Dr. Huaying Liu on New Treatment Options for Relapsed/Refractory Pediatric B-ALL

Summary: Dr. Huaying Liu of the GoBroad Chunfu Institute of Hematology & Oncology provides a practical overview of treatment for relapsed/refractory pediatric B-cell acute lymphoblastic leukemia (B-ALL). She discusses disease classification and risk stratification, immunotherapy strategies such as CAR-T cell therapy and bispecific antibodies, indications for hematopoietic stem cell transplantation, and the principles and potential clinical advantages of TCRαβ+ cell-depleted hematopoietic stem cell transplantation (TDH). The article is designed to help families better understand current treatment options and the role of newer approaches in pediatric ALL.

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a key treatment for serious hematologic diseases. TCRαβ+ cell-depleted hematopoietic stem cell transplantation (TDH) selectively removes αβ T cells from the graft outside the body because these cells can contribute to graft-versus-host disease (GVHD), while preserving CD34+ stem cells and large numbers of immune cells such as NK cells and γδ T cells. The aim is to support earlier hematopoietic recovery, reduce the risk of GVHD, and improve long-term quality of life. This approach has shown encouraging outcomes in both international and Chinese practice, but it remains unfamiliar to many families. Here, Dr. Huaying Liu of the GoBroad Chunfu Institute of Hematology & Oncology explains how CAR-T therapy and TDH transplantation may be used together in children with ALL.

Leukemia is one of the most common cancers in children, accounting for around 30% of pediatric cancer diagnoses. Nearly three-quarters of childhood leukemia cases are acute lymphoblastic leukemia (ALL). B-ALL accounts for about 85% of ALL, with the remainder made up of T-ALL and other rare subtypes.

Diagnosis of ALL is first established through an integrated MICM assessment - morphology, immunology, cytogenetics, and molecular genetics. Patients are then assigned to risk groups according to relevant clinical and biological factors. The exact criteria vary slightly among cooperative groups, but the overall principles are similar.

Risk assessment generally considers the child's age at diagnosis, peripheral white blood cell count, presence of extramedullary leukemia, immunophenotype, cytogenetic findings, and response to treatment.

International systems used by groups such as the Children's Oncology Group (COG) and St. Jude generally classify children into low-, standard-, high-, and very-high-risk groups. In China, risk stratification more commonly uses three groups: low, intermediate, and high risk.

Factors associated with a more favorable prognosis include B-ALL, age 1-9.9 years, an initial white blood cell count below 50 × 10^9/L, hyperdiploidy, and ETV6-RUNX1 (TEL-AML1), among others.

Factors associated with a less favorable prognosis include age ≥10 years, an initial white blood cell count ≥50 × 10^9/L, T-ALL, certain genetic abnormalities such as BCR::ABL1, KMT2A rearrangements, IKZF1 deletion, low hypodiploidy, and TCF3-HLF fusion, as well as central nervous system leukemia (CNS-3) or testicular leukemia at diagnosis.

An increasing number of genetic markers are being incorporated into risk assessment. Their clinical significance should be interpreted in context, taking into account coexisting genetic changes and the patient's response to treatment.

With standardized diagnosis and treatment, the 5-year overall survival (OS) rate for childhood ALL in China is now close to 90%. However, some children still develop refractory or relapsed disease and face a less favorable prognosis.

Defining Relapsed/Refractory ALL and Choosing a Treatment Strategy

Relapsed ALL: leukemia that returns after complete remission (CR), with >5% blasts again detected in the bone marrow or peripheral blood, or leukemia reappearing at any extramedullary site.

Refractory ALL: failure to achieve complete remission (CR) after standard induction therapy, two or more relapses, persistent extramedullary leukemia, or persistently positive measurable residual disease (MRD).

For relapsed/refractory B-ALL - particularly early relapse, relapse after transplantation, or multiple relapses - conventional chemotherapy often has limited efficacy, and immunologically targeted therapies are now commonly prioritized.

According to the 2025 CSCO Guidelines for Malignant Hematologic Diseases, commonly used immunotherapies include:

1. CAR-T cell therapy: The main targets are CD19 and CD22. Treatment may use a single target, dual targets, or sequential or combined infusion of different CAR-T products.

2. Bispecific antibody: Blinatumomab, which targets CD3 and CD19.

3. Antibody-drug conjugate (ADC): Inotuzumab ozogamicin, which targets CD22.

Before CAR-T therapy, patients require a comprehensive assessment of disease status, genetic background, and any extramedullary involvement. During treatment, complications such as cytokine release syndrome (CRS), immune effector cell-associated neurotoxicity syndrome (ICANS), and infection require close, full-course management. After treatment, continued monitoring is needed for CAR-T cell expansion, B-cell recovery, MRD, and changes in tumor-associated genetic markers.

