Regulation of the lectin galectin 9 (Gal-9) was investigated for the first time during human cytomegalovirus (HCMV) infection. Gal-9 transcription was significantly upregulated in transplant recipients with reactivated HCMV in vivo. In vitro, Gal-9 was potently upregulated by HCMV independently of viral gene expression, with interferon beta (IFN-) identified as the mediator of this effect. This study defines an immunoregulatory protein potently increased by HCMV infection and a novel mechanism to control Gal-9 through IFN- induction. P rimary human cytomegalovirus (HCMV) infection is followed by lifelong latency (1). Reactivation from latency is associated with severe morbidity and mortality in the immunocompromised, especially in the allogeneic hematopoietic stem cell transplant (HSCT) setting, where donor or recipient HCMV seropositivity is associated with adverse outcomes. In addition, HCMV is the leading infectious cause of birth defects in the developed world (2).Galectins are a family of lectins that preferentially bind -galactosides. Galectin 9 (Gal-9) can modulate diverse biological activities, including cell adhesion, proliferation, apoptosis, and cell cycle progression (3). Despite such varied functions, regulation of Gal-9 is very poorly understood. Functionally, Gal-9 is best characterized as an immunoregulatory molecule controlling T-cell activity via interaction with its receptor, Tim-3 (4), although Tim-3-independent functions have also been described (5). Gal-9 can play an important role in virus life cycles. It modulates human immunodeficiency virus type 1 (HIV-1) entry (6, 7), while Gal-9-knockout mice exhibit more potent antiviral T-cell responses than wild-type mice (8, 9), and infection with Epstein-Barr virus (EBV) modulates Gal-9 expression to evade immune clearance, inducing apoptosis of EBV-specific CD4 ϩ T cells (10, 11). We investigated the expression of Gal-9 in the context of HCMV infection both in vivo and in vitro, identifying a novel, virally induced mechanism to promote Gal-9 expression.Galectin 9 is upregulated in hematopoietic stem cell transplant recipients with reactivated HCMV infection. We hypothesized that Gal-9 is upregulated in patients with active HCMV replication. Blood samples were drawn from HSCT recipients before peripheral blood mononuclear cells (PBMCs) were isolated by Ficoll density gradient centrifugation from patients with or without HCMV reactivation at a range of time points posttransplant (detailed in Tables 1 and 2). Patients were monitored for HCMV reactivation by quantitative PCR (qPCR; Roche Cobas Amplicor CMV Monitor test), with a sustained rise in serum HCMV genome copies above assay detection limits over at least two time points used to define HCMV reactivation.RNA was then isolated from PBMCs using an RNAqueous kit (Ambion). RNA was converted to cDNA using random primers and SuperScript III reverse transcriptase (Life Technologies). cDNA levels were assayed by qPCR (StepOnePlus real-time PCR system; Life Technologies) using 2ϫ Brilliant II SYBR g...