Where do we end and viruses begin?
Today's post is a short piece about tiny, tiny pieces of cells called "extracellular vesicles". It's primarily inspired by this very well written paper from the group of a collaborator that I work with. I really recommend giving it a look through if you don't mind some scientific jargon!
Anyways, let's get down to business. What are these so-called extracellular vesicles?
Extracellular vesicles (EVs) could be described as any and all of the following: cell vomit, a part of the cell's postal system, or an escape route from the cell for things like viruses. The thing is, we don't really know all of the functions of EVs yet. However, we do know more or less what they look like.
EVs are very tiny spherical structures that are released from pretty much every known type of cell - including yours! In essence, they kind of look like mini-cells. They have a membrane made of fat that encloses whatever is contained inside. And part of the excitement about EVs is that we don't yet fully understand what is inside this membrane, and why.
When cells are just doing their cell thing (growing, eating, recycling, the usual), they will release EVs. We now think this might be a way for cells to chat with each other. And, even more interesting for microbiologists like us, we now know that EVs also play a role during viral infection.
For instance, sometimes EVs sent out by infected cells can be really helpful to their little cellular neighbours. These EVs might contain little warning signals that say, "HEY!! Attention!! Virus in town, prepare your defences!" Such a message can help these surrounding cells prepare to fight off incoming viruses.
However, there is also evidence that EVs can help the virus instead of the cells. For instance, viruses can hijack the EV postal system, and get EVs to carry molecules that actually mess with the immune system. This makes the cells that these EVs are addressed to even less able to fight off any incoming viruses than usual. To make matters even worse, EVs can also be infectious. In other words, EVs might actually carry viruses, or bits of viruses, that are able to kick off a new viral infection in surrounding cells.
Now, before we go any further, I would like to point out something I find particularly intriguing. Remember how EVs are delimited by a membrane? Well, the same thing goes for many viruses. We call these 'enveloped viruses': the membrane creates a sort of envelope around the rest of the virus. This envelope is always stolen from the host cell of the virus, and often can help the virus hide from your immune system. Because the membrane that makes up the envelope looks like the membrane around your cells, the virus blends in. To put it simply, the envelope can act like Harry Potter's invisibility cloak: it allows the virus to blend in with its surroundings.
Many viruses have a membrane surrounding them, which can help them hide from the immune system
It is exactly here lies a big question in the cross-over field of EVs and virology. EVs have a membrane, can spread between cells, and be infectious. Enveloped viruses get their membrane by stealing it from the host cell, meaning that the membranes surrounding EVs and viruses can be extremely similar. Taking all of this into consideration, where then do we draw the line between EVs and enveloped viruses?
On this Microbial Mondays post, I'm not giving out any satisfying answers, because we haven't totally worked this out yet! Currently, we tend to think of there being a kind of gradient from EV to virus. Basically, tiny spherical membranes floating about outside of cells can just look more virussy or more EV-like. It can be difficult to find a cut-off where we can convincingly say that one little sphere is strictly an EV, or strictly an enveloped virus.
In conclusion, the take-home message from this week isn't a fun fact or an actionable piece of knowledge. It's a reminder that we still have many questions left in science to answer!
Until next time, quest(ion) on to look for those answers!