Viruses, small infectious agents that replicate only inside the living cells of other organisms, have been around for billions of years. They can infect all types of life forms, from microorganisms, plants and animals to humans. It has been estimated that there are 10 to the power of 31 viruses on Earth, but humans don’t just live in a viral world, we are also part virus ourselves. Viruses actually comprise up to the 8% of the human genome and these so called endogenous retroviruses (ERVs) represent ancient viral infections that became integrated into the human genome. Scientists had previously known the importance of a specific ERV in the development of the placenta, but now they have discovered a subset of ERVs that help regulate our immune system.
When a cell gets infected it releases chemical signals called interferons (IFN). These signalling proteins stimulate the expression of certain genes that are important for mounting an immune response against the invading pathogen. A number of ERVs from the MER41 family were found adjacent to several of these immune response genes, increasing their expression. When these ERVs were deleted using a gene-editing system (CRISPR-Cas9), the expression of these IFN-stimulated genes was decreased. Furthermore, in the case of one ERV (MER41.AIM2) the mutant cell’s ability to mount an inflammatory response following viral infection was markedly reduced.
Taking into account both, the new function of ERVs in immune regulation and the fact that ERVs make up 8% of the human genome, the next question is what are all these ERVs doing? For example, in the case of the 18 intact ERV envelope open reading frames in the genome, “only three are competent for fusion” according to Joe Timpona, a graduate student in the Virology Program at Harvard who studies endogenous viral envelope proteins. As for the other 15? “They could be involved in signaling, but they could also be involved in receptor blocking by occupying receptors used by exogenous retroviruses.”
These are only speculations and it is obvious that a lot of research still needs to be done in the field of ERVs. However, the discovery that ERVs can help muster our immune response to extant viruses encourages continued research into finding other examples of ancient rogue viruses gone good.
Katarina Kovac, PhD, BioSistemika LLC
The original article has been published by SITN