Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an important therapeutic approach for hematologic malignancies such as leukemia and has been widely adopted both domestically and internationally. Among the various modalities, T-cell-depleted transplantation (TCD-HSCT) is a specialized form of allo-HSCT in which donor T lymphocytes are removed from the graft to reduce the incidence of graft-versus-host disease (GVHD), thereby improving transplant success rates and patient survival. Despite significant progress in TCD-HSCT, many patients still have questions regarding its indications, pre-transplant preparation, procedural risks, post-transplant care, and prognosis.

In this article, Professor Chunfu Li, Director of the Nanfang Chunfu Institute of Hematology, provides a detailed overview of TCRαβ+ T cell-depleted hematopoietic stem cell transplantation (TDH), explaining its mechanisms, evolution, advantages, and applications in hematologic and immune disorders.

I. Mechanism and Development of TDH Transplantation

Currently, clinical transplantation protocols are broadly divided into two categories:

1. T-cell-replete transplantation (TCR-HCT)
2. Ex vivo T-cell-depleted transplantation (TCD-HCT)

• T-cell-replete HCT involves grafts that retain a high number of T cells and includes matched sibling donor (MSD) transplants, matched unrelated donor (MUD) transplants, and haploidentical transplants (Haplo-HCT). Haplo-HCT includes the ATG-based (Beijing protocol) and PTCy-based (U.S. protocol) approaches. Both methods aim to suppress T cells in vivo through anti-thymocyte globulin (ATG) or post-transplant cyclophosphamide (PTCy) to reduce GVHD.
• Ex vivo TCD-HCT removes T cells from the graft prior to infusion and has undergone four developmental stages:

Stage 1: In the 1990s, physicians attempted CD34+ stem cell selection, infusing only CD34+ cells. Outcomes were suboptimal.

Stage 2: Negative selection techniques emerged, depleting CD3+ (T) cells. Although outcomes improved, efficacy remained limited.

Stage 3: CD45RA depletion targeted naïve T cells, but results were still unsatisfactory.

Stage 4: The current TDH approach enables selective depletion of αβ T cells while retaining γδ T cells and natural killer (NK) cells, which are critical for antitumor and anti-infective immunity.

In fact, TDH technology represents a step forward toward precision transplantation.

II. Rationale for αβ T Cell Depletion

While T cells (CD3+ cells) are generally considered the primary mediators of GVHD, they are composed of αβ and γδ subsets. αβ T cells are the main drivers of GVHD, whereas γδ T cells and NK cells contribute to tumor surveillance and infection control. Thus, selectively removing αβ T cells mitigates GVHD risk while preserving beneficial immune functions.

Immunomagnetic bead sorting is commonly used to deplete αβ T cells. Magnetic beads specifically bind to αβ T cells, forming bead-cell complexes, which are then removed by magnetic columns, leaving behind a graft enriched in CD34+ stem cells, γδ T cells, and NK cells. The αβ T cell content is reduced to less than 1×10⁵/kg.

These enriched grafts help reconstruct hematopoiesis (via CD34+ cells) and provide immune protection (via NK and γδ T cells).

Since around 2008, TDH technology has evolved. Initially, CD3+ T cell depletion yielded limited efficacy (6–8×10⁵), requiring immunosuppressive therapy. By 2014, depletion efficiency improved to 1–5×10⁵ for TCRαβ+ cells, but immunosuppressants were still necessary. Post-2015, the technique matured, reducing residual αβ T cells to <5×10⁴, eliminating the need for immunosuppressive therapy and minimizing GVHD risk while maintaining robust hematopoietic recovery and antitumor effect.

III. Advantages of TDH Technology

Let us examine how TDH transplantation reduces risks and improves outcomes in allo-HSCT.

Major risks of allo-HSCT include:

• Graft failure (implantation failure, incidence <5%)
• GVHD (usually >20%)
• Relapse (approximately 20%, varies by disease and transplant conditions)
• Infection (nearly universal, with ~10% mortality)

IV. How does TDH technology reduce these post-transplantation risks and thereby increase the success rate of transplantation?

1. Enhancing Graft Success Rate

In diseases like thalassemia, which have higher graft failure rates, TDH transplantation has reduced failure rates to below 2% in our center. In other diseases, the failure rate is even lower.

2. Reducing GVHD and Infection

TDH enables infusion of a large number of CD34+ stem cells, expediting hematopoietic recovery. Faster recovery of neutrophils and platelets shortens inpatient time, reduces hemorrhagic complications, and lowers infection risks. International studies show TDH is associated with the lowest incidence of both acute and chronic GVHD.

3. Lowering Relapse Rates

Relapse remains the most challenging issue post-HSCT. Once relapse occurs, survival drops below 20%. Thus, prevention is key. Why the rate of relapse is high? In haploidentical transplants using ATG or PTCy, T cell and NK cell recovery is hindered, impairing graft-versus-leukemia effects. Furthermore, T-cell-replete transplants require immunosuppressive therapy (IST), which suppresses T/NK cell function and contributes to relapse.

Contrary to early beliefs that TCD increases relapse risk (due to immature CD34+ selection methods), modern TCRαβ+ depletion allows infusion of large numbers of NK and γδ T cells with strong antitumor activity.

Studies have shown that a graft NK cell count >6.33×10⁶/kg reduces relapse risk by 90%. In our 83 TDH transplant recipients with leukemia, the median NK cell count reached 110.5×10⁶/kg, well above this threshold, with high γδ T cell counts as well.

In cord blood transplantation, NK cells are the first to recover. Their robust reconstitution reduces relapse. Similarly, early post-transplant recovery of γδ T cells within 30 days correlates with improved survival and lower relapse. Critically, TDH eliminates the need for IST, and does not require preconditioning with ATG/PTCy, allowing for rapid T cell recovery and enhanced antileukemic effect.

The trend in HSCT is moving toward minimizing or eliminating ATG. Previously, limited understanding of immunotherapy led to reliance on ATG. Now, as immunotherapy advances, ATG is increasingly seen as unnecessary. TDH serves as a platform for integrating immunotherapy, helping further reduce relapse and improve long-term survival.