Internalised viruses are recognised by pathogen recognition
receptors (PRRs) which sense Pathogen-associated Molecular Patterns (PAMP).
Generally, PRRs are located within the endosomes -as it the case for Toll-like
receptor (TLR)-7 or MyD88 - or alternatively in the cytoplasm, as it the case
for retinoic acid inducible gene I (RIG-I) like receptors, and melanoma
differentiation associated gene 5 (MDA5). In the case of RNA viruses, viral RNA
is recognised by proteins belonging to the cytoplasmic RNA-induced silencing
complex (RISC), which consists of RIG-1, MDA-5, protein kinase RNA activator
(PACT), transactivation response RNA binding protein (TRBP), and Dicer. RIG-1,
PACT, and MDA-5 recognize viral RNA and induce the expression of various
cytokines, in particular Interferon(s) (IFN) and ultimately a broad array of
IFN stimulated genes (ISG) as part of the antiviral response. In contrast, TRBP
and Dicer process pre-microRNAs into mature microRNAs (miRNAs) by recruiting
Argonaute 2 (Ago2) that target specific mRNA species -such as viral RNA- for
regulation and targeting steroid-responsive promoters.
PACT, allowing the
degradation of viral RNA as well as stimulating an immune response, can induce
both TBRP and Dicer and thus targets viral microRNAs, viral small RNAs, and viral dsRNA.
ssRNA or dsRNA can induce an antiviral Interferon or NF-κB response via TLR or MAVS |
In order to prevent host cell derived mRNAs from being
recognised by cellular PRRs, cellular mRNA molecules not only possess a 5’ cap
structure but are also methylated at their 5’ end; any RNA contained no 5’ cap,
a non-methylated, or incompletely methylated 5’cap can therefore be recognised
by cellular PAMP receptors and induce an antiviral response, in addition to
viral dsRNA intermediates. In this system, RIG-1 preferentially recognizes
short blunt end or 5’ triphosphate RNAs whereas MDA-5 recognizes dsRNA
intermediates as well as non-methylated RNAs lacking 2’O methylated ribose.
Antiviral signaling by these pathways can be inhibited by viruses using either
the cellular machinery for capping viral RNAs, and thus prevent them from being
recognised by the RISC, or alternatively encoding enzymes which cap the viral
RNA in a manner similar to cellular enzymes.
In addition to stimulating the expression of antiviral
genes, the recognition of single-stranded viral RNA by TLR-7 also induces the
formation of autophagy vesicles via the interaction between the TLR adaptor
MyD88 and the Beclin-1, the latter being required for the formation of the
Phagosome/Autophagosomes.
Autophagy and the antiviral response |
The formation of autophagy vesicles has been
postulated to facilitate the presentation of viral antigens by the cellular MHC
Class I (e.g. HSV-1 gB), MHC Class II complex (e.g. EBNA-1) and target viral
RNA to endosomes (and thus being recognised by TLR-7) as well as facilitating
the degradation of viral components. Indeed, infecting cells with a HSV-1
mutant deficient for the viral autophagy inhibitory ICP 34.5 protein not only
fails to inhibit autophagy but also has decreased infectious titers, suggesting
that autophagy degrades HSV-1 components in addition to facilitate the
presentation of viral antigens.
Coronavirus RTC and the antiviral response
In the case of Coronavirus infected cells, the major
pathogen associated pattern recognised by the PARRs is the dsRNA intermediate
and the viral ssRNA both which are located with the replication-transcription
complexes (RTCs) derived from the ER and induced by the expression of the viral
nsp-3,-, and -6 proteins as discussed earlier. In these RTCs, the viral RNAs
are not only synthesized but also modified in order to prevent the induction of
the antiviral response. In order to prevent the viral RNA from being recognised
by RIG-1 or MDA-5, the Coronavirus RNA contain a 5’cap structure which is added
to newly synthesized viral RNA within the RTC by the viral RNA-triphosphatase
(nsp13), 2’O-methyltransferase (nsp16), as well as a N7-Methyltransferase
(nsp14). Failure to methylate viral RNA in cells infected with a nsp16
deficient virus induces a IFN type 1, MDA-5 dependent, antiviral response
concomitant with elevated levels of ISGs. In addition to bind MDA-5, uncapped
viral RNA binds IFN induced protein with tetratricopeptide repeats (IFIT)-1,
thus preventing the translation of viral RNA. So far the precise contribution
of the viral N7-Methyltransferase remains unknown although the equivalent in
West Nile Virus has been shown to be required for 2’O-methyltransferase
mediated methylation of WNV viral RNA. The N-terminus of nsp14 encodes for the
viral 3’ -to-5’ exoribo-nuclease (ExoN) which hydrolyses both ssRNA and dsRNA
and excises nucleotide mismatches in dsRNA intermediates, thus providing
proofreading of newly synthesized RNA. Accordingly, inhibiting or deleting ExoN
increases the potency of antivirals such as 5-Fluorouracil. Since both ExoN and 2’O-methyltransferase
activity positively regulated by nsp10, inhibiting nsp10 might be an
interesting target for antiviral therapy. In addition to ExoN, both the
Arteriviridae and the Coronaviridae express another enzyme that can hydrolyze
RNA, EndoU. In contrast to ExoN the function of EndoU (nsp15) is less well
defined and it has been proposed that it might be involved in cleaving free
-“mislocalised”-RNA in order to prevent the recognition by PRRs, although EndoU
can be found both in RTCs as well as in the cytoplasm. One possibility, which
the author of these lines suggests, is that EndoU might be associated with Endosomes
that contain viral RNA (see below). These Endosomes might contain viral RNA as
a result of the induction of the formation of autophagy vesicles by nsp-3,-4,
and -6.
