A knee joint in a jar. Sounds like something from an Edgar Allan Poe novel. In fact, this was what I received in the lab one summer evening from our clinician.
Why a knee joint? This is one of the affected areas in types of arthritis, such as rheumatoid arthritis and psoriatic arthritis. Inflammatory arthritis of the joint is an autoimmune disease, an occurrence when the body’s immune system triggers a harmful response against itself.
Cell-signalling proteins called cytokines or chemokines can increase blood flow to the joint and cause the entry of fluid and immune cells into the knee joint, causing redness, pain and swelling; this is termed inflammation. Whether you’re sitting, standing, or (hopefully not) running while reading this, your knee joint is essential for support. If you hold your knee you can feel the thighbone, shinbone and kneecap. To reduce friction during movement, we have a small amount of fluid within the small space between the bones. However, when arthritis flares, inflamed knees swell with larger volumes of fluid in this small space; doctors can remove the fluid and provide medication to ease the swelling and pain.
A cytokine called interleukin-17 is known to increase inflammation in inflammatory arthritis. Interleukin-17 activates cells called fibroblasts in the knee joint tissue which produce inflammatory proteins. Interleukin-17 also triggers cells called osteoclasts to cause bone break-down in the knee. Interestingly, Secukinumab, a form of treatment which blocks interleukin-17 has recently been found to be effective in treating psoriatic arthritis.
Once removed from the knee by doctors, psoriatic arthritis patients can choose for the fluid to be donated to the lab. It is fascinating how we can isolate a range of cell types from the yellow, viscous fluid to study their characteristics. By using flow cytometry, we can distinguish different cell types based on the proteins they are known to produce or have on their cell surface. Studies over decades have used this technique to show that different cell subsets can produce interleukin-17; however, the focus of my PhD project is on a less-studied producer of interleukin-17, termed Tc17 cells.
By using flow cytometry, our lab previously showed that there are an increased number of Tc17 cells in the fluid of psoriatic arthritis patients; whereas not present in rheumatoid arthritis. Interestingly, as Tc17 cell numbers increase, the severity of disease also increases. Therefore, we hypothesise that Tc17 cells may help cause inflammation in psoriatic arthritis.
We can analyse other inflammatory proteins that Tc17 cells could produce in addition to interleukin-17 using flow cytometry. We have shown that Tc17 cells co-express another inflammatory cytokine, TNF-α. Both IL-17 and TNF-α synergise to cause enhanced inflammation, so we hypothesise that Tc17 cells are pro-inflammatory. We have also optimised a method called a Cytokine Secretion Assay in which we can isolate Tc17 cells based on their ability to produce IL-17. Subsequently, using genome-wide techniques such as RNA-sequencing, we can also detect inflammatory genes that Tc17 cells express highly or downregulate. In this manner, both flow cytometry and RNA-sequencing can determine whether Tc17 cells display the characteristics to advance joint inflammation. It is also essential to investigate how Tc17 cells interact with other cells to cause inflammation, as there are multiple cell types in the joint causing inflammation; here is why the knee joint in the jar is important. When a patient’s severe arthritis has caused bone erosion to an extent which has disabled the knee, the joint must be replaced with an artificial one. If the patient wishes to donate the damaged knee to research, we can use it in the lab to isolate immune cells from the tissue. After some time under lab conditions, fibroblast cells crawl out of the tissue and can be isolated for experiments. We can then investigate how cells in the joint and tissue interact with fibroblasts to cause further inflammation. By receiving human knee joints and fluid through consenting patients with clinicians’ help, we are one step closer to understanding the role of Tc17 cells in psoriatic arthritis.
Although Tc17 cells are a newly-found exciting cell type, there are multiple cell types which also produce interleukin-17. The question is, are Tc17 cells adding to the inflammation caused by multiple interleukin-17 producers, making them the straw that breaks the camel’s back? Or if Tc17 cell numbers increase with severity of psoriatic arthritis does this indicate a clinical importance of these cells in which their levels can predict the occurrence of psoriatic arthritis?
There is still a lot to be discovered about Tc17 cells, which I am exploring during my PhD. But it is important to understand that other immune cells, even cells that do not produce interleukin-17 such as fibroblasts and osteoclasts are key in arthritis. Cells alone cannot cause inflammation without the effect of proteins produced by other cells. Like a jigsaw puzzle, you need to put the pieces together to contribute to the picture – to cause inflammation; it really is a joint effort.
Written by Ushani Srenathan