Supramolecular peptide hydrogels are promising materials for a wide variety of biomedical applications. Since they are composed of the same 20 natural amino acid building blocks that comprise all human proteins, they have the potential for enhanced biocompatibility and modular bioactivity. Their supramolecular nature endows them with shear-thinning and self-healing materials properties that enables their minimally invasive administration through small-bore needles. These viscoelastic materials can be over 99% water by mass. Their high-water content mimics the natural extracellular matrix and facilitates the facile loading of hydrophilic drugs and proteins while avoiding denaturation from organic solvents. Due to these promising properties, many groups have focused on developing these materials as tissue engineering scaffolds, drug delivery platforms, vaccine adjuvants, materials for cancer immunotherapy, hemostatic agents, antimicrobial wound dressings, etc.

Multidomain peptide (MDP) hydrogels, developed in the Hartgerink lab, are a class of self-assembling peptides that form hydrogels under physiologically relevant conditions. These peptide sequences follow a general ABA structural pattern. The “A” motifs contain a charged amino acid selected from arginine, lysine, aspartic acid, and glutamic acid. The B domain is an amphiphilic region that promotes self-assembly into β-sheets. This region of the sequence typically contains repeating serine-leucine residues. For over a decade, these materials have been investigated for use in a plethora of different biomedical applications. In this thesis, I describe my investigations into using MDP hydrogels as vaccine adjuvants and drug delivery vehicles. I additionally explore designing novel peptide hydrogels with non-canonical motifs to improve their adjutancy and capacity to control the release of small and macromolecular payloads.

Chapter 1 serves as an introduction to adjuvants and immunomodulatory materials. While each chapter relating to vaccine adjuvants contains a targeted introduction, this chapter particularly provides critical context for the importance of these materials in today’s world for pandemic preparedness. In Chapter 2, I investigate the use of differently charged MDPs to adjuvant the model antigen ovalbumin. I found that lysine containing MDPs are the best suited MDPs for vaccine adjuvant applications and that, interestingly, MDPs strongly bias a humoral immune response. I build on this work in Chapter 3 where, by modifying MDPs with a toll-like receptor 7 agonist, we further enhance the adjuvant potential of these peptide hydrogels. Further, we illicit a more balanced cellular and humoral immune response, which could be useful for a wide variety of infectious disease vaccines and cancer immunotherapies.

Chapter 4 is a bridge chapter that links my work in immunomodulation to drug delivery leveraging dynamic covalent interactions. In this chapter, I aimed to control the release of the carbohydrate adjuvant mannan by forming dynamic imine bonds between the oxidized carbohydrate that the primary amines on a lysine containing MDP. I found that imine bonds can delay the release of oxidized mannan in vitro, but in vivo reduced and oxidized mannan is cleared much more rapidly than reduced mannan. The release of this oxidized carbohydrate can be reduced to the same rate as the reduced carbohydrate through imine bonding the MDP. The differences in reduced and oxidized mannan have been underappreciated in the literature, and I describe how MDPs may be useful for investigating the biological consequences of the different behaviors of these two materials in vivo.

Chapter 5 focuses on peptide-based hydrogels that leverage boronic acid dynamic covalent chemistry for drug delivery. A thorough review of related drug delivery literature is included in the introduction of Chapter 5. In this part of my thesis, I describe my work developing new MDPs capable of forming dynamic covalent interactions with boronic acids (BAs). I found that these modified MDPs can significantly delay the release of BA-containing small molecules and BA-labeled proteins in vitro and in vivo. These materials can be used for both the local systemic delivery of therapeutics and improve the pharmacokinetic profiles of the drugs tested. I evaluate the potential application of this system by delivering basal insulin to diabetic mice. After a single injection of BA-modified insulin in this drug delivery platform, diabetic mice remain normoglycemic for 144 h. In comparison, a single injection of the maximum safe dose of insulin without hydrogel only maintained healthy blood glucose levels for 4 h.

The work I present in this thesis aims to motivate the development of the next generation of peptide materials that leverage non-canonical modifications to improve upon the 20 amino acids nature provides. I show that including these modifications can alter both the immunological activity and drug delivery capabilities of MDPs and enhance their performance in in vivo models beyond what is possible with unmodified MDPs. This work shows the potential for next-generation MDPs that include non-canonical motifs to build on the past success of these materials and further their development towards clinical translation.