Human herpesvirus 8 (HHV-8) viral
interleukin-6 (vIL-6) is a
cytokine that is poorly secreted and localized largely to the endoplasmic reticulum (ER). It has been implicated, along with other HHV-8 proinflammatory and/or angiogenic
viral proteins, in HHV-8-associated
Kaposi's sarcoma,
primary effusion lymphoma (PEL), and
multicentric Castleman's disease (MCD), in addition to an MCD-related disorder involving systemic elevation of proinflammatory
cytokines, including vIL-6 and human
IL-6 (hIL-6). In these diseases, lytic (productive) replication, in addition to viral latency, is believed to play a critical role. Proreplication activity of vIL-6 has been identified experimentally in PEL and endothelial cells, but the relative contributions of different vIL-6 interactions have not been established. Productive interactions of vIL-6 with the
IL-6 signal transducer, gp130, can occur within the ER, but vIL-6 also interacts in the ER with a nonsignaling receptor called
vitamin K epoxide reductase complex subunit 1 variant 2 (VKORC1v2),
calnexin, and VKORC1v2- and
calnexin-associated
proteins UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT1) and
glucosidase II (GlucII). Here, we report the systematic characterization of interaction-altered vIL-6 variants and the lytic phenotypes of recombinant viruses expressing selected variants. Our data identify the critical importance of vIL-6 and its ER-localized activity via gp130 to productive replication in inducible SLK (epithelial) cells, absence of detectable involvement of vIL-6 interactions with VKORC1v2, GlucII, or UGGT1, and the insufficiency and lack of direct contributory effects of extracellular signaling by vIL-6 or hIL-6. These findings, obtained through genetics-based approaches,
complement and extend previous analyses of vIL-6 activity.IMPORTANCE Human herpesvirus 8 (HHV-8)-encoded viral
interleukin-6 (vIL-6) was the first viral
IL-6 homologue to be identified. Experimental and clinical evidence suggests that vIL-6 is important for the onset and/or progression of HHV-8-associated endothelial-cell and B-cell pathologies, including
AIDS-associated
Kaposi's sarcoma and
multicentric Castleman's disease. The
protein is unusual in its poor secretion from cells and its intracellular activity; it interacts, directly or indirectly, with a number of
proteins beyond the
IL-6 signal transducer, gp130, and can mediate activities through these interactions in the endoplasmic reticulum. Here, we report the characterization with respect to
protein interactions and signal-transducing activity of a panel of vIL-6 variants and utilization of HHV-8 mutant viruses expressing selected variants in phenotypic analyses. Our findings establish the importance of vIL-6 in HHV-8 productive replication and the contributions of individual vIL-6-protein interactions to HHV-8 lytic biology. This work furthers understanding of the biological significance of vIL-6 and its unique intracellular interactions.