Assessment of human platelet survival in the NOD/SCID mouse model: technical considerations.

BACKGROUND: The NOD/SCID mouse model is a unique and sophisticated method to study the survival of human platelets (PLTs) in vivo. Meanwhile, several research groups adopted this model to analyze a wide range of PLT antibodies. Differences exist between the research groups regarding the method of PLT injection, the amount and route of antibody injection, and the preparation of blood samples collected from the animal, making it difficult to compare results between studies. STUDY DESIGN AND METHODS: We compared the survival of human PLTs infused into NOD/SCID mice via the tail vein or the retro-orbital plexus. The percentage of circulating human PLTs in the mouse circulation was determined by flow cytometry. Murine blood samples were prepared using two different methods: 1) direct fixation of whole blood samples and 2) isolation of PLTs by density gradient centrifugation. RESULTS: Recovery of human PLTs after tail vein injection was comparable to retro-orbital injection (13% vs. 11% of all circulating PLTs, p = 0.401). However, the survival rate of tail vein-infused PLTs was higher than that of retro-orbitally injected PLTs (median PLT survival after 5 hr 84% vs. 56%, p = 0.025). Moreover, we observed that determination of circulating human PLTs in directly fixed murine whole blood samples shows better reproducibility compared to the density gradient centrifugation method. CONCLUSIONS: Tail vein injection of human PLTs into the NOD/SCID mice is superior to retro-orbital injection in terms of human PLT survival. Direct fixation of whole blood samples allows better reproducibility of results compared to the density gradient centrifugation method.

Partially desulfated heparin modulates the interaction between anti-protamine/heparin antibodies and platelets.

Protamine (PRT) is the standard drug to neutralise heparin. PRT/heparin complexes induce an immune response similar to that observed in heparin-induced thrombocytopenia (HIT). Partially desulfated heparin (ODSH) was shown to interfere with anti-platelet factor 4/heparin antibodies (Abs), which are responsible for HIT. In this study, we analyse the impact of ODSH on the interaction between anti-PRT/heparin Abs and platelets. The ability of ODSH to prevent anti-PRT/heparin Ab-induced platelet destruction in vivo was investigated using the NOD/SCID mouse model. ODSH improved platelet survival in the presence of PRT, heparin and anti-PRT/heparin Abs (median platelet survival after 300 minutes (min) with 20 µg/ml ODSH: 75%, range 70-81% vs without ODSH: 49%, range 44-59%, p=0.006). Furthermore, when ODSH was applied 60 min after Ab injection platelet survival was improved (median platelet survival after 300 min with ODSH: 83%, range 77-93% vs without ODSH: 59%, range 29-61%, p=0.02). In in vitro experiments ODSH inhibited platelet activation at concentrations >16 µg/mL (p<0.001), as well as PRT/heparin complex binding to platelets (mean fluorescence intensity [MFI] without ODSH: 85 ± 14 vs with ODSH: 15 ± 0.6, p=0.013). ODSH also displaced pre-bound complexes from the platelet surface (MFI without ODSH: 324 ± 43 vs with 32 µg/ml ODSH: 53 ± 9, p<0.001). While interfering with platelet activation by anti-PRT/heparin Abs, up to a concentration of 16 µg/ml, ODSH had only minimal impact on neutralisation of heparin by PRT. In conclusion, our study shows that ODSH is able to inhibit platelet activation and destruction suggesting a potential clinical use to reduce anti-PRT/heparin Ab-mediated adverse effects.

Anti-protamine-heparin antibodies: incidence, clinical relevance, and pathogenesis.

Protamine, which is routinely used after cardiac surgery to reverse the anticoagulant effects of heparin, is known to be immunogenic. Observing patients with an otherwise unexplained rapid decrease in platelet count directly after protamine administration, we determined the incidence and clinical relevance of protamine-reactive antibodies in patients undergoing cardiac-surgery. In vitro, these antibodies activated washed platelets in a FcγRIIa-dependent fashion. Using a nonobese diabetic/severe combined immunodeficiency mouse model, those antibodies induced thrombocytopenia only when protamine and heparin were present but not with protamine alone. Of 591 patients undergoing cardiopulmonary bypass surgery, 57 (9.6%) tested positive for anti-protamine-heparin antibodies at baseline and 154 (26.6%) tested positive at day 10. Diabetes was identified as a risk factor for the development of anti-protamine-heparin antibodies. In the majority of the patients, these antibodies were transient and titers decreased substantially after 4 months (P < .001). Seven patients had platelet-activating, anti-protamine-heparin antibodies at baseline and showed a greater and more prolonged decline in platelet counts compared with antibody-negative patients (P = .003). In addition, 2 of those patients experienced early arterial thromboembolic complications vs 9 of 584 control patients (multivariate analysis: odds ratio, 21.58; 95% confidence interval, 2.90-160.89; P = .003). Platelet-activating anti-protamine-heparin antibodies show several similarities with anti-platelet factor 4-heparin antibodies and are a potential risk factor for early postoperative thrombosis.