Pharmacologic Reduction of Platelet Count Attenuates Whole Blood Hypofibrinolysis in a Mouse Model of Sickle Cell Disease

Increased risk of venous thrombosis is a hallmark of sickle cell disease (SCD). We previously reported that SCD dramatically alters the properties of venous clots, including resistance to tissue plasminogen activator (tPA)-mediated fibrinolysis. Using an internal fibrinolysis assay (tPA added to whole blood before clot formation), clot lysis time (CLT) was significantly (p<0.01) prolonged in clots formed ex vivo from whole blood of SCD patients (SS) (651±34 min) compared to race-matched controls (AA) (370±75). We then investigated how therapeutic RBC exchange affects CLT in SCD patients. Interestingly, despite hemoglobin A levels of approximately 50%, CLT for the pre-exchange SS clots (639±81 min) was similar to CLT recorded for blood clots from SS patients who did not undergo RBC exchange (see above). However, CLT for post-exchange SS clots was significantly shortened (240±34; p<0.05 vs pre-exchange) to the level observed for clots from AA controls. We noticed that the RBC exchange procedure not only changes the proportion of AA and SS RBCs, but also reduces both platelet counts and serum plasminogen activator inhibitor-1 (PAI-1) levels by approximately 50-60%. Importantly, ex vivo restoration of the platelet number in post-exchange samples to the level observed in pre-exchange blood resulted in restoration of the hypofibrinolytic phenotype and a partial restoration of serum PAI-1 levels.

Changes in platelet count after RBC exchange are quickly normalized. Therefore, in the current study, we investigated the effect of sustained reduction of platelet count on whole blood hypofibrinolysis in SCD mice. Using an external fibrinolysis assay (2.5 nM tPA added after clot formation) as described by Bonnard et al. (Sci Rep, 2017) we determined if hypofibrinolysis is also observed in Townes sickle cell mice (SS). Consistent with human SCD, the CLT was profoundly increased in ex vivo clots formed from whole blood of SS mice (n=13) compared to non-sickle (AA) control mice (n=12) (Figure 1A; first two bars). Next, we demonstrated that the cellular fraction of SS blood was responsible for resistance of the clots to tPA-mediated fibrinolysis, regardless of whether the plasma was from SS or AA mice (Figure 1A; last four bars).

We then used a pharmacologic strategy to reduce platelet counts. Thrombopoietin (TPO) is a growth factor that promotes maturation of hematopoietic stem cells into megakaryocytes and thereby increases platelet production. We previously showed that silencing liver TPO expression using a liver-specific antisense oligonucleotide (TPO-ASO) resulted in a sustained 50% reduction in both plasma TPO levels and platelet counts without affecting hemostasis in both baboons and mice. In the present study, TPO-ASO (20 mg/kg subcutaneous) or vehicle control were administered to 3-month old AA and SS mice (n=3-6) on days 0, 3, 5 and 7, followed by weekly maintenance injections for the next 3 weeks. 4 weeks after initiation of treatment, mice were euthanized, and blood was collected to evaluate plasma levels of TPO and external whole blood fibrinolysis. Platelet, but no other cell count was significantly reduced in both AA (by 62%, p<0.05) and SS (by 47%, P<0.05) mice treated with TPO-ASO. Baseline plasma levels of TPO were reduced in SS mice (101±20 pg/ml) compared to AA controls (182±10; p<0.05). TPO-ASO treatment resulted in significant reduction of TPO plasma levels in both AA (21±16; p<0.01) and SS (17±9; p<0.05) mice. Interestingly, TPO-ASO-mediated reduction of platelet count did not affect CLT of ex vivo clots formed from whole blood of AA mice (Figure 1B). In contrast, TPO-ASO treatment dramatically reduced CLT in SS clots (p<0.05) (Figure 1B).

In conclusion, we demonstrated that: (i) the resistance of ex vivo formed clots to tPA-mediated fibrinolysis observed in SCD patients can be reproduced in a mouse model of the disease; (i) the external fibrinolysis assay provides a robust and reliable model for studying whole blood hypofibrinolysis in SCD; (iii) hypofibrinolysis in SCD appears to be predominantly driven by the cellular fraction of blood; and (iv) platelets play an important role in the hypofibrinolytic phenotype in SCD, demonstrated by mitigation of the hypofibrinolytic phenotype in SS mice subjected to long-term reduction of platelet count. Further studies investigating the effect of platelet count reduction on venous thrombosis and vascular stasis in SS mice are currently ongoing.