The Presence of Moderate to High Donor-Specific Anti-HLA Antibody Titers Is Associated with a High Risk of Graft Failure in Haploidentical Stem Cell Transplantation

Background and Rationale: Stem cell transplantation using alternative sources of stem cells from haploidentical relatives is an established therapy for high risk leukemia patients who lack a matched related or unrelated donor. Although efforts have been made to improve the engraftment rate in haploidentical stem cell transplantation, approximately 10–20% of these patients fail to achieve primary engraftment. The causes for this higher rate of graft failure are currently unknown. We hypothesized that anti-HLA antibodies directed against donor specific antigens (DSA) are a possible cause for graft failure in such patients.

Materials and Methods: Twenty-two consecutive patients who received a T-cell depleted haploidentical graft at our institution after 9/2005 were evaluated prospectively for presence of DSA. The patients received a conditioning regimen consisting of fludarabine, melphalan and thiotepa previously described, and megadoses of CD34+ cells. The presence of antibodies against DSA was determined by testing the patients’ sera with a panel of fluorescent beads coated with single HLA antigen preparations using a Luminex^™ platform; results were interpreted as fluorescence intensity (FI) against DSA mismatch. FI less then 500 was considered negative, while positive FI ranging 500–1500, 1500–3000 and >3000 were classified as weak, intermediate and strong, respectively. HLA A, B, C, DRB1, DRB3/4/5, DQB1 and DPB1 were typed by high resolution methods.

Results: Of the 22 treated patients, 6 experienced primary graft failure (PGF). Five of these patients, all females with median age 39 years (range 26–50), were identified to have intermediate or strong FI anti DSA. The anti-HLA antibodies identified were directed against both HLA class I and II antigens, most commonly anti-HLA DRB1 (Table 1). After the first patient was identified to have graft failure in the presence of DSA, treatment with rituximab and plasma exchange was initiated in an attempt to decrease DSA titers and achieve engraftment. Subsequently, four patients were treated in this fashion, two of which had a significant decrease in antibody titers and achieved engraftment. The remaining 2 patients experienced also PGF. All 3 patients who had PGF underwent a second transplant from the same donors. In one of these patients who experienced two graft losses, the DSA was strong before the first transplant decreased significantly a few days after transplantation and returned to the intermediate FI range before the second transplant. Overall, 4 of 6 (67%) haploidentical transplants performed in the in the presence of DSA, and 4 of 4 (100%) transplants performed in patients with moderate to high DSA titers failed to achieve engraftment. Rituximab and plasma exchange appeared to decrease the DSA in 2 of 4 patients with moderate titers, enough to allow engraftment (Table 1). No significant differences were found in the number of CD34+ cells infused, number of allele mismatched, and number of bone marrow blasts present at the time of transplant between the PGF and the control group. The fact that all patients with DSA were females prompted a review of the pregnancy history. The median number of pregnancies was 3 in the PGF group (N=5) as compared with 0 in the control group (N=6) (P=0.1). In addition, a review of the transfusion history to explain a possible alloimmunization at least in one patient without prior pregnancies, identified a median of 37 units (range 17–65) transfused in the PGF group (N=5) as compared with 15 the control group (range 4–38) (N=16) (P=0.007).

Conclusion: Donor specific anti-HLA antibodies appear to play an important role in preventing engraftment in patients receiving a (T-cell depleted) haploidentical stem cell transplant.

Figure 1. Relationship between HLA antibody titers and engraftment in 5 patients who received a total of eight haploidentical transplants at MDACC.