When we get a bacterial infection in our skin, our immune system responds. It's been thought that inflammatory cells called monocytes are crucial to eliminating infection, although different immune cells called neutrophils have been shown to have a more robust response to affected tissue. Monocytes (also known as white blood cells) can convert to macrophage cells, which have a range of important roles, while neutrophils cannot. In a new study reported in Nature, scientists studied bacterial infections in skin tissue, and learned more about how neutrophils and monocytes clear those infections. The findings could offer a new approach to treating bacterial infections that occur in tissue or wounds, and change how we see the process.
"While translating our research from bench to bedside will require many more experiments and involve a model more closely related to human disease, it is exciting that we have made a fundamental discovery that could improve infections and tissue repair in humans, especially hard-to-treat cases," said first study author Dr. Rachel Kratofil, PhD, of the University of Calgary.
Neutrophils and monocytes are both important to clearing a bacterial skin infection, but this study has suggested that monocytes can heal wounds faster than neutrophils. Monocytes encourage healing after converting to macrophages, because they persist for weeks after the infection, controlling levels of a hormone called leptin.
The study has revealed "a paradigm shift challenging the current thinking that neutrophils and monocytes clear bacteria," and has elevated monocytes' role in wound repair, noted Kratofil.
Monocytes were also found to generate a hormone called ghrelin near the site of infection, which helps control the growth of blood vessels as wounds are repaired, and promotes healing.
Ghrelin is known to be released by the stomach when we get hungry, and fat cells release leptin once we have eaten and are full; these hormones have crucial roles in hunger and satiety, and are known to maintain a balance in metabolism. But this research has revealed a novel role for ghrelin in immunity and tissue repair.
Staphylococcus aureus is commonly found on our skin or in our noses. It can exist there harmlessly, but it can also cause serious infections. Both neutrophils and monocytes are recruited after an S. aureus infection, with neutrophils eliminating bacteria and monocytes facilitating tissue repair. When there aren't enough monocytes on the scene, however, leptin levels are high, which can cause the growth of blood vessels in the infection, which delays healing and causes scarring. Monocytes can stop that excessive blood vessel growth by generating ghrelin at the infection site.
This work may opens the door to using the metabolic hormones ghrelin and leptin in immunology or microbiology. Now the investigators want to know more about how neutrophils and other immune cells are responding to infection.
Dr. Kratofil can be seen in this three-minute thesis explainer video.