Eltrombopag is currently the only US Food and Drug Administration–approved thrombopoietin receptor agonist for the treatment of chronic immune thrombocytopenia (ITP) in children. This oral, once-per-day therapy has shown favorable efficacy and adverse effect profiles in children. Two multicenter, double-blind, placebo controlled clinical trials (PETIT [Efficacy and Safety Study of Eltrombopag in Pediatric Patients With Thrombocytopenia From Chronic Idiopathic Thrombocytopenic Purpura (ITP)] and PETIT2 [Study of a New Medication for Childhood Chronic Immune Thrombocytopenia (ITP), a Blood Disorder of Low Platelet Counts That Can Lead to Bruising Easily, Bleeding Gums, and/or Bleeding Inside the Body]) demonstrated efficacy in raising platelet counts, reducing bleeding, and reducing the need for concomitant ITP therapies with relatively few adverse effects. The most commonly reported drug-related adverse effects include headache, nausea, and hepatobiliary laboratory abnormalities. Long-term safety data in children are limited, and studies in adults have not revealed a clinically significant increased incidence of thrombosis, marrow fibrosis, or cataract formation. Eltrombopag has also been approved for treating refractory severe aplastic anemia (AA) and has potential for expanded use in ITP and severe AA as well as in other conditions associated with thrombocytopenia.

Immune thrombocytopenia (ITP) is a common platelet disorder in pediatric patients. Pediatric and adult ITP have been associated with sialic acid alterations, but the pathophysiology of ITP remains elusive, and ITP is often a diagnosis of exclusion. Our analysis of pediatric ITP plasma samples showed increased anti-Thomsen-Friedenreich antigen (TF-antigen) antibody representation, suggesting increased exposure of the typically sialylated and cryptic TF-antigen in these patients. The O-glycan sialyltransferase St3gal1 add sialic acid specifically on the TF-antigen. To understand if TF-antigen exposure associates with thrombocytopenia, we generated a mouse model with targeted deletion of St3gal1 in megakaryocytes (MK) (St3gal1MK-/-). TF-antigen exposure was restricted to MKs and resulted in thrombocytopenia. Deletion of Jak3 in St3gal1MK-/- mice normalized platelet counts implicating involvement of immune cells. Interferon-producing Siglec H-positive bone marrow (BM) immune cells engaged with O-glycan sialic acid moieties to regulate type I interferon (IFN-I) secretion and platelet release (thrombopoiesis), as evidenced by partially normalized platelet count following and inhibition of interferon and Siglec H receptors. Single cell RNAseq determined that TF-antigen exposure by MKs primed St3gal1MK-/- BM immune cells to release IFN-I. Single cell RNAseq further revealed a new population of immune cells with a plasmacytoid dendritic cell (pDC)-like signature and concomitant upregulation of immunoglobulin re-arrangement gene transcripts Igkc and Ighm, suggesting additional immune regulatory mechanisms. Thus, aberrant TF-antigen moieties, often found in pathological conditions, regulate immune cells and thrombopoiesis in the BM, leading to reduced platelet count.-

Background: Although chiefly a B-lymphocyte disorder, several research groups have identified common variable immunodeficiency (CVID) subjects with numeric and/or functional T H cell alterations. The causes, interrelationships, and consequences of CVID-associated CD4 1 T-cell derangements to hypogammaglobulinemia, autoantibody production, or both remain unclear. Objective: We sought to determine how circulating CD4 1 T cells are altered in CVID subjects with autoimmune cytopenias (AICs; CVID1AIC) and the causes of these derangements. Methods: Using hypothesis-generating, high-dimensional single-cell analyses, we created comprehensive phenotypic maps of circulating CD4 1 T cells. Differences between subject groups were confirmed in a large and genetically diverse cohort of CVID subjects (n 5 69) by using flow cytometry, transcriptional profiling, multiplex cytokine/chemokine detection, and a suite of in vitro functional assays measuring naive T-cell differentiation, B-cell/T-cell cocultures, and regulatory T-cell suppression. Results: Although CD4 1 T H cell profiles from healthy donors and CVID subjects without AICs were virtually indistinguishable, T cells from CVID1AIC subjects exhibited follicular features as early as thymic egress. Follicular skewing correlated with IgA deficiencyassociated endotoxemia and endotoxin-induced expression of activin A and inducible T-cell costimulator ligand. The resulting enlarged circulating follicular helper T-cell population from CVID1AIC subjects provided efficient help to receptive healthy donor B cells but not unresponsive CVID B cells. Despite this, circulating follicular helper T cells from CVID1AIC subjects exhibited aberrant transcriptional profiles and altered chemokine/cytokine receptor expression patterns that interfered with regulatory T-cell suppression assays and were associated with autoantibody production. Conclusions: Endotoxemia is associated with early commitment to the follicular T-cell lineage in IgA-deficient CVID subjects, particularly those with AICs.

Although the most common front‐line therapies for immune thrombocytopenia (ITP) have been in use for decades, it is still not possible to predict an individual patient's clinical course and response to therapy. Patients are managed with a trial‐and‐error approach and often suffer side effects of therapies which could have been avoided if response prediction were possible. Corticosteroids are the most frequently used upfront therapy for adults and children with ITP. Our group performed whole exome sequencing on a cohort of pediatric ITP patients, and identified two missense single nucleotide variants (SNV) in Toll‐like receptor 4 (TLR4). These coding variants in TLR4 had an increased frequency in Caucasian patients with poor response to upfront steroid therapy. Both TLR4 (D299G; rs4986790) and TLR4 (T399I; rs4986791) had a minor allele frequency (MAF) of 20.7% in those patients unresponsive to steroids, but were present at lower allele frequencies of 2.3% and 3.4% in responders respectively (P < .001). These findings were consistent with the trend identified in an independent cohort of pediatric ITP patients treated with corticosteroids who underwent direct genotyping for both SNVs. This study identified two candidate genetic variants in two unique cohorts of ITP patients which may contribute to steroid response and have prognostic implications for treatment response in ITP.