Insights into Sensing of Murine Retroviruses

Abstract

Retroviruses are major causes of disease in animals and human. Better understanding of the initial host immune response to these viruses could provide insight into how to limit infection. Mouse retroviruses that are endemic in their hosts provide an important genetic tool to dissect the different arms of the innate immune system that recognize retroviruses as foreign. Here, we review what is known about the major branches of the innate immune system that respond to mouse retrovirus infection, Toll-like receptors and nucleic acid sensors, and discuss the importance of these responses in activating adaptive immunity and controlling infection.

Keywords: ALR; PAMP; PRR; TLR; mouse mammary tumor virus; murine leukemia virus; nucleic acid sensor.

Figure 1

Retroviral reverse transcription. (I) Cellular tRNA binds to the primer binding site (PBS) in the Unique 5′ Region (U5) region of the RNA, initiating minus-strand DNA synthesis. (II) Synthesis proceeds to the 5′ end of the RNA, generating minus-strand strong-stop DNA (-sssDNA). (III,IV) RNaseH degrades the RNA template, enabling -sssDNA transfer to the 3′ R region (identical to the 5′ R region) where DNA synthesis and RNaseH digestion proceed. (V) RNaseH is unable to digest the polypurine tract (PPT), leaving a segment of RNA which primes synthesis of plus-strand DNA. Synthesis proceeds until reaching the tRNA primer, creating plus-strand strong-stop DNA (+sssDNA). (VI) A second strand transfer occurs, where the +sssDNA PBS anneals to the 3’ minus-strand PBS. (VII) The two strands act as templates for each other to complete dsDNA synthesis, generating identical long terminal repeats (LTR) at both ends of the dsDNA, each containing U3-R-U5 sequences. Shown to the right are the PRRs that recognize the nucleic acids generated at the different reverse transcription steps. Red, RNA; blue, minus-strand DNA; purple, plus-strand DNA.

Figure 2

Retroviral replication and innate immune sensing. Murine retroviruses bind cell surface entry receptors, fuse with the cell membrane, either at the plasma membrane (MLV) or in endosomes (MMTV), and release their viral capsids into the cell cytoplasm, where reverse transcription takes place. Cytosolic sensors such as DDX41, IFI203, and cGAS detect viral reverse transcripts and upregulate transcription of type I IFNs and cytokines through STING-dependent signaling, which results in the phosphorylation of TBK1 and the transcription factors IRF3 and IRF7, or in NF-κB activation. TREX1 degrades reverse transcripts and endogenous retroelement DNA and hinders immune responses to these ligands. IFI205 stimulates an immune response to endogenous retroelement DNA; however, AIM2 blocks this STING-dependent immune response by sequestering IFI205 from STING. APOBEC3 blocks murine retroviral reverse transcription in the cytosol and diminishes innate immune responses. However, some fraction of newly synthesized viral DNA enters the nucleus via the viral PIC, and viral DNA integrates into the host cell DNA. The integrated proviral DNA is transcribed to generate viral mRNAs and new copies of the viral genome for packaging. Sensors such as IFI204, which can localize to the nucleus, are capable of inhibiting retroviral gene transcription. The cytosolic RNA sensor, ZAP, recruits viral mRNAs and exosome components to RNA-containing stress granules, resulting in degradation of retroviral mRNAs. Viral mRNAs that avoid restriction enter the cytoplasm to begin translation of viral proteins. TLRs also detect viral ssRNA (TLR7), dsRNA (TLR3), and dsDNA (TLR9) in endosomes and initiate cytokine responses via the adaptors MyD88 and TRIF. Packaging of new virions occurs at the cell membrane (MLV) or in structures in the cytoplasm which are later transported to the membrane (MMTV). MMTV incorporates LPS-binding proteins such as MD-2, TLR4, and CD14 into its membrane, which subsequently bind to bacterial LPS and facilitate virus transmission to pups via milk.