Dynamic Imine Bonding Facilitates Mannan Release from a Nanofibrous Peptide Hydrogel

Recently, there has been increased interest in using mannan as an immunomodulatory bioconjugate. Despite notable immunological and functional differences between the reduced (R-Man) and oxidized (O-Man) forms of mannan, little is known about the impact of mannan oxidation state on its in vivo persistence or its potential controlled release from biomaterials that may improve immunotherapeutic or prophylactic efficacy. Here, we investigate the impact of oxidation state on the in vitro and in vivo release of mannan from a biocompatible and immunostimulatory multidomain peptide hydrogel, K₂(SL)₆K₂ (abbreviated as K₂), that has been previously used for the controlled release of protein and small molecule payloads. We observed that O-Man released more slowly from K₂ hydrogels in vitro than R-Man. In vivo, the clearance of O-Man from K₂ hydrogels was slower than O-Man alone. We attributed the slower release rate to the formation of dynamic imine bonds between reactive aldehyde groups on O-Man and the lysine residues on K₂. This imine interaction was also observed to improve K₂ + O-Man hydrogel strength and shear recovery without significantly influencing secondary structure or peptide nanofiber formation. There were no observed differences in the in vivo release rates of O-Man loaded in K₂, R-Man loaded in K₂, and R-Man alone. These data suggest that, after subcutaneous injection, R-Man naturally persists longer in vivo than O-Man and minimally interacts with the peptide hydrogel. These results highlight a potentially critical, but previously unreported, difference in the in vivo behavior of O-Man and R-Man and demonstrate that K₂ can be used to normalize the release of O-Man to that of R-Man. Further, since K₂ itself is an adjuvant, a combination of O-Man and K₂ could be used to enhance the immunostimulatory effects of O-Man for applications such as infectious disease vaccines and cancer immunotherapy.