Self-Assembling Protein Nanoparticles Deliver Gene Editors Into Cells Without Toxicity
Harvard-developed ENTER nanoparticles outperform lipid transfection reagents in delivering mRNA, CRISPR, proteins, and siRNA to diverse cell types.
Summary
Researchers at Harvard and Beth Israel Deaconess Medical Center engineered a new class of protein-based nanoparticles called ENTER (Elastin-based Nanoparticles for Therapeutic delivERy) capable of delivering gene editors, mRNA, proteins, and siRNA directly into cells. Built from recombinant elastin-like polypeptides (ELPs) fused to computationally optimized endosomal escape peptides, these particles self-assemble into pH-sensitive micelles that disassemble inside acidic endosomes to release cargo. Through four generations of iterative design and in silico screening of α-helical peptide libraries, the team identified EEP13 — a novel peptide boosting protein delivery efficiency by 48% over a benchmark. The system worked across cell lines and primary cells and achieved lung epithelial gene editing in reporter mice via intranasal administration.
Detailed Summary
Safe and efficient delivery of biological macromolecules into cells remains one of the central challenges in gene therapy and precision medicine. Current leading technologies — lipid nanoparticles (LNPs) and viral vectors — carry significant limitations including immunogenicity, manufacturing complexity, and difficulty incorporating active cell-targeting domains. This paper from researchers at Harvard Medical School and the Wyss Institute describes ENTER, a fully recombinant, protein-based nanoparticle platform designed to overcome these barriers using elastin-like polypeptide (ELP) building blocks.
The ENTER system was developed through four iterative design generations. First-generation ELP micelles could encapsulate cargo but lacked endosomal escape capability. The second generation introduced histidine residues into the hydrophobic core, enabling pH-responsive disassembly via proton sponge effect at endosomal pH below ~6.8. Third-generation constructs placed endosomal escape peptides (EEPs) at the particle surface, improving delivery but causing cytotoxicity. The fourth-generation breakthrough relocated EEPs to the particle's interior C-terminus, shielding them from plasma membrane interaction until endosomal acidification triggered release. This V4-ELP architecture formed monodisperse ~70 nm spherical micelles and achieved near-complete Cre recombinase delivery to HEK293-RFP reporter cells with no measurable cytotoxicity.
Critically, the team used machine learning-guided in silico screening of an α-helical peptide library to discover EEP13, a novel endosomal escape peptide achieving 48% greater protein delivery efficiency than the benchmark S10 peptide. EEP13 was identified through a two-stage process: computational scoring of structural and physicochemical properties followed by iterative experimental validation. V4-ELP-EEP13 outperformed or matched Lipofectamine 3000 and other lipid-based transfection reagents in delivering Cre recombinase protein, mRNA-encoded Cre, plasmid DNA-encoded Cre, Cas9 and ABE8e ribonucleoproteins (RNPs), and siRNA across HEK293, HeLa, and Jurkat cell lines as well as primary human fibroblasts, macrophages, T cells, and hematopoietic stem cells.
The platform's versatility was demonstrated across multiple therapeutic modalities. For siRNA delivery, ELP-EEP13 achieved potent gene knockdown at low doses. For CRISPR applications, Cas9 RNP and ABE8e base editor delivery enabled efficient genome editing in hard-to-transfect primary cells. A particularly notable application was delivering IRF5, an M1 macrophage transcription factor, as a protein directly into primary macrophages, significantly upregulating key pro-inflammatory cytokines — demonstrating the potential to reprogram innate immune cells. V4-ELP-S10 achieved 50% RFP+ cells at ~1.5 μM versus ~32 μM for free S10 peptide, a greater than 20-fold potency advantage, with zero cytotoxicity versus significant cell death with free peptide.
In the most translational experiment, intranasal administration of ELP-EEP13 complexed with Cre recombinase protein achieved efficient editing of lung epithelial cells in Rosa26 reporter mice, confirmed by histological analysis of lung tissue. This in vivo result positions ENTER as a potential vehicle for inhaled gene therapies targeting pulmonary diseases including cystic fibrosis and lung cancer. The fully recombinant, non-viral, non-lipid nature of ENTER offers potential immunogenicity and manufacturing advantages over existing platforms, though direct clinical comparisons remain to be conducted.
Key Findings
- V4-ELP-EEP13 achieved 48% improved protein delivery efficiency vs the benchmark S10 peptide, identified via machine learning-guided in silico screening of an α-helical peptide library
- V4-ELP-S10 required ~1.5 μM to achieve 50% Cre+ cells vs ~32 μM for free S10 peptide — a >20-fold potency improvement — with zero cytotoxicity at effective doses
- ENTER nanoparticles matched or outperformed Lipofectamine 3000 across delivery of mRNA, plasmid DNA, protein, siRNA, and CRISPR RNPs in multiple cell types
- pH-responsive micelle disassembly confirmed below pH ~6.8, corresponding to endosomal acidification, enabling triggered cargo release without plasma membrane disruption
- Intranasal ELP-EEP13 + Cre protein achieved efficient in vivo editing of lung epithelial cells in Rosa26 reporter mice, confirmed by histological lung tissue analysis
- IRF5 transcription factor protein delivery to primary macrophages significantly upregulated pro-inflammatory cytokines, demonstrating functional intracellular protein reprogramming
- Sortase-mediated conjugation achieved ~50% functionalization of accessible GGG motifs on ELP nanoparticles; anti-CD117 antibody conjugation enabled selective binding to CD117+ hematopoietic stem cell lines
Methodology
This was an iterative experimental design study using HEK293-RFP Cre reporter cells as the primary functional readout for intracellular protein delivery efficiency. The team screened across four ELP design generations, multiple EEP peptides, and six protease-cleavable linkers in cell lines (HEK293, HeLa, Jurkat) and primary cells (fibroblasts, macrophages, T cells, HSCs). In vivo validation used Rosa26 reporter mice with intranasal delivery and histological lung tissue analysis. EEP discovery used machine learning-guided in silico screening followed by iterative experimental validation; statistical comparisons were made against Lipofectamine 3000 and free peptide controls.
Study Limitations
The study is preclinical, with in vivo results limited to a single mouse reporter model using intranasal administration — systemic delivery efficacy, biodistribution, and immunogenicity in larger animal models have not yet been evaluated. The sortase conjugation efficiency of approximately 50% for surface protein functionalization may limit precise dosing of surface-displayed cargo in clinical translation. No formal conflict of interest disclosures were extractable from the provided text, and long-term safety, stability under physiological storage conditions, and scalable manufacturing have not been addressed.
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