Virally encoded interleukin-6 facilitates KSHV replication in monocytes and induction of dysfunctional macrophages.

Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic double-stranded DNA virus and the etiologic agent of Kaposi's sarcoma and hyperinflammatory lymphoproliferative disorders. Understanding the mechanism by which KSHV increases the infected cell population is crucial for curing KSHV-associated diseases. Using scRNA-seq, we demonstrate that KSHV preferentially infects CD14+ monocytes, sustains viral lytic replication through the viral interleukin-6 (vIL-6), which activates STAT1 and 3, and induces an inflammatory gene expression program. To study the role of vIL-6 in monocytes upon KSHV infection, we generated recombinant KSHV with premature stop codon (vIL-6(-)) and its revertant viruses (vIL-6(+)). Infection of the recombinant viruses shows that both vIL-6(+) and vIL-6(-) KSHV infection induced indistinguishable host anti-viral response with STAT1 and 3 activations in monocytes; however, vIL-6(+), but not vIL-6(-), KSHV infection promoted the proliferation and differentiation of KSHV-infected monocytes into macrophages. The macrophages derived from vIL-6(+) KSHV infection showed a distinct transcriptional profile of elevated IFN-pathway activation with immune suppression and were compromised in T-cell stimulation function compared to those from vIL-6(-) KSHV infection or uninfected control. Notably, a viral nuclear long noncoding RNA (PAN RNA), which is required for sustaining KSHV gene expression, was substantially reduced in infected primary monocytes upon vIL-6(-) KSHV infection. These results highlight the critical role of vIL-6 in sustaining KSHV transcription in primary monocytes. Our findings also imply a clever strategy in which KSHV utilizes vIL-6 to secure its viral pool by expanding infected monocytes via differentiating into longer-lived dysfunctional macrophages. This mechanism may facilitate KSHV to escape from host immune surveillance and to support a lifelong infection. Author summary: Kaposi's sarcoma-associated virus (KSHV) is the causative agent of highly inflammatory diseases that include Kaposi's sarcoma and KSHV-inflammatory cytokine syndrome (KICS). Macrophages are important immune cells that regulate inflammation by stimulating both innate and adaptive immune systems. A small fraction of monocytes differentiates into macrophages to acquire a longer life span and migrate to residential areas. Deregulation of macrophage functions weakens host immune defense mechanisms, allowing secondary infections resulting in prolonged inflammation. Here, we demonstrate that KSHV infection to monocytes facilitates their transition into macrophages; however, those infected macrophages have impaired immune stimulatory function. Such tradition depends on the expression of KSHV vIL-6, a virally encoded IL-6 homolog. Prolonged vIL-6 stimulation in culture also induced a similar phenotype in monocytes. These results suggest that continuous stimulation by KSHV-derived vIL-6 deregulates host macrophage functions. This mechanism may be associated with hyperinflammatory phenotypes seen in KSHV-associated diseases. [ABSTRACT FROM AUTHOR]