Christopher T. method that uniquely pairs a covalent, monovalent linkage between genotype and phenotype with a relatively high fusion yield.[10,11] Furthermore, it becomes possible to use very large combinatorial libraries because of the lack of an obligate in vivo step during iterative rounds of enrichment, and mRNA-display libraries can encompass 103C105 -fold more unique sequences than typical phage- or cell-based display experiments.[11] Larger libraries often produce higher-affinity binders, [12] and can also yield diverse pools of target-specific ligands with unique properties.[5] Large libraries are especially beneficial for evolving rare functionalities including enzymatic activity,[13] and the capacity to target nonstructured biological targets.[6] However, effective isolation of desired molecules from high-complexity mRNA-display libraries typically requires many rounds of selection, which is often resource intensive. Many iterative selection cycles may also result in the enrichment of suboptimal ligands as a result of compounding biases from selective constraints other than binding efficiency (e.g. PCR or translation efficiencies). Thus, novel technologies that can accelerate and automate the selection process are urgently needed. We report herein a rapid, low-cost, highly efficient method for generating high-affinity antibody mimetics using small-scale, continuous-flow magnetic separation (CFMS). Unlike previous microfluidic approaches, which have required fabrication of specialized devices,[14] CFMS SJFδ can be performed within a small section of perfluoroalkoxy (PFA) tubing to achieve highly stringent selection with minimal background. This low background directly contributes to the efficiency of selection, and continuous flow improves the washing efficiency while promoting selection for low off-rates; together, these factors contribute to the rapid convergence of high-affinity ligands. We also describe the implementation of an improved 10Fn3 library with enhanced expression (e10Fn3). Previously, we utilized an in vivo expression screen based on a green fluorescence protein (GFP) folding SJFδ reporter[15] to improve the expression of a phospho-specific SJFδ IB-binding Fn3 variant.[6] We demonstrate here that these framework mutations plus an additional rational mutation enhance expression of four unrelated 10Fn3 variants both in vivo and in vitro (see Figure S1 in the Supporting Information). The enhancement in expression may be due to the replacement of three solvent exposed hydrophobic residues, which are localized on the three-dimensional structure, with polar residues, as well as replacing a buried Leu for Ile, which may enhance stability because of a significantly higher -sheet-forming propensity.[16,17] The substantial 2C4-fold expression increase for individual e10Fn3 clones in rabbit reticulocyte lysate (Figure S1c) is also seen for the na?ve e10Fn3 library as a whole as expected (data not shown). One additional change includes limiting variation of the final BC loop random position to the hydrophobic residues Leu, Ile, and Val, as this position is a buried core residue in the wild-type 10Fn3 structure and may interact in the transition-state folding nucleus.[18] We chose to target interleukin 6 (IL-6) as a model to generate high-affinity e10Fn3-based ligands using CFMS mRNA display. This cytokine contributes to the regulation of the immune response and hematopoiesis, [19] and aberrant IL-6 serum levels are implicated in various inflammatory diseases and cancers.[20] We show that CFMS selection offers significantly improved (ca. 30-fold) partition SJFδ efficiencies compared to conventional methods, and report the generation of a high-affinity IL-6 ligand (KD = Rabbit Polyclonal to PITX1 21 nM) with an excellent off-rate (8.8 10?4 s?1). This high-affinity IL-6 ligand is capable of inhibiting signaling through gp130, thus indicating the molecules potential value and demonstrating the effectiveness of CFMS for rapidly identifying clinically relevant molecules. A key advantage of our method is the ability to perform highly stringent molecular selections on very small scales using readily available materials while maintaining high recovery of target binders. The selection process is shown.