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.