Frequent CD7 Antigen Loss in Aggressive Natural Killer-Cell Leukemia Can Be a Useful Diagnostic Marker.

Aggressive natural killer-cell leukemia (ANKL) is a rare neoplasm characterized by systemic proliferation of NK cells with rapidly progressive clinical course and fatal outcome. Because of the aggressive clinical course, rapid and accurate diagnosis of ANKL is critical. However, the differential diagnosis of NK cell lymphoproliferative disorders including hemophagocytic lymphohistiocytosis is still challenging in the absence of a distinct diagnostic hallmark. Furthermore, cases with a low burden of malignant cell polpuation makes it more difficult. To find any diagnostic markers in ANKL, we analyzed clinical data and laboratory findings from bone marrow studies in Korean patients with bone marrow involvement of ANKL. From January 2000 to July 2007, a total of 20 cases were diagnosed with ANKL based on morphologic and immunophenotypic findings from bone marrow studies. The leukemic cells were surface CD3–CD16/56+ large granular lymphocytes with pale or lightly basophilic cytoplasm containing azurophilic granules. We retrospectively analyzed clinical features and laboratory findings including complete blood count (CBC), Epstein-Barr virus (EBV) status, serum lactate dehydrogenase (LDH) level, immunophenotype, and cytogenetic results from medical records. There were 6 (30%) women and 14 (70%) men with a median age of 44 years (range, 2–70 years). Hepatomegaly (70%), splenomegaly (60%), and lymphadenopathy (30%) were frequently observed. Peripheral blood counts were variable; anemia (hemoglobin <10g/dL) was predominant in 14 patients and thrombocytopenia (platelet <100×10⁹/L) in 16. The proportion of leukemic NK cells ranged 3∼70%. EBV was detected in 15 of 18 cases (83%) by EBV in situ hybridization or EBV quantitative PCR. Cytogenetic studies were performed in 18 cases, and karyotypic abnormalities were observed in 50% (9/18). There were no recurrent cytogenetic abnormalities, except 6q abnormalities observed in 4 cases (4/18, 22%). The immunophenotype of the leukemic NK cells by flow cytometry was cytoplasmic CD3+, surface CD3−, CD16/56+, CD2+, and CD5−. Most cases were CD4− (13/16, 81%) and CD8− (11/14, 79%). Of note, loss of CD7 antigen was observed in 10 patients (10/20; 50%) (normal NK cells: CD2+, CD7+, and CD5−). There were no significant differences in clinical or laboratory parameters between the CD7+ and CD7− groups. All three cases with deletion of 6q revealed absent expression of CD7. When the CD7 loss was combined with cytogenetic abnormalities, clonal markers could be identified in 75% of ANKL cases. We observed frequent CD7 antigen loss in our series of Korean patients with ANKL. This characteristic immunophenotypic finding can provide a reliable and timely information as a diagnostic marker in ANKL along with cytogenetic findings. Therefore, immunophenotypic analysis of the expression of CD7 should be included in the diagnostic workup of NK cell neoplasms.