The devastating clinical consequences associated with human cytomegalovirus (HCMV) infection and reactivation underscores the importance of understanding triggers of HCMV reactivation in dendritic cells (DC). Here we show that ERK-mediated reactivation is dependent on the mitogen and stress activated kinase (MSK) family. Furthermore, this MSK mediated response is dependent on CREB binding to the viral major immediate early promoter (MIEP). Specifically, CREB binding to the MIEP provides the target for MSK recruitment. Importantly, MSK mediated phosphorylation of histone H3 is required to promote histone de-methylation and the subsequent exit of HCMV from latency. Taken together, these data suggest that CREB binding to the MIEP is necessary for the recruitment of the kinase activity of MSKs to initiate the chromatin remodelling at the MIEP required for reactivation. Thus the importance of CREB during HCMV reactivation is to promote chromatin modifications conducive for viral gene expression as well as acting as a classical transcription factor. Clearly, specific inhibition of this interaction between CREB and MSKs could provide a strategy for therapeutic intervention.

Platelets are anuclear cells that are essential for blood clotting. They are produced by large polyploid precursor cells called megakaryocytes. Previous genome-wide association studies in nearly 70,000 individuals indicated that single nucleotide variants (SNVs) in the gene encoding the actin cytoskeletal regulator tropomyosin 4 (TPM4) exert an effect on the count and volume of platelets. Platelet number and volume are independent risk factors for heart attack and stroke. Here, we have identified 2 unrelated families in the BRIDGE Bleeding and Platelet Disorders (BPD) collection who carry a TPM4 variant that causes truncation of the TPM4 protein and segregates with macrothrombocytopenia, a disorder characterized by low platelet count. N-Ethyl-N-nitrosourea–induced (ENU-induced) missense mutations in Tpm4 or targeted inactivation of the Tpm4 locus led to gene dosage–dependent macrothrombocytopenia in mice. All other blood cell counts in Tpm4-deficient mice were normal. Insufficient TPM4 expression in human and mouse megakaryocytes resulted in a defect in the terminal stages of platelet production and had a mild effect on platelet function. Together, our findings demonstrate a nonredundant role for TPM4 in platelet biogenesis in humans and mice and reveal that truncating variants in TPM4 cause a previously undescribed dominant Mendelian platelet disorder.

Studies from a number of laboratories have shown that the myeloid lineage is prominent in human cytomegalovirus (HCMV) latency, reactivation, dissemination, and pathogenesis. Existing as a latent infection in CD34؉ progenitors and circulating CD14 ؉ monocytes, reactivation is observed upon differentiation to mature macrophage or dendritic cell (DC) phenotypes. Langerhans' cells (LCs) are a subset of periphery resident DCs that represent a DC population likely to encounter HCMV early during primary infection. Furthermore, we have previously shown that CD34؉ derived LCs are a site of HCMV reactivation ex vivo. Accordingly, we have utilized healthy-donor CD34 ؉ cells to study latency and reactivation of HCMV in LCs. However, the increasing difficulty acquiring healthy-donor CD34؉ cells-particularly from seropositive donors due to the screening regimens used-led us to investigate the use of CD14 ؉ monocytes to generate LCs. We show here that CD14 ؉ monocytes cultured with transforming growth factor ␤ generate Langerin-positive DCs (MoLCs). Consistent with observations using CD34 ؉ derived LCs, only mature MoLCs were permissive for HCMV infection. The lytic infection of mature MoLCs is productive and results in a marked inhibition in the capacity of these cells to promote T cell proliferation. Pertinently, differentiation of experimentally latent monocytes to the MoLC phenotype promotes reactivation in a maturation and interleukin-6 (IL-6)-dependent manner. Intriguingly, however, IL-6-mediated effects were restricted to mature LCs, in contrast to observations with classical CD14؉ derived DCs. Consequently, elucidation of the molecular basis behind the differential response of the two DC subsets should further our understanding of the fundamental mechanisms important for reactivation. Human cytomegalovirus (HCMV) represents an opportunistic pathogen that is a major cause of disease in a number of immunocompromised patient populations (30,60). A significant contribution to morbidity in the clinical setting results from the reactivation of latent HCMV, particularly in seropositive bone marrow transplant recipients (40). Although the mechanisms that govern HCMV reactivation have not been fully elucidated, work from a number of laboratories has shown that CD34 ϩ hematopoietic cells, and the early granulocyte-macrophage progenitors derived from them, which are normally resident in the bone marrow are an important site for the carriage of HCMV latency in vivo (27,34,53) and that reactivation is intrinsically linked with the differentiation to a more mature myeloid cell phenotype (19,37,45,56,59,64).Langerhans' cells (LCs) are a unique population of dendritic cells (DCs) resident in the epidermis and a number of mucosal tissues (e.g., nasal, oral, vaginal, and corneal). They are derived from bone marrow progenitors (26) and exhibit a capacity for self-renewal (11, 36), as well as exhibiting prodigious longevity for a DC, with a half-life of up to 78 days documented (62) and, in one case, a donor's LCs were observed to persis...