Nature Communications | CF2H: A Cell-Free System for Rapid Protein Interaction Screening Enables Efficient Discovery of High-Affinity PD-L1 Binding Proteins

This study provides a high-throughput, low-cost experimental validation pathway for the development of immune checkpoint inhibitors, significantly accelerating the iteration cycle from computational design to functional validation, offering direct guidance for PD-L1-targeted drug discovery.

 

Literature Overview

The paper 'CF2H: a cell-free two-hybrid platform for rapid protein binder screening,' published in Nature Communications, systematically explores how cell-free systems can enable rapid, sensitive, and cloning- and purification-free protein interaction detection. The authors developed a novel platform called CF2H (Cell-Free Two-Hybrid), which fuses the target protein to the DNA-binding domain (DBD) of the λ phage CI protein. Upon binding, dimerization is induced, activating reporter gene expression and enabling real-time monitoring of protein interaction events. This method is compatible with linear DNA templates for expression, greatly simplifying experimental procedures, and is suitable for screening various protein binders, including single-domain antibodies, DARPins, and de novo designed proteins. The study further demonstrates its potential in discovering high-affinity PD-L1 binders and small-molecule PPI modulators.

Background Knowledge

The PD-1/PD-L1 signaling pathway is currently one of the core targets in cancer immunotherapy, and its inhibitors have shown significant efficacy in multiple cancers. However, developing novel blocking antibodies or protein therapeutics still faces bottlenecks such as long development cycles, high costs, and reliance on complex expression and purification processes. Traditional methods like yeast display or SPR analysis require substantial resources and specialized expertise, limiting accessibility for non-specialized laboratories. Moreover, PD-L1 tends to form inclusion bodies in E. coli, making soluble expression in prokaryotic systems difficult and posing a major obstacle to high-throughput screening. Therefore, there is an urgent need for a fast, flexible, and cell culture-free screening platform. This study addresses this need by proposing a cell-free system to replace traditional cell-dependent methods, using a designed binary interaction detection system with inducible transcription to overcome technical barriers between protein expression and functional validation, offering a new pathway for experimental validation of de novo protein designs.

 

 

Research Methods and Experiments

The authors employed an E. coli lysate-based cell-free expression system (CFS), combined with a strategy that separates the DNA-binding domain and dimerization domain of the λ phage CI protein, to construct the CF2H detection platform. The target protein is expressed as a fusion with CI-DBD; when a binding partner is present, CI dimerization is induced, thereby activating the sfGFP reporter gene driven by the pRM promoter. This system directly uses linear DNA templates amplified by PCR, eliminating the need for cloning or sequencing. To address the difficulty of PD-L1 expression, the authors adopted two strategies: first, fusing PD-L1 with a multimerization domain (e.g., p53 tetramer) to enhance its valency and stability; second, using biotinylated PD-L1 assembled with streptavidin into a tetrameric complex as an exogenously added antigen, effectively circumventing the challenges of endogenous expression. The study also verified the correlation between binding affinity and signal intensity via alanine scanning and tested the interference effects of small-molecule PPI inhibitors (e.g., NVP-CGM097, Venetoclax) on interactions.

Key Conclusions and Perspectives

• CF2H completes the entire workflow from DNA to functional validation within 24 hours, significantly shortening the multi-week cycle of traditional methods and providing an efficient solution for high-throughput screening.
• The platform successfully identified multiple high-affinity candidates from 36 computationally designed PD-L1 binders, with DBP035 exhibiting a Kd of 0.78 nM and blocking PD-1/PD-L1 interaction at an IC50 of 28 nM in cellular models, demonstrating its functionality and translational potential.
• By using exogenously added multimeric antigens, CF2H overcomes expression challenges associated with difficult targets like PD-L1, expanding the platform’s applicability to traditionally hard-to-express proteins.
• CF2H can not only detect protein–protein interactions but also evaluate the regulatory effects of small molecules on PPIs, such as Venetoclax’s inhibition of the BCL2/LD3 complex, indicating its utility in drug screening and mechanistic studies.
• The platform's signal intensity positively correlates with binding affinity and is not solely dominated by expression levels, aiding in the prioritization of candidate molecules with both high affinity and favorable physicochemical properties.

Research Significance and Prospects

This study provides an accessible and user-friendly screening tool for the immuno-oncology field, particularly suitable for laboratories with limited resources or lacking experience in protein expression to rapidly validate de novo design outcomes. The cell-free nature avoids cytotoxicity and expression bottlenecks, enhancing screening success rates. Future integration with automated liquid handling and multiplexed detection could enable even higher throughput. Additionally, the CF2H platform could be extended to develop binding proteins for other immune checkpoints such as CTLA-4, CD47, or used to build cell-free biosensors responsive to specific antigens, advancing point-of-care diagnostics and personalized therapies.

 

 

Conclusion

The CF2H platform established in this study marks a paradigm shift from traditional cell-dependent screening to cell-free rapid validation. By cleverly leveraging the transcriptional activation mechanism of the CI protein, it enables protein interaction detection to begin within minutes in a test tube, compressing the screening timeline from weeks to under 24 hours. This technology not only solves the validation challenges of difficult-to-express targets like PD-L1 but also provides strong experimental support for de novo protein design. The successful discovery and functional validation of DBP035 demonstrate that CF2H is capable of identifying high-affinity, functional binding proteins with direct potential to become preclinical candidates. More importantly, the modular design of the platform allows broad application in various protein interaction studies, including small-molecule modulator screening and biomarker detection. For cancer immunotherapy, CF2H has the potential to accelerate the discovery of next-generation immune checkpoint inhibitors, lower development barriers, encourage broader participation in innovative drug development, and ultimately promote the construction of a precision medicine ecosystem.

 

Julien Capin, Pauline Mayonove, Angelique DeVisch, Julien Espeut, and Jerome Bonnet. CF2H: a cell-free two-hybrid platform for rapid protein binder screening. Nature Communications.