Anti-HLA Antibody Specificities Are Differentially Associated with Immune Platelet Transfusion Refractoriness in Patients at Lower Calculated Panel Reactive Antibody (cPRA) Levels

Introduction: Platelet transfusion refractoriness (PTR) is a major challenge in multiply transfused patients. We previously ascertained the impact of cumulative donor-specific HLA-antibody thresholds and other factors on immune PTR, however altogether these factors only explained about 1/3 of variation in platelet transfusion response in PTR patients. It is not well-understood what other factors influence development of immune PTR, particularly at lower cPRA thresholds. We hypothesized that occurrence of HLA-mediated immune PTR and HLA antibodies vary by cPRA range and that individual anti-HLA antibody specificities are differentially associated with immune PTR at lower cPRA levels.

Study design: We studied a prospectively curated and unbiased dataset of 2052 patients with hematologic malignancies who underwent screening tests for hematopoietic stem cell transplantation from March 2020 through December 2023 at our institution. All patients were HLA typed (CareDx AlloSeq Tx17) and screened for HLA antibodies (One Lambda). Antibody strength was determined by mean fluorescence intensity (MFI). Immune PTR was defined as 0-2-hour post-transfusion corrected count increment (CCI) <7500 in 2 consecutive platelet transfusions within 1 week. Non-immune PTR was defined as 2-24-hour post-transfusion CCI <7500 in 2 consecutive transfusions within 1 week.

Results: 3498 HLA antibody screens were performed on 2052 patients. Peak cPRA was 0% in 1544 (75.2%) patients, 1-50% in 237 (11.2%) patients, 51-70% in 67 (3.3%) patients, and 71-100% in 204 (9.9%) patients. Of these patients, 861 received 23,266 platelet transfusions of which 93% were random donor platelets. Among factors known to negatively affect CCI, splenomegaly, platelet additive solution, major/bidirectional ABO incompatibility, and transfusion reactions were present in 25%, 45%, 21%, 4.2% of transfusions, respectively. Immune PTR at any point in the study period was observed in 278 of 755 patients (36.8%), and non-immune PTR at any point was observed in 111 of 672 patients (16.5%).

Trends of immune PTR at varying cPRA levels were then examined. Immune PTR was observed in 204 patients and 1742 of 7729 transfusions (23%) at 0% cPRA, 25 patients and 272 of 976 transfusions (28%) with 1-50% cPRA, 13 patients and 205 of 595 transfusions (34%) with 51-70% cPRA, and 37 patients and 355 of 1436 transfusions (25%) with 71-100% cPRA. Relative frequency of specific anti-HLA antibodies (MFI ≥1000) in transfusions with limited cPRA (0-50%) was compared between 2042 immune PTR transfusions and 8611 transfusions without immune PTR. This range was selected as HLA-selected platelets are typically ordered at our institution when cPRA >50%. By Fisher exact testing with correction for multiple comparisons, significantly enriched (p < 0.05) antibodies in immune PTR transfusions relative to transfusions without immune PTR included anti-B27 (3.4-fold enriched with mean MFI 4895 in immune PTR transfusions and 5016 in transfusions without immune PTR), anti-B73 (3.1-fold enriched, mean MFIs 1729 and 2538), and anti-A30 (3.1-fold enriched, mean MFIs 1011 and 1470). Significantly enriched antibodies in transfusions without immune PTR included anti-B44 (16.7-fold enriched with mean MFI 2566 in transfusions without immune PTR and 2525 in immune PTR transfusions), anti-B67 (12.1-fold enriched, mean MFIs 2946 and 1611), and anti-B8 (8.7-fold enriched, mean MFIs 2491 and 1556) among others.

Discussion: In this cohort, HLA alloimmunization occurred in about 1/4 of patients and immune PTR occurred in about 1/3 of these patients. Non-immune factors that negatively affect transfusion response were also frequently present. The frequency of immune PTR increased with increasing cPRA up to about 70%. Beyond this cPRA range, the use of HLA-selected products likely confounded the estimation of immune-PTR. In a regime of limited cPRA (≤50%) we identified anti-HLA antibodies that are differentially associated with increased or decreased occurrence of immune PTR. Notably, antibodies against B8 and B44 were associated with decreased immune PTR, and prior work has shown their corresponding HLAs have reduced surface expression on platelets in some donors, mitigating immune clearance. Further studies are needed to determine whether certain antibody/epitope specificities and avidity definitively influence PTR and what mechanisms are involved.