Human cytomegalovirus productively infects lymphatic endothelial cells and induces a secretome that promotes angiogenesis and lymphangiogenesis through interleukin-6 and granulocyte-macrophage colony-stimulating factor

Endothelial cells (EC) are a site of human cytomegalovirus (HCMV) productive replication, haematogeneous dissemination and persistence and are assumed to play a critical role in the development of HCMV associated vascular diseases. Although early reports have shown the presence of HCMV antigens and DNA in lymphoid tissues, the ability of HCMV to infect lymphatic endothelial cells (LECs) has remained unaddressed due to the lack of a suitable in vitro system. Here, we provide evidence that a clinical isolate of HCMV (retaining its natural endotheliotropism) productively infects purified lymph node-derived LECs and dysregulates the expression of several LEC genes involved in the inflammatory response to viral infection. Qualitative and quantitative analysis of virus-free supernatants from HCMV-infected LEC cultures revealed the virus-induced secretion of several cytokines, chemokines and growth factors, many of which are involved in the regulation of EC physiological properties. Functional assays, in fact, demonstrated that the secretome produced by HCMV-infected LECs stimulates angiogenesis in both LECs and blood ECs, and that neutralisation of either IL-6 or GM-CSF in the secretome causes the loss of its angiogenic properties. The involvement of IL-6 and GM-CSF in the HCMV-mediated angiogenesis is further supported by the finding that the recombinant cytokines reproduce the angiogenic effects of HCMV secretome. These findings suggest that HCMV induces hemangiogenesis and lymphangiogenesis through an indirect mechanism that relies on the stimulation of IL-6 and GM-CSF secretion from infected cells.