Negative Depletion of B Cells and T Cells Expressing the αβ Chain of the T-Cell Receptor (TCR) for Haploidentical Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) from a HLA-haploidentical relative is a suitable option for patients (pts) lacking a compatible donor, either related or unrelated. The two main approaches for overcoming the obstacles of HLA barriers are based either on the infusion of large numbers of T-cell-depleted HSC or on intensive pharmacological prevention of graft-versus-host disease (GVHD). While for many years T-cell depletion (TCD) of the graft has been based on either immunomagnetic positive selection of CD34⁺ cells or on physical removal of all subsets of T cells by virtue of mAb, we and other groups have recently developed a novel method of ex vivo TCD based on the selective elimination of αβ⁺ T cells through labeling with a biotinylated anti-TCRαβ Ab, followed by incubation with an anti-biotin Ab conjugated to paramagnetic beads (Miltenyi Biotec, Germany). This approach also allows the removal of B cells to prevent post-transplant EBV-associated lymphoproliferative disease (PTLD). Here, we report the results of graft manipulation using this approach.

Twenty-two children entered the study, 15 with hematological malignancies and 7 with non-malignant disorders. No post-transplant GvHD prophylaxis was employed. HLA-haploidentical family donors received G-CSF (12–16 μg/kg of body weight) to mobilize HSC prior to large-volume leukapheresis, which was commenced when circulating CD34⁺ HSC were >20 cells/μl. Cell therapy products containing up to 60×10⁹ white blood cells (WBC) were processed according to the manufacturer's protocol. In some cases, leukapheresis bags were stored overnight at 4°C in appropriate media at a WBC concentration <200×10⁶/ml. Aliquots of the graft were used to enumerate residual αβ T cells and B cells, as well as other immune effector cells, including type 1 and 2 dendritic cell (DC) precursors, invariant NKT cells, classical CD14⁺CD16⁻ and non-classical CD14⁺CD16⁺ monocytes. All TCR αβ⁺/CD19⁺cell-depleted grafts were re-infused within 2 hours from the end of processing.

Median recovery of CD34⁺ HSC and median number of infused CD34⁺ HSC were 99.3% (range 55.4–100) and 14.7×10⁶/kg (range 7.9–37), respectively. The graft contained 3.9×10⁶ CD3⁺ T cells/kg (range 0.9–30.9) and 0.08×10⁶ B cells/kg (range 0.002–0.32). The log-depletion of αβ⁺ T cells was 4.5 (range 3.2–6.1), with a median number of transplanted αβ⁺ T cells equal to 36×10³/kg (range 1–139.9). Patients received 36×10⁶ CD56⁺ NK cells/kg (range 1.6–80.3) and 4.1×10⁶ γδ⁺ T cells/kg (range 0.9–30.8). A median of 40.8% NK cells (range 30.3–60.2) co-expressed Tim-3, an activating co-receptor promoting IFN-γ production and being scarcely expressed on NK cells that reconstitute after allogeneic HSCT. The αβ/CD19-depleted grafts also contained DC1 and DC2 precursors, which were identified based on BDCA-1 (0.23% of all nucleated cells, range 0.02–2.4), BDCA-3 (0.93%, range 0.57–9.4) and BDCA-2 (0.66%, range 0.49–1.28) or BDCA-4 expression (0.69%, range 0.02–1.5), respectively. Invariant NKT cells were enumerated using mAb reacting against the TCR Vα24-Jα18 chain and were detected in minute percentages (<0.05% of all nucleated cells) within the graft. The αβ/CD19-depleted grafts were also enriched in non-classical CD14⁺CD16⁺ monocytes (13.7% of all CD14⁺cells, range 9.3–29.4), which have been reported to predict a reduced incidence of GVHD after allogeneic HSCT. All patients rapidly engrafted, the median time to reach 500 neutrophils and 50,000 platelets per μl of blood being 12 days (range 10–16) and 13 days (range 12–18), respectively. Only 2 pts developed skin grade I/II acute GVHD, while no pt had visceral acute GVHD. With a median follow-up of 6 months (range 2–8), no pt experienced EBV-related PTLD or died of transplant-related complications.

The immunomagnetic removal of αβ⁺ T cells and B cells was effective and gave reproducible results. In all cases, a remarkable depletion of unwanted T and B cells was attained. The depleted graft contained high numbers of CD34⁺ HSC, as well as of immune effector cells implicated in the control of leukemia growth and GVHD, such as γδ⁺ T cells, NK cells, DC1, DC2 and non-classical CD14⁺CD16⁺ monocytes. From a clinical standpoint, the infusion of αβ/CD19-depleted grafts from HLA-haploidentical family donors was safe, resulting into sustained donor engraftment without life-threatening complications.

No relevant conflicts of interest to declare.