Cytomegalovirus pulls strings behind NK cells

Tyrosine kinase inhibitors (TKIs) targeting the BCRABL1 kinase have revolutionized the treatment strategy for Philadelphia chromosome-positive (Ph+) chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (ALL). Among the TKIs, dasatinib is distinctive in that it inhibits a broad spectrum of tyrosine kinases, including key regulators of immune responses [1]. In agreement with this profile, dasatinib suppresses activity of various immune cells, such as T cells, NK cells, and plasmacytoid dendritic cells in vitro. Meanwhile, a unique immunological phenomenon, expansion of large granular lymphocytes (LGLs), has been frequently observed in Ph+ leukemia patients treated with dasatinib. Such expansion does not occur with other TKIs, imatinib or nilotinib. Importantly, LGL expansion during dasatinib treatment is associated with superior therapeutic responses [2]. Thus, elucidating the mechanisms underlying the LGL expansion is instrumental in improving the treatment outcome for Ph+ leukemia. Here is an enigma; dasatinib is immunosuppressive in vitro, whereas it is immunostimulatory in vivo. How can we reconcile these apparently opposite phenomena? A previous study implicated cytomegalovirus (CMV) in the LGL expansion, since it is predominantly observed in CMV-seropositive (CMV+) patients [3]. However, CMV reactivation is observed only in a small fraction of patients, and therefore, underlying factors in the lymphocytosis were uncertain. Thus, we aimed to identify them. LGLs are composed of CD8+ T cells, γδT cells, and NK cells, collectively cytotoxic lymphocytes. We first identified NK cells as the dominant LGLs expanding in dasatinib-treated patients. All the patients with NK cell expansion were CMV+. NK cells express a complex mosaic of inhibitory and activating receptors with large variation among individuals. In order to assess the accurate status of such variegated cells, we performed multiparametric phenotyping of NK cells from healthy donors and Ph+ leukemia patients treated with imatinib, nilotinib or dasatinib. We then analyzed the data using principal component analysis (PCA), a mathematical algorithm that reduces dimensionality of multiparametric data by defining a novel parameter (principal component) as a combination of the parameters to preserve the most essential characteristics of the dataset. PCA revealed that NK cells from CMV+ dasatinib-treated patients underwent phenotypic progression that reflects CMVassociated differentiation (such as NKG2Chigh CD57high LIR-1high NKp30low NKp46low). NK cells from CMV+ imatinibor nilotinib-treated patients and CMV+ healthy donors had an intermediate phenotype, and those from CMV-seronegative individuals did not have the CMVassociated phenotype. Seven of 30 CMV+ dasatinibtreated patients developed CMV reactivation as defined by an elevation of CMV-IgM or a positive result of PCR. Notably, the CMV-associated highly differentiated status of NK cells was already observed at leukemia diagnosis in the majority of CMV+ patients, and was further enhanced after starting dasatinib in virtually all CMV+ patients. CMV reactivation was detected in 8 of 20 CMV+ patients at leukemia diagnosis. Importantly, the patients with a higher degree of NK cell differentiation at diagnosis underwent significantly greater NK cell expansion and faster decreases in BCR-ABL1 mRNA after starting dasatinib. Thus, pretreatment differentiated status of NK cells predicts robust expansion of NK cells and an earlier clinical response after starting dasatinib in CMV+ patients. In this study, the PCA revealed that virtually all the CMV+ patients exhibited enhancement of CMV-associated NK cell differentiation during dasatinib treatment, regardless of the presence or absence of documented CMV reactivation. We assume that a low level of continuous Editorial