Double Negative T-Cells in Children with Aplastic Anemia.
Authors: Bennett Russell Thomas , Bakke Antony C , Kurre Peter 
Published: October 2019

Abstract
Abstract 3195
Poster Board III-132

Background
Aplastic Anemia (AA) is characterized by an acquired, progressive loss of hematopoietic tissue from the bone marrow thought to result from an immune reaction targeting the stem cell. Lymphocytes expressing CD8 and cytokines, including tumor necrosis factor alpha and interferon gamma, are known key factors in the pathophysiology. More recently, the role of other T cell populations has been invoked. We undertook the current retrospective study of 19 consecutive pediatric patients to understand the scope of immune dysregulation seen in children with AA. Patients between the ages of 2 and 17 (10 girls, 9 boys) were referred to our center for evaluation of pancytopenia and underwent a staged comprehensive work up to exclude heritable, infectious, infiltrative, or nutritional etiologies for bone marrow failure. Patients were categorized as severe (n=16), or moderately severe (n=3) according to accepted criteria. Association with hepatitis was noted in one patient. Laboratory studies were performed in a certified clinical laboratory, with reference ranges established according to established protocols.

Methods
In a subset of 9 patients, we performed detailed T- cell immunophenotyping by flow cytometry. As previously described in adult AA patients, 5 of our 9 patients showed decreases in CD56/CD16 co-expressing NK cells (% range: 1.9-7.5% with reference range of 5-15%) whereas CD2 /CD25 co-expressing T-regulatory cells were decreased in 1 of 9 patients and increased 3 of 9 patient without correlation with demographic, or clinical parameters (% range: 0.6-15.3% with reference range of 2-10%). Further detailed analysis of T-cell subsets revealed an increase in CD3/CD4 double-negative T-cells (DNT) in 6 of 9 patients (% range: 2.3-21.7% with reference range of 0-4%). This population is widely considered diagnostic of autoimmune lymphoproliferative syndrome (ALPS), a rare immune mediated disease commonly involving lymphadenopathy, splenomegaly, fevers and variable cytopenias. While ALPS often arises from mutations in components of fas apoptotic signaling pathway, none of the nine patients tested showed abnormalities outside the reference range in cell surface expression of fas-, or fas ligand. Finally, clones of cells lacking expression of glycosylphosphatidyl-inositol (GPI) linked proteins have been reported in adult patients with AA, where clone size fluctuates with response to immunotherapy treatment. Among our patients 4 of 9 showed red cell (% clonal size range 0.1-14.6) or white blood cell (% clonal size range 0.2-38) clones.

Conclusion
In conclusion, our study in a pediatric AA cohort, while small, confirms key findings of immune dysregulation previously seen in adult patients and for the first time demonstrates the existence of sizable populations of immature double-negative T-cells. Understanding the full spectrum of immune dysfunction is critical in understanding the pathophysiology of AA and may help in guiding immunotherapy.

Disclosures
No relevant conflicts of interest to declare.