Bellevue Faculty and Undergraduate Student Publish Research on Virus Immunity

Dr. Tyler Moore and Bellevue University alum Jennifer Mather recently published a paper on their research into how viruses can avoid being eliminated by the immune system in order to persist in their hosts. The paper is published in mBio, an open access journal within the American Society of Microbiology, and can be viewed here: https://mbio.asm.org/content/10/1/e02578-18. The work was accomplished in collaboration with Dr. Kim Hasenkrug of the National Institutes of Health (NIH) and Dr. Ulf Dittmer of the Institute for Virology at the University of Duisberg-Essen in Germany. Jennifer, a Bellevue University Biology undergraduate at the time, spent the summer learning new techniques and conducting experiments alongside her faculty mentor, Dr. Moore, in the NIH Laboratory for Persistent Viral Diseases at the Rocky Mountain Laboratories in Hamilton, Montana. If you are interested in learning more about undergraduate research opportunities at Bellevue University, check out the Student Research page. If you are not yet a student, go to https://www.bellevue.edu/ and apply!

Here is description of the paper, including why we thought this was an interesting problem, how we approached solving the problem, and what we learned from our experiments that could be used to solve additional problems.

T cells kill cells that are infected by viruses

apc-tcell

We have known for a while that special immune cells called T cells have the ability to kill cells infected with viruses. Because viruses need healthy cells to live in and reproduce, killing these cells prevents the virus from spreading. In order to make sure our T cells only kill infected cells, special cells called Antigen Presenting Cells (or APCs) train the T cells what to kill. It is kind of like showing a picture to a search party.

What happens when APCs are infected by viruses?

apc-tcell-viruspersistence

We know that some viruses can actually infect APCs. In some cases, this makes the APCs less able to instruct T cells. Without proper instructions, T cells do not become fully activated and can’t kill infected cells. We thought this might be why some potentially acute viruses become persistent. We study a particular mouse virus known as Friend virus (named after Charlotte Friend, who discovered it). We can use this virus as a model to learn more about virus infections and the immune system in general. This particular virus infects many types of cells, including APCs. However, the immune system doesn’t fully eliminate the virus, causing a chronic infection. We were particularly interested in the fact that a particular type of APC, known as “B cells”–these are the cells that make antibodies, are one of the main types cells infected during the chronic infection.

Are APCs more or less activated when they are infected with Friend virus?

figure1

Friend virus infects many types of cells, but one of them is a type of APC known as B cells. We were curious if there was a difference in activation between B cells infected with Friend virus compared to B cells from an infected mouse that didn’t actually have the virus growing inside of them. In order to test this, we used a technique known as FACS. This allows us to identify which cells are infected with a virus and analyze them separately from cells that are not infected.

We infected a mouse with Friend virus. Then, we analyzed all the B cells by FACS. We compared the B cells that had virus growing inside of them to B cells that came from an infected mouse but were not infected with a virus. We were surprised to find that the B cells infected with the virus were actually much more activated than uninfected B cells. This suggests that Friend virus infection of B cells didn’t impair their function as APCs.

Are Friend virus-infected B cells better at activating antiviral T cells?

figure2

We know that B cells can be APCs. We also know T cells need APCs to become activated and proliferate. Our new data show that infected B cells are more activated than uninfected B cells. Because of these data, we wondered if infected B cells would be better at activating T cells. To test this question, we infected mice with Friend virus and purified out infected B cells from uninfected B cells using FACS. We then could use these cells as APCs to activate T cells. To make sure the T cells were given the same chance to be activated in both conditions, we used T cells that only recognize a special protein from eggs known as ovalbumin (“OVA”). We gave both groups of APCs OVA and tested how well each could activate the T cells. Even though both APCs had OVA (think of it as the search party picture to show the T cells), the infected B cells were much better at activating the T cells. This means infected B cells more activated (like we showed in Fig. 1), but they are also better at activating antiviral T cells.

Regulatory T cells inhibit antiviral T cell responses

tregsuppression

One thing that is interesting about Friend Virus is that it induces a type of cell known as a Regulatory T cell (or Treg). Tregs are important because they make sure our immune system doesn’t cause too much damage to our tissues. However, when Tregs are induced during Friend virus infection, they prevent T cells from being fully activated. This is a major factor contributing to the persistence of Friend virus.

We can specifically deplete Regulatory T cells using special mice

tregdepletion

We were interested in how Tregs contribute to the activation and function of APCs during Friend virus infection. In order to test this, we had to use a mouse system where we can specifically get rid of the Tregs in a mouse. We are lucky that Tregs are the only cells that turn on a particular protein known as FOXP3. A group of scientists already developed mice that have the receptor for diphtheria toxin on every cell that turns on FOXP3. Mice don’t normally have the receptor for diphtheria toxin. With these mice, we can inject diphtheria toxin to kill Tregs without harming the other cells or the mouse.

Does depleting Regulatory T cells make the APCs better at activating T cells?

figure6

In order to see if Tregs change the function of APCs, we specifically depleted Tregs in mice infected with Friend virus. Then, we purified out virus-infected APCs from mice with Tregs (left) or mice with Tregs depleted (right). We found more activation in the T cells that were cultured with APCs from Treg-depleted mice. This suggests that suppressing APC function is one way Tregs are preventing T cell activation during Friend virus.

Final Conclusions

  • When Friend virus infects B cell APCs they become more activated and more able to activate antiviral T cells
  • Regulatory T cells (Tregs) prevent APCs from becoming activated and reduce their ability to activate T cells
  • From an evolutionary standpoint, sometimes getting rid of a virus isn’t as important as protecting our tissues
  • If we want better immune-based clinical therapies, we need to have a better understanding of how the immune system balances getting rid of things that could cause disease while also not damaging our own bodies

Remaining Questions

  • Some viruses infect APCs and make them less able to activate T cells–what is happening in Friend virus that is causing the immune system to respond differently?
  • If Friend-virus infected B cells are better at activating T cells, and T cells kill virus-infected cells, why are B cells the main cells infected during chronic Friend virus infection?

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