Murine Studies to Test the Efficacy of Platelet Transfusions in the Management of Platelet Function Disorders

Background -In platelets, the nucleotide exchange factor CalDAG-GEFI (Cdg1) is critical for efficient integrin-mediated aggregation upon cellular stimulation. Mutations in Cdg1 have recently been shown to cause a novel platelet function disorder, which is characterized by partially impaired platelet aggregation and moderate-to-severe bleeding. Interestingly, platelet transfusions were successful in the treatment of bleeding in some but not all of these patients.

Aims -In this study we investigated the hypotheses that (1) the hemostatic efficacy of transfused platelets is diminished by circulating malfunctional platelets, and (2) a certain ratio of "good" to "bad" platelets has to be reached in order to correct the hemostatic defect observed in these patients.

Methods - A recently developed adoptive platelet transfer model was utilized to establish defined levels and ratios of wild-type (WT) to Cdg1^-/- platelets in thrombocytopenic recipient mice. Additionally, WT platelets were transfused into mice lacking Cdg1, talin-1 or the extracellular domain of GPIbα (IL4α/GPIbα-tg). Hemostatic plug formation was visualized using intravital microscopy following laser ablation injury to the saphenous vein.

Results - Our studies demonstrate that WT platelets are less effective in forming hemostatic plugs when co-transfused into thrombocytopenic mice with large numbers of Cdg1^-/- platelets. Consistently, transfusion of WT platelets to a final level of 5 x 10⁷ platelets/mL in circulation had only a minimal effect on the markedly prolonged bleeding time observed in Cdg1^-/- or talin1^-/- mice, while the same number of platelets rescued the hemostasic defect seen in platelet depleted or non-depleted IL4α/GPIbα-tg mice. Full recovery of hemostasis in Cdg1^-/- mice required a final concentration of 2.5 x 10⁸ WT platelets/mL or a ratio of not more than three Cdg1^-/- platelets for every WT platelet transfused into these mice.

Conclusion - Our studies in mouse models suggest limited efficacy of platelet transfusion therapy in the treatment of platelet function disorders due to competition between "good" and "bad" platelets for von Willebrand factor adhesion sites along the damaged endothelium . To correct the bleeding associated with the platelet function disorder, the fraction of transfused WT platelets has to reach ~25% of all platelets in circulation.