Low Density Lipoprotein Apheresis Reduces PF4 on the Surface of Platelets: a Possible Protective Mechanism Against HIT

Abstract 2530

Heparin is the anticoagulant used when performing low density lipoprotein (LDL)-apheresis using the Liposorber® system for patients with hypercholesterolemia. This system removes LDL particles via adsorption to a dextran sulfate cellulose bead column (LA-15 column). Despite the exposure to large amounts of heparin and the predisposition of patients with vascular diseases such as atherosclerosis to heparin induced thrombocytopenia (HIT), HIT in patients undergoing LDL-apheresis is very rare. We, therefore, investigated the possibility that the LDL apheresis procedure decreases platelet factor 4 (PF4) adsorbed to cell surfaces and/or plasma HIT antibody levels, either of which would disfavor HIT. We enrolled 25 patients undergoing LDL apheresis (Liposorber®) in the Apheresis and Infusion Clinic at the Hospital of the University of Pennsylvania. Patients were 28–77 years old with high levels of LDL-cholesterol or Lp(a). Whole blood samples were drawn before (pre-treatment), immediately after (post-treatment), and 30 minutes after treatment (30' post-treatment). Plasma samples were also drawn proximal (pre-column) and distal (post-column) to the LA-15 column after ∼350-500ml (∼15-30 minutes) of plasma volume had been processed. PF4, anti-PF4-heparin complex (HIT) antibodies and soluble CD40 ligand (sCD40L; as a marker of platelet activation) were measured using ELISA. Serotonin release assay was performed on samples that were positive for HIT antibodies. Platelet surface PF4 and P-selectin were measured by flow cytometry. Heparin levels were measured using the heparin anti-Xa chromogenic assay. The Student's t test, ANOVA and Wald tests were used to compare results. There were more males than females. None of the patients had any clinical symptoms of HIT. The LA-15 column was found to efficiently remove PF4 (pre-column mean ± SD=133.8 ± 88.26 IU/ml, post-column mean ± SD=0 IU/ml; P<0.0001, 2-tailed). The PF4 level in peripheral blood plasma did not change significantly after LDL apheresis (pre-treatment mean ± SD=116.5 ± 79.14 IU/ml, post-treatment mean ± SD=110.8 ± 67.28 IU/ml; P=NS). However, the amount of PF4 on the platelet surface was significantly decreased by LDL apheresis (pre-treatment mean fluorescence intensity (MFI) mean ± SD=46.32 ± 20.98, post-treatment MFI mean ± SD=31.37 ± 10.11, 30' post-treatment MFI mean ± SD=29.83 ± 10.35; P<0.001 for pre-treatment vs post-treatment and pre-treatment vs 30'post-treatment, 2-tailed). Heparin concentration in the plasma sample increased during treatment and remained elevated 30 minutes after treatment ended (pre-treatment mean ± SD=0.05 ± 0.10 U/ml, post-treatment mean ± SD=0.89 ± 0.09 U/ml, 30'post-treatment mean ± SD=0.79 ± 0.17 U/ml, P<0.0001 for pre-treatment vs post-treatment and pre-treatment vs 30'post-treatment, 2-tailed). HIT antibodies were found in only 2 patients and these antibodies were nonfunctional as assessed by the serotonin release assay. Platelet surface P-selectin, a marker of platelet activation, did not change during treatment (pre-treatment MFI mean ± SD=49.07 ± 27.24, post-treatment MFI mean ± SD=45.14 ± 25.77; P=NS) nor did sCD40L levels which were low before and after treatment. These data provide a potential explanation for the near lack of HIT in hypercholesterolemic patients undergoing LDL apheresis. They also suggest the possibility that the Liposorber® system (or the dextran sulfate adsorption column) may be useful for lowering PF4 in patients with HIT and may have therapeutic potential in its treatment as surface-bound PF4 is the pathologic target of HIT antibodies.