Significance: The newest generation of small-molecule vaccine adjuvants aims at triggering specific receptors expressed by dendritic cells, the working horses of our immune system. Unfortunately, owing to their small size, upon administration these molecules rapidly enter systemic circulation and cause systemic inflammation. We report on a nanotechnology-based solution for this issue by covalent ligation of a potent immunostimulatory small molecule to hydrogel nanoparticles. This approach allows for lymph node-restricted immune activation and avoids systemic dissemination. Importantly, relative to soluble immunostimulatory compound, nanoparticle ligation yields increased immune activation in the draining lymph nodes and results in strongly increased antibody titers and T-cell responses against an admixed vaccine antigen.

Analogous to an assembly line, we employed a modular design for the high-throughput study of 1,536 structurally distinct nanoparticles with cationic cores and variable shells. This enabled elucidation of complexation, internalization, and delivery trends that could only be learned through evaluation of a large library. Using robotic automation, epoxide-functionalized block polymers were combinatorially cross-linked with a diverse library of amines, followed by measurement of molecular weight, diameter, RNA complexation, cellular internalization, and in vitro siRNA and pDNA delivery. Analysis revealed structure-function relationships and beneficial design guidelines, including a higher reactive block weight fraction, stoichiometric equivalence between epoxides and amines, and thin hydrophilic shells. Cross-linkers optimally possessed tertiary dimethylamine or piperazine groups and potential buffering capacity. Covalent cholesterol attachment allowed for transfection in vivo to liver hepatocytes in mice. The ability to tune the chemical nature of the core and shell may afford utility of these materials in additional applications.

The focus of this review is on the class of transiently thermoresponsive polymers.