Producing the right proteins at the right time

Fiamma Salerno will defend her PhD thesis on February 2^nd, 2018 at the University of Amsterdam with her thesis: “Walking the wire: Post-transcriptional regulation of T cell effector functions in health and disease”.  She studied how T cells can be active against pathogens and tumors, and what makes them able to produce the right proteins at the right place and time. In the future this could help to improve T cell therapy.

What makes T cells so fascinating? 

"T cells provide protection against recurring infections. They circulate through our body in a constant quest for pathogenic targets. This includes cells that are infected with viruses or bacteria, or cells that are becoming tumor cells. T cells can also provide local protection from infections by scanning the organs in which a specific infection occurred before, such as the skin or the lung. While patrolling our body, T cells are silent, however they maintain a ready-to-respond state. This enables them to clear infected cells early during infection. T cells therefore prevent the spreading of pathogens, and we do not get sick and perhaps even do not realize that we were “under attack”."

What did you study in your PhD? 

"I studied how and when T cells produce cytokines. T cells make these effector molecules to help protecting us from infections or to kill tumor cells. But the cytokines are also highly toxic and can cause harm. Even though we know a lot on what these cytokines do, much less is known on how and when T cells make them.

Let me take you one step back to basic biology. Our genes, including those for cytokines, are encoded in our genome (the DNA). We only have two gene copies per cell, making it difficult to produce high amounts of cytokines when a T cell encounters a pathogen. So nature found a trick to amplify the templates for protein production, by making sometimes thousands of copies of the DNA in the form of messenger RNA (mRNA). The mRNA is then used as a blue print for the production of proteins.

Both of these steps - making mRNA from DNA and making protein from mRNA - are tightly controlled. Whereas a lot is known about the mechanisms that control the passage from DNA into mRNA, when and how much protein is made from mRNA is not so well understood. That this step is very important becomes more and more evident. In fact, we and others observed that the number of mRNA molecules that a cell contains does not always directly correlate with the amount of protein that is generated from it.

A T cell can also express cytokine mRNA, but fail to make the protein from it. Thus, the so-called “post-transcriptional regulation” - the regulation of the process that converts mRNA into protein - determines the actual protein output of a T cell. I studied how these post-transcriptional events determine how much and for how long T cells produce cytokines."

What do you consider the highlight of your thesis? 

"We found that T cells patrolling our body for infections are pre-armed with mRNA for cytokine. However, this mRNA is most of the time silenced. And this is important since cytokine production in the absence of the infection is harmful. We discovered how T cells maintain this pre-formed mRNA in stall. We found that a regulatory protein binds to the mRNA and blocks the use of mRNA to make cytokines.

Interestingly, this block of protein production only occurs in the absence of infection. As soon as a T cell meets an infected cell, the regulatory protein releases the pre-formed mRNA and T cells can now use this mRNA as a template to quickly produce large quantities of cytokines, without losing precious time to first amplify the DNA."

Why is it so important to have such a tight regulation on T cells?

"Pro-inflammatory cytokines are critical to clear infections and tumor cells. However, I also mentioned that they are very toxic. When cytokines are produced at the wrong time or place, this can induce severe tissue damage and/or lead to the development of auto-immune disease. But also the opposite can occur: that T cells produce too little of these cytokines.

During chronic infections with HIV or Hepatitis C for instance, T cells lose their capacity to produce the proteins that kill infected cells. A similar loss of function of T cells can be observed in tumors. Thus, understanding post-transcriptional mechanisms that fine-tune cytokine production in T cells is essential to in the future rectify aberrant T cell responses."

Is post-transcriptional regulation a common feature for blood cells?

"Yes, it is. You will find this in pretty much every cell type. As a matter of fact, post-transcriptional regulation controls brain cells, gut cells, and evidence for it is found in each cell of our body. However, we are only at the beginning of fully understanding their cell-type specific roles. So there is a lot of work to do for us!"

Do your findings have implications for therapies?

"Although we are still far away from the clinic, our findings have a clear therapeutic potential. For example, T cells that infiltrate tumors are often shut off and do not produce the levels of cytokines that they normally would to kill the tumor cells. We observed that the T cells in the tumor still contain cytokine mRNA. Just by manipulating sequences in the cytokine mRNA of T cells to remove post-transcriptional regulation, we improved the cytokine production, and thus the efficacy of T cells to block the tumor growth. Making use of similar strategies in human cells could also help to improve T cell therapy."

Ms Salerno performed her research in the T cell differentiation group of Dr. Monika Wolkers at the department of Hematopoiesis at Sanquin Research.