Virus Basics
Viruses are tiny little packages of protein and genetic material (DNA or RNA). They seek out our cells then invade, hijack, and convert them to miniature virus factories churning out thousands upon thousands of new viruses.
The viral proteins act as a shell that surrounds the genetic material, protecting it when the virus is not infecting a host. Some of the viral proteins also stick out from the surface of the virus and can attach to specific cells within the body. This is why doctors call a virus like influenza a respiratory virus; it has proteins that only allow it to grab onto and infect our respiratory cells. When a viral protein hooks to a human cell, it can dump the genetic material into that host cell and hijack it.
Okay, now back to virotherapy. There are currently three main areas of virotherapy: oncolytic virotherapy, viral gene therapy, and viral immunotherapy. While they all use viruses to treat disease, they do so by different strategies. Let's look at the key features of each.
Oncolytic Virotherapy
Oncolytic virotherapy uses viruses to target cancer cells and destroy them. We can break down the term oncolytic into onco which refers to cancer and lytic which refers to destruction.
Remember that viruses have surface proteins that attach to specific human cells. In addition to the normal human cells that are targeted by a virus, the proteins on many viruses will also preferentially attach to and invade tumor cells. Once a virus invades a tumor cell, it can still force the tumor cell to make more viruses, but when it is done making new viruses, the tumor cell dies. This is a good thing! The defining characteristic of cancer or tumor cells is the ability to continue growing and dividing out of control. The viral infection can break that cycle and limit the spread of the cancer.
Beside the 'natural' viruses that invade tumor cells, scientists are also trying to engineer or design viral proteins that can attach to specific tumor cells. This could make the process more targeted and more efficient.
Viral Gene Therapy
Viral gene therapy uses viruses to deliver beneficial or therapeutic genes into human cells, where the genes can be used to change the cell in some way. In this form of virotherapy, the virus is just a shuttle to get the cargo (the therapeutic gene) to where it is needed.
Remember we said that a virus was just a protein shell that surrounds the viral genetic material. Normally, the viral genetic material gets delivered to the host cell, where it causes problems (damage, disease, etc.). But, if we replace the viral genetic material with a beneficial gene, the protein shell will still deliver that cargo to human cells. If we have a human cell that has a defective gene that is causing the cell to misbehave, the virus can deliver a new, non-defective copy of the gene to replace the bad one.
Viral Immunotherapy
Viral immunotherapy uses viruses to deliver molecules that stimulate the immune system to attack a specific target. The target could be an infection from something like a bacteria or virus or it could be defective human cells, like cancer cells.
So let's see how this might work with something like cancer cells. Viral immunotherapy uses the same premise as the oncolytic approach (the idea that viruses can be used to target cancer cells) but it doesn't just rely on the virus to destroy the tumor cells. Instead, this approach gets the human body involved too. An additional molecular component is added to the oncolytic virus so that once it reaches its destination in the cancer cells, this extra component acts as a signal to the body's immune system. It basically says 'Hey, immune system! Over here! Attack these cells!' This gets the whole body involved in attacking and destroying the cancer cells.
Advances in Modern Medicine
Hopefully, now you can see how scientists are reinventing viruses into something beneficial. You might think this all sounds pretty sci-fi, but really the original idea go back to at least the mid-1900s. Today there are clinical trials testing all of these therapies and even recommended Food and Drug Administration approval for specific approaches. Meanwhile, scientists continue to work to develop new approaches and engineer viruses that will be even more effective than those currently being tested.
Lesson Summary
Virotherapy uses viruses as tools to help treat disease. There are three main categories of virotherapy.
- Oncolytic virotherapy uses viruses that target and destroy cancer cells.
- Viral gene therapy uses viruses as delivery vehicles to get therapeutic genes into human cells. *Viral immunotherapy uses viruses to deliver molecules that stimulate the body's immune system to attack.
