Scientists at St. Jude Children’s Research Hospital have solved an immunological puzzle. A CD8 + T cell may have two functionally different daughter cells after division, even though the cells are genetically identical. The researchers explained how by revealing one method that the immune system uses to provide immediate and long-term protection. The study is today in Molecular cell.
The researchers showed how a specific protein complex controls the translation of an important immune transcription factor in one area of the mother T cell. If a cell divides because the transcription factor is only in one region, it is then asymmetrically inherited into two daughter cells. The transcription factor controls the expression of a set of genes in one daughter cell, causing it to become an effector cell and the other a memory cell.
“Our results suggest that events that occur very early in T cell life may affect cell function much later,” said Corresponding Author Doug Green, Ph.D., Department of Immunology, St. Jude. “We have found one way in which the immune system ensures that when T cells are activated, the response will be varied: some cells, the effects, begin a rapid attack on the invader, while others remain in reserve for later as memory. cells ”.
Two very different daughters with the same DNA
The immune system has many different cell types with different functions. One of the main cell types are CD8 + T cells. These cells are responsible for the direct destruction of infected and tumor cells. They are activated by a special cell that contains on its surface a little virus or tumor cells called antigens. The point of contact between T cells and antigen-presenting cells is called the immune synapse. Upon activation, T cells divide into genetically identical daughter cells.
Many of the daughter cells become effector cells that also destroy infected or cancer cells. However, some of the daughter cells become memory cells to help protect against future infections or the same cancer. Prior to this study, it was unclear how effector cells and memory cells could originate from the same parent T cells.
Unstable protein lost without translation
As a first clue, Green’s group previously showed that the first two daughter cells from an activated parent T cell have different levels of c-Myc protein. This is important because c-Myc is a transcription factor known to stimulate the expression of genes that cause T cells to become effector cells. However, c-Myc is unstable, half of all c-Myc in the cell disappears within 20 minutes. So how then is c-Myc present long enough and in the right position to be predominantly divided into a single daughter cell?
Typically, the response involves mRNA. mRNA serves as a template that cells use to make protein. When an unstable protein is concentrated in one part of a cell, it is because its mRNA pattern is confined to that location. However, c-myc The mRNA transcripts appeared to be uniformly distributed throughout the cell.
Instead, the researchers found that the protein complex that makes up c-Myc was present only next to the immune synapse. The specific complex responsible for c-Myc translation is called the 4F (eIF4F) translation initiation factor complex of eukaryotes. The eIF4F complex is a translation mechanism that receives mRNA messages and converts them into proteins, in this case c-Myc.
The c-myc mRNA has a complex structure at one end. Only the eIF4F complex can use a complex structure c-myc mRNA to begin the process of translation into protein. Thus, c-Myc is formed only where eIF4F is present, which rejects c-Myc to one side of the cell.
This is the first time that the location of translation mechanisms has been described as the reason why protein is present in only one part of the cell.
Search for a molecular platform
While the location of eIF4F explains why c-Myc is only in one part of the cell, it has revealed a new mystery: how did eIF4F focus on one end of the cell?
The scientists used a special microscopy technique called expansion microscopy to “blow up” a T cell to learn how eIF4F moves through the cell. This is roughly equivalent to an elephant-sized mouse blast. This study is the first time the technique has been used with a primary T cell. Green’s group saw eIF4F move through the cell to one end via an immune synapse, including both the mechanisms of trade and the final placement of eIF4F on the molecular “platform” associated with the synapse.
Genetic “bar coding” confirms the fate of sister cells
Green’s group confirmed that pairs of “sister” cells – daughter cells from the same parent – began expressing genes from two different lines, effector or memory. The group adopted a genetic system of “barcodes” to track which individual cells were directly related. It was an extraordinary technical feat, as outside of the bar code sequence rearrangement there were many cells that were genetically identical. But the group was able to sequence the mRNA transcripts of these cells. Transcripts showed genetically identical sister cells with matching barcodes, expressing different genes known to be associated with their respective T cell subtype.
“This study is the first time we can say with confidence that two sister cells may have very different patterns of gene expression,” Green said. “Research also shows that there are basic principles of cellular architecture that create platforms on which intracellular events can be localized. Asymmetry in the distribution of these platforms can lead to diversification of cell fate. Details may not match for other cell types, but the principles are rather above all, will be preserved. “
The study was partially supported by the National Cancer Institute (P30 CA021765), the National Institute of Allergy and Infectious Diseases (RO1AI123322, R01AI154470 and R01AI136514), the German Research Foundation (LI 2967 / 1-1) and Fundraising and ALSAC. Of St. Jude.