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.