Varicella-zoster virus (VZV) is a clinically important alphaherpesvirus that causes both varicella (chickenpox) and herpes zoster (shingles), conditions usually separated by multiple decades. To achieve this long-term latency within a human host, VZV employs extensive immune evasion strategies to avoid early detection and clearance during primary infection. One of the mechanisms commonly used by alphaherpesviruses is to subvert programmed cell death pathways in host cells by conferring apoptotic resistance against death ligands, which is advantageous for production of viral progeny. It is currently unknown if VZV possesses this ability and to investigate this we set out to first assess if the expression or function of death receptors, gatekeepers of these pathways, were modulated.
To investigate this, VZV-permissive human foreskin fibroblasts were infected with VZV strain rOka via a cell-associated infection model, where inoculum cells were CFSE labeled to allow exclusion from analysis. Viral antigen-positive target cells (as determined by VZV gE:gI+) were analysed for surface expression of cell death receptors Fas (CD95), TRAIL-R2 (CD262) and TNF-R1 (CD120a) by flow cytometry. Expression of these receptors was observed to significantly decrease over time as the percentage of VZV antigen positive target cells increased. This was not a global effect on surface receptors as surface levels of the transferrin receptor (CD71) remained unaffected. Total cellular expression of Fas was also observed to be downregulated at later timepoints, both at the transcript and protein level as assessed by qPCR and Western blot, respectively.
This study begins to describe additional mechanisms whereby VZV may confer host apoptotic resistance by modulating molecules upstream of caspases in the external apoptotic pathway, with implications for the clearance of VZV infected cells by death ligand-wielding immune cells.