Breakthrough in antigen recognition: Tracer-I amplifies the immune target

Breakthrough in antigen recognition: Tracer-I amplifies the immune target

The study introduces Tracer-I, a protein platform with broad HLA compatibility, paving the way for advanced immune response engineering and disease-specific targeting.

In a recently published study innatural biotechnology,Researchers describe the molecular structure of the targeted recognition of the antigen-MHC complex reporter for MHC I (Tracer-I), a protein platform that can be used to engineer immune responses.

Significance of MHC I peptide presentation

Diseased cells accumulate several aberrant proteins over time, including tumor-associated antigens or neoantigens as well as pathogen-derived antigens, which are ultimately degraded in proteosomes and lysosomes. Some of these antigenic peptide fragments, which are between eight and 12 lengths, are represented on the cell surface by major histocompatibility complex (MHC I) class I proteins.

MHC 1 presentation is crucial for the immune response as it enables innate immune cell-mediated killing of diseased cells and stimulates adaptive immunity. Adaptive immunity allows T cells to recognize antigens and undergo activation, triggering cell-mediated killing and antibody production.

The potential of T cell receptors

T cell receptors (TCRs) bind to peptide-MHC complexes (PMHC) through a combination of six flexible complementarity completion regions (CDR). The diversity introduced by the combinatorial mechanism as well as variations in docking angles and binding orientations enables significant specificity of antigen recognition.

Scientists are currently developing TCR CDRs to produce MHC I binding molecules with binding specificity against disease antigens. However, there are several challenges associated with these studies.

For this purpose, the development of TCRs from cells that have low affinity for the antigen is very slow. Furthermore, TCRs are associated with an inherent polyspecificity that facilitates immune surveillance for many pathogen-derived epitopes with a relatively limited TCR repertoire while limiting their specificity as therapeutic agents.

The HLA genes encoding MHC I peptides have over 38,000 allotypes across populations and genetic groups. TCRs are restricted to PMHC targets in only a few HLA versions, limiting their usefulness in different genetic contexts.

What is Tracer-i?

The authors of the current study took advantage of the limited number of backbone conformations on MHC I antigens to generate a platform compatible with a variety of HLA allotypes.

A single docking orientation can be used by most binding agents to interact with MHC I if it covers the full length of the antigen. This PMHC I binder scaffold has a surface that can be tailored to specifically bind to multiple disease-related peptides, which has the potential to be a very cost-effective and rapid system.

Tracer-I's scaffold uses a modified form of their previous peptide-oriented PMHC II binding platform Tracer-II to create an MHC I counterpart. Tracer-II has a concave surface feature that normally binds perpendicularly to the peptide binding groove on extended peptide structures of MHC III.

This platform was adapted by introducing a directed mutation to include the triggered peptide conformation on MHC I in a parallel orientation. Using computer modeling, a sequence compatible with the MHC I surface was identified. Variations were then introduced in the sequence in the concave feature to optimize the binding mode.

Binding specificity

To test the platform, three different PMHC-I targets were used, including peptides derived from esophageal squamous cell carcinoma, severe acute respiratory syndrome coronavirus 2 (SARS-COV-2) and B*08 allotypes.

All three targets were specifically bound by tracers, the peptide-focused binding interface binders, indicating a generalized binder-generating platform that specifically targets multiple HLA alleles.

The specific binding ability of divergent PMHC-I targets indicates that this platform can bind molecules across a wide range of targets and populations for multiple applications. Despite their bacterial origin, tracers do not provoke robust immune or cytotoxic responses in mice.

This simplified approach enables the rapid and easy creation of peptide-targeted PMHC binders for a variety of antigens. “”

Investigating the effectiveness of Tracer I

The Tracer I platform was incorporated into a humanized antibody fragment in a bispecific T cell engager format (Bite). In the tested cancer cells from patients, effective T cell activation was observed in killing the target at the nanomolar concentration.

Molecular mechanism

X-ray crystallography showed that tracer engages the PMHC I target along its entire length through shape complementarity. The interface has a set of eight residues that remain constant across different HLAs, eliminating the need for variable PMHC -I recognition modes through the invariant binding mode.

Further variation between the eight-win set enabled binding to a wide range of epitopes across multiple diseases presented by different HLA allotypes. This point-substitution resolution shows that tracers can be developed for different antigens without losing specificity.

A monomeric form of the tracer was then created to be compatible with chimera antigen receptor T cells (CAR-T). This form was found to effectively bind cars in high affinity and induce cancer cell killing at the target.

Conclusions

Our platforms have high peptide-oriented specificity, broad compatibility with a variety of antigens, and ease of development, significantly expanding the accessibility of targeted MHC biomarkers.”

Further development of Tracer-I could help develop targeted antigens. However, further research is neededIn vivo.

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