Viruses need to infect a cell in order to replicate and survive since, by themselves, they do not have the necessary tools. During the infection process, the immune system detects and eliminates it, although some viruses are able to hide for a long time .
How do we defend ourselves against viruses?
Our immune system is the product of thousands of years of evolution, and has given rise to two types of immune responses: innate and adaptive. The innate immune response has specialized cells to detect and identify the main markers of viruses, without paying attention to variants or subclasses. The important thing is that it is removed quickly .
Once we have finished with the threat, our defenses “learn” what type of virus it is in case it infects our body again. This long-term, specialized response is the adaptive immune response .
The combination of both types of response makes our immune system an almost impenetrable barrier when it is working well, especially when we are dealing with pathogens that have been circulating among humans for thousands of years.
Viruses also have defense strategies
However, just as evolution has given rise to increasingly effective defenses, viruses have also undergone a parallel evolution . In this way, those viruses that presented strategies to escape from our immune system have endured due to their survival success.
This is how we find today some types of viruses that are able to hide inside our cells without being detected by our strong defense. Among them, we find the HIV or herpes virus , as classic examples.
There are several camouflage mechanisms that depend on its composition and mechanism of infection. In today’s article we are going to focus on a single category of viruses : those that carry their genetic information in the form of DNA .
Herpes virus: a big family
There is a family of viruses that contain the instructions in the form of DNA and are called Herpesviridae . All viruses of this type infect animal cells and have a very similar structure and mechanism of infection.
Within this family, there are three genera or subfamilies where we find some examples of the viruses that cause many infections in humans :
- herpes simplex virus
- Varicella-zoster virus
- Epstein–Barr virus
- Cytomegalovirus
When a virus infects a cell , it introduces its instruction book to make all the necessary molecules that make up a new copy of itself, using the machinery of our cells. These instructions are in the form of DNA, which codes for the components of the virus.
Although the origin of viral DNA is external, once inside, our cell begins to make molecules no matter where the instructions come from. Once a new copy of the virus has been generated, the genetic information is duplicated and it leaves the cell, ready to infect.
Due to the format of its genetic information, viral DNA can remain inside our cells without triggering an immune response. Our body’s defenses are designed to recognize the molecules (proteins) of the virus, not its instruction books .
It is this characteristic that presents a window of opportunity for the virus to hide by integrating its DNA into the DNA contained in our cells. This piece of genetic code is much smaller than our own content, so it is not so strange that it can go unnoticed.
Hidden without making a sound
Within a cell, there are large portions of DNA that are naturally silenced . The reasons are several. For example, a neuron does not need to make molecules that break down food. Just as a liver cell does not create all the specialized proteins for muscle contraction.
For this reason, many genes remain silent, but remain in the nucleus. However, if the cell splits in two, it will have to make a copy of all of its DNA , even the silenced parts.
When a virus introduces its DNA into our nucleus, it can be silenced in the same way as many other genes, and go through several generations of cell divisions, without a copy of the virus appearing.
Why does herpes wake up?
The main problem with this type of virus is the moment in which this DNA returns to produce viral molecules and copies of the virus. The exact mechanism by which a virus can reawaken after being “sleeping” is unknown . However, some patients have recurring factors.
One of the best studied cases is the cold sore virus , which causes an infection in the form of blisters or rashes on the skin, where outbreaks can be associated with sun exposure or stress .
Another factor is the aging itself and the decline of the immune system associated with age, as occurs with varicella-zoster . This virus can remain hidden in the body for decades, and reawaken after the age of 50 or 60 . In these cases, you can opt for a preventive vaccine.
The study of these camouflage mechanisms can help us understand how to “deactivate” an infection or design new vaccination strategies for other types of viruses, even those that do not have their genetic information in the form of DNA.
References:
Mody et al. 2020. Herpes Simplex Virus: A Versatile Tool for Insights Into Evolution, Gene Delivery, and Tumor Immunotherapy. Virology (Auckl). doi: 10.1177/1178122X20913274
