Gaining Insights Into Chronic Natural Killer Cell Leukemias Through Extensive Characterization of an Individual Case

Abstract

To the Editor: Chronic lymphoproliferative disorder of natural killer cells (CLPD-NK) is a recently recognized provisional entity characterized by abnormal peripheral blood natural killer (NK) cell expansions with associated cytopenias.1 We report a 46-year-old man who presented to his local physician with fatigue, lethargy, and unexplained weight loss associated with mild to moderate pancytopenia and modest lymphocytosis consisting of cells with small round nuclei and abundant granulated cytoplasm. Physical and radiologic examinations revealed prominent splenomegaly. Bone marrow examination and splenectomy revealed the marrow interstitium and splenic red pulp to be infiltrated by lymphocytes strongly positive for CD2, TIA-1, and granzyme B; variably positive for CD56; and negative for CD3, CD5, CD7, and CD8. Peripheral blood flow cytometry revealed that 90% of the lymphocytes were brightly CD16 positive; CD3 negative NK cells with abnormal loss of CD7, NKp46 (CD335), and KIRs (CD158a, CD158b, and CD158e); and diminished CD56 and CD57 expression. The NK cells strongly expressed activation markers, NKp80, CD94/NKG2a heterodimer, and NKG2D (CD314). The plasma level of the NK cell activating cytokine interleukin (IL) 12 was more than 5 times above the upper limit of normal; the levels of interferon γ, IL-2, and IL-15 were within normal limits. In vitro, the leukemic cells had abnormally high levels of cytotoxicity through the activating receptors CD16, NKG2D (CD314), and NKp80 in the absence of normally requisite cellular priming or exogenous cytokines (Figure). This cytotoxicity was completely abrogated by simultaneous ligation of the inhibitory CD94/NKG2A receptor complex. To measure their cytolytic activity against potential physiologic targets, cytotoxicity assays were performed using freshly isolated normal human neutrophils as the cellular target. In these assays, the leukemic NK cells demonstrated more than 5-fold higher levels of neutrophil cytotoxicity than normal activated NK cell controls (Figure). FIGURE. Cytotoxic activity of the leukemic natural killer (NK) cells. Left, The ability of freshly isolated patient peripheral blood lymphocytes (PBLs) to lyse target cells through receptor ligation was measured and compared to freshly isolated normal PBLs (reverse ... After splenectomy the patient had a period of symptomatic improvement followed by disease progression requiring prednisone and methotrexate therapy. At last follow-up, 18 months after splenectomy, the patient was asymptomatic while receiving continued methotrexate therapy. The features in this case, including the prolonged course, associated cytopenias and infection, lack of Epstein-Barr virus positivity, and loss of NKp46, are diagnostic of CLPD-NK.1,2 The loss of NKp46 seen in CLPD-NK is intriguing when considering the similar entity T-cell large granular lymphocytic leukemia because studies suggest that it is derived from NKp46-positive T cells highly responsive to IL-15.3 Our serologic, phenotypic, and functional studies indicate that the leukemic NK cells are in a persistent, highly activated state with an abnormal capacity to mediate cell lysis through ligation of either NKG2D alone or NKp80 not associated with commensurate loss of responsiveness to inhibitory signals.4 Importantly, the potent ability of the leukemic cells to lyse normal neutrophils provides strong evidence that direct cytotoxicity is a cause of the disease-associated cytopenias; this activity may be mediated by NKp80 because its ligand, AICL, is expressed on myeloid cells.5 Our observations support the hypothesis that CLPD-NK is a disorder of activated NK cells and that in-appropriate cellular cytotoxicity may be responsible for many of the clinicopathologic features. Hopefully, these insights will provide the opportunity to further understand these rare NK cell leukemias and lead to therapies targeted at specific cellular pathways.