Treatment of Refractory BK Infections With Related Donor BK Specific Cytotoxic T-cells (CTLs)

This study tests BK-specific cytotoxic T lymphocytes (CTLs) — immune cells from a related donor that are trained to fight BK virus — to treat severe BK virus infections that haven't responded to standard antiviral therapy. BK virus is common and harmless in healthy people, but in immunocompromised patients (such as those who have received a stem cell transplant or organ transplant), it can cause serious bladder and kidney damage. Standard antivirals often don't work well in these patients, making this cell therapy a critical experimental option. Participants are patients with persistent or worsening BK infection after a transplant who have failed or are intolerant to conventional antiviral treatments. Donor-derived T cells are infused to help the immune system control the virus. You may be eligible if... - You have a BK virus infection after an allogeneic stem cell transplant, solid organ transplant, or due to a primary immunodeficiency - BK viral loads are rising or persistent despite at least 2 weeks of appropriate antiviral therapy, or you cannot tolerate antiviral medications - You are between approximately 1 month and 79 years old - Your performance status is above the minimum threshold (Lansky/Karnofsky >30%) You may NOT be eligible if... - You have acute graft-versus-host disease (GVHD) grade 2 or higher - You are on high-dose steroids (>0.5 mg/kg prednisone) at the time of infusion - You have had a donor lymphocyte infusion in the last 4 weeks - You are pregnant or breastfeeding - You have known HIV infection Talk to your doctor to see if this trial is right for you.