Prevention of Alloimmune Platelet (Plt) Refractoriness In a Dog Model Requires Both Removal and Inactivation of Contaminating Donor White Blood Cells (WBCs)
Authors: Slichter Sherrill J. , Pellham Esther , Bailey S. Lawrence , Christoffel Todd , Gaur Lakshmi , Latchman Yvette , Nelson Karen , Bolgiano Doug 
Published: October 2019

Abstract
Abstract 3343

Background:
The TRAP Trial (NEJM 1997;337:1861) evaluated filtration leukocyte-reduction (F-LR) and UV-B irradiation (UV-BI) to prevent plt alloimmunization in AML patients (pts) receiving chemotherapy. Inclusion of UV-BI was based on a 45% rate of preventing plt alloimmunization in our dog plt tx model. UV-BI was 79% successful in the TRAP Trial. The lower success rate in the dog was probably because we use normal immunocompetent recipient dogs versus chemotherapy-induced immunosuppressed pts. The residual rate of alloimmunization in the TRAP Trial using either F-LR or UV-BI was still 17% to 21%, suggesting that better prevention methods are needed.

Methods:
In our dog plt tx model, we evaluated two leukoreduction filters to remove antigen presenting WBCs (APCs) and γ-irradiation (γ-I) or UV-I combined with riboflavin (Mirasol pathogen reduction technology) to inactivate APCs. Crossmatch-negative, DLA-DRB incompatible donor/recipient pairs were used. Txs from the same donor were given weekly for up to 8 weeks or until the donor's 51Cr-labeled plts were rejected based on two sequential txs with donor plt recoveries ≤5% at 20 hours post-tx.

Results:
Residual WBC counts for standard, γ-l, or Mirasol-treated plts averaged 2,860/μl ± 1,800/μl, and, for F-LR plts using the PL1-B or the PLS-5A filter, they were 80/μl ± 10/μl and 110/μl ± 270/μl, respectively. Residual WBCs were significantly reduced after F-LR compared to non-leukoreduced plts (p<0.001), but there was no significant difference for WBC removal using either filter (p=0.86).

Conclusions:
Except for the results with the PLS-5A filter, a single modification of the donor dog's plts is only minimally successful. Although residual WBC counts were similar for both filters, the better results achieved with the PLS-5A filter compared to the PL1-B filter (66% versus 20% success rate, respectively) suggest that the types of WBCs removed may be as important as simply a quantitative reduction in WBCs. Unfortunately, adding γ-l to F-LR did not improve the results using either filter. In contrast, Mirasol treatment markedly improved the results with both filters, suggesting that the effects of F-LR and Mirasol treatment are synergistic with F-LR removing APCs, while Mirasol treatment inactivates residual APCs. The effect is particularly striking for the PL1-B filter that is only minimally effective when used alone. The fact that a combined F-LR Mirasol treatment approach is so successful in our immunocompetent dog model using both good and poor performing filters may suggest two things: 1) the combined approach will be equally successful in both immunosuppressed as well as non-immunosuppressed pts; and 2) using any available leukoreduction filter combined with Mirasol treatment will give results similar to the two filters already tested. Based on our prior experience with UV-BI, data from our dog model may be directly transferable to man.

Disclosures:
Slichter: CaridianBCT Biotechnologies, LLC; Lakewood, CO: Research Funding; US Army Medical Research and Materiel Command/Dept of the Army – USAMRAA: Research Funding.