In utero exposure to alloantigens primes alloimmunization to platelet transfusion in mice

Platelet transfusions remain a mainstay of treatment for many patients with thrombocytopenia, but can lead to alloantibodies to Human Leukocyte Antigens (anti-HLA) resulting in inadequate responses to subsequent platelet transfusions (refractoriness), as well as complicate transplantation. Despite substantial decreases in alloimmunization with the implementation of leukoreduction, a significant percentage of patients still become alloimmunized following platelet transfusions. It remains unclear why some patients make anti-HLA antibodies, but others do not make anti-HLA antibodies even with chronic transfusion. Antecedent pregnancy correlates with risk of alloimmunization due to platelet transfusion in humans - however, isolation of pregnancy as a single variable is not possible in human populations.

A tractable murine model of pregnancy and transfusion was engineered by breeding C57BL/6 (H-2^b ) dames with BALB/c (H-2^d ) sires. After pregnancy, female mice were transfused with leukoreduced platelets from F1 (H-2^b/d ) donors that expressed the same paternal major histocompatibility complex (MHC) H-2^d alloantigens as the sires. Control groups allowed isolation of pregnancy or transfusion alone as independent variables. Alloimmunization was determined by testing serum for antibodies to H-2^d MHC alloantigens.

No alloantibodies were detected after pregnancy alone, or in response to transfusion of platelets alone; however, significant levels of alloantibodies were detected when pregnancy was followed by transfusion.

These findings isolate antecedent pregnancy as a causal contribution to increased frequencies of alloimmunization by subsequent platelet transfusion in mice and provide a platform for ongoing mechanistic investigation.