Human immune cells are remarkably adaptable. These foot soldiers of the immune system can be harvested from the body, retrained to recognize and fight different diseases, and then redeployed in the body. This process, known as adoptive cell transfer, has incredible potential for treating cancer or chronic infections–particularly in individuals whose immune systems have been weakened by diseases like AIDS. There is, however, a big problem with this technique. Immune cells that have been harvested and engineered to fight specific diseases can often go haywire and turn their weapons on host cells. As a result, healthy cells and tissues in the body that aren’t supposed to be attacked by immune cells get caught up in the crossfire and end up as casualties. As a solution, biologists want to keep these immune cells in line by reprogramming their behavior using the very tools that bacteria employ to slip past our immune system.
All immune cells have a network of proteins that alerts the cell to the presence of pathogenic intruders. For all of you LoTR fans, this network works a lot like the Warning Beacons of Gondor:
On the surface of immune cells are receptor proteins that act like guardians at an outpost. At the first sign of the enemy—usually, bacterial proteins that bind to the receptors—these guardians sound the alarm by “lighting” a beacon. This signals to the other beacons to be lit as well, thereby alerting the immune cell of impending danger.
Inside of immune cells, however, this “relay of beacons” is actually made up of proteins that chemically modify other proteins in a cascade. Each time a protein modifies another protein, “word” of the intruder is spread throughout the cell until it finally triggers an immune response. As a result, immune cells release a volley of helpful molecules—some recruit reinforcements, some mark the bacteria as the enemy, while others make immune cells more effective soldiers. Finally, when the alarm reaches the nucleus—the cell’s central command—orders are sent out for the cell to rapidly divide, making even more immune cells to help in the battle.
Many disease-causing bacteria, however, have evolved a vast array of smoke bombs, stealth cloaks, and other weapons to sabotage our defenses. For instance, when the diarrhea-inducing bacteria Shigella flexneri infects the body it plants a bomb to disrupt the warning beacons. S. flexneri produces a protein called OspF that interacts with the protein MAPK (mitogen activated protein kinase) and prevents MAPK from modifying other proteins. With MAPK unable to “light its beacon,” the alert system is short-circuited leaving the immune cell in the dark and unaware of an attack. While conventional wisdom is to make drugs that target OspF, some researchers see an opportunity to exploit the use of OspF. Since OspF can sabotage the immune response, scientists now want to use this “bomb” as a way to prevent immune cells from attacking other cells in the body during adoptive cell transfer.
In a Letter to Nature, Wendell Lim and colleagues described two ways in which OspF could be used to protect the body’s cells from unwarranted attacks by immune cells. The first way involved reducing the immune cell’s firepower. The scientists genetically-engineered T-cells (a type of immune cell) harvested from the body to produce the bacterial protein OspF. This was accomplished by introducing into the cell DNA that contained not only the OspF gene but also regulatory DNA, which carries instructions for when and where the gene should be expressed. The specific regulatory DNA they used instructs the cell to produce OspF only when the immune response is activated. The scientists then tricked the engineered T-cells into thinking they were under bacterial attack by exposing them to antibodies that bind to the receptor proteins (the outpost guardians) and trigger them to light their beacons. Lim and colleagues observed that the expression of genes turned on by the immune response was reduced in the OspF-producing immune cells compared to normal immune cells. This suggested that OspF weakened the magnitude of the immune response by virtue of its ability to inhibit MAPK. By reducing the firepower of immune cells scientists hope to limit the number of casualties due to “friendly fire.”
Lim and colleagues were also able to institute a “cease fire” in immune cells. The team of scientists genetically-reprogrammed T-cells to produce OspF, but only in the presence of the drug doxycycline. This time, the scientists used regulatory DNA that requires doxycycline to turn on gene expression. These immune cells were also tricked into thinking they were under attack, but doxycycline treatment halted the immune response—the immune cells stopped dividing and stopped releasing immunity-boosting molecules. This means that at the first sign of reprogrammed immune cells attacking other cells in the body, scientists can use doxycycline to order the immune cells to “cease fire.”
Finally, the researchers were able to manipulate OspF to interact with and sabotage different proteins by tinkering with the OspF gene and expressing it in yeast cells. Proteins interact with one another through domains that function like clothing fasteners. Proteins also have different interaction domains just as clothing have different fasteners like Velcro, zippers, and buttons. By swapping the “fastener” domains in OspF, the researchers were able to “stick, zip, or button” OspF to different yeast proteins. Although this experiment was done in yeast, the yeast proteins the researchers targeted for sabotage can also be found in the “warning beacon relay” of immune cells. This opens the door for scientists to specifically target OspF to different proteins in immune cells. Since the alert system triggers different responses in immune cells, this strategy can be used to pinpoint specific beacons in the relay and shut down either the entire immune response or only specific branches.
The work done by Lim and colleagues demonstrate that OspF can be used to reduce the firepower of immune cells and order a ceasefire. These results highlight the potential for using bacterial proteins to reprogram the behavior of immune cells in adoptive cell transfer therapy. Changing the behavior of immune cells, however, is one thing. Effectively mounting a counterattack against pathogens without harming the body remains another. Since these experiments were performed in immune cells outside of the body, the next step in this research is to deploy these reprogrammed foot soldiers back into the body. Then we’ll see how effective they are against actual bacterial invaders while limiting innocent bystander casualties due to friendly fire.
Wei P, Wong WW, Park JS, Corcoran EE, Peisajovich SG, Onuffer JJ, Weiss A, Lim WA. Bacterial virulence proteins as tools to rewire kinase pathways in yeast and immune cells. Nature. 2012 Aug 16;488(7411):384-8. doi: 10.1038/nature11259. PubMed PMID: 22820255; PubMed Central PMCID: PMC3422413.
Featured image credit: Lord of the Rings – Return of the King - Blue Ray Edition.