Arnold Caplan | Photo by Tammie Lee

Arnold Caplan, PhD, was a pioneering scientist and international legend.

That’s why, nine years ago, a middle-school teacher in São Paulo, Brazil, handed 13-year-old Franco Kraiselburd a Caplan study and said: “‘You cannot talk about mesenchymal stem cells (MSCs) without talking about Arnold Caplan.’”

Five years later, Kraiselburd became a Case Western Reserve undergraduate because of Caplan, and later began working in his Skeletal Research Center on campus.

“He taught me how to be a better person,” said Kraiselburd, a senior. “And that made me a better scientist than I could have ever imagined.”

Caplan, a biology professor and trailblazing researcher in the field of regenerative medicine and cell-based therapies, died in January after an illustrious career that included 54 years at CWRU.

He was the first to discover and describe MSCs, specialized cells with the ability to provide neighboring cells signals to remedy a range of problems. Caplan liked to call them the “drugstores for sites of injury and inflammation.” His work has improved the treatment of human diseases, ranging from COVID-19, multiple sclerosis and osteoarthritis to spinal cord injuries and cancer. He also held secondary appointments at CWRU’s engineering and medical schools.

Long supported by the National Institutes of Health and other government, nonprofit and for-profit agencies, he published nearly 500 peer-reviewed articles.

“He would remind me that only a small number of his grant applications or peer-reviewed manuscripts were accepted on the first try,” said Stephen Haynesworth, PhD (GRS ’87, biology), an associate professor of biology who was a PhD student in Caplan’s lab and later worked there as a research associate before joining the faculty. After receiving a rejection, Caplan “allowed himself one day to sulk,” Haynesworth added, “then it was back to work to revise the grant or manuscript for re-review. He was tenacious.”

Caplan also was supportive, inspiring, demanding, creative and charismatic. He was a legend who kept his door open to everyone; a brilliant scientist who crafted relatable stories to explain the most complicated research; and a supportive boss whose employees stayed 30 years or longer.

“Over 150 graduate students, postdoctoral fellows, medical students and visiting researchers from all over the world trained with Arnold,” Haynesworth said. “Arnold never recruited them; they would always seek him out.”

In the late 1980s, Caplan zeroed in on MSCs and their potential for making and repairing tissues, cartilage, bone and bone marrow.

One day, he walked into the campus office of Stan Gerson, MD, now dean of the CWRU School of Medicine, wondering if— just as certain stem cells could differentiate into different types of blood cells (red cells, white cells, and platelets)—could MSCs differentiate to make all of the skeletal and connective tissue of the body, such as cartilage, bone, or the fat cells in bone marrow?

The two quickly realized the enormous clinical applications. In 1995, they, along with Haynesworth and Hillard Lazarus, MD, now a CWRU professor emeritus, conducted the first clinical trial to determine the safety and feasibility of transplanting the cells into patients to regenerate bone marrow destroyed during cancer treatments.

Since then, more than 1,400 clinical trials have been conducted across the globe based on the MSC technology that Caplan and his colleagues developed, according to the U.S. National Library of Medicine.

“Arnie’s efforts across the world were incredible,” Gerson said.

In 1992, Caplan and Haynesworth founded a company, Osiris Therapeutics Inc., to develop stem-cell products to regenerate damaged or diseased tissue and treat inflammatory diseases. It was sold to a global medical-technology firm in 2019.

“Using Arnold’s MSC technology, Osiris developed the first stem-cell based drug to receive regulatory approval anywhere in the world,” Haynesworth said.

Caplan’s enormous impact continues. Kraiselburd, for example, co-founded a startup, Asclepii, using the stem-cell knowledge he’s gained to improve wound care.

“Everything I do with this company is largely thanks to [Caplan] and to his work,” Kraiselburd said.