Man Learned His Cancer Was in Remission Before Officiating Wedding of Leukemia Survivor He Saved (Exclusive)

"He saved my life and now he's literally a family member,” Chris Costello, now 35, tells PEOPLE of Ed Suslovic

A life-saving bone marrow transplant from a stranger saved then-10-year-old Chris Costello when he had leukemia. When that man — who became a close family friend — was diagnosed with a different type of leukemia years later, Costello stepped up to return the favor.

“He was a complete stranger, and he saved my life, and now he's literally a family member,” Costello, now 35, tells PEOPLE of Ed Suslovic.

Although he never had to take Costello up on his offer, Suslovic learned his cancer was in remission in September, shortly before officiating Costello's wedding.

"It was just the most perfect day," says Costello.

Related: 9-Year-Old Who Had Leukemia Meets the Bone Marrow Donor Who Saved His Life

When Costello was diagnosed with acute myelogenous leukemia (a cancer of the blood and bone marrow) at the age of 8 in 1997, he says that "nobody in my family was a match" — nor were any of the 1,000 people in his hometown who volunteered to be tested.

However, as it turned out, Suslovic — who joined the bone marrow donor registry years earlier — was a match, and the transplant took place on June 2, 1998.

"If you're in a position to help someone out and give them a second chance at life, why wouldn't you?” Suslovic says.

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A year after the transplant, they were allowed to meet and became fast friends. “We clicked,” Costello says.

The two families visited each other multiple times a year and Suslovic’s children grew up thinking of Chris and his siblings as older brothers.

“There was just a bond, a connection,” says Suslovic, now 63, a former mayor and city council member of Portland, Maine. “We gained this whole other family.”

Related: Organ Donor Becomes Best Friends with Stranger Who Received Her Kidney: 'Most Amazing Journey'

As a sign of how close they are, shortly after Costello met future wife Chelsie Gardner in February 2018 he introduced her to Suslovic.

“From the first time I met him, he kind of just became my family as well,” says Gardner, 32.

After getting engaged, Costello asked Suslovic to serve as co-best man (with his brother Kevin) and officiate their wedding on Sept. 29, 2023.

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As their wedding day approached, Suslovic also leaned on Costello and his family after receiving a cancer diagnosis of his own.

Suslovic was attempting to donate a kidney to a friend two and a half years ago when doctors discovered a low platelet count during the screening process. On Jan. 13, 2021, he was diagnosed with hairy cell leukemia (a cancer of the white blood cells) and immediately reached out to the Costello family.

“They were just so there for me, and I felt the love,” Suslovic says. “Chris called me up right after he heard and basically said, 'Hey, I know a guy who's got a pretty good match for bone marrow if you need. We’re identical.'”

Fortunately, shortly before the wedding, Suslovic learned that his cancer was in remission.

Although Gardner tells PEOPLE that her husband wouldn't be here without Suslovic, he says that being called a “hero” embarrasses him.

“I didn't run into a burning building to save this guy. I just donated some bone marrow,” Suslovic says, going on to note how much it meant for him to be a part of the couple's special day. “For 25 years, to be part of his life, and then to walk down the aisle with him and then stand there and marry him and Chelsie, it's like a fairytale — only it's true.”

Related: Family of 5-Year-Old Girl with Rare Disease Fights to Get Her Life-Saving Bone Marrow Transplant

The lifelong friends are now sharing their story with the public in order to encourage others to join the DKMS registry and become stem cell donors.

Costello’s mother founded the non-profit Christopher’s Challenge, which has registered 4,000 people as potential blood stem cell donors through DKMS — and about 30 have actually become donors.

"We also want people to know that a cancer diagnosis, especially a leukemia diagnosis, is not a death sentence," says Suslovic. "Look at the two of us.”

To learn more about registering to be a blood stem cell donor or to request a free swab kit, go to www.dkms.org.

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