*Purpose: Screening for anti-HLA antibodies in heart transplantation candidates is complicated by transfusions as well as by the presence of ventricular assist devices (VADs), which have been shown to be an independent risk factor for developing anti-human leukocyte antigen (HLA) antibodies. As a result, these patients require frequent antibody screens to monitor for the development of anti-HLA antibodies following these sensitizing events. The goal of this study is to define the frequency of HLA alloimmunization in patients that have received VADs.

*Methods: We performed a retrospective analysis of heart transplantation candidates at our center who received a VAD from 1/2015-10/2018, had at least one negative antibody screen pre-VAD (median of 13 days pre-VAD), and were screened at least once post-VAD (first screen performed at a median of 26 days post-VAD). Patients were screened an average of four times post-VAD, and screens after transplantation were not included.

*Results: Of the 52 patients that had a negative pre-VAD antibody screen, 30 remained negative on all subsequent screens post-VAD, while 22 had at least one positive antibody screen post-VAD. The two groups were similar in terms of age, sex and VAD type. On detailed analysis, 15/22 patients with positive post-VAD screens had antibodies that did not fall into the well-characterized epitope group patterns observed in post-transplant or post-pregnancy alloimmunization. All 15 had antibodies with MFI < 3000 on the first positive screen (average MFI ~1700), which would not lead to a positive virtual crossmatch, and 9/15 were inconsistently present on subsequent screening. Furthermore, a subset of these antibodies were detected only by Luminex single antigen beads and not by Luminex screens or panels, suggesting that they may be binding to cryptic epitopes exposed on the recombinant HLA proteins present on single antigen beads. The 15 patients with non-epitope grouped antibodies received a median of 2 transfusions, and the 7 with canonical epitope patterns received a median of 8 transfusions.

*Conclusions: Taken together, these results suggest that the epitope grouped antibodies arose due to transfusion, while the non-epitope grouped antibodies could represent solid phase HLA cross-reactivities related to the presence of a VAD. In conclusion, the implantation of a VAD appears to lead to canonical HLA alloimmunization in <15% of patients, likely due to multiple transfusions, while frequently leading to development of low-level antibodies of unclear clinical significance.

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