Targeting macrophages prevents alloantibody-mediated platelet clearance in a murine model of transfusion refractoriness

HLA Class I-immunized patients can experience a serious complication known as platelet transfusion refractoriness (PTR). This issue becomes especially relevant in onco-hematology departments where platelet transfusions are at the heart of patient care. While the transfusion failure is evidenced by a rapid elimination of allogeneic platelets from the recipient's bloodstream, the mechanisms behind it remain poorly characterized. The aim of this study was to better define these mechanisms in order to improve therapy for PTR. Using a murine model of MHC Class I incompatibility to mimic PTR, we first established that antibodies, but not NK or CD8 cells, mediated platelet clearance. However, blocking Fcγ receptors with IVIg or a monoclonal antibody, or complement depletion did not correct refractoriness in alloimmune mice. We therefore investigated other alternatives beyond antibody-dependent mechanisms. Flow cytometric and microscopic analysis, showed that Kupffer cells in the liver and red pulp macrophages in the spleen phagocytose allogeneic platelets during PTR. Moreover, intravital microscopy revealed allogeneic platelets retained in close interaction with macrophages in the red pulp only in alloimmune animals. Splenectomy or Kupffer cell depletion with clodronate in alloimmune mice suggested the existence of compensatory elimination mechanisms in the liver and spleen. Thus, simultaneous removal of both macrophage populations was an effective strategy to abrogate PTR. Our study provides an insight into mechanisms of platelet clearance in alloimmune pathologies and opens up new perspectives for therapeutic targets.