This study lasts about six months. Before starting, participants will have health checks, including a physical exam, blood tests, and other assessments. Some participants may also have a pregnancy test.

Participants will be placed into one of two groups by chance. One group will receive a treatment called remestemcel along with a medicine called ruxolitinib. The other group will receive a placebo, which has no active medicine, along with ruxolitinib. Neither the participants nor the study doctors will know which group each person is in.

Remestemcel or the placebo is given through a vein twice a week for four weeks. Some people may receive extra weekly doses depending on how well they respond. Ruxolitinib is taken by mouth twice a day for up to 56 days. During the study, participants will have regular visits, blood tests, health checks, and quality of life questionnaires. The goal is to learn if this treatment can help people with severe graft versus host disease after a blood or bone marrow transplant.

This study is for adults who have a serious condition called acute graft versus host disease. This can happen after a blood or bone marrow transplant when the new immune cells attack parts of the body. The study focuses on people whose disease is severe and has not improved with treatment using steroid medicines. Researchers are looking for better ways to help people whose graft versus host disease does not respond to standard treatment.

People can join this study if they are 18 years old or older and have a serious condition called acute graft versus host disease after a blood or bone marrow transplant. Their disease must be severe, Grade III or Grade IV, and must not have improved with steroid treatment. Researchers are studying new treatment options to help people whose graft versus host disease is not responding to standard medicines.

The purpose of this study is to learn if adding a treatment called remestemcel to a medicine called ruxolitinib works better than using ruxolitinib alone for people who have severe graft versus host disease after a blood or bone marrow transplant. This condition happens when donated immune cells attack the body and does not get better with steroid treatment. Researchers will compare people who receive remestemcel and ruxolitinib with people who receive a placebo and ruxolitinib. The goal is to find out if the combination treatment can better control the disease and improve health after transplant.