A growing body of research and expert consensus in China and internationally suggests that bridging to hematopoietic stem cell transplantation after CAR-T therapy can reduce relapse risk and improve long-term leukemia-free survival in some children with relapsed/refractory ALL.

When Is Hematopoietic Stem Cell Transplantation Considered, and Which Transplant Platform May Be Used?

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective treatment for children with relapsed/refractory ALL. Both international and Chinese guidelines and consensus recommendations are available. Current Chinese transplant recommendations (referencing the 2022 edition) outline specific indications for low- and intermediate-risk patients in Tables 1 and 2, while allo-HSCT is recommended for all children classified as high risk.

Table 1

Table 2

I. Philadelphia Chromosome-Negative ALL (Ph- ALL)

1. First Complete Remission (CR1)

① Failure to achieve CR after induction therapy; or achievement of CR after induction with measurable residual disease (MRD, assessed by flow cytometry [FCM]) remaining ≥10^-2. For patients with post-induction MRD between 10^-2 and 10^-3, close monitoring of bone marrow MRD is recommended.

② Persistent MRD by FCM ≥10^-4 after ≥3 months of standardized chemotherapy. For children who are FCM-negative after ≥3 months of standardized chemotherapy but remain positive for a fusion gene, close quantitative monitoring of the fusion gene is recommended; allo-HSCT is recommended if the fusion-gene level shows a progressive increase.

③ MRD becomes positive on two occasions at any point during treatment, based on FCM or RQ-PCR for a specific fusion gene (FCM MRD ≥0.01%). Based on MRD monitoring, allo-HSCT at an experienced transplant center is recommended.

④ For patients with MLL rearrangements, allo-HSCT at an experienced center is recommended for those with MLL-AF6 positivity.

⑤ Presence of a TCF3-HLF fusion gene.

⑥ Hypodiploidy (<44 chromosomes).

2. Second Complete Remission (CR2)

① For children with very early relapse (within 18 months of starting treatment) or early relapse (more than 18 months from diagnosis but within 6 months after completion of first-line therapy, or within 36 months of starting treatment), allo-HSCT is recommended in CR2. For late bone marrow relapse (>36 months), allo-HSCT is recommended after CR2 if MRD remains positive (>10^-4).

② Extramedullary relapse occurring within 18 months of treatment; extramedullary relapse occurring ≥18 months after treatment when remission has not been achieved after 4-8 weeks of therapy; or bone marrow relapse accompanied by extramedullary relapse.

3. All Patients in CR3 or Beyond

II. Philadelphia Chromosome-Positive ALL (Ph+ ALL)

① Children with Philadelphia chromosome-positive ALL or BCR/ABL fusion-positive ALL who do not achieve remission after standard induction chemotherapy.

② After 3 months of regular chemotherapy combined with an oral tyrosine kinase inhibitor (TKI), such as imatinib or dasatinib, the quantitative BCR/ABL fusion level has fallen by less than 3 log levels from baseline.

③ Presence of the ABL T315I mutation.

1. First Complete Remission (CR1)

① Failure to achieve bone marrow CR after a standard induction regimen; or achievement of CR with MRD by FCM remaining ≥10^-2.

② At the end of intensification therapy (approximately 2.5-3 months after treatment begins), MRD remains >10^-3, or extramedullary leukemia persists, including disease in the mediastinum, lymph nodes, central nervous system, or other sites.

③ MRD by FCM becomes positive on two or more occasions at any point during treatment.

④ Isolated extramedullary relapse within 18 months of starting treatment; for extramedullary relapse occurring ≥18 months after treatment begins, treatment response and bone marrow MRD should be closely monitored. Allo-HSCT is required for patients with a poor treatment response or concomitant bone marrow MRD relapse.

2. Second Complete Remission (CR2)

Any bone marrow and/or extramedullary relapse. Donor selection should take into account the patient's disease status, donor availability and characteristics, and the transplant center's experience.

In recent years, as transplant techniques have continued to improve, related haploidentical transplantation has increasingly become a preferred option in leukemia treatment. It expands donor availability - virtually every patient can identify a potential haploidentical family donor - while modern transplant strategies focus not only on successful engraftment, but also on preserving the graft-versus-leukemia (GVL) effect to help prevent relapse.

Three major haploidentical transplant platforms are widely used internationally. A key difference among them is when and how T cells are depleted to prevent GVHD:

The first is αβ T-cell-depleted transplantation, an ex vivo T-cell depletion approach. TDH is one example. Before stem cell infusion, magnetic bead selection is used outside the body to selectively remove αβ T cells from the graft because these cells can contribute to GVHD, while preserving stem cells, NK cells, and other immune cells.

The second is an antithymocyte globulin (ATG)-based transplant platform, often referred to as the “Beijing protocol,” which uses in vivo T-cell depletion. ATG is given intravenously during conditioning before transplantation to deplete T cells in the patient's body and reduce the risk of GVHD.

