Donor-Specific Antibody Mean Fluorescence Intensity Threshold Predicts Platelet Transfusion Response in HLA-Alloimmunized Patients

Introduction: Patients with platelet transfusion refractoriness (PTR) due to HLA-alloimmunization respond to HLA-matched platelets. However, 4/4 HLA-matched platelets or platelets without donor-recipient antigenic mismatches are rarely available. Further, in severe HLA-alloimmunization, it is difficult to find donor products which do not contain antigens to which the recipient has developed antibodies (i.e. antibody specificity prediction, ASP method). Hence, studies have evaluated platelet transfusion responsiveness in the presence of ≥1 donor specific antibodies (DSAs) to widen donor pools, a strategy termed permissive mismatching (PERMM). Using established semi-quantitative techniques for HLA-antibody detection, we sought to identify a cumulative DSA threshold which predicted platelet corrected count increment (CCI) failure despite PERMM.

Methods: We retrospectively reviewed platelet transfusions among patients with HLA-alloimmunization and suspected PTR at the University of Washington/Fred Hutchinson Cancer Center from 2021-2023. HLA Class I antibodies were detected using a solid-phase single antigen bead assay (One Lambda) on the Luminex platform with individual antibody strengths measured by the mean fluorescence intensity (MFI). The calculated panel reactive antibodies (cPRA) were defined as the proportion of donors against whom recipients had antibodies. Transfusions were categorized as HLA matched, PERMM (DSA MFI>0), or random donor platelets (HLA-type unknown). CCI was calculated as described by Cohn ASH 2020.

Results: Of 420 patients suspected to have PTR, 75 had cPRA > 50% and received ≥3 HLA-selected platelet transfusions. Of the 2726 transfusions these patients received, 1198 were excluded due to unavailability of post-transfusion platelet counts within 2 hours or due to multiple transfusions occurring between platelet counts. Ultimately, 1528 transfusions were analyzed (Panel A).

In 1031 PERMM transfusions, the cumulative MFI was a stronger predictor of CCI at 2 hours than cPRA or the number of mismatched DSA (coefficients of determination or R² of 7.3%, 2%, and 0.4%, respectively). We then examined the spectrum of cumulative MFIs to identify a threshold MFI predicting transfusion success/failure among the PERMM products. PERMM platelet transfusions in recipients with DSA MFIs of up to 5000 generated CCIs similar to HLA-matched platelets. However, CCI was significantly different between MFI <10,000 (median CCI 17,783) and MFI ≥10,000 (median CCI 3753) (Panel B). We then stratified data by splenomegaly, a significant non-immunologic cause of PTR (23.2% of transfusions, R² 15.8% for CCI prediction). Of the transfusions not affected by splenomegaly, 9.6% did not generate an adequate platelet response (defined as a CCI >5000) across all MFIs. This failure rate increased to 33.3%, 40.8%, and 52.8% among transfusions in recipients with MFIs >5000, >7500, and >10,000, respectively.

After multiple testing correction, MFI of several individual DSAs (A1, A2, A24, A68, B8, B37, B39, B44, B51, B57, and B62) were also found to be significant predictors of CCI using linear regression (n ranging 32-193, R² ranging 6-33%).

Conclusions: This is the largest study to date evaluating optimal MFI thresholds of DSAs for permissive HLA mismatching in the context of platelet transfusions. Cumulative DSA MFIs demonstrated a dose response effect with lower MFIs generating better CCIs. In our patient cohort overall, and when stratified by splenomegaly, we identified a threshold for cumulative DSA MFIs of 10,000, under which >50% of transfusions resulted in an adequate CCI. This MFI threshold (>10,000) has been implicated in HLA mismatched solid organ transplant rejections (Kallon, Transf. Med. 2020) as well as in a pooled analysis of smaller HLA-selected platelet transfusion studies conducted by our group (unpublished). If incorporated into a transfusion strategy, this MFI threshold may perform better than units chosen based on the number of mismatched DSAs alone or random donor platelets and widen the donor pool. Identifying the impact of specific HLA antibodies on CCIs needs further study. Studies are also underway to correlate cumulative DSA MFIs with complement activation (C1q levels) as a possible mechanistic explanation for the differences noted across the MFI threshold of 10,000 in our study.