New immune cells discovered by a research team revealed the identities of new subsets of immune cells at the frontline of the body's defense mechanism against infection.

The research team's discovery expands the understanding of immune cells called granulocytes – immune cells that can trigger inflammation and engulf microbes to slow an infection's progress. As well as protecting against infection, granulocytes contribute to inflammatory conditions such as asthma and allergy.

The research, led by Walter and Eliza Hall Institute researchers Dr. Carolyn de Graaf, Dr. Kirsten Fairfax and Dr. Jessica Bolden, extends to the team's extensive 'atlas' of blood cells, called Haemopedia. The new findings were published in two articles in the Journal of Leukocyte Biology.

Distinguishing new cell types

Our immune system is a complex network of cells working together to prevent infection and keep us healthy. However, excessive or misdirected immune cell activity can drive inflammatory conditions such as asthma and allergies.

Granulocytes are critical 'front line' cells in the immune system, serving as a rapid defense against infection by engulfing invading microbes, triggering inflammation and alerting other immune cells to the threat.

The surface of granulocytes is covered with molecules that influence the cells' function, Dr. de Graaf said. "Our team focused our attention on a surface molecule called Siglec-F, which had previously been implicated in 'switching off' a type of granulocyte called eosinophils," she said.

Eosinophils are important triggers of inflammation in response to parasitic infections but are also key players in inflammatory diseases such as asthma and allergies.

"We discovered we could divide eosinophils into two 'populations' based on how much Siglec-F they had on their surface," Dr. de Graaf said. "These two populations appeared to differ in their maturity, and thus in their function within the body. This is the first time differences in gene expression in these two populations have been documented."

Unraveling immune cell development

The team also investigated the development of eosinophils and other related granulocytes. These cells develop in the bone marrow, originating from blood stem cells, said Dr. Fairfax.

"The developing granulocytes transition through immature stages known as progenitor cells before becoming different populations of mature granulocytes, such as eosinophils," she said.

"Using flow cytometry, we could follow the development of progenitors through different stages in the bone marrow. We discovered the amount of Siglec-F on the surface of cells varied as the cells developed," she said.

This allowed us to identify a never-before-identified precursor marked by Siglec-F, which could also give rise to macrophages, another immune cell closely related to granulocytes," Fairfax said.