Folding and Refolding Protocols for pMHC Generation

Properly folded peptide–major histocompatibility complexes (pMHCs) underpin reliable T cell assays, structural immunology projects, and therapeutic screening. The pMHC folding process involves expressing major histocompatibility complex (MHC) components in E. coli, isolating them as inclusion bodies, and refolding them with peptide ligands under tightly controlled conditions to form stable complexes. Correct peptide binding within the binding groove is essential for accurate T cell recognition and downstream immune responses. This guide explains each stage in detail, identifies common challenges, and offers practical solutions to improve yield, stability, and reproducibility.

Overview of pMHC Complex Components

A class I MHC pMHC contains three parts: an MHC class I heavy chain, β2-microglobulin (β2m), and an antigenic peptide, typically eight to ten amino acids long. Because bacterial expression systems do not support natural folding, heavy chain and β2m accumulate as insoluble inclusion bodies. Refolding these proteins with the peptide in vitro recreates the native peptide–MHC complex that a T cell receptor (TCR) would recognize through its peptide binding groove.

In living cells, MHC class I molecules assemble in the endoplasmic reticulum, where peptide loading and antigen processing pathways determine which peptides reach the cell surface for T cell recognition. Reproducing that interaction in vitro allows structural biologists to study peptide MHC specificity, while immunologists can characterize antigen-specific T cells or design T cell vaccines and adoptive T cell therapies.

Expression and Isolation of MHC Heavy Chain and β2-Microglobulin

After defining the target HLA-A*02:01 or another allele and the desired peptide, the next step is producing each subunit in a form suitable for efficient pMHC folding. This involves bacterial expression, isolation of inclusion bodies, chemical solubilization, and partial purification to remove impurities that could compromise yield and folding fidelity.

Bacterial Expression

To express the MHC heavy chain and β2m, their respective genes are cloned into separate expression vectors optimized for Escherichia coli (E. coli). Each vector is transformed into a suitable E. coli strain. Protein expression is induced using isopropyl β-D-1-thiogalactopyranoside (IPTG), typically at 25 - 30 °C. The reduced temperature helps minimize protein misfolding and aggregation during overexpression. Despite this, these proteins generally form inclusion bodies, making further processing necessary.

Harvesting Inclusion Bodies

Following the expression, bacterial cells are harvested and lysed to release the contents. Lysis can be achieved mechanically (e.g., sonication or high-pressure homogenization) or chemically using lysozyme and detergents. The lysate is then centrifuged to separate soluble proteins from insoluble material. The inclusion bodies, which contain the overexpressed MHC heavy chain or β2m in a misfolded but concentrated form, are recovered from the pellet fraction.

Solubilization

To denature and solubilize the inclusion bodies, the pellets are resuspended in a strong denaturant, typically 6–8 M urea or guanidine hydrochloride (GnHCl). A reducing agent, such as 10 mM dithiothreitol (DTT), is included to break disulfide bonds and ensure complete unfolding. The suspension is stirred at room temperature until all visible solids have dissolved, indicating complete solubilization of the proteins. This stage yields a uniform solution ready for refolding with synthetic peptide or high-affinity peptides that form bound peptide complexes during later steps.

Partial Purification

Before attempting refolding, partially purifying the denatured proteins helps improve folding efficiency and final yield. Ion-exchange chromatography or size-exclusion chromatography can be used under denaturing conditions to remove bacterial contaminants, degraded protein fragments, or other impurities. This step ensures that the proteins entering the refolding process are as pure and homogenous as possible, reducing the likelihood of aggregation and misfolding during subsequent steps.

In Vitro Refolding Protocol (MHC Class I Complexes)

Once purified, the denatured chains must fold together with peptide ligands under carefully balanced buffer conditions.

Buffer Composition

Step-by-Step Workflow

• Buffer preparation – Chill all reagents and glassware. Prepare the folding buffer on ice and degas for ≈ 10 min to minimize adventitious oxidation.

• Protein assembly – Combine denatured heavy chain, β2-microglobulin, and peptide at a 1 : 1.2 : 3 molar ratio.

• Controlled dilution – While gently stirring, slowly dilute the protein mix into cold folding buffer until the overall protein concentration is 25–50 µg/mL. This step preserves thermal stability and avoids aggregation while encouraging peptide binding to the peptide-receptive MHC class I molecules.

• Incubation – Hold at 4–10 °C for 24–48 h. This temperature range supports correct folding and the formation of native disulfide bonds.

• Cleanup – Concentrate and buffer-exchange (ultrafiltration or dialysis) to remove urea, arginine, and glutathione derivatives.

