. The equivalence of these hybrids with biological composites is striking.