Improved Immune Recovery after Transplantation of TCRαβ/CD19 Depleted Allografts from Haploidentical Donors in Pediatric Patients

Transplantation of haploidentical stem cells has become an accepted option for pediatric patients and adults with high risk malignancies who lack a matched related or unrelated donor. In recent years, the majority of pediatric transplant centers chose the CD34 positive selection of peripheral stem cells, which allowed minimizing GvHD by effective reduction of T cells in the graft. However, infectious complications caused by delayed immune recovery were a major reason for transplant related mortality (TRM). In order to improve the immune recovery, we have established a new T-cell depletion method which removes αβ+ T-lymphocytes via a biotinylated anti-TcRαβ antibody followed by an anti-biotin antibody conjugated to magnetic microbeads while retaining γδ+ T-lymphocytes, natural killer (NK) cells and other cells in the graft. In addition, CD19+ B-lymphocytes were concomitantly depleted for the prevention of post-transplant EBV-associated lymphoproliferative disease.

Immune recovery was retrospectively analyzed in a cohort of 41 patients with acute leukemia, MDS and non-malignant diseases, who received αβ T and B cell depleted allografts from haploidentical family donors. Conditioning regimens consisted of fludarabine or clofarabine, thiotepa, melphalan and serotherapy with OKT3 or ATG-Fresenius^®. Graft manipulation was carried out with anti TCRαβ and anti CD19 antibodies and immunomagnetic microbeads. γδ T cells and NK cells remained in the grafts. Primary engraftment occurred in 88%, acute graft versus host disease (aGvHD) grade II and III-IV occurred in 10% and 15%. Immune recovery data were available in 26 patients and comparable after OKT3 (n=7) or ATG-F^® (n=19). Median time to reach > 100 CD3+/µl, > 200 CD19+ cells/µl and > 200 CD56+ cells/µl for the whole group was 13, 127 and 12.5 days. Compared to a historical control group of patients with CD34 positive selected grafts, significantly higher cell numbers were found for CD3+ at days +30 and +90 (267 vs. 27 and 397 vs. 163 cells/µl), for CD3+4+ at day +30 (58 vs. 11 cells/µl) and for CD56+ at day +14 (622 vs. 27 cells/µl). The clinical impact of this accelerated immune recovery will be evaluated in an ongoing prospective multi-center trial.