*Purpose: Little is known about the development of Human Leukocyte Antigen antibodies with the use of the new Impella 5.5 temporary mechanical circulatory assist device.

*Methods: The prevalence and strength of HLA Class I and II antibodies were assessed prospectively from 6 patients with the Impella 5.5 and 10 control patients with no device support. Single antigen beads (One Lambda) were used to detect HLA antibodies in serum samples pre- and post-implantation of the device up to the time of heart transplantation. 6-month analysis for de novo HLA antibodies, rejection, rehospitalization and deaths were analyzed.

*Results: Baseline characteristics are shown in table 1A. 3/10 and 2/6 patients had pre-transplant HLA antibodies in the control and Impella groups, respectively. Additionally cross match results are shown in Table 1B. There was no increase in the prevalence of HLA antibodies detected post-transplant. None of the patients were admitted for concern of rejection, nor required outpatient optimization of immunosuppression. In the control group, 3 patients were hospitalized within 6 months post-transplant for non-rejection (COVID infection, pericardial effusion and right ventricular failure). There were no re-admissions within the Impella group. There was one death in the Impella group prior to discharge at index admission for transplant due to CMV viremia and stenotophomonas maltophilia infection post-transplant. There were no deaths in the control group. (Table 1C).

*Conclusions: The use of the new Impella 5.5 MCS assist device does not appear to increase the risk of development of de novo HLA antibodies nor appear to increase the risk of allograft rejection. Larger studies are needed to validate these preliminary findings.

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