Comparison Of Lymphocytic Immune Profiles Of Aplastic Anemia and Hypocellular Myelodysplastic Syndrome
Authors: Pu Jeffrey J. , Rivera Guillermo Rangel , Sido Abigail , Berg Arthur , Boyer Cinda , Bayerl Michael G. 
Published: October 2019

Abstract

Background
Aplastic anemia (AA) and hypocellular myelodysplastic syndrome (MDS) are two common acquired bone marrow failure diseases. AA is mostly an acquired bone marrow disease caused by cellular and humoral mediated immune attack of hematopoietic stem cells (HSC) due to dysregulation of lymphocytic system, which leads to hematopoietic progenitor cell apoptosis and bone marrow failure. MDS is a group of heterogeneous acquired clonal HSC disorders with ineffective hematopoiesis. Approximately 10% to 20% of MDS manifests a reduced bone marrow cellularity, which comprises hypocellular MDS. There is increasing experimental and clinical indication that an immune-mediated damage to hematopoietic HSCs and changes in the hematopoiesis-supporting microenvironment contribute to the pathogenesis of hypocellular MDS. Because of the similarity of their bone marrow manifestation, hypocellular MDS and AA are often hard to distinguish. Mounting evidence indicates that abnormal activation of cytotoxic T cells plays a crucial role in the pathophysiology of these diseases. One study showed that AA patients have an abnormally activated subpopulation of CD4+ helper cells and a decreased number and function of T regulatory cells in the bone marrow. GVHD mouse models further demonstrated that self-reactive T cells were capable of recognizing non-polymorphic tissue or commensally-derived antigens. Recent literature suggests that immune dysregulation plays a major role in pathogenesis of acquired bone marrow failure disease. However immune profiles of these two diseases have not been thoroughly studied, specially the role of B lymphocyte population. Our study aims to find lymphocytic surface marker expression patterns of hypocellular MDS and AA in both immature cell and lymphocyte populations.

Methods
This retrospective study analyzed flow cytometry lymphocytic antigen expression profiles from patients diagnosed as AA and hypocellular MDS as per standard criteria. A total of 31 AA and 26 hypocellular MDS patient cases were recruited. The bone marrow aspirate/biopsy data, bone marrow aspiration flow cytometry reports, and Complete Blood Counts (CBC)s from individual patients were analyzed. Using side scatter (SSC) vs. CD45 gating flow cytometry panels, we identified immature cell population (SSClow/CD45low) and lymphocyte population (SSClow/CD45high). We then quantitatively analyzed the expression patterns of 33 cluster differentiation (CD) molecules on individual sample. Finally, we compared the CD expression patterns between AA and hypocellular MDS in both cell populations respectively.

Results
CD19 expression was significantly higher in AA than in hypocellular MDS in both SSClow/CD45low cell population (P=0.001) and SSClow/CD45high cell population (P=0.003). Hypocellular MDS contains significantly higher CD34high cells than AA in SSClow/CD45low populations (mean:28.5% vs 8.5%; range; 1% to 94% vs 2% to 27%; P=0.04). However, patients with both diseases similarly contains very few CD34high cells in SSClow/CD45high cell population (mean: 0.6% vs 2.6%; range: 0.0% to 2% vs 0.0% to 32%; P=0.99).

Conclusion
1. In AA, B cells are highly proliferative in both immature stage and mature stage. This data indicates that B cells which may play a unique role in AA pathogenesis but not in hypocellular MDS.
2. In both AA and hypocellular MDS, the majority of lymphocyte population are mature cells. This data suggests that the pathogeneses of both diseases caused by a persistently dysregulated immune microenvironment, not by an acute insult.
CD19 expression pattern may be a useful marker to distinguish AA and hypocellular MDS.

Disclosures:
No relevant conflicts of interest to declare.