The Role of PERVs in Immune System
Multiple immune stimuli have been shown to induce ERV expression. In mice, lipopolysaccharide (LPS), lipoprotein (LP) and 5-Bromo-2’-deoxyuridine (BrdU) were able to induce ERV expression (Stoye & Moroni, 1983). ERV assessment in gut tissues from different conditions revealed that expression of certain ERVs depended on the presence of the gut microbiota. Recently, it was shown that the host immune system could use the ERVs to communicate with the exogenous microbiota that modulated tissue repair or inflammation (Lima-Junior et al., 2021). At the same time, an increased expression of PERVs was found in mitogen-stimulated porcine lymphocytes (Tacke, Specke, & Denner, 2003), indicating that immune stimulation may also activate PERV expression in vivo.
Most exogenous retroviruses have been described to suppress the host’s immune system. By analyzing the mechanism of immunosuppression by retroviruses, it was shown that the Env protein has a highly conserved so-called immunosuppressive domain (ISD) in the transmembrane unit that is responsible for the immunosuppressive activity (Denner, 2014). Exposure of peripheral blood mononuclear cells (PBMCs) to ISD-derived peptides increased the expression of different cytokines meanwhile inhibited mitogen-driven cell proliferation (Cianciolo, Copeland, Oroszlan, & Snyderman, 1985; Morozov, Dao Thi, & Denner, 2013). PERVs remain the intact Env proteins and the ISD-related domain as a molecular relic of the ancient retroviruses. It was shown that purified inactivated-PERV virus and the ISD-related peptide inhibited proliferation of human PBMCs and induced IL-10 production in PBMCs (Denner & Tönjes, 2012).
The impact of ERVs on the immune response is very complicated. On the one hand, the conserved ISD domain in the Env protein inhibits the host innate immune system. On the other hand, ERVs’ replication intermediates can stimulate the innate sensors and trigger the interferons (IFNs) response.
In the innate immune system, the pattern recognition receptors (PRRs) are key components that can directly recognize pathogen-associated molecular patterns (PAMPs) and trigger the innate immune defence line. There are two main families responsible for recognizing the nucleic acids of ERVs. One is toll-like receptor (TLR) family which monitors the contents of endolysosomal compartments. The other is cytosolic PRRs recognizing nucleic acids in the cytoplasm. The PRRs include the RIG-I-Like receptors (RLRs) and analogous DNA sensing receptors such as cGAMP synthase (cGAS), etc . (Barbalat, Ewald, Mouchess, & Barton, 2011). Once ERVs are activated by stimuli like 5-Azacytidine (AZA), which promotes ERVs expression, PRRs are triggered following signal cascades and lead to type I IFN production (Chiappinelli, Zahnow, Ahuja, & Baylin, 2016). In the same way, the activation of ERVs could stimulate anti-tumour immunity by promoting the viral recognition and interferon response pathway (Roulois et al., 2015).
However, the association between PERVs and the porcine immune system remains poorly understood. Interestingly, there was an increased expression of PERVs in the tumours compared with that in the normal pig cells. The PERV viral particles had been isolated from lymphoma cells, radiation-induced leukemia cells and PK-15 cell lines (Denner & Tönjes, 2012). It indicated PERVs might play a beneficial role in the porcine innate immune system.
Potential Risk of PERVs inxenotransplantation
PERV-A and PERV-B are found to be able to infect various kinds of human cells in vitro , therefore PERVs represent a potential risk for xenotransplantation (Güell et al., 2017; Kimsa et al., 2013). Although there has been no report of PERV transmission in clinical trials, some rare cases like antibodies against PERV structural proteins have been found (Denner & Tönjes, 2012), indicating the impact of PERVs in xenotransplantation need to be further explored.
Like other retroviruses, PERVs are able to integrate their viral cDNA into the genome of the host cell. As the result of random insertion, PERVs may destroy the normal genes and irreversibly break the balance between the expression of oncogenes and tumour suppressor genes. Somein vitro studies had shown that PERVs-infected human cells were able to produce LTR sequence-altered PERVs, and these mutated viruses not only increased reverse transcriptional activity but also enhanced the infectivity (Wilson et al., 2000). Moreover, Patience et al.found that PERVs released by the infected human cells were resistant to complement-mediated lysis (Patience et al., 1997), further demonstrating the potential risk of PERVs in xenotransplantation.
Human endogenous retroviruses (HERVs) account for 5-8% of the human genome. Same as PERVs, HERVs are remnant relics of the exogenous retroviruses in ancient germline infections. Although most HERVs are inactivated after million years of evolution, several of these proviruses have retained partial coding capacity. And many recent studies have shown that HERVs are not only related to cancer and neurodegenerative diseases (Garcia-Montojo et al., 2018; Li et al., 2015; Zhou et al., 2021), but also involved in shaping cell types and chromosome structures (Zhang et al., 2019). In addition, HERV-K related proteins participate in the immune pathway in the early stage of human embryonic development. In the human genome, there are a large number of gamma-retroviruses, such as HERV-T, HERV-E and HERV-I, that are closely related to PERVs (Jern, Sperber, & Blomberg, 2005). Once the recombinations between PERVs and HERVs occur, it might possibly result in a recombinant pathogenic strain, posing a new threat to human society.
Several studies have demonstrated that transcripts and proteins of HERVs are up-regulated in patients with retrovirus HIV-1 infections. It has been reported that the HIV-1 Tat protein is able to activate the HERV-K LTRs directly (van der Kuyl, 2012). Whether PERVs would influence HERVs’ expression like HIV still remains unknown. Using the CRISPR/Cas9 system targeting the catalytic core of RT enzyme of PERVs, we successfully produced PERVs-inactivated PK-15 cells and live pigs (Niu et al., 2017). Caused by the RT knock-out, these virions were no longer infectious (Niu et al., 2017; Yang et al., 2015), and the usage of the PERVs-inactivated donor pigs in xenotransplantion would eliminate most threats caused by the random integration of PERV proviral cDNAs into host genome. Recent studies have further checked our RT-inactivated PK-15 cells, and confirmed the PERVs relased by the cells were impaired non-infectious viral particles (Godehardt et al., 2020). However, due to the intact sequence of PERV env and gag genes, the RT-inactivated PERVs still carry normal viral proteins such as env protein which is required for viral adherence and uptake. It raises new questions that whether the intact env or gag protein would upregulate the expression of HERVs and trigger a negative influence on the recipients.