Engineering synthetic vaccines using cues from natural immunity

Abstract

Vaccines aim to protect against or treat diseases through manipulation of the immune response, promoting either immunity or tolerance. In the former case, vaccines generate antibodies and T cells poised to protect against future pathogen encounter or attack diseased cells such as tumours; in the latter case, which is far less developed, vaccines block pathogenic autoreactive T cells and autoantibodies that target self tissue. Enormous challenges remain, however, as a consequence of our incomplete understanding of human immunity. A rapidly growing field of research is the design of vaccines based on synthetic materials to target organs, tissues, cells or intracellular compartments; to co-deliver immunomodulatory signals that control the quality of the immune response; or to act directly as immune regulators. There exists great potential for well-defined materials to further our understanding of immunity. Here we describe recent advances in the design of synthetic materials to direct immune responses, highlighting successes and challenges in prophylactic, therapeutic and tolerance-inducing vaccines.

Figure 1. Pathogen sensing by the immune system and immune context during the priming of an adaptive immune response

DCs are a central interpreter in distinguishing between foreign and self-antigens in the context of microenvironmental cues, and play a major role (along with other innate immune cells) in determining the outcome of antigen recognition by T- and B-cells. A, At steady state, immature DCs (iDCs) throughout the periphery constantly sample their environment and encounter 1) immunogenic signals from infected or immunized, dying cells, accompanied by triggering of danger sensors (TLRs, CLRs, NLRs, RLRs, SRs) or 2) tolerogenic signals from dying self-cells or cellular debris generated by homeostatic turnover; these produce a continuous spectrum of output responses ranging from strong induction of effector phase immunity to strong induction of tolerance, with the exact outcome determined by the integration of inputs by the DC. Pathogen detection occurs via a conserved suite of danger sensors relies on detection of “danger signals,” microbe-associated products with distinct molecular motifs. Different sensors are present in endosomes (TLRs, SRs), the cytosol (RLRs, NLRs), the ER (SRs) and the plasma membrane (TLRs, CLRs). Each danger sensor recognizes a different motif that is present in a class of microbes but absent from host tissues. In response to these “danger” or tolerizing signals, DCs (and other innate cells) create the immunological context for antigen recognition by secreting cytokines, expressing diverse adhesive, costimulatory or regulatory receptors that provide cues to responding lymphocytes. B, In immunogenic contexts, responding B-cells can subsequently enter germinal centers to undergo somatic hypermutation, become short-lived plasmablasts, or differentiate into long-lived memory B-cells or plasma cells. T-cells can differentiate into effector cells or memory cells with distinct homing and functional capacities; effector cells can have diverse functions (Th1, Th2, Th17, etc.) depending on the context set by DCs. Notably, regulatory feedback loops are engaged even in highly inflammatory contexts, as part of the natural control system regulating immunity, and primed effector cells can be driven to anergic/exhausted states similar to tolerance at later stages of an immune response. C, Peripheral tolerance is maintained by a distinct set of signals, e.g., apoptotic cells that die during homeostatic turnover contain ligands that activate the plasma membrane-expressed Tyro-3, Axl, and Mer (TAM) family receptor tyrosine kinases, inhibiting DC activation and maturation. Many additional APCs also participate in tolerogenic signaling. In tolerogenic contexts, T-cells are driven into several different states of non-responsiveness (anergy, exhaustion, deletion, or regulatory fates) that prevent effector responses against self or harmless environmental antigens.

Figure 2. Structural and compositional features of microbes and their mimicry in synthetic biomaterials-based vaccines

Microbes and microbial products are particulates spanning length scales from tens to thousands of nm in size, with distinct structural and chemical features that are sensed by the immune system. A rich strategy in biomaterials-based vaccines is to design nanoparticles and microparticles that mimic key features of microbes to invoke similar signaling pathways and immune responses elicited by native microbes, without the danger of infection or uncontrolled inflammation.

Figure 3. Effects of particulate size on tissue, cell, and intracellular targets after entry into interstitial tissue

A, After injection into the interstitium (i.e., intramuscular, intradermal, subcutaneous, etc), particles (whose definition here includes molecules) will disperse and convect with interstitial flow, driven by transient pressure gradients that arise from the injection as well as the natural small pressure gradient between blood and lymphatic capillaries. Very small particles (red), whose diffusion velocity is greater than convective velocity, can readily diffuse and will rapidly dilute in local concentration, which limits the effective lymphatic concentration. Larger, intermediate-sized (blue) particles have smaller diffusion speeds and furthermore are transported within the more permeable regions of the ECM (as in size-exclusion chromatography), and thus their transport is governed more by convection and they are more efficiently directed into the lymphatic vessels. However, as size increases, steric hindrance becomes limiting, and particles that are too large (~ >500 nm although this depends on tissue, level of hydration, and experimental conditions) remain mostly trapped in the interstitial space. B, Once inside the lymphatic vessel, lymph node retention positively correlates with particle size. Larger (or opsonized) particles are readily taken up by subcapcular macrophages, while intermediate-sized particles can directly access the T cell zone and associated DCs. The B-cell zone conduits, however, which are formed by follicular dendritic cells, restrict access to particles <~3 nm. C, Size also affects antigen concentration and dose upon intracellular uptake by the APC. If the antigen is a free protein, then the effective “particle concentration” is equal to the antigen concentration, and this is also equal to the concentration within macropinocytotic vesicles after uptake. However, if antigen is adsorbed or incorporated into a nanoparticle, then the concentration of antigen “units”, or particles, is less than the antigen concentration by the number of antigen molecules adsorbed per particle. Larger particles of e.g., 250 nm can contain 1000 antigen molecules per particle, and thus reduce the effective antigen concentration 1000-fold. On the other hand, upon uptake, antigen should be 10-fold or 1000-fold more concentrated inside the phagosome when taken up in nanoparticulate form vs. free antigen form. It is unknown how such differences in antigen delivery (i.e., more vesicles with fewer antigens each vs. fewer vesicles with more antigens each) affect cross-presentation efficiency.