In response to the shortage of HLA-matched donors between siblings, the donor population has been expanded to include HLA-matched unrelated people in the last decade or so. Since 1988, when Gluckman E reported the first successful case of umbilical cord blood transplantation (UCBT), which proved the potential of clinical application of umbilical cord blood transplantation, many foreign countries have established public umbilical cord blood banks to address the shortage of donor sources. Since Gluckman E reported the first successful cord blood transplantation (UCBT) in 1988, many foreign countries have established public cord blood banks to address the shortage of donor sources, while clinical research on cord blood HSCT has been widely carried out and has made great progress. In China, cord blood banks have been established in Beijing, Guangzhou, Jinan, Tianjin, Shanghai and other cities since the early 1990s, with a total of about 40,000 cord blood in stock and an estimated 300 UCBT cases nationwide, mainly unrelated UCBT. Most previous clinical studies have shown that cord blood matched at 4-6 loci can reconstruct the hematopoietic and immune systems of pediatric patients with hematologic tumors, achieving better therapeutic outcomes. However, the use of cord blood transplantation in adults has been limited by its high transplant-related mortality. Recent clinical data show that very good results have been achieved in large-weight children and adults as well as in non-malignant diseases, and the number of cord blood transplants is increasing year by year. Although cord blood transplantation has made great clinical progress, low UCBT implantation rates and delayed hematopoietic recovery and slow immune reconstitution with increased patient infections and early transplant-related deaths are prominent problems due to the low number of cord blood cells and their immune immaturity. Status in pediatric and adult allo-HSCT The status of allo-HSCT in children is well established, and the clinical efficacy of both sibling and unrelated UCBT is comparable to that of the corresponding sibling and unrelated BMT. The former conclusion comes from a comparative study of sibling UCBT and BMT by Rocha et al. who analyzed the clinical outcomes of 113 HLA-compatible children with UCBT and 2052 HLA-compatible children with BMT. The focus was on comparing the incidence of acute and chronic GVHD, implantation rates and survival in the two transplantation groups. It was found that the rate of failure to implant was higher in the UCBT group than in the BMT group, and the incidence of GVHD was lower in UCBT patients than in the BMT group for both acute and chronic GVHD, with 3-year survival rates of 64% and 66%, respectively, with no statistically significant differences. For pediatric bloodless UCBT, Rocha et al. retrospectively analyzed the results of a multicenter study of 541 pediatric acute leukemia patients applying bloodless UCBT and bloodless BMT. Univariate analysis showed that neutrophil and platelet recovery was delayed in UCBT patients compared to BMT patients; the incidence of greater than grade II acute GVHD and chronic GVHD was significantly lower in UCBT and T-cell removal BMT patients compared to BMT patients; and early TRM was higher in UCBT patients. relapse rate after 100 days, mortality were comparable in the 3 groups of cases. 2-year relapse rate, overall survival and disease-free survival (DFS) were not significantly different among the 3 groups. To further elucidate the advantages and disadvantages of UCBT, Barker et al. applied a prospective, randomized, paired approach to study neutrophil and platelet recovery, GVHD occurrence, and survival in pediatric HLA loci 0-3 incompatible unrelated UCBT and HLA fully matched unrelated BMT patients. The results showed that neutrophil recovery was delayed in UCBT patients, while platelet recovery time was similar in both groups, and the neutrophil implantation rate was not significantly different between the UCBT and BMT groups. Despite the large HLA difference between the UCBT group and the recipient, there was no increase in GVHD compared with BMT, no significant difference in early mortality, and no significant difference in 2-year survival rates, 53% and 41%, respectively. This study shows that despite the large HLA difference in the UCBT group, there was no significant difference in implantation rate, GVHD, or survival rate compared with matched BMT, suggesting that cord blood is a good substitute for unrelated bone marrow for HSCT in children. The main reason affecting the widespread use of UCBT in adults is the increased risk of graft failure and delayed hematopoietic recovery due to the low number of nucleated cells in cord blood, which is only 1/10th of the number of bone marrow grafts. therefore, early cord blood transplantation is only used as a last option for the treatment of high-risk hematologic tumors. Two recent multicenter bulk case reports from Europe and North America analyzed in detail the results of clinical studies of unrelated BMT and unrelated UCBT in adults with acute leukemia. The European data [6] compared the clinical outcomes of 682 adult acute leukemia unrelated hematopoietic stem cell transplants from 