Developing an in Vitro Functional Model of Allergic Transfusion Reactions

Allergic transfusion reactions (ATRs) occur in ∼2–3% of platelet transfusions, but their etiology remains largely unclear. Both donor and recipient factors have been shown to contribute to ATR risk. An in vitro model of ATRs would enable mechanistic studies.

To develop a basophil activation test as a model of ATRs to apheresis platelets (AP) by measuring histamine release (HR) and leukotriene C₄ (LTC₄) production after exposing basophils to AP supernatants.

Basophil-enriched suspensions (∼5–15% basophils) from healthy donors were prepared using double-Percoll density centrifugation. After washing and counting basophil numbers using Alcian blue staining, cells were resuspended in buffer containing 2 ng/mL IL-3 and 5mM Ca⁺⁺/Mg⁺⁺ such that 20–30×10⁴ basophils were added in 0.025mL aliquots. IL-3 is a potent basophil priming agent and was used to model the increased recipient susceptibility to allergic reactions that has been observed clinically. Reaction tubes (1.5 mL) were set-up containing 0.025mL of 2× (twice the final conc.) of ATR/control supernatant (final titer 1:2–1:8), negative (buffer alone), and positive control reagents (100 ng/mL anti-IgE antibody, 50 ng/mL C5a, 1 μM FMLP). Cells and reaction tubes were pre-warmed separately to 37°C for 15' before aliquoting 0.025mL cell suspension to reaction tubes for a final volume of 0.050mL. After 30′ incubation at 37°C, 0.950mL of cold buffer (without IL-3 or Ca⁺⁺/Mg⁺⁺) was added to each reaction tube. Tubes were centrifuged 10" at ∼300g (4°C) before removing 0.900mL for histamine detection by automated fluorimetry and LTC₄ detection by ELISA. AP supernatants that were (n=10) or were not (n=10) associated with ATRs were tested. HR values were determined as a percentage of the total histamine content by lysis with perchloric acid (1.6%). Results used in analysis are net HR or LTC₄ production (AP sample minus IL-3 primed negative control) normalized to HR or LTC₄ production observed with positive control.

IL-3 priming of basophils was necessary in order to evoke a response after exposure to AP supernatant. HR in response to AP incubation with unprimed basophils was <3% of positive controls. Maximal HR and LTC₄ production in response to AP supernatants occurred at a final titer of 1:4 using IL-3 primed basophils.

HR and LTC₄ production within individual reactions were correlated (Spearman rho=0.83, P<0.0001). Mean net HR among all AP supernatant samples was 55±28%, 60±49%, and 53±29% of c5a, anti-IgE, and FMLP positive controls, respectively. LTC₄ production was 19±26%, 14±22%, and 15±20% of c5a, anti-IgE, and FMLP positive controls.

No statistically significant differences were observed in HR or LTC₄ production when ATR and control groups were compared (P>0.1). ATRs were stratified into clinical categories of mild (pruritus/urticaria, n=3), moderate (angioedema/dyspnea, n=5), and severe (any symptom with hypotension, n=2). There were no differences or trends in HR and LTC₄ production among the categories (P>0.2). There was no correlation between pre/post transfusion tryptase elevations in recipient plasma (n=4) and HR or LTC₄ production. HR and LTC₄ production was similar among blood types of AP products (A: n=12, O: n=5, AB: n=3), (P>0.7).

We previously reported that concentrations of C5a are higher in AP products associated with ATRs than controls. C5a content in AP supernatant modestly correlated with basophil HR release (rho: 0.47, P=0.04), but less with LTC₄ production (rho: 0.27, P=0.3).

An in vitro basophil activation test that measures HR and LTC₄ production was developed to study ATRs. We find that 1) IL-3 priming of basophils was necessary to elicit responses to AP supernatants; 2) LTC₄ and HR are correlated within each reaction; 3) there are no differences in HR and LTC₄ production between ATR and control AP supernatants were observed, regardless of clinical ATR severity; and 4) HR is associated with C5a content in the AP supernatant.

The observations of 1) an overall requirement for basophil priming and 2) the lack of correlation between the clinical severity of ATRs and basophil HR and LTC₄ release implicate recipient atopic priming as a risk factor for ATRs. However, the association of AP C5a content with HR suggests a donor/product role in ATRs, as well. Studies focusing on the nature of recipient susceptibility may further elucidate mechanisms of ATRs.

No relevant conflicts of interest to declare.