The patient was a 3-year-old girl. In January 2020, because of pale complexion and petechiae over the entire body, she was admitted to a local hospital. She had a full-term natural birth and normal growth and development. Her father was in good health, and her mother had hepatitis B virus infection. They were not in a consanguineous marriage. The parents denied a family history of similar or other cancers or hereditary diseases. Her white blood cell count was 209x10^9/L. She was diagnosed with acute lymphocytic leukemia by MICM of the bone marrow. Since January 15, 2020, according to the CCLG-ALL 2015 protocol, she was evaluated as having intermediate risk and underwent induction chemotherapy. The reexamination showed remission in the bone marrow morphology and MRD<0.01%. After that, she was given regimens of CAM and HD-MTX+6-MP. On August 7, 2020, morphological evaluation of her bone marrow showed 4.5% lymphoblasts and prolymphocytes; flow cytometry indicated 9.85% abnormal naive T lymphocytes. Thus, she was suspected to have a recurrence of acute lymphocytic leukemia. Peripheral blood smears indicated 4% naive T lymphocytes. BLOCK3 chemotherapy was given. On September 18, 2020, morphological evaluation of bone marrow showed 24.5% lymphoblasts and prolymphocytes, and flow cytometry showed 19.98% abnormal naive T lymphocytes. She was given VDS+DEX and VDIP regimens successively, but with no remission. During this period, no abnormal lymphocytes were found in multiple lumbar punctures + sheath injections of cerebrospinal fluid. On November 10, 2020, the patient was admitted to our hospital for the first time. MICM confirmed the previous diagnosis of acute T-lymphocytic leukemia. Gene mutations identified Notch1 p.S2467Pfs * 12, STAT5A p.T6285, and IL7R p.L243_T244insCP, but HBV DNA>1*10^8 IU/ml suggested associated hepatitis B. She was given entecavir 0.25 mg once per day and an intravenous injection of freeze-dried hepatitis B human immunoglobulin 2500 IU once per day for antiviral treatment. The changes in HBV DNA during treatment are illustrated in  Figure 1 . On November 19, 2020, the FLU + idarubicin chemotherapy regimen was given to eliminate lymphocytes; on November 25, 2020, she was transfused with humanized CD7 CAR-T cells from a donor (her father). The donor's physical examination results were normal, and his HBV DNA was measured at 0. On Days 15 and 30 after CD7 CAR-T therapy (2020-12-15 and 2020-12-24), the bone puncture reexamination showed morphological remission, with residue negativity. The treatment process is outlined in  Table 1 .
Four days after CAR-T transfusion, a CRS reaction (grade 2 at maximum) occurred, which was relieved after hormone therapy. She had a rash 3 weeks after CAR-T therapy, which was considered aGVHD (skin grade II) and was relieved after intense anti-GVHD treatment. Two weeks after CD7 CAR-T transfusion, the white blood cell count was lower than 0.5x10^9/L. After CAR-T transfusion, she still received anti-hepatitis B virus therapy. The HBV DNA copy number increased after CAR-T transfusion and peaked (3.9*106 IU/ml) on the 14th day. After consultation with a liver disease specialist, since there was the possibility of the activation of the hepatitis B virus, tenofovir disoproxil fumarate tablets were given at a dosage of 75 mg once per day, in addition to entecavir and hepatitis B globulin. After that, her hepatitis B HBV DNA copy number fell to 7.3x10^4 IU/ml before transplantation. The BU/FLU/ATG/Me-CCNU conditioning regimen started on December 26, 2020. The donor of CAR-T was also used for the transplant. She subsequently received HLA 5/10-matched, blood type-mismatched (donor O+, recipient B+) haploidentical stem cell transplantation from her father on January 4, 2021. Graft-versus-host disease (GVHD) prophylaxis was performed with cyclosporine, short-term methotrexate and mycophenolate mofetil. Seven days after transplantation, the child had diarrhea and persistent bloody stools, but no pathogenic bacteria were found in the culture; engraftment syndrome was excluded. Eighteen days after transplantation, the neutrophil count was 0.25x109/L, suggesting that the neutrophil were not engrafting. The patient had gastrointestinal bleeding during the neutropenic period soon after the myeloablative conditioning regimen. The bleeding may have been due to the side effects of the anti-hepatitis B drugs and thrombocytopenia. The bleeding symptoms gradually improved after reducing the doses of tenofovir disoproxil fumarate and entecavir and transfusing platelets. The time of neutrophil engraftment was on days 22 after transplantation. The time of platelets engraftment was on days 91 after transplantation. The plasma CMV DNA and EBV DNA of the patient were all negative after transplantation. Bone marrow puncture and lumbar puncture at 1, 2 and 3 months after transplantation showed that the bone marrow morphology was in remission, with residue negativity, and the chimerism rates of bone marrow blood and peripheral blood showed it was of the complete donor type. After transplantation, PCR detected CD7 CAR-Ts again, and stem cell implantation had no effect on hepatitis B virus replication ( Figure 2 ). Four and a half months after transplantation, abnormal T lymphocytes expressing CD7-CD5+ were detected by flow cytometry, but the patient was still in complete remission with full donor chimerism in both bone marrow and CD3+ cells in peripheral blood. Half a month after reducing immunosuppressive agents, the flow cytometry of bone marrow turned negative ( Figure 3 ). As of August 2021, eight months after transplantation, the patient showed complete remission and was HBV DNA negative.
In this case, the patient reached complete remission after CD7 CAR-T therapy, and then bridging HSCT was performed. HBV DNA increased temporarily after CD7 CAR-T therapy and then gradually decreased. Blood indicators (white blood cells, platelets), biochemical indicators (AST, ALT, DBIL, TBIL), and cytokines (interleukin 6, tumor necrosis factor alpha, interleukin 10, soluble CD25, interferon gamma) were all monitored ( Figure 4 ). White blood cells increased to a maximum of 14.93*109/L 11 days after CD7 CAR-T therapy, dropped to less than 0.5*109/L, and remained in continuous agranulocytosis. Engrafted leukocytes survived 18 days after transplantation ( Figure 4A ). The time of neutrophil engraftment was on days 22 after transplantation. Since the patient had hepatitis B virus replication, liver function was monitored (AST and ALT are illustrated in  Figure 4B , TBIL and DBIL in  Figure 4C ). There was a transient increase in transaminase and bilirubin after transplantation. At that time, there were no other GVHD clinical manifestations such as rash, diarrhea. It has been improved with strengthening liver protection medicines without addition of anti-GVHD treatment. Therefore, the diagnosis of abnormal liver function was considered to be drug-induced liver damage rather than GVHD. The concentrations of cytokines (interleukin 6, tumor necrosis factor alpha, interleukin 10, soluble CD25, interferon gamma) peaked four days after CD7 CAR-T treatment, consistent with the CRS response, then increased to another peak after HSCT, and fell after that ( Figure 4D ).