1998-2002, 98 in the UCBT group and 584 in the BMT group. Multivariate analysis showed a lower risk of acute GVHD in UCBT, but a significant delay in neutrophil recovery. There were no significant differences in the incidence of chronic GVHD, TRM, or DFS between the two groups. Therefore, the authors of this study suggest that UCBT is an acceptable treatment for adult leukemia patients without an HLA-matched bone marrow donor. The North American data compared the clinical outcomes of 600 adult leukemia cases undergoing unmatched HSCT in the International Bone Marrow Transplant Registry (IBMTR), which reached similar conclusions to those in Europe. Impact of cord blood cell count and degree of HLA compatibility on cord blood transplantation outcomes The main factor limiting the clinical use of cord blood transplantation is its nucleated cell count. Most studies have demonstrated that low cord blood cell counts are associated with low implantation rates, high TRM and low survival, especially in large-weight pediatric and adult cord blood transplant patients. For example, Wagner et al. reported that TRM exceeded 70% if the input CD34+ cell count was less than 1.7 x 105/kg, and in 2006 the European Umbilical Cord Blood Transplantation Collaboration (Eurocord) recommended that the cord blood cell count should be higher than 3 x 107/kg [2]. In addition to cord blood cell count, the degree of HLA compatibility also significantly affects cord blood implantation and patient survival; Eurocord reported an increase in transplant-related mortality with increasing HLA incompatibility. The New York Blood Center (NYBC) analyzed large numbers of cord blood transplantation cases with HLA 6/6 to 3/6 loci compatibility and found that the degree of HLA incompatibility was strongly associated with transplantation outcome [9]. However, the New York Blood Center (NYBC) reported no significant correlation between transplantation outcome and the number of nucleated cells transfused (0.7->10 x 107/kg) in cord blood transplantation cases with HLA 6/6 loci compatibility. The authors suggest that this may be due to HLA compatibility compensating for the lack of cord blood count. Therefore, for cord blood screening, both the number of cord blood cells and the degree of HLA compatibility should be taken into account; the greater the degree of HLA incompatibility, the greater the number of cells required. Therefore, Eurocord recommends that the TNC should be greater than 3 x 107/kg, 4 x 107/kg and 5 x 107/kg for HLA 6/6, 5/6 and 4/6 matches, respectively, in order to obtain a “satisfactory” single copy of cord blood. This recommendation for cord blood selection still needs to be confirmed by prospective studies. Strategies to address cord blood implantation failure and delayed hematopoietic reconstitution As mentioned above, in addition to the number of cells involved, the degree of donor-recipient HLA compatibility also has an important impact on implantation. Although HLA 1-2 loci incompatibility is allowed in cord blood, a high HLA match may improve the implantation rate of cord blood stem cells. Therefore, in order to improve cord blood stem cell implantation, it is most important to select cord blood that is as HLA compatible as possible, increase the number of cord blood stem cells, and promote hematopoietic cell proliferation and differentiation. Selecting cord blood with high nucleated cell count and CD34+ cell count: Among the many influencing factors, the number of nucleated cells and CD34+ cells in cord blood is the key to successful transplantation. After considering the degree of HLA compatibility, Wagner et al. suggested that the number of nucleated or CD34+ cells should be considered first when multiple copies of cord blood with HLA 0-2 loci incompatibility are available. . In vivo promotion of hematopoietic cell maturation: In addition to the number of HSC, the “immature” nature of cord blood stem cells may also contribute to their delayed implantation. It has been shown that the differentiation time of cord blood into megakaryocytes is significantly longer than that of bone marrow HSCs, and early application of cytokine IL-11 can accelerate hematopoietic reconstitution. Early (0-7 days) application of G-CSF significantly accelerates neutrophil hematopoietic recovery and may improve survival. Our application of recombinant human IL-11 starting on day +1 of cord blood transplantation significantly accelerated platelet recovery, and the time to platelet >20 x 109/L was shortened from the generally reported 40-60 days to an average of about 25 days. In addition, since the differentiation and maturation of hematopoietic stem cells is regulated by other cells, it remains to be investigated whether hematopoietic reconstitution can be accelerated by applying non-hematopoietic or immune cells in vivo or in vitro to promote the maturation of umbilical cord blood stem cells, including mature blood cells, lymphocytes, antigen-presenting cells and mesenchymal stem cells. Umbilical cord blood stem cell expansion: Increasing the amount of HSC transfused through in vitro expansion of umbilical cord blood HSC will hopefully shorten the time to hematopoietic reconstitution, but so far an effective expansion