Investigating the Relationship between CD34⁺and CD3⁺ Cell Doses, One-Year Graft-Versus-Relapse-Free-Survival, Graft-Versus-Host Disease, and Overall Survival in Haploidentical Hematopoietic Stem Cell Transplantation: A Single Center Experience

Background:

Haploidentical hematopoietic cell transplantation (haplo-HCT) has emerged as a popular alternative to traditional HLA-matched hematopoietic cell transplant. As the number of haplo-HCT's rises, investigating the factors that may affect outcomes is necessary in order to improve overall survival and reduce transplant-related mortality. The optimal dose of CD34⁺ cells used during haplo-HCT to ensure favorable outcomes using PTCy has not yet been reported though a range of 2 to 5.00x10⁶ cells/kg is commonly used.Furthermore, the optimal dose of CD3+ cells is unknown however recent data has suggested less than 3.00x10⁸ cells/kg may prevent the development of acute GVHD. The importance of studying the impact of CD34⁺/CD3⁺ cell dosing may help to improve outcomes in this setting.

Methods:

We retrospectively analyzed adult patients at USC Norris Cancer Hospital (age ≥ 21) who received haplo-HCT from 2014 to 2019. The primary end-point assessed was 1-year GVHD-free/relapse-free survival (GRFS) defined as grade 3-4 acute GVHD, systemic therapy-requiring chronic GVHD, relapse, or death in the first post-HCT year. Secondary end-points included 1-, 2-, and 3-year relapse-related mortality (RRM) and overall survival (OS) in addition to 1-year transplant related mortality (TRM) and incidence of both acute and chronic GVHD.

Results:

A total of 67 adult haplo-HCT recipients were reviewed. Of the patients evaluated, approximately 50% (n = 33) were male and 49% (n = 32) were female. The age range was 21-71 years old (median = 44), and the most common underlying hematologic disorders included AML (40%), ALL (38%), aplastic anemia (7.7%), and others (MDS, lymphoma, myelofibrosis, and HLH) (13.8%). 67% of patients received myeloablative conditioning regimens while 33% received reduced intensity regimens. 70% (n = 47) of patients received peripheral blood as a stem cell source with 30% (n = 20) receiving bone marrow. The mean CD34⁺ dose infused was 6.07x10⁶ cells/kg and the mean CD3⁺ dose was 2.94x10⁸ cells/kg. The mean time to recovery of platelets, neutrophils, and lymphocytes was 25, 18, and 37 days respectively. CD34⁺ stem cells ≥5.00x10⁶ cells/kg was significantly associated with shorter time to lymphocyte recovery (p = 0.0265) though recovery less than 30 days was not significantly associated with OS (p = 0.5268). Incidence of 1-year GRFS was 71% (n= 46) and 1-, 2-, and 3-year RRM were 4.6%, 6%, and 7.7% respectively. 1-year TRM was 15.3% with 50% of deaths from acute GVHD. 1-, 2-, and 3-year OS were 80%, 78%, and 77% respectively. Factors significantly associated with increased mortality included use of RIC regimen (p = 0.004) and disease status at time of transplant (p = 0.04). Cumulative incidence of GVHD was 63% (n = 42) with 33% (n = 22) and 30% of patients (n = 20) with acute and chronic GVHD respectively. Lack of mild chronic GVHD was associated with increased mortality (p = 0.0029) and use of a myeloablative regimen (p = 0.0029) was significantly associated with GVHD. Subgroup analysis of those who received CD34⁺ dose ≥7.00x10⁶ cells/kg (n = 24) and ≥10x10⁶ cells/kg (n = 7) were found to have 1-year OS of 87.5% and 85.7% compared with 77% and 80% in those that received lower doses (p= 0.2229 and p = 1.00) respectively however this was not found to be significantly associated with increased incidence of GVHD, relapse, or mortality.

Discussion:

Our results demonstrate improved outcomes specifically 71% survived 1 year without experiencing at least 1 GRFS event compared with 24-35% reported by CIBMTR, Holtan et al 2015, and Solh et al 2016 with 3-year OS of 77% when compared with a previously reported 48%. The mean CD34+ cell dose of our population is higher than the standard range which may account for the improved outcomes however the dosing of CD34+/CD3+ cells were not significantly associated with our primary and secondary end-points. It was significantly associated, however, with shorter time to lymphocyte recovery, a factor that has been reported to be associated with decreased RRM and therefore improved OS. Furthermore, subgroup analysis of higher CD34+ dose did show a better 1-year OS though this was not statistically significant. Limitations of this study include small sample size and short follow-up period. Further research with a prospective study identifying the optimal CD34+/CD3+ cell dose in addition to comprehensive evaluation of immune recovery is warranted in order to improve haplo-HCT outcomes.

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