The purpose of this study is to test whether Lomecel-B™ works in treating patients with hypoplastic left heart syndrome (HLHS) and to gather additional information about the safety of Lomecel-B. Lomecel-B contains human mesenchymal stem cells (MSCs) as the active ingredient. MSCs are special cells in the body that are able to change into other types of cells, such as heart, blood, and muscle cells. MSCs are found in various tissues of the body, such as the bone marrow, which is the spongy tissue inside of your bones. Lomecel-B uses MSCs from bone marrow of unrelated young healthy donors. These are called "allogeneic", and do not require donor matching to the patient.