Although the Coronaviral nsps-3,-4, and -6 induce the
formation of autophagy like (LC3-II negative) vesicles, so far the formation of
autolysosomes and subsequent degradation of viral components in infected cells
has not been demonstrated. Since the formation of LC3-II positive vesicles has
been demonstrated in cells transfected with nsp6 derived from SARS-CoV, MHV,
and avian IBV it might be possible that viral RNA derived from autophagy
vesicles can be found in endosomes and induce TLR-7 mediated antiviral
signaling and/or viral components transferred to multivesicular class II loading
compartments. If this is the case, TLR-7/-8 mediated anitviral signaling might
be inhibited by the orf4a protein (in the case of MERS-CoV and SARS-CoV)
whereas the MHC- Class II mediated activation of cytotoxic T lymphocytes might
not be inhibited and indeed contribute in particular to the disease outcome of
SARS-CoV or MERS-CoV infected patients.
Coronavirus nsp-3,-4, and -6 might contribute to the induction of autophagy vesicle induced antiviral response counteracted by orf4a |
The infection of microglia cells with a
neurotropic strain of MHV, MHV-JHM, indeed lead to a sustained up-regulation of
both MHC Class I and Class II molecules not only during viral induced
inflammation but also following viral clearance, similar to patients which have
recovered from SARS. The formation of LC3-II positive vesicles therefore might
induce an antiviral response that might be partially blocked by viral proteins.
Some evidence suggests that the formation of LC3-II is more pronounced in cells
transfected with SARS-CoV derived nsp6 compared to IBV derived nsp6 but if this
has any implication for inducing an antiviral response has to be investigated.
Last but not least, a short note on STAT1 and 2 mediated
signaling, STAT 1 and 2 signaling can be inhibited by SARS-CoV orf6 protein
which in contrast to the above mentioned proteins is not residing within the
RTC but at the ER and Golgi membrane where it sequesters STAT1 and 2 and thus
prevents nuclear entry of these. In this case, orf6 therefore prevents the
activation of IFN-β stimulated genes.
The emerging MERS-CoV has been shown to infect a wide
variety of cells of the immune response, including dendritic cells, macrophages
and T-Lymphocytes. It remains to be seen if the antiviral genes encoded by
MERS-CoV are differ in their ability to block antiviral signaling compared to
those found in SARS-CoV or HCoV-NL63. It also remains to be seen if the
antiviral signaling in dromedary camels or in bats infected with MERS-CoV is
less vulnerable to these proteins or not - not an easy task in dromedaries- and
if the differences in the antiviral response account for the relatively benign
outcome in these animals. In humans, it would be interesting to see if TLR-8
(restricted to myeloid dendrite cells, monocytes, and monocyte derived
dendritic cells) is differently affected by MERS-CoV derived proteins than
TLR-7 (plasmacytoid dendrite cells). One might argue that these are marginal
questions, but I would argue that it might shed some light on the pathogenesis
of MERS.
The importance of antagonizing the antiviral response does not end at preventing the viral RNA from being recognised. A recent study indicated that the HCoV-OC43 Nucleocapsid protein binds microRNA9 and prevents a NF-κΒ dependent antiviral response and thus TLR-8 antiviral signaling; . In addition, the MERS-CoV derived orf4b and SARS-CoV N protein also block NF-κΒ. Moreover, the SARS-CoV nsp1 papain-like protease inhibits IRF3 induced expression of IFN-β. It remains of course to be seen if the mechanisms described for SARS-CoV or HCoV-OC43 also apply to MERS-CoV and more importantly if these mechanisms differ among various host species.
