This Phase 1 clinical study is designed to examine the safety and feasibility of using anti-CD3/CD28 activated marrow infiltrating lymphocytes (MILs) as treatment of relapse after allogeneic hematopoietic cell transplantation (alloHCT) for patients with hematologic malignancies with bone marrow involvement of their relapsed disease. These MILs will be derived from the bone marrow of the relapsed patient who had previously received post-transplantation cyclophosphamide (PTCy) as graft-versus-host disease (GVHD) prophylaxis (PTCy-MILs). A bone marrow aspiration will be performed on the patient to collect ~200ml of marrow for ex vivo expansion. During this expansion process, T cells will be activated and expanded by co-stimulation with anti-CD3/anti-CD28 monoclonal antibodies covalently attached to super-paramagnetic microbeads. Patients will be treated with salvage therapy while this ex vivo expansion is ongoing. After the simultaneous salvage therapy and ex vivo expansion, the activated PTCy-MILs will be reinfused. Patients will be monitored with the primary objective being the feasibility of expanding to targeted dose levels activated PTCy-MILs that do not cause grade III-IV acute GVHD within the first 90 days after PTCy-MIL infusion.