Kaposi's sarcoma-associated herpesvirus (KSHV) gB dictates a low-pH endocytotic entry pathway as revealed by a dual-fluorescent virus system and a rhesus monkey rhadinovirus expressing KSHV gB

Interaction with host cell receptors initiates internalization of Kaposis sarcoma-associated herpesvirus (KSHV) particles via endocytosis or macropinocytosis. Fusion of viral and host cell membranes, which is followed by release of the viral capsid into the cytoplasm, is executed by the core fusion machinery composed of gH, gL, and gB, that is common to all herpesviruses. KSHV infection has been shown to be sensitive to inhibitors of vacuolar acidification, suggestive of low pH as a fusion trigger. To analyze KSHV entry at the single particle level we developed single- and dual-fluorescent recombinant KSHV strains that incorporate tagged glycoproteins, capsid proteins or a combination thereof. In addition, we generated a hybrid rhesus monkey rhadinovirus (RRV) that expresses KSHV gB in place of RRV gB to analyze gB-dependent differences in infection pathways. Our data demonstrate lytic reactivation and infectivity of dual-fluorescent KSHV and incorporation of fluorescently-tagged proteins into viral particles. Confocal microscopy was used to quantify co-localization of fluorescently-tagged glycoproteins and capsid proteins. By using the ratio of dual-positive KSHV particles to single-positive capsids as an indicator of fusion events we established the KSHV fusion kinetics upon infection of different target cells. We measured marked differences in the "time-to-fusion" between different cell types. Inhibition of vesicle acidification prevented virus-cell fusion, implicating low vesicle pH as a requirement for the KSHV fusion step. These findings were corroborated by comparison of RRV-YFP reporter virus with wildtype gB and RRV-YFP encoding KSHV gB in place of RRV gB. While RRV wt infection of receptor-overexpressing cells was unaffected by inhibition of vesicle acidification, RRV-YFP expressing KSHV gB was sensitive to Bafilomycin A1, an inhibitor of vacuolar acidification. Single- and dual-fluorescent KSHV strains eliminate the need for virus-specific antibodies and enable the tracking of single viral particles during entry and fusion. Together with a hybrid RRV expressing KSHV gB, these two novel tools identify low vesicle pH as an endocytotic trigger for KSHV membrane fusion.