The release of porcine endogenous retrovirus (PERV) particles from pig cells is a potential risk factor during xenotransplantation by way of productively infecting the human transplant recipient. Potential countermeasures against PERV replication are restriction factors, such as SAMHD1, that block retroviral replication. SAMHD1 is a triphosphohydrolase that depletes the cellular pool of dNTPs in non-cycling cells preventing retroviral reverse transcription. Restriction factors and innate immune responses are the first line of defense against invading pathogens. Sensing of viruses in a cell usually results in the production of type I IFNs that lead to the activation of IFN-stimulated genes. The first part of this study focuses on the analysis of the antiviral activity of SAMHD1 against PERV-A/C in human immune cells. To study that, PERV-A/Cs originating from activated porcine PBMCs was used to infect human primary cells from healthy human donors. In parallel, primary cells were transduced with virus-like particles (VLPs) lacking or containing the Vpx protein from SIVmac239. The result showed that SAMHD1 potently restricts PERV-A/C reverse transcription in human monocytes, monocyte-derived dendritic cells (MDDC), macrophages (MDM) or resting-CD4+ T-cells and in monocytic THP-1 cells. Degradation of SAMHD1 by SIVmac Vpx or CRISPR/Cas9 knock-out of SAMHD1 allowed for PERV reverse transcription. Addition of deoxynucleosides alleviated the SAMHD1-mediated restriction suggesting that SAMHD1-mediated degradation of dNTPs restricts PERV replication in these human immune cells. The aims in the second part of this study were to investigate the IFN response by monitoring induction of IFN stimulated genes following infection by PERV-A/Cs, and to identify the signaling pathways, which play a role in the type I IFNs response. The results showed that PERV-A/C increased (over 100-fold) CXCL-10 production in human MDDCs, monocytes and MDMs compared with non-infected cells or heat-inactivated virus. Treatment with VLP+Vpx or empty VLP did not cause more CXCL-10 induction compared with untreatment. The JAK-inhibitor (AT9283) reduced the CXCL-10 induction in infected cells indicating that the JAK/STAT pathway is activated by PERV-A/C infection of primary human myeloid cells. In conclusion, the findings in this work highlight SAMHD1 as a potential barrier to PERV-A/C transmission from pig transplants to human recipients during xenotransplantation.
