What Zebrafish Can Teach Us About Our Own Immune System

The human immune system is a miraculous and mysterious place.

It is staggeringly complex, as tens of thousands of genes within several different cell types interact in our blood to fight germs and other invaders. The system’s behavior is impacted by anything from diet and age to stress levels and genetics.

The exact mechanisms of a variety of immune system cells remain puzzling. One person’s system may contain cells associated with developing diabetes, while another person with the same type of cells may not develop the disease.

A new study from the lab of Jeffrey Yoder, associate professor of innate immunology at the NC State College of Veterinary Medicine, is shedding more light on one human gene, TRIM9. TRIM9 is known to play a role in how the brain functions, but it had never been studied in an immune response context. The study, funded by the National Institutes of Health among other entities, was published in the Journal of Leukocyte Biology.

What did you notice happening with the TRIM9 gene after stimulation of the immune system?

The initial goal of this project was to identify genes that respond to immune stimuli, as these genes may play important roles in battling infections. Initially, we used zebrafish embryos as our animal model. We exposed embryos to reagents that mimic either bacterial or viral infection and then quantified resultant changes in gene expression in the whole animal. The expression of over 1,000 genes changed, either went up or down, after exposure to either mimic, but only about 100 genes responded to both mimics. One of those genes was TRIM9.

Our investigation showed that, after infection, TRIM9 expression increases in the macrophages, a population of immune cells that provides one of the first lines of defense against infection. Importantly, we subsequently found a similar increase in TRIM9 expression in human macrophages exposed to immune stimuli.

This result was very satisfying as it indicated that, despite about 400 million years of evolutionary divergence, the TRIM9 response to infection is highly conserved from fish to humans, suggesting that its function could be fundamentally important in vertebrate immunity.

What does that tell us about the role of TRIM9 in the human immune system?

Previous research on the role of TRIM9 in neurons demonstrated that it was required for the movement of neuronal cells. Based on this observation, we hypothesized that TRIM9 may be playing a similar role in macrophages, which migrate to wounds and sites of infection.

We used transgenesis to express a fluorescent protein in the macrophages of a living zebrafish embryo. With fluorescent microscopy, we made movies showing that the disruption of TRIM9 dramatically reduced macrophage motility and changed macrophage shape. These remarkable observations suggest that TRIM9 normally mediates macrophage shape and motility, important features for normal immune function.

What does the study potentially reveal about how the human immune system responds to pathogens?

In all vertebrates, macrophages are highly motile cells and their motility is essential for identifying, battling and removing pathogens. Our study in the zebrafish model suggests that TRIM9 plays a crucial role in controlling the ability of macrophages to migrate to sites of infection. As the sequences of the human and zebrafish TRIM9 proteins are 84 percent identical, and our experiments suggest that the expression response of TRIM9 to infection is

conserved in human macrophages, we suspect that TRIM9 is also required for critical aspects of human macrophage motility and immune response.

Why was the zebrafish used as a model for the study?

In contrast to mammalian embryos that develop inside their mother, zebrafish lay eggs in the water so the embryos develop outside their mother. We can also collect 100-200 embryos from a single mating pair of adults. These features allow us to incubate embryos in aquarium water in small Petri dishes and run multiple immune stimulation experiments simultaneously.

Another major advantage is the ease of generating transgenic zebrafish with fluorescently labeled cell populations. Finally, the transparent nature of the zebrafish embryos makes them ideal for visualizing, in real time in the embryo, the movement and behavior of their immune cells. Such studies would have been very challenging, if not impossible, to perform in a mammalian model.

You’re planning other studies looking at TRIM9. What interests you most about the gene and what does research like this mean for the development of future therapies?

We are now moving into using human cell lines to determine if TRIM9 plays a role in other cellular functions, like engulfing pathogens or releasing immune-signaling molecules. Both of these functions require dynamic alterations to the cell shape and underlying cellular architecture. We are also very excited to determine what proteins and cellular pathways are regulated by TRIM9. Understanding what proteins control macrophage motility and shape will have important implications for future drug development geared to boost or suppress the immune response.

The read the study and see a complete list of research authors, go here.

~Jordan Bartel/NC State Veterinary Medicine