Prevalence and Significance of Pre-Formed Anti-HLA Antibodies in Hematopoietic Stem Cell Transplant Recipients.

The presence of sensitization to HLA antigens has been an important consideration in solid organ transplantation. It is considered a standard process to check for donor-specific allogeneic (allo) antibodies (DSA) and monitor formation of such antibodies post-transplant which could predict early and late graft failure. Most of the current data regarding the importance of anti-HLA (human leukocyte antigen) antibodies is available from renal transplant where presence of HLA antibodies is clearly associated with an increased risk of early graft loss up to the magnitude of 21%. It is routine to perform desensitization to alleviate these antibodies in an effort to enhance their chances of engraftment. The role of and approach to prior sensitization in the hematopoietic stem cell transplantation (HSC) setting is far less clear. This is of unique importance as a wider range of donor cell sources and transplant applications are utilized to treat hematologic diseases. Many of our patients have had multiple transfusions in the past, been pregnant or have had prior HLA mismatched allograft, all of which predispose to development of anti-HLA antibodies. Here we analyze the prevalence of Class I and Class II antibodies as a primary goal and also see if they correlate with graft survival.

52 patients were followed between July 2008 and July 2009 with hematologic malignancies including leukemia's, lymphoma's, multiple myeloma and others. 37/52 underwent transplantation of which 14 were unrelated donor (URD), 5 cord blood (CB) and 8 sibling (sib) transplants. Donors with corresponding HLA were excluded. Post-transplantation with day 100 antibody testing was performed in eligible patients. Antibody determination was done by testing the patients' sera with a panel of fluorescent beads coated with single HLA antigens using a solid-phase Luminex™ platform. Cut-point of 1500 [mean fluorescence intensity (MFI) ≥ 1500 defined as positive] was used for performing statistical analyses. The prevalence of positive antibody levels was compared among the transplant groups using a Fisher's exact test. Level of expression of antibodies was evaluated with MFI <500 considered negative, 500-1500 weak, 1500-3000 intermediate and >3000 strong. High resolution HLA typing was performed.

Class I antibodies were positive in 24 out of 52 total (46%) with 95% CI: 32% to 61%.14/37 (38%) who underwent transplantation (95% CI: 22% to 55%), 12/27 (44%) undergoing allo transplant, CB (20%), sib (38%), and URD (57%) were positive. The prevalence did not differ significantly among the transplant groups (p=0.3). Class II antibodies were positive in 8 out of 52 total (15%) with 95% CI: 7% to 28%. 5/37 (14%) who underwent transplantation (95% CI: 5% to 29%), 4/27 (15%) undergoing allo transplant, Sib (0%), CB (20%) and URD (21%) were positive. The prevalence did not differ significantly among the transplant groups (p=0.6). In females, 18/28 (64%) were positive for Class I or Class II antibodies of which 5/6 (83%) underwent URD transplants. Persistent antibody levels were detected in 3 of 4 patients tested at day 100 post transplant.

Based on this limited pilot study we conclude that there is a high prevalence of anti-HLA antibodies present in recipients at the time of HSC transplantation. However detection of such antibodies did not jeopardize engraftment from various donor sources when HLA donor specific reactions are excluded. Bray et al showed higher incidence of graft failure associated with DSA. Takanashi et al showed that in CB transplants, antibodies were not significant unless the corresponding HLA was present in the CB unit. Based on these studies, we excluded donors with corresponding HLA. All but one patient, in whom donor specific anti-HLA antibodies were identified, achieved sustained marrow engraftment. The long term implications of antibody evolution and specificity to sustained marrow engraftment, graft vs. host and graft vs. tumor effects remain to be clarified. A larger prospective study will need to be conducted to definitely evaluate these relationships including our own which is currently under way.

No relevant conflicts of interest to declare.