Designing logical cells to be disease sensors

Modern medicine often relies on taking pills for extended periods of time, if not indefinitely. This is likely caused by needing a small yet constant supply of medicine. Using synthetic biology, cells could be engineered to produce and release molecular drugs. Even better would be if these cells were able to understand when it should start and stop treatment. If you are dealing with a disease that has a biological marker, some measurable indicator of sickness like disease-specific protein levels, you could in theory design a biosensor that recognizes that marker. Psoriasis is an autoimmune disease characterized by red, itchy skin and increased skin growth. With it comes an increased risk to other immune diseases and some cancers. Psoriasis is thought to be caused by improper crosstalk between skin cells named keratinocytes and immune messenger cells named dendritic cells. This miscommunication causes increased immune cell recruitment to the skin. The immune cells then release specific cytokines to induce inflammation in an attempt to heal the body. This leads to both increased proliferation of keratinocytes, and increased cytokine receptors, both causing psoriasis symptoms. Because of the increased immune response, people suffering from psoriasis have increased levels of the cytokines interleukin-22 (IL22) and TNF in their blood. Elevated levels of these cytokines can serve as biomarkers for psoriasis. Currently, there is no cure available for psoriasis, although there are drugs, like the cytokines IL4 and IL10, which do efficiently treat the disease. However, they need to be taken daily as the disease is chronic. Instead of constantly taking medicine, scientists in Zurich thought it would be helpful to design a treatment that can be implanted into the body, and responds to the psoriasis biomarkers by producing drugs.

To attack this problem, the researchers engineered human cells (HEK cells specifically) to respond to increased levels of TNF and IL22 by producing IL4 and IL10 in a form of ‘immunomodulatory cytokine therapy'. The way it was engineered is quite ingenious. They designed a promoter in the HEK cells' genome to respond to TNF receptor signaling, which goes through a protein named NFkB. When TNF levels increase, TNF receptor signaling will increase leading to increased NFkB. The promoter responds to increased levels of NFkB in the nucleus by transcribing a gene for IL22 receptor. This IL22 receptor binds with an IL10 receptor at the cell surface, and when exposed to IL22 cytokine, will activate a signaling cascade that results in increased Stat3 protein. Stat3 then goes to the nucleus and binds to promoters engineered to respond by producing IL4 and IL10. This design means that both TNF and IL22 must be present together to induce IL4 and IL10 production. A brief outline is shown at the top of the post. This system acts as a boolean 'AND' gate, making sure IL4 and IL10 are produced only when both TNF and IL22 are present. This is shown in the graph to the right Implanting an AND gate into these cells is important because TNF and IL22 are naturally expressed in the body, and it wouldn't be ideal to increase the production of cytokines unless it was necessary. These designer cells were named ‘cytokine converters' as they seemingly transform one set of cytokine signals into another. Cytokine converters also respond to IL22 and TNF in a dose dependent manner, meaning the amount of IL4 and IL10 they secrete is dependent on the levels of IL22 and TNF they are exposed to.

After finding the cells work in a dish, the researchers wanted to see whether these cells would work in a mouse model of psoriasis. They implanted their engineered HEK cells into mice and administered a chemical called imiquimod (IMQ in the figure) which induces psoriasis-like symptoms. This experiment acts as a way to test whether their cells could prevent psoriasis flare-ups from occurring. In the mice that were treated with the cytokine converter cells (CC in the figure) they saw significant reductions in the circulating levels of IL22 and TNF. They also saw increases, although nonsignificant, in IL10 and IL4 levels. Implanting cytokine converter cells also greatly reduced the thickness of the skin, and the amount of cells at the epidermal surface. Combining this data with a reduction in the redness of the skin (called erythema, data shown below), it demonstrates that the implanted designer cells can drastically alleviate psoriasis symptoms. They are even more effective than a common treatment, prednisolone (PDS in the figure). Not only do the cytokine converter cells help reduce psoriasis flare-ups, they can also treat already present symptoms. When they implanted the cells into mice already suffering from psoriasis-like symptoms, they saw similar results. It reduced circulating levels IL22 and TNF, especially compared to either no treatment or the prednisolone treatment. The engineered HEK cells successfully prevent psoriasis flare-ups, and alleviate already present symptoms.

This research represents a way to get rid of taking pills by designing cells to be biosensors and implanting them into the body. In this scenario, when levels of the cytokines that mark psoriasis increase, the engineered cytokine converter cells respond by increasing their own cytokines to diminish the symptoms. It worked in mice to both prevent symptoms from occurring and reduce already established psoriasis-like symptoms. This work sets the stage for creating designer cell lines that could be used to treat other diseases that have biomarkers associated with them. Instilling Boolean logic into a cell, like the ‘AND' gate, implies that biological engineering may be able to deal with complicated biological scenarios we may come across in the future. Hopefully, this will open the door for more engineered bio-medicines to be used as treatments for diseases.

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References:

Schukur, L., Geering, B., Charpin-El Hamri, G., Fussenegger, M. Implantable synthetic cytokine convert cells with AND-gate logic treat experimental psoriasis. Science Translational Medicine 7, 1-11 (2015)

Image credits:

All images are augmented from Schukur et al. paper cited above.