Coronavirus derived proteins block the antiviral signaling at various stages |
As the reader of these lines can see, there is much to be learned from a virus family which until 2003 has been considered to be only a marginal virus by many.
Further reading
Kindler E, & Thiel V (2014). To sense or not to sense viral RNA-essentials of coronavirus innate immune evasion. Current opinion in microbiology, 20C, 69-75 PMID: 24908561
Zinzula L, & Tramontano E (2013). Strategies of highly pathogenic RNA viruses to block dsRNA detection by RIG-I-like receptors: hide, mask, hit. Antiviral research, 100 (3), 615-35 PMID: 24129118
Redfern AD, Colley SM, Beveridge DJ, Ikeda N, Epis MR, Li X, Foulds CE, Stuart LM, Barker A, Russell VJ, Ramsay K, Kobelke SJ, Li X, Hatchell EC, Payne C, Giles KM, Messineo A, Gatignol A, Lanz RB, O'Malley BW, & Leedman PJ (2013). RNA-induced silencing complex (RISC) Proteins PACT, TRBP, and Dicer are SRA binding nuclear receptor coregulators. Proceedings of the National Academy of Sciences of the United States of America, 110 (16), 6536-41 PMID: 23550157
English, L., Chemali, M., Duron, J., Rondeau, C., Laplante, A., Gingras, D., Alexander, D., Leib, D., Norbury, C., Lippé, R., & Desjardins, M. (2009). Autophagy enhances the presentation of endogenous viral antigens on MHC class I molecules during HSV-1 infection Nature Immunology, 10 (5), 480-487 DOI: 10.1038/ni.1720
English L, Chemali M, & Desjardins M (2009). Nuclear membrane-derived autophagy, a novel process that participates in the presentation of endogenous viral antigens during HSV-1 infection. Autophagy, 5 (7), 1026-9 PMID: 19556870
Paludan C, Schmid D, Landthaler M, Vockerodt M, Kube D, Tuschl T, & Münz C (2005). Endogenous MHC class II processing of a viral nuclear antigen after autophagy. Science (New York, N.Y.), 307 (5709), 593-6 PMID: 15591165
Cavignac Y, & Esclatine A (2010). Herpesviruses and autophagy: catch me if you can! Viruses, 2 (1), 314-33 PMID: 21994613
Taylor GS, & Rickinson AB (2007). Antigens and autophagy: the path less travelled? Autophagy, 3 (1), 60-2 PMID: 17102586
Züst R, Cervantes-Barragan L, Habjan M, Maier R, Neuman BW, Ziebuhr J, Szretter KJ, Baker SC, Barchet W, Diamond MS, Siddell SG, Ludewig B, & Thiel V (2011). Ribose 2'-O-methylation provides a molecular signature for the distinction of self and non-self mRNA dependent on the RNA sensor Mda5. Nature immunology, 12 (2), 137-43 PMID: 21217758
Chen Y, Su C, Ke M, Jin X, Xu L, Zhang Z, Wu A, Sun Y, Yang Z, Tien P, Ahola T, Liang Y, Liu X, & Guo D (2011). Biochemical and structural insights into the mechanisms of SARS coronavirus RNA ribose 2'-O-methylation by nsp16/nsp10 protein complex. PLoS pathogens, 7 (10) PMID: 22022266
Cao J, & Zhang X (2012). Comparative in vivo analysis of the nsp15 endoribonuclease of murine, porcine and severe acute respiratory syndrome coronaviruses. Virus research, 167 (2), 247-58 PMID: 22617024
Li T, Xie J, He Y, Fan H, Baril L, Qiu Z, Han Y, Xu W, Zhang W, You H, Zuo Y, Fang Q, Yu J, Chen Z, & Zhang L (2006). Long-term persistence of robust antibody and cytotoxic T cell responses in recovered patients infected with SARS coronavirus. PloS one, 1 PMID: 17183651
Hamo L, Stohlman SA, Otto-Duessel M, & Bergmann CC (2007). Distinct regulation of MHC molecule expression on astrocytes and microglia during viral encephalomyelitis. Glia, 55 (11), 1169-77 PMID: 17600339
Frieman M, Yount B, Heise M, Kopecky-Bromberg SA, Palese P, & Baric RS (2007). Severe acute respiratory syndrome coronavirus ORF6 antagonizes STAT1 function by sequestering nuclear import factors on the rough endoplasmic reticulum/Golgi membrane. Journal of virology, 81 (18), 9812-24 PMID: 17596301
Lai FW, Stephenson KB, Mahony J, & Lichty BD (2014). Human coronavirus OC43 nucleocapsid protein binds microRNA 9 and potentiates NF-κB activation. Journal of virology, 88 (1), 54-65 PMID: 24109243
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