Profile of Warren J. Leonard

The immune system is the human body’s natural defense against disease. Many molecules and cells take part in the immune response, including cytokines, a critical group of intercellular signaling molecules that influence the development and actions of immune cells.

Warren J. Leonard. Image courtesy of Bill Branson (National Institutes of Health, Bethesda, MD).

Immunologist Warren J. Leonard, at the National Institutes of Health (NIH) in Bethesda, Maryland, has dedicated much of his career to studying the molecular aspects of the immune system, in particular, cytokines. Leonard’s early work on cloning a component of the human receptor for the cytokine IL-2 was the foundation for his later work dissecting cytokine pathways, and led to the discovery of the genetic factors underlying the immune disorder X-linked severe combined immunodeficiency (XSCID), or “Bubble Boy” disease, as well as other forms of human immunodeficiency. Moreover, it also served as the basis for a range of other fundamental contributions by his laboratory over the years.

Leonard’s research has earned him multiple awards and accolades, including election to the US National Academy of Sciences in 2015.

Focused on Math and Science

Leonard grew up in Bethesda. His father, Fred Leonard, was a physical and polymer chemist, and his mother worked in retail sales. Leonard’s father directed a biomedical research laboratory at the Walter Reed Army Medical Center Forest Glen Annex from 1948 to 1972, and later became the dean of research at George Washington University School of Medicine in Washington, DC. Among his father’s accomplishments were major contributions to prosthetics and to the discovery of α-cyanoacrylate adhesives, the basis of superglue, for which he held the patent for medical and dental applications. For these efforts, President Richard Nixon awarded Leonard’s father the President's Award for Distinguished Federal Civilian Service.

As a child, Leonard was inspired by his father and enjoyed visits to his laboratory at Walter Reed. His teachers at Walt Whitman High School also reinforced his innate interest in math and science.

Following high school graduation, Leonard enrolled at the Massachusetts Institute of Technology, but transferred a year later to Princeton University, drawn to its liberal arts allure. At Princeton, Leonard pursued mathematics and was propelled by the “intellectual high” that he says comes with proving theorems or understanding mathematical concepts. However, he became interested in biology after working summers with late statistician Samuel Greenhouse, who studied biometry and epidemiology at the NIH, and molecular biologists Max Gottesman, Sankar Adhya, and Ira Pastan in the Laboratory of Molecular Biology at the National Cancer Institute.

Leonard was also influenced during his senior year by Sir Robert May, a theoretical physicist-turned-biologist who helped establish the field of theoretical ecology. Along with mathematician Michael Reed, May served as an advisor on Leonard’s senior thesis. Under their mentorship, Leonard tackled an area of mathematical biology involving Lotka-Volterra equations, which describe predator–prey relationships. With May, Leonard uncovered nuances in Lotka-Volterra mathematical properties that regulate predator–prey population dynamics in three-species systems (1).

Leonard received an AB degree in mathematics in 1973, and considers May an early inspiration in his scientific career. “Meeting with him weekly while doing my senior thesis was an extraordinary experience,” he says. “May was always very excited intellectually and quick-witted.”

Following graduation from Princeton, Leonard entered medical school at Stanford University in California. After receiving an MD degree in 1977, Leonard trained as a medical intern at George Washington University Hospital in Washington, DC, and as a resident at Barnes Hospital in St. Louis, Missouri. He then completed a year of postdoctoral training in biochemistry and molecular biology, working with Jeffrey Gordon and Arnold Strauss at Washington University in St. Louis before moving to the National Cancer Institute in Bethesda in 1981.

Branching into Immunology

At the NIH, Leonard trained under Warner Greene, an immunologist and virologist now at the University of California, San Francisco, and immunologist Thomas Waldmann.

During this time, Leonard focused on IL-2, which had been discovered as a T-cell growth factor but was renamed IL-2 in 1979. IL-2 regulates T cell and natural killer (NK) cell activity, among other functions. But the protein’s membrane receptor had not been characterized.

In a 1982 article in Nature (2), Leonard and his colleagues reported that a monoclonal antibody, denoted anti-Tac, bound to the human cell surface receptor for IL-2 and blocked the binding of IL-2. Subsequently, in 1984, Leonard and his colleagues reported the cloning, sequencing, and expression of IL-2 receptor (IL-2R) cDNA (3). However, it soon became clear that the cDNA-encoded protein did not recapitulate the physiological behavior of the normal cell surface receptor.

“What we learned immediately from the cDNA sequence was that this protein had a significant extracellular domain and a hydrophobic putative transmembrane domain, but only a very short cytoplasmic domain, which suggested that it could not by itself transmit the signal conferred by IL-2 into the cell,” Leonard explains.

In fact, the researchers had cloned a protein subunit of the IL-2R, the IL-2Rα chain. The work led to the discovery of IL-2Rβ in 1986, and an additional IL-2R subunit, IL-2Rγ, in 1992; the receptor chains form three classes of IL-2R: low-, intermediate-, and high-affinity receptors, with high-affinity receptors containing all three chains. The identification of IL-2Rγ paved the way for a major finding made in Leonard’s laboratory in the ensuing months.

Interleukins and Immunodeficiency

Shortly after IL-2Rγ was cloned, Leonard and his colleagues identified the location of the IL2RG gene at the same chromosomal region of the X chromosome as the gene responsible for the most common version of SCID, X-linked SCID (XSCID), which is characterized by a lack of T cells and NK cells, as well as nonfunctional B cells; XSCID is also known as the “Bubble Boy” disease, after David Vetter, a boy with XSCID who lived in a plastic, germ-free enclosure for 12 years.

