Date Published: November 28, 2016
Publisher: Public Library of Science
Author(s): Samantha J. Williamson, Samantha M. Nicol, Michael Stürzl, Shereen Sabbah, Andrew D. Hislop, Dirk P. Dittmer.
Kaposi sarcoma-associated herpesvirus (KSHV) is linked with the development of Kaposi sarcoma and the B lymphocyte disorders primary effusion lymphoma (PEL) and multi-centric Castleman disease. T cell immunity limits KSHV infection and disease, however the virus employs multiple mechanisms to inhibit efficient control by these effectors. Thus KSHV-specific CD4+ T cells poorly recognize most PEL cells and even where they can, they are unable to kill them. To make KSHV-infected cells more sensitive to T cell control we treated PEL cells with the thymidine analogue azidothymidine (AZT), which sensitizes PEL lines to Fas-ligand and TRAIL challenge; effector mechanisms which T cells use. PELs co-cultured with KSHV-specific CD4+ T cells in the absence of AZT showed no control of PEL outgrowth. However in the presence of AZT PEL outgrowth was controlled in an MHC-restricted manner. To investigate how AZT sensitizes PELs to immune control we first examined BJAB cells transduced with individual KSHV-latent genes for their ability to resist apoptosis mediated by stimuli delivered through Fas and TRAIL receptors. This showed that in addition to the previously described vFLIP protein, expression of vIRF3 also inhibited apoptosis delivered by these stimuli. Importantly vIRF3 mediated protection from these apoptotic stimuli was inhibited in the presence of AZT as was a second vIRF3 associated phenotype, the downregulation of surface MHC class II. Although both vFLIP and vIRF3 are expressed in PELs, we propose that inhibiting vIRF3 function with AZT may be sufficient to restore T cell control of these tumor cells.
Kaposi sarcoma-associated herpesvirus (KSHV) is an oncogenic human γ-herpesvirus which infects endothelial cells and establishes a latent infection in B lymphocytes. It is associated with the endothelial cell malignancy Kaposi sarcoma (KS) and the B lymphocyte disorders primary effusion lymphoma (PEL) and multi-centric Castleman disease (MCD). The immune response is important in controlling infection and disease caused by KSHV as seen by the higher frequency of KSHV-associated disease found in immunosuppressed patients such as HIV or transplant patients. Restoration of immune competence in KS patients can lead to resolution of this malignancy [3,4], implying an important role for the cellular immune response in the control of KSHV-infection and disease.
This work was prompted by our observations that although KSHV-specific CD4+ T cells could recognize PELs expressing MHC class II  or PELs engineered to express surface class II, they were unable to control them despite killing other cell types . To make PELs more sensitive to T cell control, we treated PELs with AZT and found that LANA- and vCyclin-specific CD4+ T cells inhibited or completely controlled the outgrowth of PELs in co-culture assays. The inhibition observed in these assays was equivalent or more efficient than that seen in similar assays using CD4+ T cells specific for latent epitopes from the related human γ-herpesvirus EBV to control outgrowth of EBV-transformed B cells .