Common Issues and Fixes

 

Quality Assessment of Refolded pMHC

Rigorous verification confirms the structural fidelity and functional competence of refolded pmhc complexes. Techniques such as SDS-PAGE under reducing and non-reducing conditions verify correct disulfide bond formation, while size-exclusion chromatography (SEC) identifies monomeric mhc class I molecules at the expected molecular weight. Functional assays test whether the peptide mhc conformation is recognized by the t cell receptor.

For example, ELISA or flow cytometry using mhc tetramers can confirm selective t cell binding and cell activation. High-affinity interactions reflect accurate peptide binding within the binding groove, a prerequisite for reliable antigen specific t cells detection and downstream cell responses. Crystallographers often follow with limited proteolysis and high-resolution SEC to ensure homogeneity before crystallization. Such structural analyses reveal the structural and kinetic basis of tcr binding, aiding interpretation of peptide mhc specificity and tcr constant domains orientation.

Special Considerations for MHC Class II and Custom Peptides

MHC class II molecules require co-refolding of both α and β chains before peptide loading. Hydrophobic or elongated single antigenic peptides may require ≤ 5 % DMSO to remain soluble. Gradual temperature increases (4 → 20 °C) improve yields and preserve the binding groove conformation.

Researchers working on peptide exchange methods can use photolabile peptide ligands or excess peptide conditions to replace a bound sequence without denaturing the entire complex. Such peptide exchange allows quick testing of variant epitopes for heightened immunogenicity or irrelevant peptide controls when comparing antigen-specific responses.

Additionally, inclusion of molecular chaperone TAPBPR or temperature-modulated peptide exchange can generate empty MHC class I molecules for further peptide synthesis and peptide loading studies, closely mimicking the antigen processing pathway in the endoplasmic reticulum.

Applications in Immunology and Therapeutics

Validated, stable peptide major histocompatibility complex molecules support a wide range of research and therapeutic applications in cell biology and cancer immunotherapy.

• Tetramer staining quantifies and phenotypes antigen-specific T cells directly from cells expressing target epitopes.

• Functional read-outs (cytokine release, proliferation) measure T cell activity in cell line models or patient samples.

• Structural analyses clarify T cell receptor–MHC class I molecules contacts, revealing precise TCR specificities that inform cell vaccines and cell therapies.

• Studies comparing empty MHC versus peptide-loaded complexes illuminate peptide-derived determinants of binding affinity and immune responses.

These data guide the rational design of adoptive T cell therapies, T cell vaccines, and cell vaccines that expand antigen-specific T cells against tumors. Folding knowledge also benefits protein engineering approaches that engineer disulfide-stabilized MHC backbones or peptide exchange scaffolds, which are compatible with fluorescence anisotropy or biophysical screening.

Empowering Discovery with KACTUS pMHC Solutions

KACTUS provides a comprehensive portfolio of MHC monomers, domains, and peptide-loaded complexes for immunological research and therapeutic discovery. Each protein undergoes strict QC testing, including SEC purity analysis and binding validation by ELISA or SPR, ensuring reliable performance in T-cell assays and structural studies.

Researchers can also request custom peptide-MHC refolding services, allowing for the precise design of complexes tailored to specific HLA alleles or peptide epitopes. Combined with KACTUS expertise in soluble TCR expression and SPR analytical services, these solutions accelerate discovery from peptide screening to immune monitoring.

References

[1] Garboczi DN, Ghosh P, Utz U, Fan QR, Biddison WE, Wiley DC. Structure of the complex between human T-cell receptor, viral peptide, and HLA-A2. Nature. 1996;384(6605):134-141.

 [2] Van Regenmortel MHV. Folding and assembly of MHC class I molecules. Curr Opin Immunol. 1992;4(1):89-95.

[3] Garboczi DN, Biddison WE, Wiley DC. Efficient refolding and crystallization of MHC class I–peptide complexes produced in E. coli. J Immunol Methods. 1994;168(1):1-12.

[4] Wang J, Sidney J, Kim Y, Sette A. Peptide–MHC binding predictions and applications in vaccine design. Immunology. 2010;130(2):213-224.

[5] Day CL, et al. Direct ex vivo analysis of human memory CD8+ T-cell responses using MHC tetramers. J Immunol Methods. 2003;268(1):51-64.

[6] Smith KJ, et al. Refolding protocols for HLA class I complexes containing non-canonical peptides. Protein Expr Purif. 2018;146:43-50.

 [7] Rudolph MG, Wilson IA. The specificity of T-cell receptor recognition of peptide–MHC complexes. Curr Opin Immunol. 2002;14(1):52-65.

[8] KACTUS Bio. Soluble TCR Expression & Affinity Analysis Services. 2025. Available at: https://kactusbio.com/services/soluble-tcr-expression-affinity-analysis 

 

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