Objective:To study the performance of immune reconstitution in patients with chimeric antigen receptor (CAR)-T cell immunotherapy bridging allogeneic hematopoietic stem cell transplantation (allo-HSCT).Methods:A total of 61 patients with acute B lymphocytic leukemia (B-ALL) who received CAR-T cell bridging allo-HSCT in Beijing Lu Daopei Hospital from August 2018 to December 2021 were enrolled, and the clinical medical records of the above patients were retrospectively analyzed. The average age was 14 (7, 30) years old, including 39 males and 22 females. 32 patients were treated with CAR-T cell immunotherapy(CAR-T Group) and 29 didn't with CAR-T cell immunotherapy(non-CAR-T group). The follow-up period was 561 (235,784) days. Multicolor flow cytometry was used to detect the peripheral blood lymphocyte subsets, i.e. total lymphocytes, T lymphocytes, helper T cells, cytotoxic T cells, B lymphocytes, NK cells, and Treg cell counts before transplantation and 1, 2, 3, 6, 8, 10, and 12 months after transplantation, to evaluate the immune reconstitution performance post allo-HSCT.Results:Serum globulin before transplantation: The IgA level in the CAR-T group was 0.18 (0.06, 0.49) g/L, which was lower than that of 1.03 (0.63, 1.56) g/L in the non-CAR-T group ( U=103.5, P<0.001). The IgG level in the CAR-T group was 5.54 (4.04, 7.09) g/L, lower than that of 6.78 (5.27, 9.26) g/L in the non-CAR-T group, ( U=1 298.5, P=0.017), and the IgM level in the CAR-T group was 0.18 (0.05, 0.30) g/L, lower than that of 0.40 (0.26, 0.71) g/L in the non-CAR-T group ( U=166.0, P<0.001). In the CAR-T group before transplantation, the absolute count of total lymphocyte in peripheral blood was 833.00 (335.00, 1 727.50) /μl, lower than that of 1 052.00 (545.75, 1 812.50) /μl in the non-CAR-T group ( U=404.0, P<0.001). The absolute count of T lymphocyte in the CAR-T group before transplantation was 686.00 (233.00, 1 307.00)/μl, lower than that of 860.00 (391.00, 1 419.75) /μl in the non-CAR-T group ( U=406.0, P<0.001). The absolute count of helper T lymphocytes in the CAR-T group was 146.00 (40.50, 327.50) /μl, lower than that of 162.50 (66.00, 384.75) /μl in the non-CAR-T group ( U=494.0, P=0.002). The absolute count of cytotoxic T lymphocytes in the CAR-T group was 343.00 (56.50, 924.00) /μl, lower than that of 478.00 (143.50, 992.25) /μl in the non-CAR-T group ( U=483.5, P=0.001). The absolute count of B lymphocytes in CAR-T group was 22.00 (6.00, 186.00) /μl, lower than that of 33.00 (8.00, 220.00) /μl in the non-CAR-T group ( U=498.0, P=0.002). And when two groups of patients were monitored after transplantation, there was no statistical difference in absolute cell counts of each immune cell subpopulation( P>0.05). Comparing the clinical features of the two groups, the pre-transplant history of the CAR-T group was 981.00 (368.50, 1 514.75) d, longer than that of 323.00 (167.50, 450.50) d in the non-CAR-T group ( U=263.0, P=0.004). The dose of rabbit anti-human thymic immunoglobulin (ATG) in the pretreatment protocol of patients in the CAR-T group was 5.00 (5.00, 7.50) mg/Kg, lower than that of 7.00 (5.00, 7.50) mg/kg in the non-CAR-T group ( U=288.5, P=0.018). The infusion dose of CD34 +cells in the CAR-T group was 5.91 (4.23, 6.02) ×10 6/kg, higher than that of 4.51 (4.00, 5.93)×10 6/kg in the non-CAR-T group ( U=291.0, P=0.012). The duration of the application of cyclosporine after transplantation in the CAR-T group was 167.00 (119.25, 299.50) d, which was shorter than that of 197.00 (102.50, 450.50) d in the non-CAR-T group ( U=421.0, P=0.001). Conclusions:For patients in CAR-T group with low immune function before transplantation, it may be possible to make them comparable to non-CAR-T group in immune reconstitution state by reducing the dose of pretreatment ATG, increasing the counts of CD34 + cells infusion in the graft, and discontinuing cyclosporine as soon as possible after transplantation.