2-OR: A CASE OF MIXED UMBILICAL CORD/ RECIPIENT CHIMERISM IN BUCCAL CELLS, FOLLOWED BY COMPETITION BETWEEN CORD AND A SECOND HAPLOIDENTICAL HEMATOPOIETIC STEM CELL TRANSPLANT

Aim Many leukemia patients referred to St. Jude for HSCT have been previously transplanted. Buccal samples may be a mixture of donor and recipient-derived cells and this may complicate HLA typing. In addition, due to HLA disparity between various types of donors, there may be competition for engraftment of subsequent transplants. Methods HLA Typing SSP. HLA Typing SSO. HLA Typing SBT. HLA Antibody ID. Results The patient is a female with infant ALL, who received a 6/6 matched unrelated cord blood transplant. Following relapse, she was referred to St. Jude for haploidentical HSCT and NK therapy. HLA typing of the patient’s buccal sample was in agreement with family studies and with referred typing at all loci, with the exception of DPB1. DPB1 typing by SSO, SBT and SSP were inconclusive. Manual analysis of SSP suggested that multiple alleles were present in this sample, two of which were not present in either parent. Indeed, FISH analysis using XY probes showed that 96.5% of blood cells were of donor origin. The patient received a haploidentical transplant and NK infusion from her mother, and exhibited early engraftment. However, the patient’s chimerism began to show a decrease in donor 2 (mother) and a reappearance of donor 1 (cord), beginning at day 41 post-transplant. She was given 3 donor lymphocyte infusions (DLI) from her mother but continued to lose the graft. Tests of the patient’s serum for anti-class I and class II HLA antibodies were negative. The patient was given a stem cell boost from her mother, but continues to exhibit mixed donor chimerism. Conclusions The DPB1 alleles complicating HLA typing of the buccal sample likely represent those derived from the previous cord donor. Although the cord unit was well-matched with the patient, it is unlikely that donor and recipient also shared both DPB1 alleles. Finally, although the patient did not have detectable antibodies to HLA, the competition between the cord donor and haploidentical donor may reflect a T cell-mediated response.