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...