Circulating Lymphoma Cells in Mycosis Fungoides/Sezary Syndrome Show Heterogeneity of the Cell-of-Origin and Variable PD-1 Expression By Flow Cytometric Analysis

Introduction: Mycosis fungoides (MF) is an epidermotropic primary cutaneous T-cell lymphoma. Its leukemic variant is recognized as Sezary syndrome (SS), although a study suggests that MF and SS may be distinct entities arising from different T-cell subsets; effector memory T-cells and central memory T-cells, respectively (Campbell JJ et al. Blood. 2010;116(5):767-771). Recent studies show bright PD-1 (T follicular helper (TFH)-like), and CD25/FOXP3 (Treg-like) expression in a subset of MF/SS cases by immunohistochemistry, and these findings raise the possibility that the cell-of-origin of MF/SS may be heterogeneous (Cetinozman F et al. Arch Dermatol. 2012;148(12):1379-1385, Prince HM et al. J Am Acad Dermatol. 2012;67(5):867-875, Satou A et al. Histopathology. 2016;68(7):1099-1108). In order to address this question comprehensively, we evaluated the expression levels of PD-1 on lymphoma cells of MF/SS patients by flow cytometry in peripheral blood (PB), and the results were compared to other T-cell lymphomas including angioimmunoblastic T-cell lymphoma (AITL). We also performed extensive flow cytometric immunophenotyping to algorithmically assess cell-of-origin of MF/SS with a subset of patients (Maecker HT et al. Nat Rev Immunol. 2012;12(3):191-200). Methods: Patients who were diagnosed with T-cell lymphoma at Memorial Sloan Kettering Cancer Center between August 2015 and August 2018, and have circulating lymphoma cells in PB were selected for this study. Diagnosis of MF/SS was confirmed by skin biopsy along with the clinical presentation. PD-1 expression levels on lymphoma cells in PB were evaluated by 10-color flow cytometry including anti-CD279 (PD-1) antibody. Immunophenotyping to assess cell-of-origin of lymphoma cells (T-cell subset analysis) was performed with 3 tube/10-color flow cytometry, using the following antibodies: CD3, CD4, CD8, CD25, CD27, CD45RA, CD45RO, CD127, CD279, HLA-DR, CCR4, CCR6, CCR7, and CXCR3. Results: Our study group is composed of 82 patients, including 34 MF/SS, 22 AITL, 2 anaplastic large cell lymphoma, ALK-negative (ALCL, ALK-), 8 adult T-cell leukemia/ lymphoma (ATLL), 5 peripheral T-cell lymphoma, NOS (PTCL-NOS), 5 T-cell large granular lymphocytic leukemia (T-LGL), and 6 T-cell prolymphocytic leukemia (T-PLL). The expression levels of PD-1 of lymphoma cells of the patients with MF/SS were widely variable (mean fluorescence intensity (MFI); Mean 949.0; range 101.0 - 3188.6) (Fig 1). 32.4% (11/34) of MF/SS cases showed h