Introduction:

Hematopoietic Cell Transplant (HCT) using a HLA-haploidentical donor greatly expands access to curative therapy for patients with sickle cell disease (SCD). Historically haploidentical HCT for SCD has been complicated by a high incidence of graft failure. Testing that could identify patients at risk of graft failure could be an important clinical tool. We investigated a novel HLA-specific Flow cytometry-based Assay (HFA) to evaluate lymphocyte donor chimerism as a potential predictive indicator of engraftment outcomes.

Objective:

In the BMT CTN 1507 multicenter study (NCT#03263559) evaluating haploidentical HCT for severe SCD using reduced intensity conditioning with post-HCT cyclophosphamide, we sought to quantify donor T-cell and Natural Killer (NK) cell populations utilizing HFA and compare these results to clinical chimerism testing.

Methods:

Peripheral blood mononuclear cells (PBMCs) were collected and analyzed from patients pre-transplants, day +28, day +100, day +180, and day +365. Donors were also analyzed when available. All samples were evaluated for donor HLA detection by assessing the single-allele HLA mismatch between the haploidentical donor and the patient based on the high-resolution HLA typing. Donor HLA detection was further supported by an HFA assay that utilizes cell surface markers (CD3, CD56 and HLA antibodies targeting a donor or patient specific HLA for each case) for lymphocyte donor chimerism.

Results/Discussion:

In the adult stratum of the BMT CTN 1507 clinical trial, research samples for HFA testing were available for 33 of the 42 transplanted patients. Clinically 32/33 (97.0%) analyzed patients had sustained donor engraftment at 1-year post-HCT and all of these patients also had evidence of donor engraftment on HFA testing. The one patient with secondary graft failure was diagnosed with graft failure at day +82. Importantly for this patient on day +28 the HFA testing showed only 7.7% T-cell and 2.8% NK cell donor chimerism, whereas clinical whole blood donor chimerism at this time was 96.0%. In addition, one other patient had low (<15%) lymphocyte donor chimerism at day +28 with HFA testing that declined with time. With clinical chimerism testing this patient initially had stable, strong donor engraftment (100% myeloid donor chimerism at day +100 and +180) but then had a notable decline in myeloid donor chimerism of 84.0% at day +365 to 47% at day +730. In both the two patients with problematic donor engraftment, the HFA assay was able to indicate the risk of graft failure at an earlier timepoint than observed clinically.

Overall, we observed the lowest lymphocyte donor chimerism for T and NK-cell (84.3 ± 19.4% and 89.0% ± 17.6% respectively) on day+28. Further post-HCT, donor chimerism on HFA testing increases with the highest chimerism at day +365 and similar results to clinical chimerism testing. At day +365 there is low inter-individual variability as compared to earlier timepoints with lymphocyte donor chimerism remaining consistently high above 95% for most patients. The overall median in both lymphocyte lineage exceeds 95% in all evaluated timepoints.

Conclusion:

Our lymphocyte donor chimerism results using a novel HFA confirms the strong donor engraftment observed clinically for almost all adult patients transplanted on BMT CTN 1507. HFA testing identified the few patients with problematic donor engraftment before they were diagnosed with secondary graft failure or had a serious decline in donor myeloid engraftment. The assay provides a rapid, phenotypically resolved, predictive measure of donor derived lymphocyte reconstitution. HFA testing may serve as valuable supporting data to current clinical monitoring and enable early intervention for patients that are risk for graft failure.

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2025
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