# The PD1 Molecule May Contribute to Lower Treatment-Free Remission Rates in Patients with Chronic Myeloid Leukemia with the e13a2 Transcript

## TL;DR

This study shows that CML patients with the e13a2 transcript have higher PD1 expression in immune cells, which may lead to lower treatment-free remission rates.

## Contribution

The study links PD1 expression in immune cells to BCR::ABL1 transcript type, suggesting a novel immunological mechanism affecting remission in CML.

## Key findings

- PCA grouped CML patients into two clusters associated with BCR::ABL1 transcript types (e13a2 and e14a2).
- Patients in Cluster 1 showed significantly higher PD1 expression in multiple immune cell populations.
- The e13a2 transcript was less frequent in Cluster 2, while e14a2 was more common there.

## Abstract

Background/Objectives: Chronic myeloid leukemia (CML) is characterized by the presence of the BCR::ABL1 fusion gene, most commonly in the e14a2 or e13a2 variants. Studies show that the transcript type in CML may be important for achieving treatment-free remission (TFR). This study aimed to immunologically characterize CML patients with e13a2 and e14a2 transcripts to search for differences that may contribute to achieving remission in patients after therapy withdrawal. Methods: Using multicolor flow cytometry, we analyzed the differences in the immune system at the time of imatinib discontinuation and the early stage of TFR in fifty-one CML patients with different transcripts. RQ-PCR and ddPCR were used to monitor the dynamics of BCR::ABL1 transcript changes. The patients were grouped using principal component analysis (PCA) based on the percentage of detected immune cells that were classified as populations consistently selected by the MCFS-ID algorithm from randomly selected data. Results: PCA separated CML patients into two groups defined by k-means clustering, indicating significant heterogeneity within the studied population. We found a significant association between Cluster metrics (Cluster 1 and 2) and BCR::ABL1 transcript types (e13a2 or e14a2) (p = 0.003, 95% CI: 0.026–0.595, OR = 0.14, Fisher test). The e13a2 transcript was less frequent in Cluster 2 than in Cluster 1, while e14a2 was more common in Cluster 2. Additionally, patients grouped into Cluster 1 had significantly higher percentages of the PD1 expressing populations cDC PD1+, CD56dimCD16+PD1+, CD8+PD1+, CD4+PD1+, and CD19+PD1+, as identified by the MCFS-ID algorithm, compared to patients in Cluster 2. Conclusions: Our results suggest that immunological differences may be related to the BCR::ABL1 transcript type, which could affect the number of active CML cells represented by the BCR::ABL1 transcript amount and thus may determine molecular recurrence.