Ellie Teng ‘21

Retroviruses such as HIV are viruses that take genes from host cells and incorporate them into their own genomes. Transposons are DNA elements that can change positions in the genome, increasing the potential for mutations and genome instability. Piwi-interacting RNAs (piRNAs) guide the immune system towards silencing the transposons during germline development. Koalas infected by the retrovirus KoRV-A virus are immunodeficient and prone to infections and cancer. This virus also infects the germline cells and affects the future generations of these cells. A team from the University of Massachusetts Medical School and the University of Queensland studied wild koala populations in Australia to see how germline cells responded to a virus. 

The host immune system recognizes specific features and components of the invading pathogen to develop immunity. For example, a feature of the proliferation cycle of gammaretroviruses is bypassed by splicing. This is recognized by the host immune system which signals the production of piRNAs. Splicing is a process that removes spacer sequences or introns to produce mRNAs that code for proteins. In retroviruses such as KoRV-A, splicing results in a protein that makes up the viral envelope. Retroviruses also produce “unspliced” mRNA; these are virus specific and are recognized by the host immune system. These are cut up into smaller piRNA sections that block the virus. 

By studying KoRV-A in koalas, scientists were able to determine that the host genome could turn a component of the retrovirus against the virus itself by turning viral mRNA into anti-viral piRNAs. KoRV-A in koalas have been thereby suggested as a model for immunity in animals; retroviruses that enter through the germline cells proliferate and insert themselves into host genomes. In response, the host immune system is altered and changes to counteract the infection. Further experiments will be conducted to determine the mechanisms involved in developing genome immunity. 

References

1. T. Yu, et al., The piRNA response to retroviral invasion of the koala genome. Cell Press 179, 632-643 (2019). Doi: 10.1016/j.cell.2019.09.002
2. Image retrieved from: https://pixnio.com/fauna-animals/koala-wood-rope-animals-leaf