More significant, however, was the demonstration by Masayuki Noguchi, a postdoctoral fellow in Leonard’s laboratory who is now a professor at Japan’s Hokkaido University, that XSCID patients all harbor mutations in IL2RG, indicating that the gene was causally related to XSCID, findings with implications for XSCID diagnosis and the development of gene therapy for XSCID (4). The findings also raised new scientific questions.

“The result was very unexpected because the phenotype in XSCID was one of dramatically defective T cell and NK cell development, whereas humans and mice with IL-2 deficiency had essentially normal T cell and NK cell development,” Leonard says. “That taught us that the γ-chain of the IL-2 receptor must be playing a broader role in order to account for the defects in T cell and NK cell development.”

Deeper understanding was soon to come. In December 1993, Leonard’s colleagues, including Noguchi and Sarah Russell, then also a postdoctoral fellow, reported in Science that IL-2Rγ, in addition to being part of IL-2R, was a functional component of the IL-7 and IL-4 receptors (5, 6), findings simultaneously reported by a separate research group in Japan. Together, the findings indicated that IL-2Rγ was a common cytokine receptor γ-chain, or γ_c, present in multiple receptors. Subsequent studies added IL-9, IL-15, and IL-21 to the family of cytokines that share γ_c.

Around the time as the XSCID discovery, Leonard was investigating mechanisms of IL-2 signaling and turned his attention toward Janus family tyrosine kinases (JAKs), which are involved in cytokine signaling. In an article in Science (7), Leonard’s group demonstrated that JAK1 and JAK3 associate with IL-2Rβ and γ_c, respectively. The findings led to other studies in which he proposed the existence of JAK3-deficient SCID, a disease that mimics XSCID. Subsequently, his group identified JAK3-deficient SCID patients and correctly speculated that inhibitors of JAK3 would be immunosuppressive (8).

Furthermore, because reduced T cell development had been identified in mice deficient in IL-7, Leonard hypothesized that the T cell deficiency observed in SCID could be explained by defective IL-7 signaling. By analyzing SCID patients who had T cell deficiency but normal NK cell numbers, Leonard and his colleagues discovered IL7R-deficient SCID and demonstrated that defective IL-7 signaling explained the T cell defect in SCID. NK cell numbers, on the other hand, were normal in IL7R-deficient SCID.

“Our findings demonstrated that the IL-7 receptor was critical for the development of human T cells, but that it wasn’t contributing to the development of NK cells, because even in the absence of the IL-7R expression, NK cells develop normally,” Leonard says.

Eventually, researchers sorted out the NK cell conundrum, with reports that mice lacking IL-15 and IL-15 components were deficient in NK cells. IL-15 was shown to contribute to the development of human NK cells.

Leonard’s work in untangling the roles of γ_c family cytokines in the immune system has been relevant to the development of gene therapy for SCID. “When things that we discover have a direct clinically related ramification, that is also very exciting,” he says.

Inspiration in Interleukins

Leonard says that the fundamental questions in cytokine biology that drew him to the field still inspire his scientific interest. He has studied all six cytokines that share γ_c, but focused mostly on IL-2 and IL-21, another γ_c family cytokine, as well as on an IL-7–related cytokine, thymic stromal lymphopoietin, or TSLP. For example, his group has demonstrated the role of IL-2 in modulating T-helper differentiation, IL-21 in regulating Ig production and B cell function, and TSLP in allergic lung inflammation. Moreover, together with Christopher Garcia, a structural biologist at Stanford University, Leonard reported partial agonists for a cytokine, IL-2, paving the way toward potential therapeutic developments (9).

Leonard’s work has also helped describe the mechanisms behind other diseases. For example, Leonard’s research has implicated IL-21 in the development of type 1 diabetes, systemic lupus erythematosus, and experimental autoimmune uveitis, based on studies in mice. His studies have helped clarify the pleiotropic actions of IL-21 on other immune cells, including T cells and dendritic cells.

Furthermore, by using next-generation sequencing techniques, Leonard’s group has helped illuminate the molecular mechanisms of cytokine signaling. The group has focused on signal transducers and activators of transcription (STAT) proteins, which transmit signals from the cytosol to the nucleus in cells. His team has also uncovered molecular mechanisms underlying IL-2 and IL-15 signaling, including the demonstration of the biological importance of STAT tetramerization in vivo. In his Inaugural Article (10), Leonard and colleagues examined the binding of STAT5 and STAT3 to IL-2 and IL-21 superenhancers, which control the expression of genes that define cell identity. In particular, the researchers identified an IL-2–activated STAT5-bound superenhancer that regulates Il2ra, expanding the researchers’ understanding of the gene that Leonard cloned more than 30 years ago.

Striking a Balance

Leonard says he is tremendously appreciative of the many talented individuals with whom he has worked over the years, and that he gains fulfillment from the journey of discovery.

Outside the laboratory, Leonard meets a group of friends from his youth to play Bridge every month, and has a longstanding family tradition of playing Anagrams on holidays, when his daughters visit home. He also enjoys swimming, cycling, tennis, and running, and has completed nine full marathons, including the Chicago, New York, and Marine Corps Marathons.

Leonard’s interests extend to classical music, opera, books, theater, and attending lectures, including at the Politics & Prose bookstore and the Carriage House of the Mathematical Association of America in Washington, DC.