Biomathematical description of synthetic peptide libraries

Exploring structural engineering approach to formulate and characterize next-generation adjuvants

It is critical to emphasize the importance of vaccination as it protects us against harmful pathogens. Despite significant progress in vaccine development, there is an ongoing need to develop vaccines that are not only safe but also highly effective in protecting against severe infections. Subunit vaccines are generally safe, but they frequently fail to elicit strong immune responses. As a result, there is a need to improve vaccine effectiveness by combining them with adjuvants, which have the potential to boost the immune system many folds. The process of developing these adjuvants requires searching for molecules capable of activating the immune system, combining these promising compounds with an antigen, and then testing this combination using animal models before approving it for clinical use. Liposomal adjuvants work as delivery adjuvants and its activity depends on certain parameters such as surface charge, vesicle size, surface modification and route of administration. Self-assembly property of peptide adjuvants and discovery of hybrid peptides have widened the scope of peptides in vaccine formulations. Since most pathogenic molecules are not peptide based, phage display technique allows for screening peptide mimics for such pathogens that have potential as adjuvants. This chapter discusses about peptide and liposome-based adjuvants focusing on their properties imparting adjuvanticity along with the methods of formulating them. Methods of adjuvant characterization important for an adjuvant to be approved for clinical trials are also discussed. These include assays for cytotoxicity, T-lymphocyte proliferation, dendritic cell maturation, cytokine and antibody production, toll-like receptor dependent signaling and adjuvant half-life.

Cell-Free Biosynthesis to Evaluate Lasso Peptide Formation and Enzyme-Substrate Tolerance

Lasso peptides are ribosomally synthesized and post-translationally modified peptide (RiPP) natural products that display a unique lariat-like, threaded conformation. Owing to a locked three-dimensional structure, lasso peptides can be unusually stable toward heat and proteolytic degradation. Some lasso peptides have been shown to bind human cell-surface receptors and exhibit anticancer properties, while others display antibacterial or antiviral activities. All known lasso peptides are produced by bacteria and genome-mining studies indicate that lasso peptides are a relatively prevalent class of RiPPs; however, the discovery, isolation, and characterization of lasso peptides are constrained by the lack of an efficient production system. In this study, we employ a cell-free biosynthesis (CFB) strategy to address longstanding challenges associated with lasso peptide production. We report the successful use of CFB for the formation of an array of sequence-diverse lasso peptides that include known examples as well as a new predicted lasso peptide from Thermobifida halotolerans. We further demonstrate the utility of CFB to rapidly generate and characterize multisite precursor peptide variants to evaluate the substrate tolerance of the biosynthetic pathway. By evaluating more than 1000 randomly chosen variants, we show that the lasso-forming cyclase from the fusilassin pathway is capable of producing millions of sequence-diverse lasso peptides via CFB. These data lay a firm foundation for the creation of large lasso peptide libraries using CFB to identify new variants with unique properties.

Isolation of Chemically Cyclized Peptide Binders Using Yeast Surface Display

Cyclic peptides with engineered protein-binding activity have gained increasing attention for use in therapeutic and biotechnology applications. We describe the efficient isolation and characterization of cyclic peptide binders from genetically encoded combinatorial libraries using yeast surface display. Here, peptide cyclization is achieved by disuccinimidyl glutarate-mediated cross-linking of amine groups within a linear peptide sequence that is expressed as a yeast cell surface fusion. Using this approach, we first screened a library of cyclic heptapeptides using magnetic selection, followed by fluorescence activated cell sorting (FACS) to isolate binders for a model target (lysozyme) with low micromolar binding affinity (KD ∼ 1.2-3.7 μM). The isolated peptides bind lysozyme selectively and only when cyclized. Importantly, we showed that yeast surface displayed cyclic peptides can be used to efficiently obtain quantitative estimates of binding affinity, circumventing the need for chemical synthesis of the selected peptides. Subsequently, to demonstrate broader applicability of our approach, we isolated cyclic heptapeptides that bind human interleukin-17 (IL-17) using yeast-displayed IL-17 as a target for magnetic selection, followed by FACS using recombinant IL-17. Molecular docking simulations and follow-up experimental analyses identified a candidate cyclic peptide that likely binds IL-17 in its receptor binding region with moderate apparent affinity (KD ∼ 300 nM). Taken together, our results show that yeast surface display can be used to efficiently isolate and characterize cyclic peptides generated by chemical modification from combinatorial libraries.

Epitope Mapping via Phage Display from Single-Gene Libraries

Antibodies are widely used in a large variety of research applications, for diagnostics and therapy of numerous diseases, primarily cancer and autoimmune diseases. Antibodies are binding specifically to target structures (antigens). The antigen-binding properties are not only dependent on the antibody sequence, but also on the discrete antigen region recognized by the antibody (epitope). Knowing the epitope is valuable information for the improvement of diagnostic assays or therapeutic antibodies, as well as to understand the immune response of a vaccine. While huge progress has been made in the pipelines for the generation and functional characterization of antibodies, the available technologies for epitope mapping are still lacking effectiveness in terms of time and effort. Also, no technique available offers the absolute guarantee of succeeding. Thus, research to develop and improve epitope mapping techniques is still an active field. Phage display from random peptide libraries or single-gene libraries are currently among the most exploited methods for epitope mapping. The first is based on the generation of mimotopes and it is fastened to the need of high-throughput sequencing and complex bioinformatic analysis. The second provides original epitope sequences without requiring complex analysis or expensive techniques, but depends on further investigation to define the functional amino acids within the epitope. In this book chapter, we describe how to perform epitope mapping by antigen fragment phage display from single-gene antigen libraries and how to construct these types of libraries. Thus, we also provide figures and analysis to demonstrate the actual potential of this technique and to prove the necessity of certain procedural steps.

Optimization of design and production strategies for novel adeno-associated viral display peptide libraries

Libraries displaying random peptides on the surface of adeno-associated virus (AAV) are powerful tools for the generation of target-specific gene therapy vectors. However, for unknown reasons the success rate of AAV library screenings is variable and the influence of the production procedure has not been thoroughly evaluated. During library screenings, the capsid variants with the most favorable tropism are enriched over several selection rounds on a target of choice and identified by subsequent sequencing of the encapsidated viral genomes encoding the library capsids with targeting peptide insertions. Thus, a high capsid-genome correlation is crucial to obtain the correct information about the selected capsid variants. Producing AAV libraries by a two-step protocol with pseudotyped library transfer shuttles has been proposed as one way to ensure such a correlation. Here we show that AAV2 libraries produced by such a protocol via transfer shuttles display an unexpected additional bias in the amino-acid composition which confers increased heparin affinity and thus similarity to wildtype AAV2 tropism. This bias may fundamentally impair the intended use of AAV libraries, discouraging the use of transfer shuttles for the production of AAV libraries in the future.