system has not been established. Perhaps more promising is the Hox gene product, of which HoxB4 is the most notable. By overexpressing HSC with HoxB4 mRNA through retroviral infection, cord blood HSC can be expanded more than 100-fold, and if purified HSC are treated with TAT-HoxB4 (a soluble HoxB4 protein), HSC can also be expanded 100-fold. Co-culture with MSC trophectoderm in vitro can also increase the expansion of HSCs. Simultaneous transplantation of two copies of umbilical cord blood: To increase the cell count of umbilical cord blood transplants, phase I clinical trials have attempted simultaneous infusion of two HLA partially matched umbilical cord blood transplants. Barker et al. at the University of Minnesota. reported the results of a clinical study of 23 patients with malignant hematologic diseases in a high-risk group receiving 2 UCBTs. Patients had a median age of 24 years (13 years C53 years), a clear marrow pretreatment regimen, and a median total nucleated cell count of 3.5 x 107/kg for both cord blood. 21 evaluable patients, 23 days of cord blood implantation, and incidence of II°CIV° and III°CIV° aGVHD of 65% and 13%, respectively; 1-year DFS was 57%, with 1-year DFS for remission transplants was 72%. Our study also demonstrated that double cord blood transplantation is safe and can overcome the problem of inadequate single-copy cord blood cell count for adult patients. Addition of low-dose hemiphasic CD34+ cells: Fernandez et al [17] showed that cord blood combined with low-dose haploidentical CD34+ cells accelerated the rate of cord blood implantation, with rapid neutrophil recovery and a significantly lower incidence of infection. Early haploidentical cell implantation predominated and was gradually replaced by cord blood, with 90% of patients transformed into complete cord blood chimeras within 100 days. 4-year DFS was as high as 65-82%. . Ibatici A et al [18] performed 29 adult umbilical cord blood transplants with a median age of 38 years, HLA matching in 18 cases 4/6, 10 cases 5/6, and 1 case 3/6. Most patients were pre-treated with the classical CY/TBI regimen. All patients surviving more than 14 days achieved 100% cord blood implantation with median time to neutrophil and platelet implantation of +23 and +38 days, respectively, a significant reduction in implantation time compared to conventional cord blood transplantation. Only 8% developed acute GVHD of degree I-II. the survival advantage cannot be evaluated yet due to the short follow-up period. . Reduced pretreatment intensity: Reduced pretreatment intensity or non-cleared marrow has been used in patients with advanced age, long-term chemotherapy and combined impaired vital organ function. brunstein et al [19] applied cyclophosphamide/fludarabine pre/200cGY pretreatment measures for 110 cord blood transplants (mostly double) for the treatment of progressive malignant hematologic disease in adults, and their 3-year survival rate could reach 45%. Ballen et al [20] applied fludarabine/marfarin/ATG pretreatment regimen for 21 double adult umbilical cord blood transplants and achieved a 100-day graft-related mortality rate of only 14% and a 1-year disease-free survival rate of 67%. These findings suggest that cord blood transplantation after non-cleared marrow pretreatment can also achieve better outcomes for patients who are not suitable for clear marrow pretreatment. Strategies to accelerate cord blood immune reconstitution Regardless of the stem cell source, the donor-derived immune system must be reconstituted after HSCT. Because of the relatively slow immune reconstitution of cord blood, the incidence of infection is high and the mortality associated with early transplantation is high. Therefore, in addition to accelerating hematopoietic reconstitution after transplantation to promote neutrophil recovery to reduce bacterial and fungal infections, accelerating cellular immune reconstitution after UCBT is also crucial to reduce infections and improve patient survival. early immune reconstitution after HSCT is mainly from donor post-thymocyte T cells, which are mature and have antigen-specific and functional specificity. However, the secondary immunity of mature T cells is not quiescent, they can be regulated by interacting with the host microenvironment (e.g. thymic or extra-thymic cytokines IL-2, IL-7, IL-15, etc.). durable T cell immune reconstitution after HSCT depends on hematopoietic stem cells developed from scratch development, which develops in the same way as fetal T cells and takes a similar amount of time. The thymic microenvironment is crucial for this developmental process of T cells. Chemotherapy, radiotherapy, GVHD, and increasing age of the recipient can damage the thymic microenvironment and affect immune reconstitution after HSCT. Immune reconstitution is delayed or diminished due to a significant reduction in the number and qualitative immaturity of HSC associated with UCBT immune reconstitution. Post-thymic T lymphocytes in cord blood are almost exclusively na?ve T, and na?ve T cells are difficult to activate, resulting in a weaker response to specific antigens in post-UCBT T cells than in memory T cells from adult grafts. In addition to T cells, antigen presenting cells (APCs) in cord blood are also different from those in adults.These immune reconstitution features of UCBT imply not only a greater tolerance to HLA incompatibility, but also a weakened ability to respond to pathogens, an increased incidence of infection, and a greater risk of death from infection in UCBT recipients. However, this does not imply a diminished graft-versus-leukemia (GVL) effect; on the contrary, the low recurrence rate of leukemia after UCBT may be related to the stronger GVL effect mediated by cord blood cells. Recent basic research and clinical practice have proposed strategies to accelerate immune reconstitution after umbilical cord blood transplantation, with measures including 1) accelerating pre-thymic T cell development, either by increasing the number of HSC or lymphoid precursor cells (CLP). Combined transplantation of CLP and HSC after mice received lethal doses of irradiation significantly reduced CMV infection than HSC alone, and a small amount of CLP significantly improved the ability of mice to resist CMV compared to transplantation of a large number of thymocytes. The difficulty of this technique is to obtain sufficient amount of CLP, and a possible solution is to activate the Notch pathway to enable HSC to expand and produce more CLP. 2) Improve thymus function and supplement thymus-secreted cytokines such as IL-7. IL-7 administration in the mouse model significantly increased the production of thymus cells after transplantation, but IL-7 also promotes the expansion of mature T 3) Application of thymoprotective agent keratinocyte growth factor (KGF), a member of the mesenchymal cell-derived fibroblast growth factor family, which binds to epithelial-specific receptors. Administration of KGF in experimental models and clinical trials resulted in reduced preconditioning toxicity, increased thymocyte production, accelerated lymphocyte recovery, and reduced occurrence of GVHD after use.4) Reduced preconditioning toxicity to mitigate thymic microenvironment injury.Duke University compared immune reconstitution in non-cleared and cleared UCBT and found faster and more stable recovery in non-cleared UCBT myeloid and gonadal lineages. This may be due to the lesser damage in the non-cleared marrow regimen or the lower incidence of GVHD in the non-cleared marrow regimen, which better protects the peripheral and thymic environment required for T cell proliferation and development.5) Minimize T lymphocyte loss in grafts: T cells in grafts have a pro-implantation effect, which accelerates the ability of HSC to differentiate to form pre-thymic T cells, which in turn produce, in response to the thymic microenvironment, T cells with T cells with immune function. Although most of the T cells in cord blood are naive T cells, they also play an important role in immune reconstitution in the early stage of transplantation. Therefore, reducing the loss of T cells in the graft has positive implications for accelerating hematopoiesis and immune reconstitution after UCBT. Therefore, pretreatment with anti-thymocyte globulin can facilitate cord blood stem cell implantation on the one hand, but the removal of T cells from cord blood has a negative impact on immune reconstitution and can prolong immune recovery for several months [26]. Clinical options for hematopoietic stem cell transplantation The development of UCBT in the last decade or so has amply demonstrated that this treatment has become an effective alternative to bloodless BMT, showing great potential for application and becoming one of the major advances in the field of hematopoietic stem cell transplantation. The status of UCBT in pediatric and adult allogeneic HSCT has been established through the analysis of large number of cases. Therefore, it is recommended that patients in clinical need of HSCT who cannot find a suitable unrelated donor within the required time frame may use UCBT with HLA 1-2 loci incompatibility. the indications for cord blood transplantation can also be relaxed as double cord blood transplantation can significantly improve clinical outcomes. Therefore, Sanz MA recommends that for patients without a suitable relative donor, both cord blood and bone marrow banks should be consulted so that a suitable donor can be found in a timely manner, which can compensate for the delay in treatment due to the long consultation time of unrelated donors. In recent years, due to the expansion of the capacity of domestic HSC database and the more mature development of HLA-matched inter-relative HSC transplantation, more patients have been able to find a more suitable donor, but UCBT still has more room for application. Due to the unique advantages of convenient and easy access to cord blood, it is the best choice for patients with unstable conditions who are in urgent need of transplantation. In the future, we should focus on improving treatment strategies to enable rapid and durable implantation of umbilical cord blood, and study measures to accelerate the immune reconstitution of UCBT and reduce transplantation-related mortality, which could lead to a greater room for improving the clinical outcome of UCBT. It is believed that the clinical application of UCBT will become more widespread in the next 5-10 years.