There is an old joke in scientific research: scientists have cured everything from the common cold to cancer a dozen times over — not in humans, though, just in a lab mouse or a petri dish of cells. But for inflammation, a condition that underlies a wide range of diseases from Alzheimer’s to diabetes, Assistant Professor of Biomedical Engineering Dr. Anjelica Gonzalez and her doctoral student Holly Lauridsen have a vision to fix this problem. Together, they have developed a new, more human-like model for studying inflammation, a major step toward ensuring that discoveries made in the lab can actually translate to insights about the human body.
“Ultimately, we’re here to improve the human condition,” said Lauridsen, first-author of the published paper. “We’re one lab. But by making more relevant models, we hope to facilitate others’ research too.”
Most people think of inflammation vaguely as the swelling, redness, and pain that accompany a sprained ankle — but these are only its symptoms. Inflammation is the body’s hallmark innate immune response, and it can be powerfully protective, damaging, or both: it is one of the earliest offensives to fight infection, yet it is also a major component of heart disease, arthritis, and obesity. This means that learning to control inflammation is vital for treating countless diseases, and it begins with understanding one of the first steps of the process: how white blood cells reach infected cells, a process known as leukocyte recruitment.
Dive into a pulsing blood vessel of your circulatory system. Typically, white blood cells — also known as leukocytes — smoothly sail through the blood stream, patrolling for damage. When an infection occurs somewhere in the body, the damaged cells cry for help by releasing chemicals that act on nearby microvessels, the smallest of blood vessels. These chemicals sink through the layers of the microvessel wall: first they reach specialized cells called pericytes, which wrap around the microvessel and embed in a layer of protein called the basement membrane; next, they reach endothelial cells that line the inner vessel wall. Along the way, these chemicals signal to the pericytes and endothelial cells to sprout proteins that are “sticky” for leukocytes. Now, instead of whizzing through the blood vessel, leukocytes are captured by these sticky proteins, roll to a stop along the layer of endothelial cells, and squeeze through the blood vessel wall to the infected cells, beginning the inflammatory cascade.
Like most of cellular and molecular biology, scientists deciphered this coordinated system with a reductionist mindset. They trimmed the system down to a few key parts, ran experiments on this simplified model, and pieced together the overall picture. For leukocyte recruitment, this meant simplifying the human blood vessel to just a single layer of endothelial cells grown on a glass slide; pericytes, which were hard to isolate from humans, were mostly ignored.
But that level of simplification has its consequences. First, it eliminates the ability to learn how endothelial cells and pericytes work together and affect each other. Second, the reduced system might be too different for its insights to actually be relevant inside a real human body.
“If you look back at seminal papers in the last ten years about how inflammation is regulated and how white blood cells get to an injury, everything is about endothelial cells,” said Lauridsen. “Granted, that’s very important — but it’s really just step one. To get a much more comprehensive picture, you have to think about the grand scheme of things.”
A New Model for Inflammation
A critical piece of this picture emerged in 2010 when Dr. Jordan Pober, Bayer Professor of Translational Medicine at Yale, published a major study on how to efficiently isolate pericytes from human tissue. Finally, human pericytes were free to be studied and characterized. But Gonzalez, who had just finished a postdoctoral position in Pober’s lab, had the foresight to see pericytes as a new tool. Trained as a biomedical engineer, she dreamed of using pericytes to build something new, like a more accurate blood vessel model.
“Being able to isolate pericytes meant that we could actually take different cells and, using our engineering techniques, start to design the human blood vessel in its entirety — instead of just using one cell type,” said Gonzalez.
Imagine taking a blood vessel section out of the body, slitting it open, and letting the circular tube uncurl and flatten. Instead of the traditional single, flat layer of endothelial cells, Gonzalez and Lauridsen’s model has three flat layers: a top layer of human endothelial cells, a bottom layer of human pericytes, and a thin, porous membrane sandwiched in the middle to represent the basement membrane layer. This membrane serves two functions: first, white blood cells can crawl from the “inside” of the blood vessel — the top layer — through the other cell layers; second, pericytes and endothelial cells can communicate through this membrane using chemical signals or by forming hundreds of intimate connections amongst themselves. The beauty of this model is that while it is a much closer imitation of a human vessel than a single slab of endothelial cells is, it is still simple enough to be easily manipulated.
“Even if we could keep human vessels alive in culture long enough for experiments, they are such complex systems that we could never decouple all of the different mediators — biochemical, mechanical, cell-to-cell contact — for evaluation of a single function,” said Gonzalez. “With this system, we can take the whole thing apart and figure out what is the most influential signal. You could never do that in a whole human vessel.”
From Bench to Bedside
This complex, multilayered model raises new questions about how the different cells of the blood vessel work together during inflammation, many of which Lauridsen describes as “mechanistic chicken-or-egg questions.” For example, the basement membranes of healthy people compared to sick patients include different proteins — but is it the disease that causes the expression of different proteins, or does the expression of different proteins cause the disease? Understanding relationships between blood vessel cells and their effect on inflammation may eventually provide therapeutic insight for various diseases, conditions, and issues related to aging.
The scientific community has high hopes for this new model. “Involvement of pericytes is crucial, because in many current models, it’s just endothelial cells,” commented inflammation expert Dr. Ruslan Medzhitov, David W. Wallace Professor of Immunobiology at Yale School of Medicine. “That system gave us a lot of information, but it is incomplete. So something like this model, if easily usable, could make a big difference.”
According to Lauridsen and Gonzalez, this is still only a “first-generation model.” Their next goals are to experiment with different materials for the membranous insert — to make it stiffer or softer, have larger or smaller pores — so that different parts of the body’s circulatory system can be modeled more specifically.
“It’s not a complete human vessel,” said Lauridsen, “but it’s one step in the right direction toward a model that can bridge that gap between what you can do in a petri dish and what you can do in a human.”
About the Author: Renee Wu is a senior Molecular, Cellular, & Developmental Biology major in Silliman College. She is the former Managing Editor and Features Editor for the Yale Scientific and works in Dr. Eric Meffre’s lab studying B cell development in humans.
Acknowledgements: The author would like to thank Professor Gonzalez, Holly Lauridsen, and Professor Medzhitov for their time and enthusiasm.
- Ayres-Sander, C. E., Lauridsen, H., Maier, C. L., Sava, P., Pober, J. S., & Gonzalez, A. L. (2013). Transendothelial migration enables subsequent transmigration of neutrophils through underlying pericytes. PLoS One, 8(3), e60025. doi: 10.1371/journal.pone.0060025
- Lauridsen, H. M., Pober, J. S., & Gonzalez, A. L. (2014). A composite model of the human postcapillary venule for investigation of microvascular leukocyte recruitment. Faseb j, 28(3), 1166-1180. doi: 10.1096/fj.13-240986
- Maier, C. L., Shepherd, B. R., Yi, T., & Pober, J. S. (2010). Explant outgrowth, propagation and characterization of human pericytes. Microcirculation, 17(5), 367-380. doi: 10.1111/j.1549-8719.2010.00038.x