Rapid desensitization through immunoadsorption during cardiopulmonary bypass. A novel method to facilitate human leukocyte antigen incompatible heart transplantation

Abstract

Background: Anti-human leukocyte antigen (HLA)-antibody production represents a major barrier to heart transplantation, limiting recipient compatibility with potential donors and increasing the risk of complications with poor waiting-list outcomes. Currently there is no consensus to when desensitization should take place, and through what mechanism, meaning that sensitized patients must wait for a compatible donor for many months, if not years. We aimed to determine if intraoperative immunoadsorption could provide a potential desensitization methodology.

Methods: Anti-HLA antibody-containing whole blood was added to a Cardiopulmonary bypass (CPB) circuit set up to mimic a 20 kg patient undergoing heart transplantation. Plasma was separated and diverted to a standalone, secondary immunoadsorption system, with antibody-depleted plasma returned to the CPB circuit. Samples for anti-HLA antibody definition were taken at baseline, when combined with the CPB prime (on bypass), and then every 20 min for the duration of treatment (total 180 min).

Results: A reduction in individual allele median fluorescence intensity (MFI) to below clinically relevant levels (<1000 MFI), and in the majority of cases below the lower positive detection limit (<500 MFI), even in alleles with a baseline MFI >4000 was demonstrated. Reduction occurred in all cases within 120 min, demonstrating efficacy in a time period usual for heart transplantation. Flowcytometric crossmatching of suitable pseudo-donor lymphocytes demonstrated a change from T cell and B cell positive channel shifts to negative, demonstrating a reduction in binding capacity.

Conclusions: Intraoperative immunoadsorption in an ex vivo setting demonstrates clinically relevant reductions in anti-HLA antibodies within the normal timeframe for heart transplantation. This method represents a potential desensitization technique that could enable sensitized children to accept a donor organ earlier, even in the presence of donor-specific anti-HLA antibodies.

Keywords: cardiopulmonary bypass; human leukocyte antigen; immunoadsorption; paediatric; transplantation.

Figure 1.

Anti-HLA Immunoadsorption Set-up. Whole blood is pumped, using the ultrafiltration pump, from the arterial limb of the bypass circuit via the plasma separator (b). The haemofilter (a) is clamped from the circuit at this stage, having been used for prebypass ultrafiltration before the start of bypass and later for conventional and modified ultrafiltration. The separated plasma is then pumped through the ADAsorb® Globaffin immunoadsorption system (c) via the immunoadsorption pump. The hemic content from the plasma separator outlet is reconstituted with the antibody-depleted plasma and returned to the circulation via the venous reservoir.

Figure 2.

Data from the HLA-IA ex vivo simulation showing the reduction over time of anti-HLA antibody as measured by Luminex assay. Lines produced by lowess smoothing of the individual allele result. The horizontal black line represents the clinical importance threshold of 500. The vertical black line represents the typical time from initiation of CPB to organ reperfusion (120 min). All samples were reduced below the threshold within the time frame described.

Figure 3.

Donor lymphocytes were matched to alleles determined by Luminex testing and then tested using flowcytometry (a) to establish T cell and B cell antibody binding. A mean channel shift of >80 (horizontal black line) was considered positive. Both T cell and B cell responses were negative following treatment. Samples were also tested using Luminex for specific DSA (b). A cumulative MFI >7000 was reduced to 0 within the experimental time period.

Figure 4.

Quantile estimation of anti-HLA antibody immunoadsorption using a GAMLSS model with SHASH distributions. Model demonstrates that all centiles are brought under the 500 MFI threshold within 120 min.

Figure 5.

Visualization of the time taken for a required reduction in MFI (80%; based on a previous patient report that was unable to undergo transplantation because of HLA incompatibility). Data are split by alleleic chromosomal region. Dot represents the median time to reduction with the error bars illustrating the 95% confidence intervals calculated using a cubic spline model. Intention is to give the clinician a clear visual guide to how long IA treatment is required to assess feasibility of completing HLA-IA process, and therefore whether patient is suitable for HLAi transplantation.