Explaining my research, part 1: Neutrophils, the devoted foot soldiers of your immune system
Neutrophils and macrophages are both myeloid cells. This cell lineage here can be roughly understood as a family tree, although in this case, the parent actually becomes one of the children, e.g. a granulocyte/macrophage progenitor can become any of the 5 different cells below it.
Every day, you walk around with an army of cells in your body, ready to defend against a wide range of foreign invaders. Fall down and scrape your knee? This could give dangerous bacteria, viruses, and fungi (a.k.a. pathogens) a route to enter your body. Without white blood cells ready to attack these invaders, you’d be in big trouble; that’s part of why immunodeficiencies and immunosuppressive treatments can be such a dangerous problem. In any healthy person, white blood cells called macrophages will immediately detect invaders and begin eating them (this is called phagocytosis) and/or secreting toxic chemicals to destroy them. These macrophages also act as sentries, sending out a distress signal in the form of chemicals called cytokines (if it helps, they are somewhat analogous to the fire used to alert China of the arrival of the Huns in Mulan). These get detected by many different cells in the body, but the first responders are white blood cells called neutrophils. I’m biased, but they’re kind of my favorite.
Neutrophils are closely related to macrophages (see the cell lineage / “family tree” here), and they are also quite adept at eating pathogens and secreting toxic chemicals. They’re always around and at the ready (at least they should be); if you take a single drop of blood from your finger and look at it under the microscope, you should find upwards of 200,000 neutrophils (although it may take a while to find them amidst the pile of 250 million red blood cells). If macrophages are the sentries, neutrophils are the foot soldiers that come flooding into the entry point to defend the city against attack. They’re the reason you have that white fluid we call pus if the wound gets too serious. They are usually able to overwhelm the invading pathogens by shear numbers.
The process I just described is only one tiny part of your immune system, but it involves fundamental components (macrophages, neutrophils, cytokines) that play key roles in all sorts of immune responses. You wouldn’t survive long if any part of this process were to go haywire. In reality, there’s A LOT more to immunology, most of which I’m not very qualified to talk about in detail. For our purposes right now, let’s focus on the above story, as it describes most of what matters for my research. As you may have guessed from my above bias, my research focuses on the foot soldiers of the immune system, neutrophils. They’re cute little guys, just look at them here. Emphasis on the little - for reference, the scale bar in that video is 1/100th of a mm, or about 1/10th of the width of one of your strands of hair. But don’t let their cuteness fool you. They really are ruthless soldiers who try their utmost to eat and kill invading pathogens.
Now, to get to the pathogens, there’s a whole process called the neutrophil recruitment cascade, which is depicted in Fig 2. Without going into too much detail, here’s what happens:
Neutrophils and the cells lining the blood vessel, endothelial cells, both sense cytokines (the distress signals sent by macrophages), which causes them to express certain molecules on their surface that allow the neutrophils to stick to the endothelial cells.
The neutrophil starts by forming temporary attachments which slow down its progress but don’t stop it altogether. This causes the cell itself to start rolling along the endothelial cells.
Eventually the neutrophil sticks to the endothelial cell layer, then begins to spread and crawl.
Once it finds a spot it likes, the neutrophil actually crawls out of the blood vessel. This process is called transmigration or extravasation.
Once in the tissue, the neutrophil follows the chemical trail of cytokines to the site of infection itself. It mainly does this by crawling through the tissue via a process known as chemotaxis. At some point, the neutrophil senses chemicals emitted from the pathogens themselves (known as chemoattractants), allowing them to course correct during chemotaxis so they veer towards the pathogens rather than the sentry macrophages.
Finally, the neutrophil sticks to the surface of the pathogen and begins to rapidly deform to spread around it and completely engulf it (phagocytosis!).
At the end of phagocytosis, the neutrophil dumps toxic chemicals on the pathogen, and often commits suicide in the end (this is called apoptosis) to make sure the job is done.
Believe it or not, my description there was vastly oversimplified. Every step in the process contains many areas of active research scientists are still working to better understand. In my research, I’m mainly focused on a few of the final steps (#5-7), and I am specifically interested in how chemical input (chemoattractants, cytokines, or molecules on the surface of pathogens) leads to mechanical output (e.g. cell deformation during crawling or during phagocytosis).
As a biomedical engineer, I often think in terms of systems which take in certain inputs, yielding specific outputs. In this case, we can think of the cell as a sort of “black box” system. The input could be cytokines sensed by receptors on the cell surface, and the output could be protruding outwards in the direction of the signal it detects. But how in the world do we get from input to output here? Answering that question is an example of “reverse-engineering” the cell. It’s remarkable to find that nature (via natural selection) has produced these tiny machines (a.k.a. cells) which are, in many ways, much more complicated than the machines we build as humans (but that’s a subject for another blog post). For now, suffice it to say that developing a comprehensive, quantitative knowledge of immune cells is a worthwhile ordeal - the more we understand about how cells do what they do, the better chance we have to intelligently intervene when something goes wrong, or even to convince immune cells to attack cancer cells (immunotherapy!).
To give you a glimpse of the sort of experiments we do in the Heinrich Lab, here are a couple videos to get you started. We use small glass straws to hold neutrophils (on the right) and “target particles” (on left), which are either pathogens or particles that mimic pathogens. Using this setup, we can hold the cell off of a surface and watch how it responds to pathogenic particles with high precision. These experiments are incredibly exciting, and I’ll describe them in a bit more detail in the future.
Now, this is all well and good for background information, but focusing on “how chemotaxis and phagocytosis work” is far too vague to make for a good PhD project. As I just mentioned, I want to understand a bit about how we get from chemical input to mechanical output. From quite a bit of research done previously, we have a good idea that the concentration of calcium inside the cell is an important player in this process in certain cases, especially during phagocytosis. For instance, when monitoring calcium concentration in the cell during phagocytosis, we consistently observe a dramatic cell-wide increase in calcium concentration, about 5-10 fold increase in less than 10 seconds. We almost always observe this during neutrophil phagocytosis, which made us start to wonder, why?
In my PhD project, I hypothesize that calcium matters during phagocytosis because it helps regulate exactly how the cell deforms. Now, what’s known about calcium in immune cells (and especially phagocytosis), and why do we think it is important for cell deformation? That will be the subject of part 2 of this blog series, which will allow me to get to my hypothesis in a bit more detail.