Persistent CAR-T donor cells drive mixed chimerism after transplant, rescued by CAR-target-knockout DLI in three cases

Patients with relapsed or refractory T-cell malignancies who receive both CAR-T cell therapy and an allogeneic hematopoietic stem cell transplant can face an unusual complication: when the CAR-T cells and the transplant come from the same donor, standard chimerism testing cannot tell the two cell populations apart. A new correspondence published in Biomarker Research on 16 October 2026 describes three such cases in which the CAR-T cells and the stem cells came from different donors, allowing a three-party short tandem repeat (STR) analysis to separate the CAR-T donor, the transplant donor, and the recipient compartments.

The report centers on whether residual CAR transgene detectable at the time of transplantation signals a risk of persistent CAR-T donor cells. Patient 2, who had 240 copies of the CAR transgene per microgram of genomic DNA at transplantation, developed mixed chimerism with cytopenia (MCC), with the CAR-T donor contributing 47.72% of the measured compartment. Patient 3, whose residual CAR transgene was far higher at 22,162 copies per microgram, also developed mixed chimerism, with a CAR-T donor contribution of 14.23%. In contrast, Patient 1, whose CAR transgene was undetectable at transplantation, showed stable complete donor chimerism and normal hematopoietic recovery.

Guided by the two-step reconstitution pattern observed in Patient 1, the team administered donor lymphocyte infusion using cells engineered to knock out the CAR target (KO-DLI). In Patient 2 this was used as rescue treatment for established mixed chimerism with cytopenia, while in Patient 3 it was given preemptively. Both patients eventually became CAR-undetectable, with restoration of complete donor chimerism and hematologic recovery. The authors note an important limitation: because the intervention was applied in only two patients and in different clinical settings, the independent effect of KO-DLI could not be isolated from other factors in their courses.

All three patients remain in complete remission, with follow-up of 713, 335, and 201 days respectively. The authors suggest that residual CAR transgene measured at the time of transplantation may serve as an early risk signal for CAR-T donor cell persistence, and they propose that KO-DLI warrants prospective validation as a transplantation-sparing salvage approach. Because this is a three-case correspondence rather than a controlled study, the findings are hypothesis-generating: the association between detectable CAR transgene and mixed chimerism is consistent across the three cases, but the numbers are too small to establish predictive value, and the reported benefit of KO-DLI cannot be causally attributed. Prospective studies with larger cohorts would be needed to confirm both the risk signal and the therapeutic role of CAR-target-knockout donor lymphocyte infusion.

Subject of Research: Persistent CAR-T donor cells causing mixed chimerism after CAR-T-bridged allogeneic stem cell transplantation, rescued by CAR-target-knockout donor lymphocyte infusion

Article Title: Persistent CAR-T donor cells contribute to mixed chimerism with cytopenia after CAR-T-bridged allo-HSCT: rescue by CAR-target-knockout DLI in three cases

Article References: Liu, Y., Wang, Z., Wu, Y., Xu, X., Hu, X., & Pan, J. (2026). Persistent CAR-T donor cells contribute to mixed chimerism with cytopenia after CAR-T-bridged allo-HSCT: rescue by CAR-target-knockout DLI in three cases. Biomarker Research, 14(1), Article 145. https://doi.org/10.1186/s40364-026-01021-y

Image Credits: AI Generated

DOI: 10.1186/s40364-026-01021-y

Keywords: CAR-T cell therapy, allo-HSCT, mixed chimerism, cytopenia, donor lymphocyte infusion, KO-DLI, CAR transgene, STR chimerism analysis, T-cell malignancy, complete donor chimerism, hematopoietic recovery, Biomarker Research