Monitoring for HLA Antibodies in Cord Blood Transplantation.

Though graft rejection in hematopoietic cell transplantation (HCT) is presumed to be mediated primarily by host anti-donor T cells and natural killer cells, host antibodies which generate antibody dependent cellular cytotoxic reactions to donor antigens may also contribute. For patients undergoing HCT with a cord blood graft which is usually markedly mismatched to the recipient, alloimmunization is a potential significant issue. Cross-matching is not able to be performed secondary to limited cell numbers available from a cord blood graft. Delayed hematopoietic recovery and graft failure are known complications of cord blood transplantation (CBT), and though likely related primarily to small graft size and presence of primarily naïve immune cells in a cord blood graft, HLA antibodies may also contribute. At our center, we investigate recipient alloimmunity in all patients undergoing CBT to guide donor selection. Patients are first screened for the presence of antibodies against HLA antigens using an ELISA-based assay in which patient serum is tested against pools of purified class I and class II HLA antigens bound in wells of a plastic microtiter plate. Serum from patients noted to have evidence of HLA antibodies prompts further testing to identify the specific HLA antibodies using panels of color coded plastic microspheres each coated with a single purified class I or class II HLA antigen. To date, 4 of 29 patients screened have had evidence of HLA alloimmunization. Further investigation of antibody specificity in one patient undergoing double unit CBT demonstrated antibodies to HLA-Bw6, an epitope known to be present on one of the donor units. Because no other donors were available, the unit was used. Following a reduced intensity preparative regimen (RIT), the patient engrafted neutrophils on day 24 and platelets on day 42. However, the HLA-Bw6 positive unit was absent on all chimerism studies (beginning day 21 post transplantation). Three other patients with HLA alloimmunization did not have identifiable antibody specificity directed against mismatched HLA antigens, and engrafted neutrophils on days 25, 29, 25 and platelets on days 29, 41, and 102 respectively. To our knowledge, we are the first to report monitoring for alloimmunization in CBT and the first to describe the outcome of grafting a cord blood unit known to be HLA antibody incompatible with the patient. When patients undergo double unit CBT, cells from both units can generally be detected in the blood of the recipient during the first month, especially following RIT conditioning, but one unit eventually and consistently prevails (though predictive factors for the winning unit have not yet been satisfactorily described). In this case the compatible unit prevailed and there was no evidence at day 21 of cells from the antibody incompatible unit. Although we cannot attribute cause and effect to the anti-Bw6 alloantibody, it is interesting to note that all seven other patients transplanted on the same RIT protocol have demonstrated at least minimal bone marrow contributions to chimerism from both units at day 28. Hence, alloimmunization may be an important factor influencing graft rejection in CBT. CBT patients should likely be screened for HLA antibodies, and positive screenings warrant further investigation to avoid whenever possible donor/recipient mismatches against which the patient is sensitized. Ongoing monitoring will help clarify the clinical significance of this issue.