Kaposi’s sarcoma-associated herpesvirus (KSHV), also referred to as human herpesvirus-8 (HHV-8), is a gamma-2 herpesvirus. KSHV is causally linked to particular human cancers including Kaposi’s sarcoma, primary effusion lymphoma, and plasmablastic multicentric Castleman’s disease. The KSHV open-reading-frame (orf) 16 encodes a viral Bcl-2 protein (KS-Bcl-2), which shares sequence and functional homology with the Bcl-2 family. Like its cellular homologs KS-Bcl-2 protects various cell types from apoptosis and can also negatively regulate autophagy. KS-Bcl-2 is transcribed during lytic infection; however, its exact function has not been determined to date. By using the full-length KSHV genome bacterial artificial chromosome 16 (BAC) clone, we have generated recombinant KSHV mutants that fail to express KS-Bcl-2 or express mCherry-tagged KS-Bcl-2. We show that KS-Bcl-2 protein is expressed at relatively low levels during lytic induction and that lack of KS-Bcl-2 largely reduces the efficiency of KSHV reactivation in terms of lytic gene expression, viral DNA replication and production of infectious particles. In contrast, the establishment of latency was not affected by the absence of KS-Bcl-2. Our findings suggest an important role for KS-Bcl-2 during initial phases of lytic reactivation and/or during its subsequent propagation. Given the known functions of KS-Bcl-2 in regulating apoptosis and autophagy, which involve its direct interaction with cellular proteins and thus require high levels of protein expression, it appears that KS-Bcl-2 may have additional regulatory functions that do not depend on high levels of protein expression.
