# UniTope & TraCR: A Universal Tool to Tag, Enrich, and Track TCR-T Cells and Therapeutic Proteins

## TL;DR

The paper introduces a new system called UniTope & TraCR to track and measure T cell receptors used in cancer therapies, ensuring they work as intended.

## Contribution

The novel contribution is a universal tagging system (UniTope & TraCR) for quantifying and monitoring recombinant TCRs in T cell therapies.

## Key findings

- Tagged rTCRs were stably expressed in human T cells with surface densities comparable to untagged rTCRs.
- The TraCR antibody bound UniTope with nanomolar affinity and no detectable cross-reactivity with endogenous proteins.
- Functional assays confirmed that UniTope-tagged rTCRs retained antigen-specific cytokine secretion and cytolytic activity.

## Abstract

Background: Adoptive cell therapy using genetically engineered recombinant T cell receptors (rTCRs) expressed in T cells (TCR-T cell therapy) provides precision targeting of cancer cells expressing tumor-associated or tumor-specific antigens recognized by the rTCRs. Standardized analytical tools are lacking to easily quantify receptor expression. Methods: To overcome this hindrance, a universal tagging system (UniTope & TraCR) was designed consisting of a minimal peptide epitope (UniTope) inserted into the constant region of the rTCR α or β chain and a high-affinity monoclonal antibody (TraCR) specific to this tag. Detailed biophysical, biochemical, and functional assays were performed to evaluate rTCR expression, folding, pairing, and antigen recognition, as well as antibody performance, using the UniTope & TraCR System. Results: Tagged rTCRs were stably expressed in human T cells with surface densities comparable to untagged rTCRs. The TraCR antibody bound UniTope with nanomolar affinity and no detectable cross-reactivity was observed for endogenous proteins expressed by human cells of diverse origin, importantly, including T cells of the natural T cell repertoires of multiple human donors. Functional assays confirmed that UniTope-tagged rTCRs preserved their antigen-specific cytokine secretion and cytolytic activity upon antigen-specific stimulation. The UniTope & TraCR System enabled robust detection of rTCR-expressing T cells by flow cytometry, and rTCR protein expression by Western blot or immunoprecipitation, supporting the quantitative assessment of receptor copy number and structural integrity. Conclusions: The UniTope & TraCR System provides a modular, construct-agnostic platform for monitoring engineered rTCRs, integrated into TCR-T cell therapies currently in development.