The migration of leukocytes from their location of origin to diverse tissues during vertebrate development is crucial for blood cell formation and immunosurveillance. In their study, Anna Huttenlocher and colleagues characterise T-cell migration within diverse tissue environments in zebrafish. To know more about their work, we spoke to the first author, Tanner Robertson, and the corresponding author, Anna Huttenlocher, Professor of Medical Microbiology & Immunology and Pediatrics at the University of Wisconsin–Madison, USA.
Anna Huttenlocher (left) and Tanner Robertson (right).
Anna, what questions is your lab trying to answer?
AH: We are interested in how chemical and mechanical cues influence leukocyte decision-making both during homeostasis in developing tissues and in response to tissue damage. We are trying to understand how cells prioritise different cues that ultimately impacts the tissue repair responses and the balance between the onset and resolution of inflammation. How these processes are altered in disease is also an area of focus for our research group. To answer these questions, we love to image leukocyte behaviour in whole animals. The transparent nature of zebrafish has enabled the discovery of new mechanisms of motility and immunity.
Tanner, how did you come to work in the lab and what drives your research today?
TR: During my PhD in The Burkhardt lab at The University of Pennsylvania, I was studying the migratory plasticity of T cells using mostly in vitro models. My project was part of a growing body of work showing that leukocytes can alter their migratory strategy depending on their physical and chemical environment. The implicit hypothesis in the field is that cells possess different modes of migration because the tissues of the body are varying and complex, and different migration strategies are needed to navigate different tissues. Towards the end of my PhD, I was looking for a way to test this directly. I saw Anna give a talk at research conference where she showed movies of neutrophils migrating in various tissues, and I realised I could use this model organism to study T cells. Anna agreed to take me on as a postdoc after I finished my PhD to study T-cell motility in zebrafish, something very few other people were doing.
Can you tell us about the background of the field that inspired your work?
AH & TR: We think the field of cell migration is currently in an interesting place. There has been a lot of elegant work done mostly in vitro showing that leukocytes can move and coordinate their movement in unexpected and fascinating ways. We think the big gap right now is understanding how this all plays out in vivo. This is the major advantage of the zebrafish system – being able to monitor cells migration within tissues at a high resolution.
Can you give us the key results of the paper in a paragraph?
AH & TR: The paper has two major findings. First, we showed that zebrafish have an epidermal population of T cells that strongly resemble the well-studied dendritic epidermal T cells that populate the epidermis of rodents. We showed that the T cells that migrate through the skin in larval zebrafish are the precursors to these dendritic epidermal T cells and their migration reflects their developmental colonisation of the skin. We then found that we could readily monitor T-cell migration in three distinct sites – the thymus, the intestines and the epidermis – and decided to compare their migration strategy across these tissues with a T-cell Lifeact reporter line that we generated for this study. This led to our second finding that T-cell migration in the epidermis is distinct from that in the thymus and intestines. While T cells in the thymus and intestines migrated with F-actin-rich leading-edge pseudopods, T cells in the epidermis migrated via stable blebs, a mode of migration numerous cell types perform under planar confinement in vitro. We correspondingly found that epidermal T cells are under apparent planar confinement in vivo, sandwiched between the basal and suprabasal keratinocyte layers.
When doing the research, did you have any particular result or eureka moment that has stuck with you?
TR: I was initially studying T-cell migration in different tissues with a simple cytoplasmic GFP reporter line, and it looked like T cells may be migrating differently in the epidermis compared to other sites, but it was tough to draw many conclusions. I was fortunate to have a very talented and dedicated undergraduate student, Zoe Zwick, who was working with me on this project. She generated the T-cell Lifeact reporter zebrafish line used throughout this study. The ‘eureka’ moment was when we first saw how differently the T-cell F-actin was organised for cells migrating in the epidermis compared to the intestines and thymus.
T cells (cyan) and macrophages (magenta) form a network in the epidermis of adult zebrafish. Image credit: Adam Horn and Tanner Robertson.
T cells (cyan) and macrophages (magenta) form a network in the epidermis of adult zebrafish. Image credit: Adam Horn and Tanner Robertson.
And what about the flipside: any moments of frustration or despair?
TR: We managed to avoid any despair for this project, but there were some frustrating roadblocks. One of the major advantages of studying cell migration in vitro is that you can carefully alter parameters such as confinement. When working in vivo, things can get tricky. While we had compelling evidence that motile T cells were confined between keratinocyte layers, disrupting this confinement was challenging. Genetic and pharmacological approaches that disrupt keratinocyte adhesions and increase the space between keratinocyte layers tend to be lethal in larval zebrafish well before T cells appear and begin migrating in the epidermis – this is just one of the (many) challenges of working in vivo!
Why did you choose to submit this paper to Development?
AH & TR: We use zebrafish to study fundamental questions about cell motility. Development is a perfect fit for fundamental biology in a model organism.
Tanner, what is next for you after this paper?
TR: I'm currently applying for faculty positions, and I hope to set up my own lab soon to continue working on T-cell migration and adaptive immune function using zebrafish. We've got a lot of exciting work in the pipeline – stay tuned!
Anna, where will this story take your lab next?
AH: We are very excited to delve further into leukocyte migration in tissue contexts: how different types of cells interact with each other to influence homeostatic motility, and the response to tissue damage and inflammation.
The ‘eureka’ moment was when we first saw how differently the T-cell F-actin was organised for cells migrating in the epidermis compared to the intestines and thymus
Finally, let's move outside the lab – what do you like to do in your spare time?
TR: I played music semi-professionally before getting into science, and that's still my major hobby. I'm also a big reader. I just finished Charles Piller's book ‘Doctored’, and I thought it was great. I think everybody conducting research would benefit from giving it a read. My fiancé and I have a border collie named Pepper who keeps us busy. We also love going to University of Wisconsin hockey games – go Badgers!
AH: I play music, hike, dance and read. One of my favourite things is hiking in the woods with my husband and dog. In Madison, I love going to the UW Union terrace to listen to music and gather with friends and colleagues.
T.R. & A.H.: Department of Medical Microbiology & Immunology, University of Wisconsin–Madison, Madison, WI 53706, USA.
A.H.: Department of Pediatrics, University of Wisconsin–Madison, Madison, WI 53792, USA.
E-mail: [email protected]
