Clinicopathological and phenotypic features of chronic NK cell lymphocytosis identified among patients with asymptomatic lymphocytosis

Summary Introduction Chronic natural killer (NK) cell lymphocytosis is currently a provisional entity. This study demonstrated NK cell lymphocytosis in patients with asymptomatic lymphocytosis and presented the hematological and phenotypic findings. Methods Flow cytometry analysis was performed for investigation of unexplained peripheral lymphocytosis. NK cells were determined by the phenotype of CD3−/CD16+/CD56+. Chronic NK cell lymphocytosis was defined by a NK cell count of ≥2 × 109/L, persistent for over 6 months, no evidence of B- or T-cell clonality and no hematologic disorders. Results Among 190 patients with peripheral lymphocytosis, 15(7.9%) patients, age 42–72 years, were identified to have NK cell lymphocytosis, with a median NK cell count of 3.1 × 109/L (range 2.1–7.3 × 109/L). Persistent NK cell lymphocytosis was confirmed with a median follow-up of 18 months. CD56bright NK cell populations were seen in eight patients and CD56dim NK cells in seven patients. CD57 co-expression was seen in both CD56dim and CD56bright cells. CD7, CD2, or CD8 expression was seen in some of the NK populations. The NK cell lymphocytosis appeared stable and no progression to NK cell leukemia during the follow-up period. Conclusion This study demonstrated that chronic NK cell lymphocytosis, similar to monoclonal B lymphocytosis or T-cell clones, may account for asymptomatic lymphocytosis. There were no identifiable causes of the NK cell expansion. The variable phenotype may represent the heterogeneity and pathological features of NK lymphocytosis.