Pivotal advance: Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded microRNA specifically induce IL-6 and IL-10 secretion by macrophages and monocytes.

Macrophages are an important source of inflammatory cytokines generated during the innate immune response,but in the microenvironment of certain tumors,macrophages promote tumor progression through their preferential secretion of cytokines that support tumor cell growth and suppress antitumoral immune responses. KSHV is the causative agent of KS and lymphomas preferentially arising in immuno compromised patients, and specific cytokines, including IL-6 and IL-10, have been implicated in KSHV-associated cancer pathogenesis. However, the contribution of KSHV-infected macrophages to the cytokine milieu within KSHV-related tumors is unclear. We found that individual KSHV-encoded miRNA induce IL-6 and IL-10 secretion independently and additively by murine macrophages and human myelomonocytic cells. Bioinformatics analysis identified KSHV miRNA binding sites formiR-K12-3 and miR-K12-7 within the 3'UTR of the basic region/leucine zipper motif transcription factor C/EBPbeta, a known regulator of IL-6 and IL-10 transcriptional activation.Subsequent immunoblot analyses revealed that miR-K12-3 and miR-K12-7 preferentially reduce expression of C/EBPbeta p20 (LIP), an isoform of C/EBPbeta known to function as a negative transcription regulator. In addition,RNA interference specifically targeting LIP induced basal secretion of IL-6 and IL-10 by macrophages.Taken together, these data support a role for KSHV miRNA in the programming of macrophage cytokine responses in favor of KSHV-related tumor progression.