The third is post-transplant cyclophosphamide (PTCy), another in vivo T-cell depletion strategy. Cyclophosphamide is given on days +3 and +4 after stem cell infusion to selectively eliminate activated T cells that might attack the recipient's tissues. The ATG- and PTCy-based approaches generally require immunosuppressive medication after transplantation to prevent GVHD. With TDH, immunosuppression may be avoided or used only briefly and at low doses, allowing other immune cells, including NK cells, to better contribute to antileukemia activity after transplantation.

Our center uses the TDH transplant platform. Because many parents are unfamiliar with this approach, the basic principle is explained below.

How TDH Works

A donor stem cell collection contains not only CD34+ hematopoietic stem cells, but also substantial numbers of immune cells, including T cells, B cells, and NK cells. T cells include both αβ T cells and γδ T cells. Magnetic bead selection can selectively remove αβ T cells while preserving abundant CD34+ stem cells, γδ T cells, NK cells, monocytes, and other components of the graft. After infusion, these cells can support rapid hematopoietic recovery and contribute to anti-infective and antitumor immunity.

With conventional haploidentical transplantation, GVHD can remain an important complication. Severe GVHD may lead to life-threatening problems and affect overall outcomes, while persistent chronic GVHD can substantially reduce a patient's quality of life.

Our center is one of the institutions in China with a large number of TDH transplants and extensive accumulated clinical data. TDH has been used at our center for a range of conditions, including nonmalignant hematologic diseases such as severe thalassemia and aplastic anemia; malignant hematologic diseases such as ALL, AML, and JMML; and immunodeficiency disorders. In particular, encouraging outcomes have been observed in children with relapsed/refractory ALL undergoing TDH transplantation.

Since 2022, our center has presented related research findings at major meetings in China and internationally, including the American Society of Hematology (ASH) Annual Meeting. Two studies of TDH transplantation in leukemia were selected for presentation as abstracts at the 2025 ASH Annual Meeting.

As of July 2025, 55 children with relapsed/refractory ALL had received CAR-T therapy followed by TDH transplantation at our center, including 51 with B-ALL and 4 with T-ALL. The median age at transplantation was 8 years, and the median follow-up was 29 months. The 2-year overall survival (OS) rate was 90.6%, leukemia-free survival (LFS) was 86.8%, the relapse rate was 4.2%, and non-relapse mortality was 9.4%.

Analysis of all children with relapsed/refractory ALL who underwent TDH transplantation showed different outcomes depending on the remission status at the time of transplant (CR1, CR2, or ≥CR3). The 2-year OS rates were 95.2% for CR1, 93.9% for CR2, and 67.5% for ≥CR3. The corresponding 2-year LFS rates were 95.2%, 83.2%, and 58.3%. The cumulative incidences of grade II-IV acute GVHD and chronic GVHD were 9.1% and 12.7%, respectively.

For children with high-risk or relapsed/refractory ALL who have an indication for transplantation, early allo-HSCT should therefore be considered. TDH transplantation may support durable remission with a relatively low rate of GVHD. Early tumor-genetic testing is also recommended to identify adverse prognostic alterations and determine whether targeted therapies may be available. Even after transplantation, MRD should be monitored regularly, and additional consolidation approaches - such as oral targeted therapy or scheduled donor lymphocyte infusions (DLI) - may be considered to further reduce the risk of relapse.

Case Example

Xiao Yu, a girl born in 2011, was diagnosed with B-ALL at age 3 in 2014. She completed standard chemotherapy but did not undergo regular follow-up. In 2020, she experienced her first relapse in the bone marrow and received an autologous murine-derived CD19 CAR-T product at another hospital. During treatment, she developed sepsis, grade 3 CRS, and grade 2 ICANS. She achieved remission and was observed afterward, but regular follow-up was not maintained.

In 2021, 14 months after CAR-T therapy, she experienced a second bone marrow relapse. After coming to our center, bone marrow flow cytometry showed MRD of 79.5%, with CD19 and CD22 positivity. Lymphoid malignancy genetic testing detected KRAS at 35.94%, with predicted sensitivity to trametinib and cobimetinib. She first received cytoreductive chemotherapy, followed by sequential infusion of humanized CD19 CAR-T and CD22 CAR-T cells. Grade 1 CRS occurred during treatment. Flow cytometry MRD and KRAS testing both became negative. She subsequently proceeded to TDH transplantation using her mother as the donor.

She is now more than three years post-transplant and returned to regular schooling two years ago.

Although childhood ALL cannot currently be prevented, it has become a highly treatable disease. When facing the diagnosis, families should focus on three pillars: standardized treatment, close partnership with the medical team, and strong social support. Continuing to pursue appropriate treatment can give each child the best possible chance of cure.