A Study to Learn About the Effects of Felzartamab Infusions in Adults With Kidney Transplants Who Have Late Isolated Microvascular Inflammation

In this study, researchers will learn more about a drug called felzartamab in people who have received a kidney transplant and later developed a condition called microvascular inflammation (MVI). MVI is a type of injury to small blood vessels in the transplanted kidney and may be a sign of rejection by the body. It can lead to serious kidney problems over time. In many cases, MVI is caused by antibodies that attack the transplanted kidney. But in some people, MVI happens without these antibodies. This type of MVI is called isolated MVI. There are currently no approved treatments for isolated MVI. The main goal of the study is to learn about the effect felzartamab has on inflammation in the transplanted kidney. The main question researchers want to answer is: • How many participants have no signs of active inflammation in the transplanted kidney after 24 weeks of treatment with felzartamab? Researchers will also study how felzartamab affects kidney function, immune activity, and overall health. They will monitor safety through kidney biopsies, lab tests, and by recording adverse events throughout the study. Adverse events are health problems that may or may not be caused by the study drug. The study will be done in 2 parts as follows: * Participants will be randomly assigned to receive either felzartamab or a placebo. A placebo looks like the study drug but contains no real medicine. * In Part A, participants will receive their assigned drug for 24 weeks. Neither the researchers nor the participants will know who is receiving felzartamab or placebo. * Part B will last another 28 weeks. All participants will receive felzartamab and both participants and researchers will know this. * All treatments will be given by intravenous (IV) infusion at the study site. * Participants will have kidney biopsies at the start of the study, at Week 24, and at Week 52 to help measure changes in inflammation. * Participants will stay in the study for about 1 year.