Research News

A puzzling experimental result recently led to the discovery of a novel “brake” that controls the development of an immune cell subset known as neutrophils, which act as the first responder to infection. The discovery was made by Jayati Basu, PhD, assistant staff in the Department of Inflammation & Immunity, and has been published in Nature Immunology.

Dr. Basu, and her collaborator, Dietmar Kappes, PhD, at Fox Chase Cancer Center, made the discovery while trying to understand how CD4 T cells develop in the thymus gland and function to regulate different parts of the body. Drs. Basu and Kappes previously showed that a protein called ThPOK acts as a master regulator for CD4 development and that this process is evolutionarily conserved in mammals since the Jurassic period. However, scientists did not know the function of ThPOK beyond CD4 T cell.

In 2017, the team engineered the ThPOK locus of the mouse genome by inserting a green fluorescent protein gene. This ‘reporter’ gene produced a protein that glows green under the microscope and showed when and where the ThPOK gene is turned on in the body.

“At the time of these experiments, everyone knew that ThPOK was selectively expressed in CD4 T cells, so we really expected to see mainly green in those cells” says Dr. Basu.  “But we were surprised when several other subsets of blood cells, including neutrophils and monocytes, were lit up strongly… like a Christmas tree! It was totally unexpected.”

While such an unexpected result might have suggested an experimental error, Dr. Basu considered the alternative possibility that it might provide a clue to a significant discovery.

“Science is all about pushing the envelope of our collective knowledge,” she says. “Sometimes, that means admitting that our current knowledge might be incomplete. But I was convinced that these results meant something.”

Drs. Basu and Kappes then reached out to  H. Leighton. Grimes, PhD, and Nathan Salomonis, PhD, at Cincinnati Children’s Hospital and Medical Center. After months of working in close collaboration, the teams obtained compelling evidence that ThPOK acts as a crucial multifaceted regulator of monocyte and neutrophil development in the bone marrow and serves as a brake for neutrophil maturation.

Individual immune stem cells develop into one of five sub-types of white blood cell, pictured above. Neutrophils are the most common sub-type. Image courtesy of my.clevelandclinic.org

What are neutrophils?

Healthy immune systems include several different types of white blood cells or leukocytes. Each type of leukocyte plays a specific and unique role – responding to a different type of infection or disease – and mounts its own response to the invaders. The most common type of leukocyte is neutrophil, which acts as the first line of defense against infection, attacking bacteria and repairing damaged tissue.

The ratio of different types of leukocytes, including neutrophils, is tightly controlled because the relative and absolute numbers strongly impact the response to invaders and a person’s overall health. Individuals with too few neutrophils, for example, develop an immunocompromised status called neutropenia and are much more susceptible to severe infection. Too many neutrophils, on the other hand, are predictive of different types of cancer.

Neutrophils and other leukocytes are made within our bone marrow. However, neutrophils and other leukocyte subsets don’t emerge fully formed. Instead, our bone marrow produces hematopoietic stem cells that have the potential to mature, or differentiate, into any immune cell. Immune cell differentiation is a tightly controlled, multistep process with multiple branch points.

“The different steps and potential outcomes are well understood, but what genes are needed to execute these steps and how they work to do it, has been trickier to figure out,” says Dr. Basu.

What does this mean for the future?

Studies in the Basu lab are currently underway to confirm the role of ThPOK in human leukemia patients and ThPOK may provide an attractive new target to develop novel Acute Myeloid Leukemia therapeutic strategies.

“If we tweak the expression of ThPOK in a patient, we can change the developmental trajectory of their immune cells,” says Dr. Basu. “Targeting ThPOK may provide a novel strategy to combat sepsis, which is major health problem worldwide.”