Defined by low platelet count in the absence of any other cause, immune thrombocytopenic purpura (ITP) affects over 4,000 US children and 8,000 adults each year. While the majority of ITP cases resolve themselves, patients with ITP have an enhanced risk of bleeding, with 10% experiencing major bleeding, and 0.5% of experiencing life-threatening intracranial hemorrhage. There is no biomarker for ITP or much less, bleeding risk, and all treatments involve significant side effects. This leaves clinicians with a significant dilemma in deciding whether or not to treat. Patients who are ultimately at high risk may not receive treatment until serious bleeding occurs, and low risk patients may be exposed to unnecessary treatment side effects. The research objective of this proposal is to investigate a novel hypothesis, namely, that the contractile force of individual platelets correlates with bleeding phenotype in ITP, independent of traditionally used biological markers or assays of hematological function. Using a newly developed high-throughput platelet contraction cytometer (PCC) to measure single platelet contractile forces in parallel, our latest results of a study of pediatric patients with primary ITP suggests that platelet forces 1) vary significantly from healthy controls, 2) strongly correlate with bleeding (n=49 patients) and 3) change over time in the same patient (n=7). Using an average force cutoff value of 26nN, we found that low forces identified bleeding in ITP with 100% sensitivity and 89.4% specificity, with a specificity improvement to 94% when only considering patients with a platelet count <40,000 uL.
Aim 1 builds on this preliminary data and proposes a rigorous investigation into the relationship between contractile force, platelet characteristics, clinical characteristics, and outcomes by studying a cohort of newly enrolled ITP patients (n=100) at a single time point and prospectively for 12 months. We will specifically see if platelet contractile force correlates with bleeding score, immature platelet fraction, platelet activation, platelet morphology, patient demographics, treatments, and time to resolution. The PCC also offers a unique opportunity to gain new insights into the function of platelets from patients with ITP and mechanistic underpinnings of low force. Previous studies were hindered as traditional tools of platelet function such as aggregometry, platelet functional analyzers, or thromboelastography are confounded by the low platelet count in ITP. We will use the PCC to test our hypothesis that both intrinsic platelet changes and extrinsic plasma factors modify platelet contractile force. From an extrinsic perspective, our data has shown that IgG correlates with more severe bleeding and lower contractile forces, and furthermore that incubating platelets in non-autologous plasma leads to lower contractile forces. From an intrinsic perspective, we have found that patients with low mean platelet volume have lower platelet force and increased bleeding. As the mechanistic underpinnings are unclear, Aim 2 seeks to perform systemic, unbiased investigation into both the intrinsic and extrinsic factors that may modulate platelet function.
Immune thrombocytopenia (ITP) is a common bleeding disorder that leads to a low platelet count and places patients at an increased risk for major (10%) or life-threatening bleeding (0.5%). Caring for these patients remains exceptionally challenging for physicians as they must weigh the significant side effects of all treatments with a qualitative estimate of the patient's risk for bleeding. Here, enabled by a tool capable of studying large numbers of individual platelet forces, this research seeks to understand if platelet force quantitatively assesses bleeding risk while also performing basic science research on the fundamental causes of the bleeding, both of which would significantly improve the quality of care for ITP patients.