# Full-Length Transcriptome Sequencing Reveals Treg-Specific Isoform Expression upon Activation

## TL;DR

This study finds that regulatory T cells (Tregs) have a unique pattern of gene isoform expression, especially when activated, which could help explain their special role in the immune system.

## Contribution

The study identifies a Treg-specific isoform repertoire using long-read sequencing and shows how activation enhances this unique expression.

## Key findings

- Tregs uniquely express full-length FOXP3 and specific isoforms of ICOS and PD-L1 upon activation.
- PD-L1 blockade reduces Treg suppressive function without affecting FOXP3 expression.
- Treg isoform expression is distinct from conventional T cells and is amplified by TCR stimulation.

## Abstract

FOXP3+ regulatory T cells (Tregs) play a central role in the regulation of the immune system. Human Tregs preferentially express a FOXP3 isoform known as delta 2, which lacks exon 2. In addition to FOXP3, Tregs also express isoforms of other Treg-related molecules, such as CTLA-4 and IKZF-2. It is hypothesized that Tregs possess a unique isoform repertoire based on their unique gene and isoform expression profiles, which include FOXP3. Here, we identified a Treg-specific unique isoform repertoire confirmed by long-read high-throughput isoform sequencing called Iso-seq, which is uniquely capable of providing data on genome-wide isoform usage. Notably, while conventional T cells (Tconvs) do not exhibit this pattern, Tregs preferentially express the full-length FOXP3 isoform. Interestingly, the preferential expression of ICOS and PD-L1 upon T-cell receptor (TCR) stimulation was noted in activated Tregs but not in Tconvs or non-activated Tregs. Moreover, using a PD-L1 antibody blockade on Tregs did not diminish FOXP3 expression; however, it significantly reduced the suppressive function. Therefore, Tregs may have a unique isoform repertoire, which becomes pronounced upon polyclonal TCR stimulation.