High Numbers of CD34+ Cells/kg and CD3+ Cells/kg Have No Negative Impact on the Incidence of Severe GvHD and Survival in a Large Series of Children Undergoing Unmanipulated Allogeneic HCT from Matched or Mismatched Unrelated Donors.

The aim of the study was to analyze the correlation between the number of transplanted CD34+ cells and CD3+ cells and the incidence of severe Graft versus Host Disease (GvHD) in children undergoing allogeneic hematopoietic cell transplantation from HLA-matched or mismatched unrelated donors.

115 patients (median age 11.4, range 0.7 – 31.6; ALL n=41, AML n=22, CML n=21, MDS/JMML n=10, NHL/HD n=4, SAA/FA n=13, SCID/WAS n=4) underwent unmanipulated allogeneic BMT (n=34) or PBSCT (n=81) from unrelated donors between 1999 and 2005. There were 88 matched- and 27 mismatched donor-recipient pairs, respectively (acc. to ALL SZT - BFM 2003 protocol). Among 88 matched pairs, 51 were 10 out of 10 HLA-allele matched (High Resolution Class I and II typed), whereas in another 37 cases 9 out of 10 alleles were matched. In the mismatched group there were 22 pairs with a 8 out of 10 allele-match and 5 pairs with 7 out of 10 allele-match, respectively. No single DRB1* mismatch was accepted. A distinct correlation between the degree of HLA-match and the risk of grades III-IV aGvHD was found (p=0.05), which occurred in 11 out of 27 pts in the mismatched group (40.7%) and in 19 out of 88 pts in the matched group (21.6%). No difference between the 2 groups was observed with regard to the incidence of extensive cGvHD. No correlation was found between the number of transplanted CD34+ cells/kg or CD3+ cells/kg and the incidence of aGvHD grades III–IV or extensive cGvHD. Median numbers of transplanted CD34+ cells/kg and CD3+ cells/kg were 7.7 × 106 (10.9 × 10⁶) and 2.6 × 10⁸ (3.3 × 10⁸) respectively in the group of patients with grades III–IV aGvHD (extensive cGvHD) and 9.3 × 10⁶ (8.8 × 10⁶) and 3.4 × 10⁸ (3.1 × 10⁸) respectively in the group without these complications. Pts in the mismatched group were given slightly higher numbers of CD3+ cells/kg (4.9 vs 3.0 × 10⁸ in the matched group) and similar numbers of CD34+ cells/kg (median 8.1 vs 8.9 × 10⁶ in the matched group). Despite a higher incidence of severe aGvHD in the mismatched group, probability of 5-year survival was in contrast slightly better in the mismatched group (pS = 0.59 vs pS = 0.48 in the matched group, p=NS). Seventeen from 27 pts in the mismatched group remain alive (62.9%). Median follow-up of all 63 surviving pts (55%) is 21 months (range 6 – 69). Neither degree of HLA match, nor the source of stem cells (PB vs BM), nor number of transplanted CD34 (less or more than 8 × 10⁶/kg) or CD3 cells/kg had an impact on survival.

In conclusion, there seems to be no correlation between the numbers of transplanted CD34+ cells/kg and/or CD3+ cells/kg and the incidence of severe GvHD in the unrelated donor setting in children. It is safe and feasible to perform allogeneic transplants from unrelated mismatched donors, provided a detailed selection of more than 1 HLA-allele-mismatched donors is performed. This conclusion remains in contrast with the ALL SZT-BFM 2003 protocol, which requires extensive T-cell depletion in case of mismatched donor-recipient pairs. High number of transplanted CD34+ cells seems to be a positive factor influencing engraftment and survival in this constellation. There seems to be not necessary to limit the number of transplanted CD34+ cells to 6 or 8 × 10⁶/kg, which had been previously suggested by studies performed in Europe or USA in adults.