RecruitingDrug trialSafety, Tolerability, and Efficacy of Immunomodulation With A Monoclonal Antibody Against CD40L in Combination With Transplanted Islet Cells in Adults With Brittle Type 1 Diabetes Mellitus (T1D)Tegoprubart (AT-1501) is a monoclonal antibody. Antibodies are Y-shaped proteins that are produced naturally by the subject's immune system to attack and fight foreign substances that cause illness. Monoclonal antibodies are man-made proteins manufactured to serve as substitute antibodies to fight diseases. Monoclonal antibodies can restore, enhance, or mimic (copy) the immune system's attack process; they can also tone down the immune system. Tegoprubart (AT-1501) is thought to work by dampening down the immune system so that it will be less likely to attack the transplanted cells. For other types of transplants, like kidney, a drug called a calcineurin inhibitor is usually used to prevent rejection. That class of drugs can be toxic to islet cells. Tegoprubart (AT-1501) is an experimental agent that is anticipated to prevent rejection without harming the islet cells.