Nuclear egress complex promotes capsid nuclear egress during Kaposi’s Sarcoma-associated herpesvirus reactivation

During herpesvirus replication, newly assembled capsids exit the nucleus by nuclear egress, in which capsids bud at the inner nuclear membrane forming perinuclear enveloped virions that then fuse with the outer nuclear membrane, releasing capsids into the cytoplasm. This process is mediated by the nuclear egress complex (NEC), a conserved viral heterodimer of UL31 and UL34 homologs. Although prior studies demonstrated that overexpression of the NEC from Kaposi’s Sarcoma-associated herpesvirus (KSHV) induces nuclear membrane vesiculation and proliferation in uninfected cells, its role during lytic replication remained undefined. Here, we examined nuclear egress in two models of lytic KSHV replication, reactivated iSLK-BAC16 and TREx-BCBL1-RTA cell lines, using transmission electron microscopy (TEM). In iSLK-BAC16 cells, loss of the NEC caused capsid accumulation at the nuclear periphery and abolished infectious virion production, demonstrating that the NEC is required for efficient nuclear egress. In both cell lines, we observed capsid egress and virion assembly, but the associated phenotypes differed considerably. KSHV reactivation in iSLK-BAC16 cells induced extensive nuclear membrane remodeling, including membrane proliferations and invaginations, whereas these features were largely reduced or absent in reactivated TREx-BCBL1-RTA cells. Moreover, KSHV nuclear egress proceeded more rapidly in TREx-BCBL1-RTA than in the iSLK-BAC16 cell line, demonstrating that KSHV lytic replication can vary across biological systems. Collectively, our data demonstrate the critical role of the KSHV NEC and show that nuclear egress can vary substantially across biological systems, underscoring the importance of the cellular context and the need to study KSHV replication in multiple model systems.